Minimalist design challenge:  200mA constant current? - Page 1
Forum section: electronics / projects

author: SilverSolder
author_rank: Super Contributor
timestamp: January 21, 2021, 06:46:25 pm
content: Any ideas for the most minimalist circuit that will limit a DC current to 200mA, with an accuracy of 20% being acceptable? The drop across the device will be max 4volts, and it should be able to keep working down to a drop in the range 0.2V - 0.5V or so.This is for a float charger project, not a commercial application.Kind of a cheapskate's challenge! Can a single transistor circuit do the job?

author: magic
author_rank: Super Contributor
timestamp: January 21, 2021, 07:42:39 pm
content: 0.2V without auxiliary power supply is going to be difficult, I think.Otherwise, TL431, two resistors and transistor easily gets you to 1%.editerm, I mean, with power supply and without the dropout requirement tooThere are lower voltage TL431 which will reduce dropuout to 1.25V and perhaps an NPN could go instead of TL431 for 0.6V and craptastic thermal stability.

author: ejeffrey
author_rank: Super Contributor
timestamp: January 21, 2021, 08:21:58 pm
content: A float charger circuit should easily have an auxillary supply available from whatever you are charging. A two terminal device is cooler but is going to cost you dropout voltage.

author: kallek
author_rank: Regular Contributor
timestamp: January 21, 2021, 08:26:45 pm
content: I used lm317 and resistor for my 200mA NiCd battery pack charger. But I didn’t have any dropout voltage requirements because I fed it from unregulated wall wart.

author: Ian.M
author_rank: Super Contributor
timestamp: January 21, 2021, 08:27:36 pm
content: You could probably abuse T.I'sTPS1H000-Q11Ω Smart High-Side Switch. It only needs one resistor to set its limiting current, with +/-15% accuracy.

author: mikerj
author_rank: Super Contributor
timestamp: January 21, 2021, 09:48:52 pm
content: A basic one transistor current source will likely work fine for this.

author: SilverSolder
author_rank: Super Contributor
timestamp: January 21, 2021, 10:11:03 pm
content: Separate supply is definitely acceptable if that simplifies things.The supply is 11VDC and charges 8x NiMH in series, which means the battery eventually reaches 11V - leaving not much voltage headroom for the regulator...

author: magic
author_rank: Super Contributor
timestamp: January 21, 2021, 11:14:40 pm
content: 11V won't charge them fully.I have built a very dumb charger for 8.4V NiMH batteries, using LM317 in constant current configuration and TL431 to turn it off at 10V. That gives 1.43V per cell and they could probably still take a bit more.I think Ian's chip looks promising.The maximum drop will be 11V, not 4V, if the cells are empty.By the way, people say that slightly overcharging NiMH batteries with multiple cells helps to "balance" them - the first one to charge turns into a "zener" and the others have a chance to reach full capacity. Not sure if it really is important. I haven't used my dummy charger much so far

author: rtm
author_rank: Contributor
timestamp: January 22, 2021, 12:43:13 am
content: How about two PNP transistors and a 47 ohm resistor? A simple current mirror as shown in the attached diagramshould work to charge the battery very close to the 11 volts of your supply.In the diagram - V2 represents your battery. LT spice shows it will be charging at about 210ma even when it is at 10.8 volts.

author: nigelwright7557
author_rank: Frequent Contributor
timestamp: January 22, 2021, 12:48:12 am
content: Simplest circuit is a short.But i have just used a resistor for a current source in an amplifier.Nowhere near perfect but worked in my application.

author: KaneTW
author_rank: Frequent Contributor
timestamp: January 22, 2021, 12:59:51 am
content: :vhttps://www.digikey.com/en/products/detail/diodes-incorporated/AL5809-100P1-7/6186966

author: SilverSolder
author_rank: Super Contributor
timestamp: January 22, 2021, 01:27:19 am
content: @nigelwright7557's single resistor has to be the winner of "simplest circuit that could possibly do the job", at the cost of giving very long charge periods. With an 11V supply for a 9.6V battery, a 22 ohm resistor might work, sort of kind of. Might be worth a try, just for fun, it is an easy enough circuit to build!@Ian.M's TPS1H000-Q1 chip suggestion looks very cool - it seems to need a 2.5V drop across it for the current limiter to work accurately (not sure what happens when the drop is smaller - may or may not be a problem). The chip looks interesting in its own right, so I might get some to play around with.@rtm's current mirror idea also looks elegantly simple, but will discrete transistors work well in this kind of circuit? The one carrying the current would get warmer than the reference, may be an issue?@kallek's suggestion of a LM317 in constant current configuration is very simple too, it requires a drop of 2V - if that is available, it looks very clean.@magic's idea of adding a TL431 to throttle a LM317 at a set voltage sounds almost high end in this context, but is still low component count and added protection.@KaneTW's suggestion of AL5809 two terminal LED driver chips is also very good, they also need a drop of 2.5V to work so are not usable without changing the supply - perhaps two in parallel would provide the 200mA needed.Thanks for the suggestions!

author: fourfathom
author_rank: Super Contributor
timestamp: January 22, 2021, 02:12:12 am
content: I used this circuit for a linear LED driver, where overhead voltage (and zero EMI) was at a premium. It's VBEsensitive, with the expected -20mV/deg C temperature variation, but it was more than adequate for my needs. It falls out of regulation at about 0.6V headroom, where the current is determined by the 3.3 Ohm emitter resistor.I have to think about what happens when the 11V supply is shut down -- it looks like the FET drain is pushed negative by the charged battery. Might be a problem, I guess it depends on the power supply off-impedance. A diode across the FET might be advisable.I played around with a single-transistor design, using a red LED (about 1.6V) for a base bias, but it fell out of regulation sooner (no surprise there).

author: ledtester
author_rank: Super Contributor
timestamp: January 22, 2021, 04:44:43 am
content: Quote from: SilverSolder on January 22, 2021, 01:27:19 amI'm thinking the voltage drop would be more like 3.25V... 2V between input and output + 1.25V between output and adjust.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: January 22, 2021, 04:46:53 am
content: And you can make do with less drop using a diffamp,but we're well into enough parts count where an integrated solution looks better. (On the ~12V supply available, no need to bother with the D1 gate voltage limiter.)Or make a dumb current mirror. Bit dubious at 200mA, but the saturation is nice and low. If it can be compensated for stability over the desired range, it'll do.Alternately, set a 13.6V limit and be done with it?Tim

author: MK14
author_rank: Super Contributor
timestamp: January 22, 2021, 05:19:07 am
content: Quote from: SilverSolder on January 21, 2021, 06:46:25 pmIf you insist, and I'm allowed to (lazily) describe a simple circuit with words.Use an old Germanium transistor from something, which switches around 200mV (approx, maybe up to 300mV), so its b.e. junction is across a current sense resistor (200mV at 200mA = 1 Ohm). The Germaniums transistor's collector then pulls a FET's gate's off. The FET (as others have drawn), has something like a 10K resistor normally turning the FET on.Hopefully giving you the desired 20% accuracy, but Germanium transistors were not particularly stable, especially as regards temperature stability. So this suggestion, is just for curiosity, rather than practicable application.Hopefully, that will give you a 200mV to 300mV min voltage dropout, before the current drops off, the 20% accuracy, and ALMOST a single transistor (ok, there would be 2).EDIT: Untested/not-simulated yet, rough diagram added.EDIT2: Could do with a schottky diode in the charge current path, to stop the under charge battery, from trying to power the charging circuit, when the main supply is off or disconnected.EDIT3: Tip: If anyone does try to experiment with this circuit. R2/10K, might need to be lowered, because Germanium transistors, can have higher leakage currents, than more modern Silicon ones. Depending on what transistor you actually use and its datasheet, obviously.

author: magic
author_rank: Super Contributor
timestamp: January 22, 2021, 07:20:49 am
content: Quote from: SilverSolder on January 22, 2021, 01:27:19 amDoubling of output current for each 10°C of Tj difference, plus some small extra from Early effect.With (many) more transistors, perhaps the LT1236 precision current mirror could be builtQuote from: SilverSolder on January 22, 2021, 01:27:19 amYou don't need voltage limiting on 11V supply but you need about -1V dropout voltage for full charge (yes, that's negative).

author: fourfathom
author_rank: Super Contributor
timestamp: January 22, 2021, 07:31:42 am
content: Quote from: MK14 on January 22, 2021, 05:19:07 amNice suggestion of the Ge transistor, otherwise it's the same circuit as in my previous post:Quote from: fourfathom on January 22, 2021, 02:12:12 amYes, a diode in series with the battery is a very good idea, but of course it eats into the voltage overhead.

author: MK14
author_rank: Super Contributor
timestamp: January 22, 2021, 07:37:01 am
content: Quote from: fourfathom on January 22, 2021, 07:31:42 amI wasn't trying to copy your circuit. But I was trying to meet the design specification, of very low dropout voltage and only using one single transistor (but I used 2).Some of the latest Schottky diodes (that I bought fairly recently), actually have surprisingly low dropout voltages. Which seem to be a nice improvement, on older ones. From memory, it was in the 60mV to 100mV range (but I could be mis-remembering and it depends on the current).But anyway, it was amazingly (to me at least) low.But you are right, it would increase the voltage drop, a (hopefully) small amount.EDIT: I'm probably mis-remembering or thinking of the typical values, rather than max Vf. I suspect it was more like 200 to 250mV.

author: Ian.M
author_rank: Super Contributor
timestamp: January 22, 2021, 07:55:56 am
content: Quote from: SilverSolder on January 22, 2021, 01:27:19 amhttps://www.ti.com/lit/an/slva859b/slva859b.pdfhas more details of the TPSxHxxx series of high side switches' current limit behavior and implementation. However hands-on testing would definitely be required, which is why I presented my suggestion as: Ithinkyou canabuse...YMMV.There's plenty of supply voltage available, so the classic OPAMP, MOSFET + low-ohm sense resistor current sink configuration shouldn't be excluded from consideration. That's only two semiconductors and six passives (including loop compensation and gate-stopper), assuming the supply stability is good enough for a simple potential divider to be used for the reference voltage.

author: fourfathom
author_rank: Super Contributor
timestamp: January 22, 2021, 08:05:13 am
content: Quote from: MK14 on January 22, 2021, 07:37:01 amI didn't think so, and it's hardly "my circuit" -- it's been used all over the place, although I first saw it with a bipolar transistor as the pass element (before MOSFETS were invented). I do appreciate the elegance of the topology.

author: MK14
author_rank: Super Contributor
timestamp: January 22, 2021, 08:32:21 am
content: Joke answer follows:$0.03 Padauk MCU, must be one with A2D input(s). 0.5 (0.47)current sense resistor, MCU via A2D pin monitors the current, and controls a FET, it also times the charge operation and times out, even if the current doesn't drop. It could even use a PWM waveform, to trickle charge it, once it is fully charged up.It could have a RGB Led, Green for charging, Red for bad batteries, flashing amber to signal an error condition.It could activate a buzzer, when the charging is completely finished.Charging dropout, only 100mV (current sense resistor) + 10mV approx, for the FET.

author: magic
author_rank: Super Contributor
timestamp: January 22, 2021, 10:04:34 am
content: Quote from: Ian.M on January 22, 2021, 07:55:56 amIf you want to venture into such crazy levels of complexity as 8 pin ICs, use LM10 and a MOSFET or some other pass transistor.

author: Zero999
author_rank: Super Contributor
timestamp: January 22, 2021, 11:26:42 am
content: Here's how I've done it in the past. This circuit gives about 350mA, with a 12V supply. It's easy to modify it for lower voltages/currents. Extra parts could be added to make the current through R2, thus the voltage drop on D1 & D2, less dependant on the supply voltage.

author: strawberry
author_rank: Super Contributor
timestamp: January 22, 2021, 01:05:34 pm
content: current mirror source

author: magic
author_rank: Super Contributor
timestamp: January 22, 2021, 03:18:43 pm
content: I think this could be improved with a resistor or twoDon't ask for plots versus input voltage, it uses it as a reference

author: strawberry
author_rank: Super Contributor
timestamp: January 22, 2021, 03:44:44 pm
content: R1 to zener

author: SilverSolder
author_rank: Super Contributor
timestamp: January 22, 2021, 03:56:41 pm
content: In LTSpice, is it possible to sweep the temperature of just one component independently, while leaving the other components at a constant temperature?Or even sweeping two different components between two different temperature end points?

author: SilverSolder
author_rank: Super Contributor
timestamp: January 22, 2021, 04:05:30 pm
content: Quote from: MK14 on January 22, 2021, 08:32:21 amIf you treat e.g. an Arduino Mini as "one component", making a charger out of one of those doesn't seem like a big project even for a one-off. Using a micro is obviously a very flexible solution in terms of easily adding additional features, as you said. It is not exactly a minimalist solution, but it is not a crazy idea - if you were making a "nice" charger, this would be an obvious way to go.

author: magic
author_rank: Super Contributor
timestamp: January 22, 2021, 04:11:21 pm
content: http://www.audio-perfection.com/spice-ltspice/parametric-sweep/individual-component-temperature-setsweep-in-ltspice-simulation.htmlAppend it to the model number to make it visible on the schematic. LTspice is pretty dumb, everything is just text as far as it cares

author: SilverSolder
author_rank: Super Contributor
timestamp: January 22, 2021, 05:03:48 pm
content: Quote from: magic on January 22, 2021, 04:11:21 pmCould be interesting to sweep Q1 and Q2 separately, to see how sensitive this is to temperature differences between the two transistors in the mirror?

author: David Hess
author_rank: Super Contributor
timestamp: January 22, 2021, 05:35:57 pm
content: A single depletion mode power MOSFET and resistor can do it. A part like the BSP129 H6906 might be suitable.

author: SilverSolder
author_rank: Super Contributor
timestamp: January 22, 2021, 05:48:58 pm
content: Quote from: David Hess on January 22, 2021, 05:35:57 pmThat is an interesting thought - the Rds of those two is too high, though - would drop > 1V at 200mA?

author: magic
author_rank: Super Contributor
timestamp: January 22, 2021, 06:08:56 pm
content: Quote from: SilverSolder on January 22, 2021, 05:03:48 pmLemme guess, 2mV/°C(Spice agrees, btw).That's why I used the conservative 1Ω shunt instead of going for something crazy like 0.1Ω.

author: SilverSolder
author_rank: Super Contributor
timestamp: January 22, 2021, 06:17:18 pm
content: Quote from: magic on January 22, 2021, 06:08:56 pmSo it doesn't get "amplified up" by the circuit? - in which case, it is obviously not an issue here!

author: magic
author_rank: Super Contributor
timestamp: January 22, 2021, 06:23:07 pm
content: If you want something completely ludicrous, I wonder if it would be possible to replace that transistor pair with LM334But then charging current will be proportional to absolute temperature of the LM334, so expect 10% variation over 30°C.By the way, didn't National produce a current source like LM334 but with built-in thermal compensation, by any chance?Quote from: SilverSolder on January 22, 2021, 06:17:18 pmIt's just a voltage divider producing 200mV across R2 and Q1,Q2 operate at roughly equal currents so offset between their emitters is maybe a few mV (and varies somewhat with temperature of Q3 because Q3 affects Q2 collector current). If voltage across the shunt is by any chance too small, it pulls more current from Q2 which is multiplied by Q3. It's unity gain.

author: David Hess
author_rank: Super Contributor
timestamp: January 22, 2021, 06:32:10 pm
content: Quote from: SilverSolder on January 22, 2021, 05:48:58 pmA lower Rds part then, but they are available. There is a tradeoff there because lower Rds parts will require a larger series resistor to provide the proper negative gate voltage.

author: SilverSolder
author_rank: Super Contributor
timestamp: January 22, 2021, 07:04:39 pm
content: Quote from: David Hess on January 22, 2021, 06:32:10 pmIt is definitely the most minimalist solution after the single resistor circuit! The downside (in this case) is requiring a higher supply voltage than what is available, but it obviously provides much better regulation than a resistor.

author: magic
author_rank: Super Contributor
timestamp: January 23, 2021, 09:30:00 am
content: Quote from: magic on January 22, 2021, 06:23:07 pmCan't find anything like that, no idea why I thought it existed.But it looks like the idea of using LM334 isn't exactly new.https://www.electronicdesign.com/technologies/analog/article/21805826/whats-all-this-lm334-stuff-anyhow

author: SilverSolder
author_rank: Super Contributor
timestamp: January 23, 2021, 07:28:34 pm
content: Quote from: magic on January 23, 2021, 09:30:00 amThe LM334 circuit has only three components, and seems to only need a headroom of 150mV to maintain full regulation. Furthermore, this circuit appears to block reverse current as well (prevents the charged battery from sending current backwards into the source when the source is powered off) without adding a diode in series with the supply.If it can be made to work at 200mA - this may be the simplest circuit that meets the entire wish list!Edit: The circuit is actually in the LM334 data sheet, which adds an extra RC filter for stability reasons. So, five components, not three!

author: magic
author_rank: Super Contributor
timestamp: January 24, 2021, 01:32:17 am
content: Quote from: SilverSolder on January 23, 2021, 07:28:34 pmYes, but I'm afraid LM334 will pull 10mA from the battery through the pass transistor in absence of external power. P-MOS eliminates this problem, but at the cost of backfeeding to the upstream rail.editPNP will also backfeed if its BE junction zeners.Quote from: SilverSolder on January 23, 2021, 07:28:34 pmI think just a capacitor may suffice if high frequency performance is unimportant.

author: SilverSolder
author_rank: Super Contributor
timestamp: January 24, 2021, 05:43:34 am
content: Quote from: magic on January 24, 2021, 01:32:17 amI'm not understanding how the LM334 will draw anything from the battery when Vin is zero (short to ground)?

author: magic
author_rank: Super Contributor
timestamp: January 24, 2021, 09:31:29 am
content: Trough the pass transistor. It does contain some parasitic diodes

author: Zero999
author_rank: Super Contributor
timestamp: January 24, 2021, 10:21:18 am
content: The LM334 does block reverse current, but the BJT will be destroyed, if the input is zero and output above 5V. If the additional voltage drop of a diode is unacceptable, two P-MOSFETs connected back-to-back, as a pass element, will provide reverse blocking.

author: magic
author_rank: Super Contributor
timestamp: January 24, 2021, 11:13:52 am
content: The discharge is going to be forward current through the LM334.LM334 notices that shunt voltage drops below 64mV (zero, negative, doesn't matter) so it sinks battery current through PNP base. The PNP will even turn on in reverse mode and drive more discharge current through any load / short on the input rail. This is in addition to its BE junction forming a shunt reference with power rail load and the battery's ESR.

author: Zero999
author_rank: Super Contributor
timestamp: January 24, 2021, 12:54:40 pm
content: Quote from: magic on January 24, 2021, 11:13:52 amWhere is the return current path through the LM334 to the PNP base? According to the internal schematic, all PN junctions inside the device will be reverse biased, if the polarity were reversed.

author: SilverSolder
author_rank: Super Contributor
timestamp: January 24, 2021, 02:14:57 pm
content: It probably depends where we connect the battery being charged?Position A:This position has the minimum overhead voltage, but it is probably destructive if +Vin is disconnected or shorted to -Vin?Position B:The other option is to connect the battery being charged in series with the supply (but opposite polarity). Here, we have a true polarity reversal of the whole circuit, so the LM334 should block the current. We still have a Zener problem and now we also have a 1Volt overhead to contend with.

author: SilverSolder
author_rank: Super Contributor
timestamp: January 24, 2021, 02:55:33 pm
content: Does this have any chance of working? The min drop would be something like 150mV in the transistor, plus 250mV across the diode, total overhead 0.4V ?

author: MK14
author_rank: Super Contributor
timestamp: January 24, 2021, 03:31:19 pm
content: Quote from: SilverSolder on January 24, 2021, 02:55:33 pmNot sure if it would work, but...What about the additional voltage drop from the Rset resistor ?Also, the 2N3906 seems to be more like (Fairchild datasheet, graphs towards bottom of it) 300mV at 200mA. Also, 200mA is its Absolute maximum rating, so probably better to choose a part which can safely handle the current (and at a lower voltage drop at that current).http://www.mit.edu/~6.301/2N3906f.pdf

author: magic
author_rank: Super Contributor
timestamp: January 24, 2021, 03:39:07 pm
content: 3906 is no good, it's gotta be TO126/TO220 for heatsinking alone.Schottky may work, but they are more leaky than silicon diodes so check the datasheet.If cost is no object, I would consider if it's possible to use LM10 such that the reference regulates current and the opamp implements ideal diode. Two back to back N-MOS (or maybe P-MOS?) would act as switch / pass element. Ideally, the IC would turn the switch off and shut itself down when backfeeding is detected. Or maybe it could be left running on battery power if 270µA isn't a big deal.

author: SilverSolder
author_rank: Super Contributor
timestamp: January 24, 2021, 03:57:56 pm
content: Quote from: MK14 on January 24, 2021, 03:31:19 pmRset only drops 64mV at the set current, that's one of the really cool things about the LM344. Rset would be something like 13 milliohms for 200mA.

author: MK14
author_rank: Super Contributor
timestamp: January 24, 2021, 04:13:59 pm
content: Quote from: SilverSolder on January 24, 2021, 03:57:56 pmThat's nice and low.I think there are a number of alternative transistors, which will give nice low saturated voltage drops, at 200mA's.

author: SilverSolder
author_rank: Super Contributor
timestamp: January 24, 2021, 04:38:53 pm
content: Here's the back-to-back PMOS version of the 200mA charging circuit.1) Parts count = 5 assuming a dual PMOS.2) Is this the lowest possible dropout solution? Could be less than 100mV with a low enough Rds(on)3) Removing the supply voltage or shorting it: No problem, no current flows (the battery is blocked by the lower PMOS M2)4) Reversing the supply voltage: No problem, no current flows (blocked by the LM334 and the lower PMOS M2)5) Removing the battery from the circuit: No problem. Current through the LM334 is limited by the 100K resistor to ~120uA at 11VOverall, pretty good performance for five parts - assuming it works!

author: magic
author_rank: Super Contributor
timestamp: January 24, 2021, 04:58:34 pm
content: LM334 will not turn off the switch if you cut power in the middle of charging, it will keep operating on battery power. 100kΩ will limit discharge current through that branch, however.And I think Rset needs to be 0.33Ω.editIt's M1 which blocks backfeeding, M2 conducts in reverse through its body diode. And in order for this to work, the gates need to be made high while the input voltage is low or open circuit (and then it's driven high by the FETs if they aren't closed yet).

author: SilverSolder
author_rank: Super Contributor
timestamp: January 24, 2021, 07:07:06 pm
content: Quote from: magic on January 24, 2021, 04:58:34 pmYes, the PMOS idea has surprisingly many problems, another issue is the maximum Vgs that they can handle. The dual PMOS transistors I have in my jumble box can't handle more than 8V.Can we crack the problem with a Sziklai pair? At least the parts for that are easy to find....1) Parts count = still 52) Dropout should be quite low, perhaps 200mV?3) Removing the supply voltage or shorting it (battery inserted): No problem, no current flows. Q2 blocks and can't zener (blocked by Q1)4) Reversing the supply voltage (battery inserted): No problem, no current flows (blocked by the LM334 and Q2/Q1)5) Removing the battery from the circuit: No problem. Current through the LM334 is self limited at 10mA.6) Reversing the supply voltage (battery removed): No problem, no current flows (blocked by the LM334)

author: magic
author_rank: Super Contributor
timestamp: January 24, 2021, 09:09:38 pm
content: Sziklai/Darlington pairs have more than 1Vbe of forward drop.You still keep forgetting that BC junctions are diodes tooI think your easiest ways of solving shorted supply are, in order:1. just let the battery boil2. fuse it (bonus: use the fuse as shunt, may or may not be possible)3. diode or schottky, pick some compromise between Vf and Ir4. dual-MOS ideal diode5. (not serious) IGBT as pass device, IIRC they withstand 11V reverse, but conduction loss is poor by low voltage standardsThe 10mA for LM334 is given as absolute maximum, I'm not sure if the device is capable of self-limiting to a safe value or of self-destruction.

author: gnuarm
author_rank: Super Contributor
timestamp: January 24, 2021, 11:34:22 pm
content: Quote from: magic on January 24, 2021, 04:58:34 pmSeems to me the easy fix for cutting the back feed is one FET and a diode instead of two FETs. Or is that too much voltage drop for the requirements?So remove M1 and insert a diode in the line from Vin

author: SilverSolder
author_rank: Super Contributor
timestamp: January 25, 2021, 12:48:14 am
content: Quote from: magic on January 24, 2021, 09:09:38 pm

author: fourfathom
author_rank: Super Contributor
timestamp: January 25, 2021, 02:33:15 am
content: So now what happens when we connect the load? (And *where* do we connect it?)Say we connect the load between batt+ and Vin- (because we want the supply and the load to share a common ground). This includes Rset in the load current loop, which may be OK (my boat has a small current-sense shunt between battery- and system ground). But if we do this, and we disconnect Vin, the load current drives the LM334 sense pin negative relative to Vin-. As long as the load current is less than the design battery charging current, the sense pin voltage probably won't go enough negative to matter, but is this a concern?We could connect the load from Batt+ and Batt- but then something has to float.And do we want the Vin supply to be able to power the load if the battery is missing or discharged?We can add steering/isolation diodes, but it's still messy. As is often the case.

author: magic
author_rank: Super Contributor
timestamp: January 25, 2021, 08:43:03 am
content: Actually, what are the ultimate objectives here, winning a bet that you will charge a battery with 5 parts?Sadly, I can think of only two transistors that can block reverse 12V. One is aforementioned IGBT (except those with freewheeling diodes), the other is "high Vebo" BJT. This thread reminded me that I have seen such things in the past and I managed to track down one PNP. It appears that Sanyo made a few more types like that. Sure enough, obsolete, like most nice transistorsBTW, the "high Vebo" transistor would still supply current to the LM334 and would still multiply LM334 current into the power supply by its reverse beta, but it would be usable with some smarter controller which detects loss of power and stops sinking. And it wouldn't withstand supply reversal if the battery is fully charged. That's something I would be tempted to solve the usual way: antiparallel diode and fuse.

author: SilverSolder
author_rank: Super Contributor
timestamp: January 25, 2021, 04:18:40 pm
content: Quote from: magic on January 25, 2021, 08:43:03 amThe backdrop to this is: I replaced all the old NiCad cells in three battery packs for a portable drill with higher spec NiMh ones, about eight years ago. The charger that came with the tool is crude, it is basically an 11V source limited by a resistance to about 400mA. Since NiMH is less tolerant of abuse than NiCad, I want to reduce the current to 200mA... in a minimalist way, as befits a minimalist system!The minimalist solution has already been presented in this thread: add a resistor in series, job done, and it is no more crude than what is already there.But then the discussion evolved into how the crude system could possibly be improved.I'm quite surprised by how difficult that actually is! The constraint of the power supply voltage itself setting the limit for the battery voltage is a difficult one to work around. Then there are all the considerations of having it react safely to user errors like reversing polarity etc., while using a "reasonable" number of parts.It struck me that given the constraints on battery voltage, maybe what we need is a SHUNT regulator - that simply bypasses any current in excess of 200mA through the battery!In this circuit, the LM334 is made to "steal" any excess current and dumping it through Rbypass, leaving 200mA to always run through the battery. No drop at all!The diode D1 protects against reverse polarity, this time without "getting in the way".Obviously this circuit needs more work, but the idea may be usable!QuoteThe "high Vebo" transistor idea sounds very cool, I didn't know they had existed! - it is sad to see how the discrete components ecosystem is shrinking... Hopefully it is made up for by all the cool ICs that exist nowadays!

author: bson
author_rank: Unknown
timestamp: January 25, 2021, 08:05:55 pm
content: Quote from: MK14 on January 24, 2021, 04:13:59 pmThe voltage drop across Q1 E-C is going to be effectively Vin+ - Vin- - Vbat - Vf1 where Vf1 is the Schottky diode's forward drop. Otherwise the emitter current can't be 200mA. This in turn tells you that Q1 isn't going to be operating in saturation. Which makes perfect sense since the voltage has to be droppedsomewhere, and it's going to be in Q1 since that's the only controllable quantity.

author: MK14
author_rank: Super Contributor
timestamp: January 25, 2021, 08:22:52 pm
content: Quote from: bson on January 25, 2021, 08:05:55 pmWhat you are saying sounds reasonable, I can believe it wasn't going to be in saturation or work properly.The earlier circuits, maybe suffered from issues, because the finer details of the specification, only seem to have been released in the last few posts by the OP.But there are now too many different circuits floating around in this thread, changes to them with text, some correct facts/interpretations and a number of mistakes/misinterpretations.Resulting in me being too confused as to which circuit(s) we were talking about, to try and give you a better and more detailed response.

author: SilverSolder
author_rank: Super Contributor
timestamp: January 25, 2021, 08:31:58 pm
content: Quote from: bson on January 25, 2021, 08:05:55 pmWhat happens as the battery voltage approaches the supply voltage? It seems to me that Q1 has to enter saturation at that point, as the LM344 draws max current in its attempt to get 200mA to appear over Rsense?

author: SilverSolder
author_rank: Super Contributor
timestamp: January 25, 2021, 08:34:20 pm
content: Quote from: MK14 on January 25, 2021, 08:22:52 pmIt seems to me he may be talking about this version:

author: MK14
author_rank: Super Contributor
timestamp: January 25, 2021, 09:34:01 pm
content: Quote from: SilverSolder on January 25, 2021, 08:34:20 pmIn that case. My response is I see Q1 as saturating, by the 10mA through its base via the current regulating IC (strictly speaking it would probably need a 10 : 1 ratio, to really reach most datasheet level of specifications to saturate, i.e. 20mA (for 200mA), but I would have thought 10mA should be near enough, some rarely specify bigger ratios for saturation, anyway, such as 20 : 1, if I remember correctly).

author: MK14
author_rank: Super Contributor
timestamp: January 25, 2021, 09:44:08 pm
content: (Another) Possible simplest, working solution.A 1.5 V, at around 300mA torch bulb. In series with the batteries.Such as these:https://www.ebay.co.uk/itm/Qty-5-1-5V-300mA-M-E-S-E10-Filament-Torch-Indicator-Light-Bulb-Lamp-/110877646220Some, only have a burn life of around 20 hours, but others can have much longer burn lifes, such as 2,000 or 5,000 hours. I'd suggest either getting one with a long life or under-volt-aging one (e.g. 2.2V bulb), to get a decent life.The idea is a bulb is almost a short-circuit, when cold. Something like a tenth of the resistance, when cold.So, the voltage drop could be less than the desired 200mV. As current begins to flow, the bulb will open up, and naturally limit the current.As the batteries charge up and use less and less current, the bulb will get cooler and cooler, constantly droping its resistance, and hence reducing the voltage drop across it.Back-feed shouldn't be a problem, because the charger should naturally limit or stop, that from happening, as the series connected bulb, should make little or no difference.In the worse case scenario, you only would have a bulb, which will naturally keep the current to safe (fire) levels, or even pop open like a fuse, if something silly occurs (the charger short-circuits itself).Careful selection of a suitable bulb, may be required.

author: exe
author_rank: Unknown
timestamp: January 25, 2021, 09:57:29 pm
content: Nobody proposed a jfet? You just need to find a right one. Small current can be trimmed down a little bit with a source resistor. I think drop of 0.5 is achievable.

author: Zero999
author_rank: Super Contributor
timestamp: January 25, 2021, 10:08:42 pm
content: Quote from: exe on January 25, 2021, 09:57:29 pmDo you know of any J-FETs rated to 200mA?How about an op-amp & reference IC, such as the NCP4300A, or TSM103W, in the usual current sink configuration? A potential divider can be added to the reference to reduce the sense voltage and if back-feeding is an issue, then use back-to-back MOSFETs.https://www.mouser.com/datasheet/2/308/ncp4300a-d-304808.pdfhttps://www.mouser.co.uk/datasheet/2/389/tsm103w-1852784.pdf

author: Gyro
author_rank: Super Contributor
timestamp: January 25, 2021, 10:19:09 pm
content: Quote from: Zero999 on January 25, 2021, 10:08:42 pmYes, you can (or could - I saw a bankruptcy reference) get SiC JFETs up to 1200V 30A from specialist manufacturers.https://www.businesswire.com/news/home/20110729005302/en/SiC-JFETs-SemiSouth-target-high-end-audioHardly the cheapest Minimalist design though!Quotehttps://www.eevblog.com/forum/projects/daves-ucurrent-in-the-uk-and-also-low-cost-precision-ohmmeter/msg3431500/#msg3431500?

author: SilverSolder
author_rank: Super Contributor
timestamp: January 25, 2021, 11:12:52 pm
content: Quote from: MK14 on January 25, 2021, 09:44:08 pmNow that's thinking outside the box!Single component, with a regulating function! If it can be made to work...

author: T3sl4co1l
author_rank: Super Contributor
timestamp: January 26, 2021, 04:18:08 am
content: Plenty!... well, you could, and still can if you don't mind ordering up a reel at a time say.https://www.digikey.com/en/products/detail/fairchild-semiconductor/J105/13510308Not that the power rating is there.But, JFETs have terrible consistency in Vpo. More useful as a low-Rds(on) switch, or low noise audio amp.Depletion MOS has already been mentioned, and does better (but still needs calibration to do well). You aren't going to get much simpler than two parts. Which, let's see, oh nice, BSP149 is even graded by voltage, so you can have a pretty tight design off the bat, considering. Pretty fast, too. DN2625 pretty good too.For bidirectional current limiting, just put two in anti-series -- the same source resistor can even be shared, saving a component.Tim

author: Zero999
author_rank: Super Contributor
timestamp: January 26, 2021, 09:52:24 am
content: Quote from: Gyro on January 25, 2021, 10:19:09 pmYes, just a little overkill. Those look interesting. It seems like they're designed to be used in enhancement mode, which is odd for a J-FET, as the gate will be forward biased, so the gate current will be relatively high.QuoteYes, that's what I was suggesting, but with an op-amp IC with a built-in reference, to reduce the part count.

author: SilverSolder
author_rank: Super Contributor
timestamp: January 26, 2021, 02:36:02 pm
content: Quote from: Zero999 on January 26, 2021, 09:52:24 amIsn't that what an LM334 essentially is? (except further optimized to not require a power supply connection)

author: magic
author_rank: Super Contributor
timestamp: January 26, 2021, 03:46:12 pm
content: Quote from: SilverSolder on January 25, 2021, 04:18:40 pmLOL, this might work, although still no protection of the battery from reverse supply. But is it really necessary if the PSU is integrated in the charger (presumably)?Is it possible to hack the PSU's feedback loop?Quote from: SilverSolder on January 26, 2021, 02:36:02 pmIt's this "optimization" why it cannot detect when power goes outeditYeah, and your shunt version still discharges the battery because of that.

author: magic
author_rank: Super Contributor
timestamp: January 26, 2021, 04:01:57 pm
content: Quote from: Zero999 on January 25, 2021, 10:08:42 pmThese are potentially useful low cost alternatives for LM10But I see you get what you pay for when it comes to ease of use and power efficiency of the reference.

author: SilverSolder
author_rank: Super Contributor
timestamp: January 26, 2021, 05:29:50 pm
content: Quote from: magic on January 26, 2021, 03:46:12 pmThat is what I was referring to when I said "needs more work"! - I'm not sure that problem can actually be solved without complicating things a lot.I will try some measurements on the supply - it is very crude, but likely does have a diode to prevent reverse flow. I doubt there is anything more sophisticated than a diode and a resistor in there, but let's see, I could be wrong!

author: SilverSolder
author_rank: Super Contributor
timestamp: January 26, 2021, 05:32:16 pm
content: Quote from: magic on January 26, 2021, 04:01:57 pmI do have a bunch of LM10 chips here, eBay bargains. - one of them could obviously be pressed into service here, but I still don't see why it would be better than an LM334?

author: exe
author_rank: Unknown
timestamp: January 26, 2021, 05:52:50 pm
content: BTW, there arehttps://www.st.com/resource/en/datasheet/ts4436.pdf, which is, afaik, like tl431, but with 0.6V reference (min supply voltage 1.7V) . Or one could make ASIC for this application....

author: magic
author_rank: Super Contributor
timestamp: January 26, 2021, 06:15:16 pm
content: Nice little chipQuote from: SilverSolder on January 26, 2021, 05:32:16 pmIf you are lucky it might prove to be possible to achieve two things:- power it from the PSU, so that it doesn't drain the battery when not charging- use the second opamp for ideal diodeThat would be an ultra high end design

author: Terry Bites
author_rank: Super Contributor
timestamp: January 26, 2021, 06:27:37 pm
content: The answer to a maidens prayerhttps://www.analog.com/en/products/lt3085.html#so far as minimalist goes but a terrifying £3.50

author: fourfathom
author_rank: Super Contributor
timestamp: January 26, 2021, 06:37:16 pm
content: Someone had to do it:A PIC10(L)F320/322 (six pins), driving a low-gate-voltage power NMOS FET on/off duty-cycle, using the PIC 8-bit ADC (internal reference) as feedback. The blue LED provides 3.2V to the PIC.The PIC has an 8-bit ADC, with a 1.024V internal reference, giving a 4 mV ADC resolution. The current-sense resistor can easily be be 0.1 Ohms or so.I accidentally drew the battery backwards. Perhaps some additional fault protection components are required.Not my preferred way to solve the problem, but what the hell...

author: MK14
author_rank: Super Contributor
timestamp: January 26, 2021, 06:45:49 pm
content: Open the charger, submit photograph of PCB electronics here, in case of difficulty.Change (probably double), charge current limiting resistor.Job done!Quote from: SilverSolder on January 25, 2021, 11:12:52 pmNow it's thinking/tinkeringINSIDEthe box!

author: RoGeorge
author_rank: Super Contributor
timestamp: January 26, 2021, 07:13:08 pm
content: 3 transistors 1 resistor.

author: BrianHG
author_rank: Super Contributor
timestamp: January 26, 2021, 07:20:17 pm
content: Why not a few JFets in parallel. It will draw 200Ma all the way right down to almost 0v. Figure 25ma each, so 8 of them for 200Ma. The circuit is simple stupid and it will work from 0v to ~15v/25v depending on the JFet.I guess the same goes for current-regulator diodes which are just a JFet with 2 pins exposed.

author: Zero999
author_rank: Super Contributor
timestamp: January 26, 2021, 07:50:08 pm
content: Quote from: SilverSolder on January 26, 2021, 05:32:16 pmThe current isn't temperature dependant.Quote from: fourfathom on January 26, 2021, 06:37:16 pmThat would need a filter on the MOSFET's gate to work, or a DAC, otherwise it'll just produce huge current spikes.

author: Ian.M
author_rank: Super Contributor
timestamp: January 26, 2021, 09:07:45 pm
content: The gate has plenty of capacitance. Use a large enough gate resistor and its self-filtering.

author: Zero999
author_rank: Super Contributor
timestamp: January 26, 2021, 09:24:54 pm
content: Quote from: Ian.M on January 26, 2021, 09:07:45 pmI agree. Note I didn't say it needed a capacitor, just a filter, which in this case could be a resistor, which should be quite big to provide plenty of filtering.

author: SilverSolder
author_rank: Super Contributor
timestamp: January 26, 2021, 10:28:53 pm
content: I made some measurements on the power supply (wall wart) to figure out what's inside. I also opened the charger itself to see what's inside.Turns out the 11V rating on the wall wart label is unusually pessimistic! No wonder the batteries are getting destroyed - this thing is still pumping 200mA when they are fully charged, LOL

author: magic
author_rank: Super Contributor
timestamp: January 26, 2021, 10:47:40 pm
content: Is it a heavy as shit linear supply or what?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: January 26, 2021, 11:07:33 pm
content: Heavy, no, those things are optimized to a tee. It's only a few VA, probably still cost competitive with a phone charger (if you could get one with as soft an output -- tons of internal resistance and leakage inductance gives the soft, current-limiting output).And yeah, those things are as bad as they look. I once got a Harbor Freight cordless drill, 18V battery -- NiCd yes indeed! This wasn't that long ago either, like 2010, not 1980 or something.Charge works fine if you can get back to it before they're overheating... forget about it at your own peril. Needless to say the batteries didn't last long.I still have that drill actually, as crappy as it is... I soldered a cable to its battery prongs, and I just use it from the bench supply now. Or a 12V gel cell when I need something portable. Thus it becomes a cordless corded cordless drill.BTW, NiMH has a particular charging curve, usually terminated by terminal voltage dropping (zero to negative dV/dt), or cell temperature rising. I think the former is because of the latter (voltage tempco) anyway, so it assumes enough charge current to make something happen (electrolysis, which I think avoids water loss thanks to a catalyst, and hence excess charge turns mainly to heat?). NiMH shouldn't be floated, IIRC.Lead acid is fine with a bulk or float charge, typically 14.3 or 13.8V respectively (for a common flooded 12V battery), with a modest current limit, and a cutoff (for bulk charge) of current falling below some fraction of ampacity. Lithium types mostly work much the same way, with the limit at 4.20V/cell, and may need a more strict current limit (LiIon <= 1C charge, usually?).In all cases, discharge should be cut at a modest fraction of nominal voltage (say 11 or 12V for lead acid, 3V/cell for LiIon, etc.), lest cumulative damage occur.NiMH I think are most tolerant of discharge? Hard to avoid anyway, the common high capacity, high leakage types are pretty much completely flat after a few months on the shelf. For infrequent use, low self discharge types are absolutely worth it.Tim

author: Ian.M
author_rank: Super Contributor
timestamp: January 26, 2021, 11:14:21 pm
content: Quote from: magic on January 26, 2021, 10:47:40 pmNo, its heavier than sh!t.I estimate the transformer alone must be at least 20VA to deliver 400mA for hours at a time, which implies a weight of over 1lb, which is around double the average daily total human stool weight in less developed countries where low protein high fibre diets are still common, and five times the average New Yorker's output!Data:https://www.gastrojournal.org/article/0016-5085(92)91435-7/pdf

author: T3sl4co1l
author_rank: Super Contributor
timestamp: January 26, 2021, 11:20:22 pm
content: Quote from: Ian.M on January 26, 2021, 11:14:21 pmI'm not sure who's full of more courics, the above subject or you!(Just teasing)Tim

author: SilverSolder
author_rank: Super Contributor
timestamp: January 26, 2021, 11:57:37 pm
content: The supply weighs 250g (an engineer was obviously told how much it was allowed to weigh!).It runs at 40C temperature while charging.The supply is able to pump out more than 1A - I stopped testing when the load resistors were getting too hot to hold!The rating of 11V / 400mA is actually close to reality.The reason I want to rescue this most modest implementation of a charger is that I have 3 of these drills, exactly the same... so, for example, when working on a project, I put different bits in all three so I don't have to waste time switching between drill bit, countersink, and screw driver.

author: Ian.M
author_rank: Super Contributor
timestamp: January 27, 2021, 12:40:22 am
content: Quote from: T3sl4co1l on January 26, 2021, 11:20:22 pmI suspect I am, as SilverSolder has just reported the PSU as sploshing in at 2.2 deciCourics. Magic was right on the money.@silverSolder,NiMH cells do *NOT* like slow dumb chargers because of the difficulty of avoiding overcharging. I doubt the transformer's beefy enough to charge the NiMH cells fast enough for effective endpoint detection, so you are left with the alternative of adding a discharge circuit so you can be reasonably certain of the initial state of charge then doing either a timed or colomb counted charge, controlled by a MCU. Fourfathom's PIC10F320/322 suggestion is starting to look sensible . . .

author: SilverSolder
author_rank: Super Contributor
timestamp: January 27, 2021, 04:43:48 pm
content: Quote from: Ian.M on January 27, 2021, 12:40:22 amOK - time to change tack:Several years ago, I bought a bigger charger from the same manufacturer (Ryobi) at a garage sale for the princely sum of $5. This charger is capable of 1.8A ... but I had given up on it because my batteries would not plug into it - the hole is the right shape, but there is a tab intentionally added to my batteries that prevented them being inserted.Last night, I dug that charger out and opened it up on the bench. It contains a big linear power supply (this unit weighs in at 1.3Kg! - not sure how many Courics that is) and contains a fairly big circuit board packed with components, including a Zilog microprocessor, an op amp, plenty of discrete silicon and passive components - it looks like a good effort was put into it.OK, first step is to get the battery into it - I simply cut the tab off the battery... it now fit into the hole. But the charger wouldn't start, blinking a red LED to indicate the battery was too hot... a bit of detective work later, it turns out the "high end" batteries have an NTC resistor in them to prevent charging of a hot battery - my batteries have nothing connected to the sense pin (open circuit). Using a pot, I found that the charger would start with any value less than 5K resistance. So I soldered a 1K resistor inside the charger to make it accept any battery.And... bingo! - It charged the battery in a couple of hours, detected the negative Delta V and the green light came on, meaning "Done".The trickle charge current when "Done" turns out to be 100mA. This is, by complete coincidence, actually correct for the 3000mAh NiMH cells (0.033C to 0.05C according to Panasonic application note).Then, I tried the next battery... cut the tab off, in he goes... red light blinking again, along with a lit "Battery Failure" LED. Now, the crude charger came into its own: ten minutes on the crude charger, and the battery was "good enough" to be accepted by the fast charger. Bingo, full charge and working great!So... the problem appears to be solved - and using only two components: a charger and a resistor!

author: Terry Bites
author_rank: Super Contributor
timestamp: January 27, 2021, 05:03:06 pm
content: Replace the diodes with an LED for improved thermal stability. Don't be cheap -make circuits you can be proud of and won't increase your insurance premiums!

author: Ian.M
author_rank: Super Contributor
timestamp: January 27, 2021, 05:08:04 pm
content: The over-temperature protection is a safety feature. You *DON'T* want to be anywhere nearby if the dV/dt end of charge detection fails, as there's a risk of cell venting spewing a highly caustic aerosol of hot concentrated Potassium Hydroxide solution. MSDS:https://www.lewisu.edu/academics/biology/pdf/KOH_liquid.pdfI strongly recommend sourcing some PTC thermistors that have a resistance of 5K at lets say 5 deg C below the highest surface temperature that's safe for the cells that your batteries have been rebuilt with, and fitting one in the core of each pack, with a little heatsink compound for good thermal contact, + a piece of closed cell foam to hold it against two adjacent cell walls, and connect it to the battery and sense contacts.

author: SilverSolder
author_rank: Super Contributor
timestamp: January 27, 2021, 05:18:00 pm
content: Quote from: Ian.M on January 27, 2021, 05:08:04 pmI thought of that - but playing with the potentiometer, I found the following behaviour:(1) Insert a hot battery - charger refuses to start.(2) Insert a cold battery that then gets hot later - the charger doesn't care, and keeps right on going!It may be reducing the current - I haven't checked that yet. It definitely doesn't go into its error state, even if the "battery" (pot) heats up to 200 degrees. Maybe the heat sensor is just a longevity feature, to prevent a heavily used, hot battery, right out of your drill, from being charged.I will definitely modify all the packs with NTCs, it looks like 10K at 25C has the right profile (they become 5K at about 40C, which is where Panasonic's app note says you shouldn't be charging any longer).

author: SilverSolder
author_rank: Super Contributor
timestamp: January 27, 2021, 05:26:23 pm
content: Quote from: Terry Bites on January 27, 2021, 05:03:06 pmA good principle is to avoid making something yourself that you can buy at a reasonable price.Another good principle is to avoid making something yourself if you can easily modify something you can buy at a reasonable price.Only if these are not possible does it make sense to build something - and to take pride in a job well done, new principles learned, etc.!

author: Ian.M
author_rank: Super Contributor
timestamp: January 27, 2021, 05:51:18 pm
content: I thought you said *LOWER* than 5K enabled charging. If you used NTC thermistors you'd therefore need to preheat your batteries before they charged.That cant be a good idea unless you are working outside in high arctic winter conditions!The other way around with a PTC thermistor that must cool to under 5K before charging starts makes *FAR* more sense!

author: SilverSolder
author_rank: Super Contributor
timestamp: January 27, 2021, 05:54:11 pm
content: Quote from: Ian.M on January 27, 2021, 05:51:18 pmI really need to drink much more coffee, or type slower... one of the two! and definitely test more!

author: Zero999
author_rank: Super Contributor
timestamp: January 27, 2021, 07:54:51 pm
content: Why not just use an LED driver? There are plenty of ICs to choose from. They're cheap and often need minimal external parts. If efficiency is important, go for switched mode.Here's a data sheet for the SC4541 I found after a couple minutes of Googling. It does boost, buck and buck-boost.https://www.mouser.co.uk/datasheet/2/761/sc4541-1278281.pdf

author: SilverSolder
author_rank: Super Contributor
timestamp: January 27, 2021, 08:53:57 pm
content: Quote from: Zero999 on January 27, 2021, 07:54:51 pmYes, I think LED drivers are good candidates for NiMH/NiCd battery chargers.Especially if "assisted" by an MCU.

author: magic
author_rank: Super Contributor
timestamp: January 27, 2021, 09:02:51 pm
content: If so, then what about... wait for it... NiMH battery charging ICs?

author: SilverSolder
author_rank: Super Contributor
timestamp: January 27, 2021, 09:09:19 pm
content: Quote from: Ian.M on January 27, 2021, 05:51:18 pmI spent a little more time with a precision pot to map out how it behaves, and try to determine what kind of PTC resistor we are dealing with here...Basically,(1) If the resistance is over a certain point when the battery is inserted, the charger starts with reduced current.(2) If the resistance drops below that threshold while in reduced current mode, the current goes up to full power- and stays there.(3) If the resistance increases while full current is applied, the charger doesn't care... it never reduces current once it has gone "all in", no matter what the temperature.The resistance at the cutoff threshold is consistent with that of a 5K PTC at around 45C, which seems a reasonable cutoff temperature on first inspection. No other PTC even comes close.I am going to order a bunch of those and see how it works out.I suspect that this design is about environmental temperature considerations, like protecting a battery pack on a hot day in Arizona, where the drill has been heavily used then laid on a black roof in the sun before the user decided to recharge...

author: SilverSolder
author_rank: Super Contributor
timestamp: January 27, 2021, 09:09:59 pm
content: Quote from: magic on January 27, 2021, 09:02:51 pmThat would be CHEATING!(Do you know of any good ones that will do 8 or more cells?)

author: exe
author_rank: Unknown
timestamp: January 27, 2021, 09:31:18 pm
content: Quote from: SilverSolder on January 27, 2021, 09:09:59 pm(Put multiple ICs together? Say, if a chip can charge two cells, put four of them.)

author: SilverSolder
author_rank: Super Contributor
timestamp: January 27, 2021, 09:51:53 pm
content: Quote from: exe on January 27, 2021, 09:31:18 pmUsually, power tool batteries only expose the + and - end of a whole bunch of cells in series. So the charging system has to be able to cope with multiple cells in series. 8 cells is at the low end of what they use... 15 cells or more are widespread.

author: magic
author_rank: Super Contributor
timestamp: January 27, 2021, 11:58:28 pm
content: Nope, if I knew something I would have posted it. But I know such things exist, including even "smart" chargers with dV/dt termination advertising support for multiple cells in series.But I don't know how good that stuff is. If you charge one cell, you can dV/dt terminate it, no problem. If you charge a pack, one cell may already be done while others are still climbing, what then?I remember reading various conflicting opinions on how to deal with large packs, some people even flat out said to just do a timed CC charge because it's hard to do better. Supposedly the cells can take a lot of overcharge over their lifetime if overcharge current is limited to C/30 or something like that. Have fun researching that.

author: SilverSolder
author_rank: Super Contributor
timestamp: January 28, 2021, 03:35:05 am
content: That's pretty much why I was looking to limit charging to 200mA (which is C/15 in this case) and then stop the whole thing with a timer.I agree with you, it cannot be too good to look at dV/dt termination on a string of 8 cells (or 15 cells in some batteries)... the optimal solution must be to expose all the cells via a connector so an external charger can maintain each cell individually, like we see with some lithium ion batteries. Or, build some circuitry into the battery pack that balances the cells. It quickly gets complicated... and then you begin to look for solutions that are "good enough for Australia"...

author: T3sl4co1l
author_rank: Super Contributor
timestamp: January 28, 2021, 03:51:36 am
content: I think NiMH packs don't usually bother, because charge efficiency is low enough, and tolerance of overcharging and reversal is good enough.Basic problem is, cells get out of sync due to diverging charge efficiency and self-discharge. A few charge-discharge cycles, smaller at first to reduce reversal and overcharge, then up to full cycles, seems to be the trick.Tim

author: Zero999
author_rank: Super Contributor
timestamp: January 28, 2021, 10:01:11 pm
content: How about using a discrete switched mode regulator, such as the one designed by Roman Black?It's primally designed to regulate the voltage, but it has current limiting, so the zener can be skipped, if you just want current regulation.https://romanblack.com/smps/a04.htmThe ASCII version, in case the website goes down.+26v -------*-----------------*--------,| | | Q1| 470 ohm R1 | PNP| | E BC32715k R1 *------B| | C (measured;)| | | L1 = 470uH (460uH, 0.7 dc ohms)| 68 ohm | || ,---RC------------------------------,| | | | | NPN| | | | C QS BC337| | 2k7 R2 | ,---B| | | | | E---,| | | Q2 | | | |100uF | | NPN BC337 | *---L1--*---RS----*---- +12.5v out63v | | C | 1.4 ohm || *----*----*-----B | || | | E--------------------------*| | | | || | | | || | '------C2--------* |zener | | 1nF | | 100uFZD1 - | - C 25v13.0v ^ C1 ^ D1 1A |400mW | | | schottky || | 3.9nF | 1N5819 || | | |Gnd --------*----*---------------------*-----------------*---- GndEDIT: Changed font so the ASCII displays properly on my work's Winblow$ machine.

author: iMo
author_rank: Super Contributor
timestamp: January 28, 2021, 10:23:36 pm
content: Quote from: magic on January 27, 2021, 11:58:28 pmWith C/30..C/15 current you may have the battery permanently connected to the charger - trickle charged.. No need to care about an overcharge, afaik..

author: SilverSolder
author_rank: Super Contributor
timestamp: January 28, 2021, 11:58:01 pm
content: Quote from: imo on January 28, 2021, 10:23:36 pmThat is kind of where I was heading with the 200mA constant current. The cells are 3000mAh.So, by calming the charger down from 400mA+ to just 200mA, the NiMH cells might have a chance of surviving the experience....Interestingly, I still have some very old NiCD batteries for these tools, that I never modified to NiMH. Surprisingly, the NiCD cells are still in good condition - despite having been discharged to 0.00V for something like ten years, they accepted a charge immediately, no cycling required, and powered the tools with no problem - for a long time too, didn't feel weak! NiCD appears to be more robust than NiMH.

author: SilverSolder
author_rank: Super Contributor
timestamp: January 29, 2021, 12:01:15 am
content: Quote from: Zero999 on January 28, 2021, 10:01:11 pmThat one is rather cute. Switch mode would mean less heat sinking too. - Might fill the area with RF emissions though?

author: iMo
author_rank: Super Contributor
timestamp: January 29, 2021, 09:42:41 am
content: A switcher dc/dc converter is a nice source of RF, indeed. Sometimes the countermeasures are much more expensive than making heat with a linear.. Especially when you want to put it into a radio..

author: T3sl4co1l
author_rank: Super Contributor
timestamp: January 29, 2021, 11:45:35 am
content: Not... really? Is shielding and additional filtering really that hard?Like, I made this thing some years ago,(also a current mode-ish design, specifically a fixed-on-time constant current controller with discrete implementation, for a higher powered (~100W) load), which made enough noise to be inoperable with my 20m SW receiver. Now, not shown on the schematic is how it's built: the power supply section is in one box and a cable leads up to a light bar (including the 4.7 ohm current sharing resistors and LEDs, but not the 0.73 ohm shunt). Generous filtering (a few X2 film caps and CMC) goes before this cable, so conducted emissions should be pretty quiet (and indeed are according to my LISN testing). Turns out the enclosure needed additional electric-field filtering -- just some conductive panels added inside, tied to DC- ("GND"), did the trick.My 20m receiver, by the way, usesbotha Meanwell SMPS for aux power, and a (discrete) SMPS for filament and B+ supply! The Meanwell is a stock module, no additional filtering; the discrete SMPS is built in a copper-clad box, with an additional stage of LC filtering tacked on its outside. Which was probably not really necessary, there's alright filtering inside too.As proof that the filtering and shielding is all doing its respective job -- I can for example, take an inductive loop probe and plug it into the radio's antenna input (yay BNC connectors!), and wave it around these various sources. The discrete SMPS module has one open side so I can probe its internal noise this way, and it definitely makes a hell of a racket across the receiver's tuning range. The Meanwell seems surprisingly well behaved (I haven't probed inside it, but its mesh enclosure can't be too fantastic for shielding, so it seems to be doing pretty well, given!). The LED lamp described above, was particularly noisy around the filter choke -- but after shielding around it, it's cool and quiet, even probing along the line or LED cables. (Nothing else in here really seems to be making much noise; another custom-made LED lamp has an SMPS with a more stable frequency, stable enough that its harmonics are apparent as obvious line-buzz channels, but it's also well enough shielded and filtered that this is only sensible with a close probing.)Tim

author: iMo
author_rank: Super Contributor
timestamp: January 29, 2021, 03:32:12 pm
content: Quote from: T3sl4co1l on January 29, 2021, 11:45:35 amTry again with it at 160m, 80m, 40m..

author: RES
author_rank: Regular Contributor
timestamp: January 29, 2021, 08:52:47 pm
content: FET+resistor ->https://www.electronics-tutorials.ws/transistor/fet-current-source.htmlorCLD / CRD ->https://en.wikipedia.org/wiki/Constant-current_diode

author: exe
author_rank: Unknown
timestamp: January 29, 2021, 09:01:29 pm
content: Quote from: RES on January 29, 2021, 08:52:47 pmIt's gonna be expensive. On mouser I see max-rated CRD is 18mA, and costs 2.5euro+VAT. We need ten of them.There a strange beast, though, this one:https://nl.mouser.com/ProductDetail/Central-Semiconductor/CMJA5050-TR13-PBFREE/?qs=7MVldsJ5UawaZfeKTd%2FXZA%3D%3D. I'm not sure what that is, doesn't look like a jfet to me.

author: Zero999
author_rank: Super Contributor
timestamp: January 29, 2021, 09:40:49 pm
content: Quote from: T3sl4co1l on January 29, 2021, 11:45:35 amExactly. Also note that heat sinking can also cost a reasonable amount. Often it's just cheaper to go switched mode.QuoteI haven't looked at it in great detail, but the main filter capacitor's voltage rating of 160V is marginal. Are you sure you didn't mean 180V? I'd go for at least 200V, for US mains voltages.Quote from: exe on January 29, 2021, 09:01:29 pmThat looks exactly like a J-FET constant current source. It's just an ordinary J-FET with the source connected to the gate.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: January 29, 2021, 09:51:06 pm
content: Quote from: imo on January 29, 2021, 03:32:12 pmI mean, I hooked it up to a spectrum analyzer in a typical conducted EMC setup, that covers a lot more than just those SW bands.Tim

author: SilverSolder
author_rank: Super Contributor
timestamp: January 29, 2021, 10:42:08 pm
content: Quote from: exe on January 29, 2021, 09:01:29 pmFour of those in parallel might work... very cheap, very minimalist!

author: KB1LQC
author_rank: Newbie
timestamp: January 30, 2021, 05:27:55 am
content: If you're willing to relax the dropout requirement, the LT3092 is about as simple as it gets IMHO for decent accuracy. Just two resistors.https://www.analog.com/media/en/technical-documentation/data-sheets/lt3092.pdf

author: exe
author_rank: Unknown
timestamp: January 30, 2021, 11:12:14 am
content: Quote from: Zero999 on January 29, 2021, 09:40:49 pmIdk, look at "current regulation vs time" plot. This device is also an outlier in terms of current. Usually jfets are not very precise, that's why they are binned and there are many devices in series. But this one is just one device. Also, look at "steady state regulator current", it gradually turns on at 6V or so.TBH, I don't like this device. I'd prefer something else, likehttps://www.onsemi.com/pub/Collateral/NSM4002MR6-D.PDF. I have this one or similar device, I measured its performance (although for much smaller currents, around 1-10mA), but can't find my records.PS this one has somewhat similar turn on characteristic and it says it's based on a self-biased bjt:https://www.onsemi.com/pub/Collateral/NSI50150AD-D.PDF

author: SilverSolder
author_rank: Super Contributor
timestamp: January 30, 2021, 04:53:23 pm
content: Quote from: KB1LQC on January 30, 2021, 05:27:55 amThat seems a nice IC, like an LM334 on steroids! Not exactly cheap, but looks like a good problem solver - with minimal parts count.

author: SilverSolder
author_rank: Super Contributor
timestamp: January 30, 2021, 04:58:49 pm
content: Quote from: exe on January 30, 2021, 11:12:14 amThere does seem to be a lot of "gotchas" with the current regulating diode (whatever it is inside). I didn't notice the big voltage drop required at first... or the slow turn-on time, several seconds?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: January 30, 2021, 10:16:42 pm
content: The voltage drop seems to be pinch-off voltage, and the "turn on time" seems to be thermal stabilization. Although, maybe it's not semiconductor at all, but purely thermal, a PTC fuse designed not to fuse (strong foldback), but to gently limit?Hell... slow response works just fine here, that's no problem. Would be a big gotcha in other applications!Tim