Switching a load with no diode drop.  Is it possible? - Page 1
Forum section: electronics / beginners

author: hgg
author_rank: Frequent Contributor
timestamp: December 11, 2016, 02:36:47 pm
content: Hi,I have a supply of 2.4V from two NiMH batteries and I would like to switch on a(flickering IC)LEDat night with a transistor but I get a diode voltage drop so the LED will not light up.I was wondering if it is possible to switch the LED without any diode drop.(I want to avoid the Joule thief solution)Thanks.

author: rob77
author_rank: Super Contributor
timestamp: December 11, 2016, 02:42:50 pm
content: with a emitter-follower you'll always have 1 diode drop - at the B-E junction.... a common emitter topology (BJT as a low-side switch) will eliminate that 1 diode drop.but anyways - you need a current limit for your LED - either a current limiting resistor or drive the diode with constant current.

author: hgg
author_rank: Frequent Contributor
timestamp: December 11, 2016, 02:54:12 pm
content: I do not need any current limiting resistor, because at 2.4V the LED draws 10mA.Don't we have a voltage drop from Collector to Emitter?

author: amyk
author_rank: Super Contributor
timestamp: December 11, 2016, 02:54:27 pm
content: You can always use a MOSFET instead.

author: hgg
author_rank: Frequent Contributor
timestamp: December 11, 2016, 02:57:53 pm
content: I've tried it with a P-Channel mosfet, high side switching, and I get the same voltage drop.

author: Gyro
author_rank: Super Contributor
timestamp: December 11, 2016, 03:06:07 pm
content: With a Mosfet you need enough gate drive voltage to turn it fully on. That might be a problem at 2.4V.

author: hgg
author_rank: Frequent Contributor
timestamp: December 11, 2016, 03:17:31 pm
content: I used the IRFR9024N with a Vgs of -2V min and -4V max.

author: orolo
author_rank: Frequent Contributor
timestamp: December 11, 2016, 03:46:54 pm
content: Try a PNP (2N3906) instead of a NPN driving the LED (maybe with a moderate base resistor to limit current). The C-E saturation voltage can be in the tens of millivolts. If your LED is white, it will not glow very much anyway with 2.4V, but the transistor will not be responsible for that.

author: Benta
author_rank: Super Contributor
timestamp: December 11, 2016, 03:48:14 pm
content: Don't know how much capacity your battery has, but the simplest solution would be to connect the LED with series resistor directly to the battery and short the LED with an NPN transistor during daylight. Eventually a resistor in the collector to limit IC.

author: 2N3055
author_rank: Super Contributor
timestamp: December 11, 2016, 03:51:00 pm
content: You have to switch on low side.. Bipolar transistor in saturation, with few mA over them will have 0.1-0.2 V drop... Mosfet can be less than that,but you will have trouble switching it on without 4-8V..

author: kosine
author_rank: Regular Contributor
timestamp: December 11, 2016, 03:53:25 pm
content: The circuit you have is almost there. You need to move the LED above Q2, add a small current limiting resistor (about 47R) and drop R1 to say 10k to give Q2 a bit more base current.When the solar cell input drops, Q1 will switch off and Q2 will switch on hard. It's Vce will be about 100mV, so nothing to worry the LED.The value of R1 will determine the quiescent current because Q1 is always on. It'll probably be 200uA or so with a 10k, but the higher you go the greater Q2's Vce. 47k might be OK.R2 will determine the LED current. Note that the circuit you show will indeed work without it due to the Vbe drop across Q2, so the LED is only seeing 1.8V. But if you move the LED above Q2 it'll see 2.3V or so, and that'll need a current limiter (R2).(Another way you could avoid R2 is to increase R1 to limit the base current of Q2. That'll also limit the collector current of Q2, but it'd dodgy since it depends on the exact gain of Q2.)

author: retrolefty
author_rank: Super Contributor
timestamp: December 11, 2016, 04:05:00 pm
content: Quote from: hgg on December 11, 2016, 03:17:31 pmKeep in mind that is the Gate Threshold Voltage, where the device just starts to conduct. So accordingto that spec your specific device, worst case may only start to conduct (250 uA) when up to 4 volts gate/source voltage. So it really can't be used in your circuit using a 2.4v Vcc.http://www.mouser.com/catalog/specsheets/international%20rectifier_irfr9024n.pdf

author: hgg
author_rank: Frequent Contributor
timestamp: December 11, 2016, 04:53:30 pm
content: Low side switching indeed worked better.Guys, thank you for your suggestions!In the BJT, is the diode seenonlybetween base and emitter?

author: KL27x
author_rank: Super Contributor
timestamp: December 11, 2016, 05:15:11 pm
content: QuoteThis might be a problem. If the LED only draws 10mA with NO limiting, it is barely functioning, at all. This is bad design, to begin with. That said, there is still something to learn and possible borderline success.+1 amyk. She is correct.QuoteYou tried it with a 55A power FET with min Gate threshold higher than 2.4V, on average. You looking for a small signal FET.QuoteYou do not need to turn it on fully, in order to beat a BJT in this instance. And FET operate at 1.8V all the time. 50% of our modern computers have hundreds of thousands of FETs in the processor operating at 1.8V. You will easily beat the BJT. Any resistive load, no matter how high impedance, is going to beat the diode drop of a BJT in this instance. Since your LED voltge drop is approaching 2.4V, this means any resistive load will have voltage drop approaching zero, which is much, much less than minimum voltage drop of a diode junction in a BJT. (Also current will approach zero; but is enough to get your LED to light, I promise.... all these "approaching" things are why this is bad design. Running LED by voltage is like dividing by zero, lol).Here is a small signal PFET, for example. For high side switching.http://www.mouser.com/ProductDetail/Infineon-Technologies/BSS816NWH6327XTSA1/?qs=sGAEpiMZZMshyDBzk1%2fWixDt2IZOmxjILFS%2fF%252bTDmNA%3dYou can just as easily use NFET for low side switching.At 2.4V, this has resistance of about 300 milliohm. This will not greatly reduce current thu your LED. Output impedance of your battery and wiring probably is greater than 300mohm.So this is bad circuit, but given directly driving the LED at 2.4V works good enough for you, this will also work good enough for you. Drive it directly, not thru a BJT. To get fuller saturation. Guarantee this will work (as good as your original design intent, anyway. LED should never be driven by unlimited current source and controlled by voltage, only, unless by active control. Even the 300mOhm resistance of your FET is not enough resistance to make this a good circuit. Very small change in voltage will make significant change in LED output or even burn it out.) But bottom line, in this particular case, I guarantee an optimal (and cheap) FET will give your LED more current than any BJT in any configuration.

author: Audioguru
author_rank: Super Contributor
timestamp: December 11, 2016, 05:28:34 pm
content: Quote from: hgg on December 11, 2016, 03:17:31 pmThat is its "threshold voltage" when it is almost turned off conducting only 0.25mA. Maybe yours had a threshold voltage near 4V.If your LED lighted without current limiting when the battery was 2.4V then how many milli-seconds will it last when the battery is fully charged at 2.9V?

author: Audioguru
author_rank: Super Contributor
timestamp: December 11, 2016, 05:38:50 pm
content: Why not buy a solar garden light for one dollar? It comes with an LED, a 1.2V Ni-MH battery cell, a solar panel and a circuit that charges the battery when there is sunlight and boosts its voltage when it detects darkness.

author: hgg
author_rank: Frequent Contributor
timestamp: December 11, 2016, 05:39:58 pm
content: I forgot to mention that the led I used was a flickering led...It works down to 2.0V but with 2.4V or even with 3.0V it did not need any current limiting.It was drawing no more than 11mA when it was directly connected to the 2.4V battery.Anyway, I might use a resistor just to be on the safe side.

author: hgg
author_rank: Frequent Contributor
timestamp: December 11, 2016, 05:41:17 pm
content: @audioguru Because they do not use any kind of overcharging protection and I needthe batteries to last for many years.

author: hgg
author_rank: Frequent Contributor
timestamp: December 11, 2016, 05:42:53 pm
content: @KL27x Thanks for the MOSFET links. I need to buy some of these.

author: KL27x
author_rank: Super Contributor
timestamp: December 11, 2016, 05:49:32 pm
content: Thx. Trust me. Gate drive voltage > 2V and load driving voltage > 0.5V (approaching zero, even) and low current, there is no way BJT will compete.*edit: And it's not even close.At 300mOhm resistance, FET will drop only 0.003V at 10mA current. Since there's that little bit o drop, perhaps your LED only draws 9.5mA, though. Ya know.

author: mariush
author_rank: Super Contributor
timestamp: December 11, 2016, 05:49:54 pm
content: People are gonna jump all over me, but here it goes.Just buy a PicKit programmer and a PIC10F222 :http://www.digikey.com/product-detail/en/microchip-technology/PIC10F222T-I-OT/PIC10F222T-I-OTCT-ND/1015706Tiny 6 pin micro, works from 2v, uses something like 100-200nA @2v when in stand-by, internal oscillator, each output pin can do up to 25mA (i think it depends on input voltage as well and you can probably connect 2 output pins in parallel for more current), built in adc to measure voltage from solar cellYou basically have a loop in the micro, measure voltage from solar cell, if it's high enough to think it's light outside turn off led, if it's too low, turn on led (connected directly to micro). sleep for 15s - 30s - 1m - 5m , repeat.Pretty good for 50 cents, and it's a tiny sot 6pin part, compared to 2 transistors and resistors taking more space and potentially costing more money.

author: hgg
author_rank: Frequent Contributor
timestamp: December 11, 2016, 06:06:49 pm
content: @bentaQuoteCan you post a schematic of your suggestion ?

author: Benta
author_rank: Super Contributor
timestamp: December 11, 2016, 06:32:12 pm
content: QuoteNah, makes no sense. Your statement that the battery needs to last for years and the LED is a type with IC makes it moot.

author: IanB
author_rank: Super Contributor
timestamp: December 11, 2016, 06:48:12 pm
content: Quote from: hgg on December 11, 2016, 02:54:12 pmJust FYI, I have 2.8 V from a pair of NiMH in series. I would not like to connect them to an LED without any current limiting.

author: Benta
author_rank: Super Contributor
timestamp: December 11, 2016, 07:00:50 pm
content: QuotePlease read reply #16. It's an LED with internal control.

author: IanB
author_rank: Super Contributor
timestamp: December 11, 2016, 07:09:56 pm
content: Quote from: Benta on December 11, 2016, 07:00:50 pmThat information wan't available at first.It's also helpful to know for anyone else reading that NiMH cells do not have an unvarying output voltage of 1.2 V.

author: hgg
author_rank: Frequent Contributor
timestamp: December 11, 2016, 07:14:27 pm
content: I know, but the flickering IC LED works fine from 2.0V up to 3.0V without any currentlimiting resistor.

author: hgg
author_rank: Frequent Contributor
timestamp: December 11, 2016, 07:16:23 pm
content: @BentaQuoteI asked for a schematic because it looks like that your suggestion will not work.I might be wrong.

author: mariush
author_rank: Super Contributor
timestamp: December 11, 2016, 07:22:29 pm
content: Not to mention you didn't say they're rechargeable. Regular NiMh alkaline batteries can be up to 1.65v when new, so you'd have 3.3v in series.There's also lithium batteries in the AA format, like the Energizer Ultimate Lithium, which could have as much as 1.7v when fresh out the box (but they quickly drop down to about 1.45v) :https://www.amazon.com/Energizer-Ultimate-Lithium-Batteries-Longest-Lasting/dp/B00003IEME?th=1They're more expensive at around 2$ a piece, but at low currents they last way longer than regular alkaline batteries. Here's a graph showing the discharge at 200mA and a comparison between a lot of batteries available commercially :http://batteryshowdown.com/results-lo.html(click on the graph picture in the bottom right to see larger)As you can see, at 200mA constant drain, they last about 50% more compared to alkaline, at only 20mA the difference will be much higher.

author: IanB
author_rank: Super Contributor
timestamp: December 11, 2016, 07:31:14 pm
content: Quote from: mariush on December 11, 2016, 07:22:29 pm1.82 V evenQuoteNot that quickly, in my experience. Even after considerable use I have still seen 1.6 V.

author: madires
author_rank: Super Contributor
timestamp: December 11, 2016, 07:39:23 pm
content: BSS138

author: hgg
author_rank: Frequent Contributor
timestamp: December 11, 2016, 07:42:29 pm
content: I have used the Lithiums. Great batteries.Now I am using the AAA Eneloops BK-4MCCE.@madires Are there any similar low signal mosfets in THT versions?

author: madires
author_rank: Super Contributor
timestamp: December 11, 2016, 07:46:26 pm
content: I'd assume so. You could try Digikey's parametric search.

author: KL27x
author_rank: Super Contributor
timestamp: December 11, 2016, 10:02:06 pm
content: BSS138 doesn't even begin to switch until about 2V. At resistance of >4ohm. Why even throw that out there?You don't need a "brand name" part. You don't need switching speed. Rise/fall times. Just do search of small signal FETs. Enter gate voltage SUB 1V. There are many with upper gate threshhold voltage (guaranteed max voltage where switching begins) of under 800mV! Another thing that will universally be the case with these guys is MAX +- gate voltage of 8V and MAX drain-source of 20V. This is because to make the gate more sensitive, dielectric must be made thinner. Already instant tipoff that BSS138 is garbage (in this context) is 50V rating and +-20V on the gate.And sort it by price. Cheaper = better.Look at price, package, and the the other things I mentioned. Then to dig a little deeper before you pull the trigger, look at the little graph in the datasheet. The one that shows drain-source resistance as a function of gate voltage. That and the price/package are really the only things you need to know for your particular problem. Every other spec for any FET on the market will more than meet all your other requirements. But you can't sort by "graph." What you want to see in this graph is a low turnon point, significantly lower than 2.4V. And a low resistance at 1.8 to 2.4V. While BSS138 might work fine for one light, if you make 5 of them and want them to match, you don't want to cut it that close for no good reason. There is considerable variation in the start of saturation between individual FETs.

author: Audioguru
author_rank: Super Contributor
timestamp: December 11, 2016, 11:20:41 pm
content: The Dollar Store sold cheap Chinese LED ornaments with a colors-changing-LED that has a controller IC inside the LED. The ornaments are powered with three 1.5V button cells in series and have a 51 ohm current-limiting resistor in series.They also sell flickering LED tea lights but I have not looked inside one to see if it uses a current-limiting resistor.

author: David Hess
author_rank: Super Contributor
timestamp: December 12, 2016, 08:26:30 am
content: Quote from: KL27x on December 11, 2016, 05:49:32 pmThey make low Vce saturation bipolar transistors which can easily meet or beat that like the ZTX618 (0.0007V at 10mA maybe?) but it is possible to do even better without saturated switches or even germanium transistors. If current gain is not an issue, then swap the collector and emitter leads of a general purpose transistor for even lower Vce saturation. They used to make special "chopper" bipolar transistors with symmetrical construction and low saturation but low gain for use where today JFETs and MOSFETs are used in chopper applications.Using a bipolar transistor as a common emitter switch to drive an LED at low voltages allows another interesting possibility. Use a pair of bipolar transistors, match them for Vbe, and configure them as a current mirror with gain using a resistor in series with the emitter of the lower current transistor. Now the transistor which is switching the LED also operates as a rough constant current source.

author: orolo
author_rank: Frequent Contributor
timestamp: December 12, 2016, 01:47:29 pm
content: Quote from: David Hess on December 12, 2016, 08:26:30 amThank you for this pearl! The incredible versatility of a humble bjt connected in unconventional ways never ceases to amaze me. I'll put high on my experiment list to explore reversed transistor characteristics. They are often dismissed as low performing, without discussion of possible merits.

author: Zero999
author_rank: Super Contributor
timestamp: December 12, 2016, 03:28:50 pm
content: Quote from: hgg on December 11, 2016, 06:06:49 pmCan you please post a schematic of what you tried originally?

author: KL27x
author_rank: Super Contributor
timestamp: December 13, 2016, 02:23:28 am
content: @ David HessMind blown. I ordered a few to test. I am sure I won't figure out a practical use for another 10 years.* Esp since they are kinda pricey. (Not because this isn't amazing; just cuz I'm dumb.) But yeah. Mind blown. I'm hardwired to think FET for efficient power switching; at lower frequencies, anyway. I guess at lower current, the script might flip!?*Well, one thing. FET driver. Wonder where I got that idea?

author: David Hess
author_rank: Super Contributor
timestamp: December 13, 2016, 03:14:32 am
content: Quote from: orolo on December 12, 2016, 01:47:29 pmThe most common albeit still very rare place I see bipolar transistors used in reverse (Jim Williams calls it "inverted mode") is when an integrating capacitor needs to be discharged to as close to zero volts as possible. This is not quite such a big deal now when we have low threshold voltage MOSFETs available but what if you want to build a high performance integrator which runs on the lowest possible supply voltage? Linear Technology application note 15 has examples in figures 3 and 6 and discusses the problems with FETs in general on page 4.

author: rob77
author_rank: Super Contributor
timestamp: December 13, 2016, 10:30:50 am
content: Quote from: David Hess on December 12, 2016, 08:26:30 aminteresting , definitely going to try itbut it will be good for small voltages only as the Vbe breakdown voltage is only few volts.

author: David Hess
author_rank: Super Contributor
timestamp: December 13, 2016, 01:59:54 pm
content: Quote from: rob77 on December 13, 2016, 10:30:50 amLow Vbe breakdown voltage is definitely a limitation. On the other hand some transistors, usually PNPs but not always, have high Vbeo breakdown voltages. In a few cases, their Vbeo breakdown voltage is higher than their Vceo breakdown voltage like the currently producedToshiba 2SC3326which is intended for inverted operation for muting in audio applications. TheCMPT404Awhich is a surface mount 2N404A is another good example designed for chopping applications.

author: KL27x
author_rank: Super Contributor
timestamp: December 17, 2016, 03:37:33 am
content: Color me a nerd. I got my transistors, a couple days ago. And since it is Friday night, I obviously have plenty of time to play with transistors, lol.I have wired up the FET vs BJT challenge.Red corner:ZTX618NPN transistor.Blue cornerBSS816NWH6327XTSA1 (this just rolls off the tongue, don't it?)This is the "PFET" which I suggested earlier, based on a 30 second Mouser search, sorted by price. And apparently, I can't tell the difference between a PFET and an NFET, cuz it's an NFET, lolSo I put 200R resistor on Collector/Drain. And for the NPN, I put a 2K resistor on the base. Gate, direct connection. Gate/base biased with Vsupply.Red corner - 30 cent BJT@ 2.4V, my PSU shows 9mA (experience tells me this reads low, but it verifies circuit working as intended) and my multimeter (Cheapo Uni-T pen meter) shows Vce 0.004V, Vsupply 2.396V@ 1.8V, my PSU can't decide if it's 4mA or 6mA... and penmeter shows Vce of 0.004VBlue corner - 8 cent FET@2.4V, 8mA, Vds 0.001V, Vsupply 2.396V@1.8V, 4ma, Vds 0.001VThe BJT is impressive to me, but the numbers don't lie. FET still wins, here. Well, THIS particular sample, anyway. Maybe next sample of same FET not as low. I'll have to try the BJT upside down, next.Real beauty of FET for low frequency power switching, though... can run them in parallel ad infinitum to decrease voltage drop even further while increasing max heat dissipation. BJT seems to bottom out, as per datasheet, 4mV is the drop at even 100mA.So, anyhow, OP has many options. Yes, you can switch without a diode drop using BJT or FET.* Using 29.3 ohm load resistor, to give ZTX room to shine, Vsupply 2.31V (voltage drop from long leads!), and 200R base resistor, Vce is 0.009V. So it is not meeting spec at this voltage. (This is only 78.8ish mA, but I didn't have right resistor to get 100mA.)Same load resistor and supply voltage, NFET has Vds of 0.009V. TIE!At 2.3V, BTS FET is right on par with graph in datasheet at this point. ~9.46mV drop is what would be expected per the graph. I'm not inclined to think this is particularly "good" example of this FET.... even though it outperformed at 10mA, the graph doesn't scale down very far. But maybe the BJT is a bad one...