Constant current dummy load, ran a gutter... - Page 1
Forum section: electronics / projects

author: bsodmike
author_rank: Regular Contributor
timestamp: August 08, 2020, 06:58:34 pm
content: Hi all,Much of the inspiration for testing this out is Dave's excellent videoI started by breadboarding the LM324N and just testing the 50K 10-turn pot, to see if it would actually give me the proper output, i.e. 0v - 2.5v (due to the double-10k divider).However, I ran a gutter with some strange behaviour. So what's going on here, let me summarise.Turning the pot to its maximum, allows me to measure ~4.99vdc at the Op amp non-inverting input pin 5. This is expected. However, somewhere in the middle/towards the upper end, the output at the double-10k divider, I get a max output of 1.88v.Let's take step back, and ignore the output divider, the voltage at pin 7 is 3.76v. It does not advance even if I go beyond this 'threashold' point, i.e. 4.99vdc at the non-inverting input, and 3.76dc at the output. This seems odd. It should be a voltage follower right?The exact point, as measured is 12.72kohm and 38.61kohm (50k pot wiper position). Anything beyond this, does not change the output.Do I need to increase the pot input voltage to "cheat" and get pin 7 to match 5vdc? This doesn't seem right, or elegant though.Thoughts please?P.S. included a shot of the PSU just to show that Chan 2 was used.Cheers!

author: bsodmike
author_rank: Regular Contributor
timestamp: August 08, 2020, 07:05:35 pm
content: I decided to just move on leaving this strange behaviour to at least see if I can drive the power MOSFET. I've used an IRL640A as this is what I have on hand.The PSU is configured to power the MOSFET using Chan 1, Chan 2 powers the LM324N.The left Fluke measures the voltage at A. The right Fluke measures the current through Drain-source.I first tried a source voltage of 1vdc, this lead to a max current draw of 0.676AAt 2vdc, 1.419V at A to give 1.303A drawnAt 3vdc, the MOSFET started to get hot and I could hit 1.5A and it possibly started to thermal throttle at that pointSo yeah, I've got a crude load that can draw ~1.5A, but the voltage follower needs sorting out.

author: MarkF
author_rank: Super Contributor
timestamp: August 08, 2020, 08:08:17 pm
content: 1) For stability, you need some resistors around the MOSFET.2) Using 5V for the power supply is probably too low and causing some of your issues. For Instance, the op-amp outputcan not go completely to the power supply rail (5V). Check the datasheet. Plus, 5V is pretty close to thevoltage required to turn on the MOSFET. A power supply between 9V and 12V would be a better choice.3) I used a 10-turn linear pot to set the current. Do you have a linear or log/audio pot?4) Here is mine (0 to 2.5A):I used the 620Ω and 5.6KΩ around my MOSFET because those were the values I used in the voltage divider.The values in 'red' are probably the best choice.

author: bsodmike
author_rank: Regular Contributor
timestamp: August 09, 2020, 02:31:54 am
content: Thanks a lot Mark!I just noticed in the Datasheet: "with V+ from 5V to 30V; and over the full input common-mode range (0V to V+ − 1.5V) for LM2902, V+ from 5V to 26V".Thanks for the other comments, I will incorporate them into my circuit!!Cheers M.

author: MarkF
author_rank: Super Contributor
timestamp: August 09, 2020, 02:06:01 pm
content: Quote from: bsodmike on August 09, 2020, 02:31:54 amAlso notice, the Output Voltage Swing (VOH) for V+ = 30V that VOH= 28V.You can only expect the output voltage to reach Vsupply - 2V.

author: MarkF
author_rank: Super Contributor
timestamp: August 09, 2020, 03:59:48 pm
content: You also may be interested in Jay_Diddy_B's Load project:https://www.eevblog.com/forum/projects/dynamic-electronic-load-project/However, notice one major difference to mine:His current set point is through the op-amp (-) negative input while I use the (+) positive input.This requires that he use a dual power supply for the op-amps.

author: rdl
author_rank: Super Contributor
timestamp: August 09, 2020, 11:22:27 pm
content: Be advised, that circuit has a tendency to oscillate.

author: bsodmike
author_rank: Regular Contributor
timestamp: August 10, 2020, 02:20:52 am
content: Hi rdl, do you mean Jay_Diddy_B's or MarkF's?I notice that Mark has added the dampening suggested by Jay_Diddy_B - he's got a 2R2 and 2.2uF on the load input. Also his use of the 100R/1k + 10nf near the MOSFET looks similar to the implementation by Jay_Diddy_B.I would assume, MarkF's doesn't oscillate as much as the poor performing examples? Of course, any PCB implementation should be tested for oscillations.

author: bsodmike
author_rank: Regular Contributor
timestamp: August 10, 2020, 02:26:01 am
content: Couple questions Mark -1) Your Ref02, what's the output voltage? I seem to get away with around 6V into the LM329N but I may need to increase this considering Vsupply - 2V.Quote from: MarkF on August 08, 2020, 08:08:17 pmYes, I've replaced my 10k/10k pair with a 10k 10-turn pot for current limit.I'm now testing against the LOAD input being a 5vdc source, so as to meet the minimum requirements of the IRL640A. I'll need to take a break till a few more parts arrive, namely a heatsink for the TO220 MOSFET; so far been testing in free-air and it heats up quickly under 5V/1A!Quote from: MarkF on August 08, 2020, 08:08:17 pmI will go with 100R/1k and the 10nF plus dampening at the load input and report back.

author: ledtester
author_rank: Super Contributor
timestamp: August 10, 2020, 02:30:00 am
content: Here's a good video exploring how to go about getting rid of the oscillations in a CC regulation circuit:https://youtu.be/8xoGhZQO29YIt is an hour long and progresses at a somewhat leisurely pace, but it's got a lot of good info in it.

author: MarkF
author_rank: Super Contributor
timestamp: August 10, 2020, 03:44:31 am
content: The REF02 is a 5V reference (See attached datasheet). The input to the 10:1 voltage divider (from the REF02 or Externally) is 5V max. This gives a control voltage to the (+) input of 0V to 0.5V. Which yields a maximum voltage of 0.5V across the 0R2Ω sense resistor. Giving my max current of 2.5A. You can select your max current by adjusting the value of the sense resistor knowing it will have a max voltage of 0.5V across it.Items of note:- I never populated the fine adjustment pot for the REF02. Never needed to adjust the REF02 output.- Also, I don't have the damping RC circuit. (Next PCB revision)- You will NEED a fan for load currents much over 0.5A (with a RA-T2X-64E or FA-T220-64E heatsink).- The IRFP250 is in a TO-247 package which is much bigger than a TO-220. Better heat dissipation in my opinion.Never had problems with oscillations. The 10nF capacitor in the feedback loop is about twice the minimum value with my op-amp. Also, the TLC272 op-amp I used can provide +/-30mA output drive current. Not sure how the LM324 will fare. I see people also using the LM358.No one has mentioned the 1KΩ resistor to 12V at the sense resistor...That's a kludge, pure and simple!The problem is that without a device-under-test(DUT) connected or the DUT that is powered 'OFF', the op-amp can NOT develop any current through the sense resistor. Therefore, it will drive VGATEof the MOSFET to the maximum possible voltage. The result is that when the DUT is then turned 'ON', the MOSFET will be fully 'ON' and present a brief short to the DUT.The 1KΩ resistor provides a minimum current through the sense resistor and will allow you to turn 'OFF' the MOSFET in this situation by setting the set current to 0A (i.e. Adjust the 10-turn pot to minimum)I based my load on a circuit from Peter Oakes. Here is a video where he is doing performance testing:

author: MarkF
author_rank: Super Contributor
timestamp: August 10, 2020, 04:09:01 am
content: Quote from: bsodmike on August 10, 2020, 02:26:01 amAlso, using a smaller sense resistor means you will have less voltage drop for the same current and will allow you to get a resistor with a smaller power rating. Therefore, you can test lower voltages.The 0R2Ω resistor at 0.5V @ 2.5A = 1.25W requirement.A 3W resistor like thishttps://www.digikey.com/product-detail/en/vishay-dale/LVR03R2000FE70/LVRB-20RTR-ND/1166251would do.

author: pqass
author_rank: Super Contributor
timestamp: August 10, 2020, 04:27:09 am
content: Because you've used a 1ohm shunt, for every amp through the shunt+MOSFET loop the voltage drop across the shunt increases by 1V. So at 1A, Vshunt_drop = 1V; at 2A, Vshunt_drop = 2V, etc.Your MOSFET gate needs to be raised above this Vshunt_drop the more amps you wish to push through the shunt+MOSFET loop. But your opamp is only being powered by 5V; an opamp that typically only can output approx. Vcc-1.25V at most. 5V - 1.25V = 3.75V at the output as you've witnessed.The IRL640A has a Vgs(th) between 1V min and 2V max which means that it starts conducting when its gate is at 1V or 2V above its source (min, max, respectively). But if you want 2A constant current, you have a Vshunt_drop of 2V, plus add the Vgs(th), and you're at 3V or 4V (min, max, respectively). To fully conduct at 11A continuously, the MOSFET gate needs to be at 5V above source. But your opamp won't go that high (given Vcc is 5V, the max output of the opamp as witnessed is 3.75V)!You have 3 options:1. raise the opamp Vcc to (Vgs+opamp output max to Vcc diff+Vshunt_drop max at 11A) 5V+1.25V+11V=17.25V; enough to allow its output to reach 5V above shunt voltage drop if it needs to.2. lower the shunt from 1ohm to, say, 0.1ohm where for each amp through it, the Vshunt_drop will be 0.1V. So now at 2A constant current will only drop 0.2V. At 2A, this will give you back 2V-0.2V or 1.8V to be used above the Vgs(th) to pump more current through the MOSFET. But you still won't get close to fully on at 11A.3. if you really really need Vcc=5V then get another op amp that can do rail-to-rail output plus use the 0.1ohm shunt.What is your maximum amps design requirement?Also, this may be of value (scroll down to my design schematic):https://www.eevblog.com/forum/buysellwanted/wtb-(uk)-electronic-dc-load/msg3140566/#msg3140566

author: bsodmike
author_rank: Regular Contributor
timestamp: August 10, 2020, 07:58:16 am
content: Quote from: pqass on August 10, 2020, 04:27:09 amSorry, but can you explain where you got 11A for Drain-source current from? We are limiting the max Ids to 2.5A?What I've understood so far (recap): with a 1ohm current sense resistor, at 1A, Vshunt_drop = 1V.So our requirements:2.5A for max Ids: Vshunt_drop = 2.5VVgs(th) is 1V to 2V (min/max) so (Vgs(th) + Vshunt_drop) = 3.5V to 4.5V (min/max). So a gate voltage of ~ 4.5V to 5V is needed to allow 2.5A current flow (Ids) - right?Therefore, option (1) is to raise the opamp Vcc to 5V+1.25V = 6.25V; enough to allow its output to reach 5V above shunt voltage drop if it needs to.I^2R for (2.5^2*1ohm) = 6.25 watts power draw on the 1ohm current sense resistor.Quote from: pqass on August 10, 2020, 04:27:09 amWe want Ids = 2.5A so with a 0.1ohm resistor, Vshunt_drop = 0.25VVgs(th) is 1V to 2V (min/max) so (Vgs(th) + Vshunt_drop) = 1.25V to 2.25V (min/max).Opamp Vcc of ~3V is enough to drive this smaller current sense resistor. Also I^2R for (2.5^2*0.1ohm) = 0.625 watts power draw on the current sense resistor.Thanks so much for going through this in detail, really appreciate it (as this is mainly a learning exercise on my part).

author: bsodmike
author_rank: Regular Contributor
timestamp: August 10, 2020, 08:07:56 am
content: Wow thanks so much, great resource and I've been following along (till around the middle). Will finish later, BUT, I tweaked my circuit to match the schematic by MarkF (well most of it). But I did not add the dampening on the load input (yet!). I wanted to watch the Drain-Source voltage across the MOSFET oscillate.See the bottom part of my sketch; I just added a 10nF between the OpAmp terminals pin 8 & 9, and a 100 ohm + 1kOhm. The output in this example is Ids = 200mA. Output oscillating at 175kHz.

author: bsodmike
author_rank: Regular Contributor
timestamp: August 10, 2020, 08:25:30 am
content: Quote from: MarkF on August 10, 2020, 03:44:31 amSweet, I was going to ask you about thatThanks for all the details!I've placed an order for a 0R1Ω sense resistor and will work with that once it arrives. For now I'm just going to play with the 1R that I have and see if I can get it stable (for kicks!).

author: SmokedComponent
author_rank: Regular Contributor
timestamp: August 10, 2020, 12:05:18 pm
content: Also, keep your shunt resistor cool. Not just by using smaller value (to generate less own heat) but also where you place it. The dialed-in current will start to drift with resistor's value. This was a minor snag I encountered during my build of a simple CC load I needed to fix in order to justify an expensive but really satisfying 10x wirewound pot.Position it away from transistor's heatsink. Or, if you are using a fan, in front of the fan, so cool air passes through shunt resistor first.

author: bsodmike
author_rank: Regular Contributor
timestamp: August 10, 2020, 01:04:00 pm
content: Haha YES, I also picked up a 50K 10-turn wire-wound pot. It is superbly sublimeThanks for the awesome tips, and seriously, thanks to everyone here!!

author: MarkF
author_rank: Super Contributor
timestamp: August 10, 2020, 01:43:14 pm
content: Quote from: bsodmike on August 10, 2020, 08:25:30 amI would not push the current levels to its maximum.Peter also used a 0R1Ω resistor with a larger IRFP064 MOSFET on a large computer heatsink. While testing a power supply, it spiked and totally destroyed his electronic load. Total meltdown.That is part of why I choose the large resistors between the MOSFET and the op-amp. To limit the current into the op-amp if the MOSFET shorts. Also, at 15V @ 5A load settings, you WILL NEED A MASSIVE heatsink with active cooling. I'm not suggesting values other than the 100Ω and 1KΩ resistors to the MOSFET (probably the best performance). Just be aware that if you push the MOSFET and it shorts, you WILL destroy all of your semiconductors.You need to carefully watch your power levels and heat dissipation.I mounted a small 1.5" 12V fan onto my RA-T2X-64E heatsink and I don't have overheating issues with loads up to 15V and 2.5A. Also, it's a good reason to use a 12V power supply for your load (i.e. 12V fans).An idea for an Arduino interface to watch power levels and limit them:

author: bsodmike
author_rank: Regular Contributor
timestamp: August 10, 2020, 02:02:59 pm
content: Quote from: MarkF on August 10, 2020, 01:43:14 pmThanks for all that info and fair warnings, yes, going with such a low-voltage current sense resistor and a shorted-MOSFET would surely wipe out the DUT and whatever's attached, eek!I've ordered this heatsink, which is almost identical to yours, just boasts (spec wise) a slightly better thermal performance (pretty much the same)https://export.farnell.com/aavid-thermalloy/6400bg/heat-sink-to-220-218-2-7-c-w/dp/1213475Just to give you a heads up, I was looking at pushing this to the extreme just for the "engineering" sake to gain more experience in this area. In reality I doubt I've ever try to push past 15V/2.5A (we'll see!).I've also already made changes to incorporate the 100Ω and 1KΩ resistors to the MOSFET. Right now I'm just passing 200mA and trying to see if I can stop the oscillations; I also plan to swap the opamp for a better high-precision one similar to the TLC272 and see what improvements I gain.I've also got one that does rail-to-rail in my parts bin, which has similar specs to the TLC.TEXAS INSTRUMENTS Operational Amplifier, 1 Amplifier, 5.5 MHz, SOT-23, 5 Pins 3117581RL OPA376AIDBVTG4Will report back on any progress made and any further questions! Thanks a lot for the Arduino idea as well, I'll look into that as well at a later stage once I get to designing the PCB.

author: MarkF
author_rank: Super Contributor
timestamp: August 10, 2020, 02:25:50 pm
content: QuoteYou miss understand. It's not the DUT but the Electronic Load that let out the magic smoke (i.e. the MOSFET and op-amp). The shorted MOSFET fed the high voltage back into the op-amp. It would also take out an Arduino if you end up going that route.

author: bsodmike
author_rank: Regular Contributor
timestamp: August 10, 2020, 02:58:06 pm
content: Interesting. Out of curiosity, when the MOSFET is shorted, what's the path current takes to destroy the Opamp, i.e. through the opamp's output (but I though that's high impedance?) or through the inverting-input (as per both our schematics). Sounds like the latter given the 5.6K or 1K resistor chosen.Assuming it is the latter, any ideas on how we can add some protection circuity to handle such a scenario? Ultimately it would be a 'current-limit' circuit, but the tricky part is "limit on short only"

author: MarkF
author_rank: Super Contributor
timestamp: August 10, 2020, 03:07:38 pm
content: Quote from: bsodmike on August 10, 2020, 02:58:06 pmHis explanation of what happened at time 12:00 (This is NOT a solution):He never did finish this design.

author: bsodmike
author_rank: Regular Contributor
timestamp: August 10, 2020, 03:23:04 pm
content: Ohh gotcha!! Thanks.

author: bsodmike
author_rank: Regular Contributor
timestamp: August 10, 2020, 04:28:54 pm
content: Quote from: MarkF on August 10, 2020, 01:43:14 pmIf I'm going to use my IRL640A, 65W @ 11A (Id=max)(Tc=100degC) = LOAD+ voltage of max 5.9Vdc65W @ 2.5A = LOAD+ voltage of max ~26Vdc (as per pqass).With regards to the use of 100R/1Kohm vs 620R/5K6, given that I've got a 10K trim-pot, any advantage to using the larger pair of resistors? i.e. they would offer better protection against any potential MOSFET shortage, but other than that I would "assume" loop-stability would be the same?Again, curiosity on my part, how have you faired with any potential PCBs you may have had fabricated? Did you run in to any issues, especially with the need for thicker traces for Vds/Ids?

author: bsodmike
author_rank: Regular Contributor
timestamp: August 10, 2020, 04:59:38 pm
content: I was just reading a comment by pqass (https://www.eevblog.com/forum/buysellwanted/wtb-(uk)-electronic-dc-load/msg3139992/#msg3139992) where he links tohttps://www.smbaker.com/raspberry-pi-controlled-dc-load.This has an image for adding MOSFETs in parallel.Again, I'm thinking for later on at least, thoughts on this approach? I'd include the 1kOhm on the feedback line.

author: pqass
author_rank: Super Contributor
timestamp: August 10, 2020, 05:02:25 pm
content: Quote from: bsodmike on August 10, 2020, 07:58:16 am11A is the max continuous current for the IRL640A.It was an assumption on my part since I didn't know what your max design current was until you just specified it.QuoteCorrect when using a 1ohm shunt.QuoteNot quite. 3.5V to 4.5V gate voltage (referenced from ground) is when the MOSFET starts to conduct (with a 1ohm shunt on its source and a design limit of 2.5A) but won't fully be on until 5V above your shunt max Vdrop. Therefore, 2.5V shunt drop + 5V fully-on gate voltage equals 7.5V. Your op amp must produce at its output at most 7.5V but since the output is always 1.25V below Vcc, therefore Vcc must be 7.5V+1.25V =8.75V.We don't really know unless we actually test it at what point the MOSFET will conduct only 2.5A. All we know is that at 5V gate to source voltage it will be fully on, conducting up to 11A comfortably (with proper heatsink of course). So the 7.5V max op amp output is actually conservatively higher than what is necessary.Why don't you experiment and give the op amp, say, 9V or 12V. Then ramp up the current to 2.5A and measure the op amp output voltage. Add 1.25V to that and now you have the minimum Vcc necessary to achieve your design goal.QuoteYou need to add the 2.5V shunt Vdrop too. So it's 8.75V.QuoteCorrect.QuoteCorrect. But that's not at 2.5A. That's just when the MOSFET starts to conduct.QuoteIncorrect. All we know is that at 5V above the 0.25V Vdrop is when the MOSFET is fully-on.So now if using a 0.1ohm shunt, Vshunt_drop+Vgs+opamp-to-Vcc output diff = 0.25V+5V+1.25V=6.5V minimum Vcc.QuoteCorrect.QuoteNo problem.I think it would have been easier for you if you had started with a typical Vcc of 9V or 12V since now you are seeing all the problems with non-ideal op amps having a max output x less than Vcc, MOSFET Vgs, Vshunt_drop that are high at high currents. It all adds up. Starting with a low Vcc gives you a very low ceiling.

author: pqass
author_rank: Super Contributor
timestamp: August 10, 2020, 05:18:38 pm
content: Quote from: bsodmike on August 10, 2020, 04:59:38 pmTypically, we want to limit each MOSFET in a TO-247 package to about 100W (for the IRFP250).But in your case, an IRL640A with a TO-220 package, you shouldn't put more than about 65W.So although you said you want to limit the current sink to 2.5A, you didn't specify what the max voltage you were designing for. So for a one device design with a max of 65W and 2.5A, gives us a max of 26V. If any more voltage, it will put more than 65W through the MOSFET. At which point, you add another device similar to the smbaker design.

author: bsodmike
author_rank: Regular Contributor
timestamp: August 10, 2020, 05:29:35 pm
content: Aha, you were referring to Rds(on) right? The datasheet states Vgs=5V. I read somewhere the MOSFET is considered full-enhanced (or fully on) and has achieved its rated Rds(on).Thanks - this is making complete sense now!Quote from: pqass on August 10, 2020, 05:02:25 pmHeh, am waiting on the heatsink to arrive; just been toying at VERY low currents.Regarding testing 2.5A draw (Ids), what's the lowest load-input voltage we can use (Vds) (voltage at LOAD+)?

author: bsodmike
author_rank: Regular Contributor
timestamp: August 10, 2020, 05:39:24 pm
content: Thanks for pointing out Id at Tc=100degC is 11.4A for the IRL640A, I was working off the 18A rating, which they misleadingly place in the title of the Datasheet and is the value used in parametric search!! Super misleading.In any case, 65W @ 11A = LOAD+ voltage of max 5.9Vdc65W @ 2.5A = LOAD+ voltage of max ~26Vdc as you've mentioned.Hmm, the IRL640A Datasheet states Pd (Tc=25degC) total power dissipation = 110W, but a linear derating factor of 0.88W/degC. Does this mean 75degC (on top of Tc=25degC) * 0.88W/degC = 66W @(Tc=100degC to match Id(max)=11A).Great!!

author: rdl
author_rank: Super Contributor
timestamp: August 10, 2020, 05:50:22 pm
content: Quote from: bsodmike on August 10, 2020, 02:20:52 amDave's original circuit.Apparently it's very dependent on layout and the actual parts used. I built the circuit on perfboard three different times using the exact same op amp and mosfet as Dave. It oscillated severely every time. I doubt I would have been able to get it working without an oscilloscope, but by following recommendations from others and some trial and error I eventually managed to get acceptable performance.

author: MarkF
author_rank: Super Contributor
timestamp: August 10, 2020, 06:07:32 pm
content: Quote from: bsodmike on August 10, 2020, 04:28:54 pmI don't see where you got your numbers.Comparison from the datasheets:IRL640A (TO-220)RDS(ON)= 0.18ΩContinuous Drain Current (TC=25°C) = 18ATotal Power Dissipation (TC=25°C) = 110WIRFP250 (TO-247)RDS(ON)= 0.085ΩContinuous Drain Current (TC=25°C) = 30ATotal Power Dissipation (TC=25°C) = 190WThe IRL640A is okay for testing. But if you intend on pushing the limits, I would seriously look at the IRFP250.As far as the PCB layout, here is what I'm using with the exception of the RC damping components:

author: Vovk_Z
author_rank: Super Contributor
timestamp: August 10, 2020, 06:39:33 pm
content: TO-220 case can safely handle about 40-50 W (for a long intime interval). TO-247 can handle up to 80-100 W.As for me I use only TO-247 MOSFETs in loads, mostly IRFP260 or IRFP460.I usually use two TO-247 in parallel for 100 W rated load (and newer had any shorted mosfet).

author: MarkF
author_rank: Super Contributor
timestamp: August 10, 2020, 06:46:48 pm
content: Quote from: bsodmike on August 10, 2020, 04:59:38 pmNo! No! No!If you want to put MOSFET's in parallel, each MOSFET needs it's own op-amp and sense resistor.Attached is also a load from HP.

author: pqass
author_rank: Super Contributor
timestamp: August 10, 2020, 06:48:37 pm
content: Quote from: bsodmike on August 10, 2020, 05:29:35 pmIf the gate is between Vgs(th) of 1V/2V (min/max) to Vgs (5V), then Rds(on) goes from very high to 0.18ohm at its minimum. The op amp will drive a voltage on the gate such that the Rds(on) at that gate voltage will produce a current of 2.5A Ids if that's what is dialed-in on the potentiometer. ie. makes the MOSFET behave like a variable resistor.QuoteYou could wait for a proper heat sink or you could just get any old hunk of sheet metal/aluminum and clamp the MOSFET to it. It should only take 10-20 seconds to set and measure the op amp output.QuoteWell, if you use a 1ohm shunt, then the voltage at the drain must be at least 2.5V (or 0.25V if 0.1ohm shunt is used). Given that Rds(on) is at max 0.18ohm, at 2.5A Ids you need a min of 0.45V Vds. Add the Vshunt_drop, then that's your min voltage that you must feed the constant current load at the +input.

author: bsodmike
author_rank: Regular Contributor
timestamp: August 10, 2020, 06:51:02 pm
content: Sorry, I corrected my post too late Mark.QuoteNote that I've referred Tc=100degC in the datasheet for a max use of 11A.pqass mentioned that TO-220 has a max-heat dissipation of ~65W.I've compiled the calculations discussed with @pqass into a summary PDF (attached). Please let me know if you spot any mistakes, thanks!

author: bsodmike
author_rank: Regular Contributor
timestamp: August 10, 2020, 07:11:10 pm
content: Quote from: pqassI also need to insulate the tab of the MOSFET and also ground whatever heatsink. Will see what I an scrummage! Thanks.Quote from: pqassYikes, so obvious now that I think aboutit but haha thanks for clarifying!! I've added the minimum-load calculations on page 3 of the PDF.

author: MarkF
author_rank: Super Contributor
timestamp: August 10, 2020, 07:18:24 pm
content: Quote from: bsodmike on August 10, 2020, 07:11:10 pmWhy? I didn't.My MOSFET is connected directly to the heatsink for better heat transfer.Then isolate both the MOSFET and heatsink.This all depends if you put in a case and how everything is mounted.I have been just using mine bare-board. Just a screw in each corner for feet.

author: pqass
author_rank: Super Contributor
timestamp: August 10, 2020, 07:21:11 pm
content: Quote from: bsodmike on August 10, 2020, 05:39:24 pmNever believe the first page intro. Before you get to the max Ids, you're going to hit the thermal wall first.The package determines how fast it can get rid of heat. You want to look up the Rθjc (junction-to-case) and Rθcs (case-to-sink), which for the IRL640A is 1.14 and 0.5 °C/W, respectively (which when added is 1.64). This means that for every Watt, there will be a rise of 1.64°C. But you can't exceed the Tj (max junction temp) of 150°C. So assuming you can maintain a 40°C heat sink temp, then you only have 150-40=110°C ceiling, which divided by 1.64 gives 67W (I rounded down to 65W).QuoteBut you forgot to also add the Watts burned in the shunt, so that raises it by another 11V (for 1ohm) or 1.1V (for 0.1ohm shunt) if you're designing for a 11A CC load.QuoteIf you run it at 11A with Tc (case temp) of 110°C, that only gives you 40°C before you hit the 150°C junction temp. Given the 1.14 °C/W Rθjc (junction-to-case), that should limit you to 35W, which at 11A only allows you to operate at 3.2V above the Vshunt_drop.Quote

author: bsodmike
author_rank: Regular Contributor
timestamp: August 10, 2020, 07:33:09 pm
content: Awesome, loving the info and detail hereIf you have time, please see my PDF and let me know if I missed anything. Going to keep it as a reference for this project log.

author: bsodmike
author_rank: Regular Contributor
timestamp: August 10, 2020, 07:40:36 pm
content: Hey Mark, thanks for the details on the PCB.(1) What connector did you use for the Ext 0-5V, looks like BNC?!?(2) PWR looks like a Sp-St switch (or Sp-Dt). Looks like you've shorted one side so it acts like an on/off switch.(3) I really like the Meter breakout. V referenced to (-), A referenced to (-) is the current in V and (+/-) is the raw 12v input.(4) FAN could use a diode to protect against back-emf maybe?(5) The raw ammeter point: what would you connect here, 4mm banana jacks and some thick wiring? Your design is 5V*2.5A = 12.5W, if I recall banana posts could work.

author: bsodmike
author_rank: Regular Contributor
timestamp: August 10, 2020, 08:20:28 pm
content: Revised calculations for 11A (see PDF), thanks!

author: MarkF
author_rank: Super Contributor
timestamp: August 10, 2020, 08:21:06 pm
content: Quote from: bsodmike on August 10, 2020, 07:40:36 pmPower switch is just ON/OFF. I just used the same SPDT switch in both places.My design is for a maximum of 24V @ 2.5A = 60WThe ammeter is for an analog meter. If you use one of the digital ones, be aware that most have the low side of their meter connected to their power supply ground. Connecting the digital ammeter to my eLoad and using the eLoads power supply will short the sense resistor. You will need an isolated power supply to power a digital meter.It's only 2.5A, so standard banana jacks would be okay.BNC Connector:https://www.mouser.com/ProductDetail/TE-Connectivity-AMP/1-1337543-0?qs=%2Fha2pyFaduhcpVJxkdxmIMsnSd2lrbI1QqLogsTxP6Y%3DSwitches (Quantity 2):https://www.mouser.com/ProductDetail/E-Switch/200AWMSP1T1A1M2RE?qs=%2Fha2pyFadujNM42l3EdMQfShiGXmWIdkR9iMGCP8DCpcgzRGBAr1dA%3D%3DFan (Haven't had a need for a diode):These are noisy. Adding a diode or two in series to lower the fan voltage to 11V or so helps a lot.https://www.amazon.com/gp/product/B01GPEQZ8A/ref=ppx_yo_dt_b_asin_title_o00_s00?ie=UTF8&psc=110-Turn Pot:https://www.mouser.com/ProductDetail/Bourns/3590P-2-103L?qs=%2Fha2pyFadui%252BXjMfCZnH9vxIImRmD8hXwKePD7irg%252BdFyxCEpRaoCg%3D%3DBanana Jacks (15Amps):https://www.digikey.com/product-detail/en/cinch-connectivity-solutions-johnson/108-0902-001/J151-ND/5929https://www.digikey.com/product-detail/en/cinch-connectivity-solutions-johnson/108-0903-001/J152-ND/5930

author: pqass
author_rank: Super Contributor
timestamp: August 11, 2020, 12:17:57 am
content: Just an FYI...An old desktop CPU heatsink with attached fan is super handy for a CC load.It's cheap (or free), readily available, and can usually sink 100W.That's what I used on my DIY CC load; it measures 60mm cubed (includes attached 50mm [between holes] fan).

author: bsodmike
author_rank: Regular Contributor
timestamp: August 11, 2020, 06:16:46 am
content: Quote from: pqass on August 11, 2020, 12:17:57 amThanks, I believe I have an old Intel heatsinks in my parts bin; will try that.I made some correction to my breadboard circuit, so it is much closer to the same circuit by MarkF. However, I noticed some odd behaviour and decided to make things REALLY simple, and test the current-limiting aspect.Code:# Notes:#- Opamp: LM342N#- MOSFET: IRL640A#- 1 Ohm shunt resistor used#- Current limit is the voltage taken from the 10kOhm trim-pot#which is tied to the second Opamp's (one connected to the MOSFET)#non-inverting input.##TEST 1LOAD input voltage:3VVcc (Opamp power):12vCurrent limit:1V (load off)Current limit:1.06V (load on)Load (Id):0.67AV(shunt):0.76VNow, we correct voltage across V(shunt) to desired 1V target (1A).Load (Id):1ACurrent limit:1.5V (corrected, load on)V(shunt):1.13VTurning the load off now,Current limit:1.41VCurrent limit (delta):+0.35V to achieve 1A across V(shunt)#TEST 2LOAD input voltage:5VVcc (Opamp power):12vCurrent limit:1V (load off)Current limit:1.09V (load on)Load (Id):0.92AV(shunt):1.02VNow, we correct voltage across V(shunt) to desired 1V target (1A).Load (Id):1ACurrent limit:1.197V (corrected, load on)V(shunt):1.10VTurning the load off now,Current limit:1.10VCurrent limit (delta):+0.097V to achieve 1A across V(shunt)(1) I noticed that as I increased the Load input supply voltage, it throws off the current-limit voltage. The idea of constant current is that once the pot value is set (say 1A), whether the Load input is 3V or 5V or 10V, it will draw a constant current of 1A.Either I've seriously screwed up somewhere (will debug this further) or this is due to the oscillations in the comparator-MOSFET loop?(2) Attached image shows:3V load input, at I(d) = 1A.Current limit = 1.481VVshunt = 1.10VYellow trace: load voltageGreen trace shunt voltage(3) Test circuit is using 10nF, 100R and 1kOhm on the second Opamp. I also tied 12V input to 1KOhm to prevent the MOSFET locking up in saturation.**UPDATE***:The YouTube video was removed as I had made a wiring mistake, so I didn't want to mislead anyone going through this in the future. However, I can see it could confuse those reading the comments by Mark/pqass where they talk about having seen the video.TL;DR - I was seeing oscillations at 3Vdc very similar to the ones discussed here (https://youtu.be/8xoGhZQO29Y) - I was seeing phase shifting and the similar oscillating output on the Gate, load and Vshunt between 62 - 95kHz.I had mis-wired the feedback loop, and connected the inverting output directly to the Rsense resistor.My latest fix (12th Aug 2020) was trying a 1uF capacitor between the MOSFET gate and ground, clearing up all oscillations.

author: bsodmike
author_rank: Regular Contributor
timestamp: August 11, 2020, 10:25:39 am
content: Hi all --Please ignore the bulk of the last post - I didn't want to delete it as it may confuse those having read it already.Today I performed some crucial testing to ensure the 'constant-current' aspect is working, and I have verified this. There is a bit of deviation as the load-input voltage increases but I'll come back to looking at that issue later on.

author: bsodmike
author_rank: Regular Contributor
timestamp: August 11, 2020, 11:31:45 am
content: Hi Mark - I was looking at the HP schematic you sent earlier and I noticed the use of fuses on both critical current paths (Opamp output and inverting input).They've gone with 62mA for the Opamp output and a suitable one at the MOSFET source. I suppose fast-blow fuses would be good and/or some applications could use HRC fuses instead? Just a thought.(2) I also digress a bit -I find the HP circuit interesting as they are using another Opamp for measuring the voltage across the shunt resistor. The only external control seems to be 'IPROG'. UNREG and TURNON are connected such it seems that if UNREG is high and TURNON is low, all the diodes will terminate current flow into the MOSFET gates. UNREG high and TURNON high should potentially allow the MOSFETs to operate.

author: pqass
author_rank: Super Contributor
timestamp: August 11, 2020, 02:22:10 pm
content: Quote from: bsodmike on August 11, 2020, 10:25:39 amTry shortening the wires (to the power source, to any meters).I just tested this on my DIY CC load and found that if I had my bench meter (with long leads) measuring DC current in the load-source loop (12V battery as source), that I had terrible oscillations of 4V@54kHz (scope between +load-in and GND) when drawing 2A. But then I replaced the meter with a short 30cm wire (passing through a DC clamp ammeter), the oscillations went away. If I elongated the pair of wires to the battery source by 60cm, the oscillation came back to 2V@54kHz when drawing 2A.This was the first time I've looked at it with my scope; was relying only on [lying, averaging] meters to inform me when it was built. I only have a 100ohm gate resistor and not the low-pass filter in the feedback loop. I'll be sure to add it later and retest. At worst, maybe this will be another condition of use like any other specification; ie. that shall use only short fat leads.Also, I think the usual bench setup (with breadboards and all sorts of meters with long leads) lends itself to generating phantom problems (oscillations and ringing). Maybe this is an opportunity to do it the Jim Williams way; air-wire prototyping, using ground planes, short fat wires with higher current lines,  sense wires, twisted pairs, etc. Damn parasites!

author: MarkF
author_rank: Super Contributor
timestamp: August 11, 2020, 03:03:42 pm
content: Quote from: bsodmike on August 11, 2020, 11:31:45 amThe main objective of the HP schematic was to show how to connect MOSFET's in parallel.You don't need most of the instrumentation for load monitoring and info display.If you want a fuse, place one in series with the drain of the MOSFET and the load.The fuse will just keep things from melting. Not from letting the magic smoke out.They have an op-amp to amplify the shunt voltage because the shunt value is very small.In watching your video, it made me wonder how you have the scope probes connected. Especially, the yellow trace and ground.Do you mean, you have one probe on the 'drain' and another on the 'source'?With the eLoad's power supply isolated from the DUT, you connect the eLoad in the same manner as you would an ammeter.I would clean-up you wiring and only probe what you need to.Maybe just an ammeter and the scope on RSENSE.Also, try a different op-amp if you have one. The inputs must be able to go to ground.

author: MarkF
author_rank: Super Contributor
timestamp: August 11, 2020, 03:16:06 pm
content: Also, your 10-turn adjustment pot output should go through an op-amp BEFORE going to the 10:1 voltage divider resistors. Otherwise, the two will interfere with each other.

author: bsodmike
author_rank: Regular Contributor
timestamp: August 11, 2020, 04:15:08 pm
content: Hey Mark / pqass --After posting the video, I went back and spent sometime going through everything and finally - a wiring screwup!! But I learned a lot in the process (more on that later). I had the inverting input STILL tied to the RSense resistor!!So, as per that video, all those issues I was showing you at 3VDC. Now, totally gone!!However, I am now seeing some oscillations at 5Vdc (with the 1Ohm resistor) and testing at 1A through the load I(d). That said, this is still the minimum requirement for V(ds), so I tried 10V.Nope, things look nice and clean now! See attached image.Other things wrong in the video(1) I had a Fluke showing V(g), yes it was the "Gate voltage", but I also have that on the 'scope (brown colour). I've replaced that scope to monitor V(gs).(2) Second photo shows the fixed circuit.However...Remember Peter Oaks' video from your earlier comment (https://www.eevblog.com/forum/projects/constant-current-dummy-load-ran-a-gutter/msg3181972/#msg3181972) - He talks about using a signal generator and he managed to get a DC-square wave from his "AWG". He mentioned running "500ms pulse, 100Hz, with 2A shift in voltage"?So it sounds like he's doing something similar to what one would call a "step-response", where we simulate an Input load turning on. Any tips on how to get this going and get _that_ square wave on to the my 'scope?I'll check my PSU manual on getting its duty-cycle feature working.Quote from: MarkF on August 11, 2020, 03:03:42 pm(1) Yellow trace is simply the LOAD input voltage, so it is connected to the PSU supply at +/-.(2) No... I cannot connect the 'scope to Drain/Source as that would cause a short due to mains-earth termination in the Scope probe. I need a differential-probe for that, so thankfully I was cautious and used the Fluke 289 (backlit display) to monitor V(ds).(3) Green trace: Rshunt.(4) Brown trace: (Orange really). MOSFET gate voltage, taken right after the 100 Ohm resistor from the Opamp output.Quote from: pqass on August 11, 2020, 02:22:10 pmThat makes sense, earlier today before I fixed my wiring blunter, I was seeing oscillation around 62 - 90kHz, possibly in the 2V region.Cheers for the tip on air-wiring, mmm that sounds interesting!I should be up in about 8 hours time and will share further info on the oscillations I'm seeing. I'll also use my free time to sort out the CPU cooler heatsink hack.

author: bsodmike
author_rank: Regular Contributor
timestamp: August 11, 2020, 04:25:59 pm
content: I replaced the 10:1 divider with a 10-turn pot, is that OK?

author: MarkF
author_rank: Super Contributor
timestamp: August 12, 2020, 01:08:53 am
content: Quote from: bsodmike on August 11, 2020, 04:15:08 pmIt's simple.Just connect a function generator to the 'external input' of my eLoad circuit.Making sure the amplitude is between 0V and 5v.Also, make sure youset the DC offset so it does not go negative.The max frequency I was able to switch my load was 5KHz.QuoteSee Peter's performance testing video at timestamp 19:35 where he shows his computer heatsink and fan.

author: bsodmike
author_rank: Regular Contributor
timestamp: August 12, 2020, 05:12:01 am
content: Awesome, thanks Mark!Here's my current circuit on the Bench (see attached).- Final Fix: Adding a 1uF capacitor between the MOSFET gate and ground,clearing up all oscillations.- This currently can drive the Opamp non-inverting (max) voltage as high as 12Vdc. This is not-needed and totally overkill as V(gs) does not need to be > 5Vdc. Note: IRL640A has V(gs) max @ 20Vdc.- Finally, the loop is oscillation free throughout the 0-15Vdc range. (Tested 0-10Vdc@2A)Changes that I will be making.- In my case, I will use my PSU's signal generator feature by providing a 5Vdc square wave with a low-current Over-current limit as the Ext. input.- Will limit Opamp non-inverting (max) voltage to 5Vdc.- I will also remove the 10K trim pot and instead use a 10:1 divider as suggested (in the final PCB).- Fuse on the Drain.Here's a quick overview:https://www.youtube.com/watch?v=TY5WDcn3OoA&feature=youtu.beAnother quick Q: Is there a difference if the Ammeter is placed in series before the Drain, rather than just before the Rsense resistor? I can't recall from my old lectures (almost a decade ago) if any current flows into the Gate (from the Drain); currentshouldonly flow Drain to Source across the channel.

author: ledtester
author_rank: Super Contributor
timestamp: August 12, 2020, 05:42:28 am
content: Hi Mike,In your schematic, what's the purpose of the 12 V -- 1 K ohm -- R sense -- GND part of the circuit?(btw, I would screw the MOSFET into one of the hard drive mounting screw holes for better thermal contact and rigidity)

author: bsodmike
author_rank: Regular Contributor
timestamp: August 12, 2020, 06:10:13 am
content: Quote from: ledtester on August 12, 2020, 05:42:28 amThanks for the tip!See original explanation by MarkF:Quote from: MarkF on August 10, 2020, 03:44:31 am

author: MarkF
author_rank: Super Contributor
timestamp: August 12, 2020, 08:08:09 am
content: Quote from: bsodmike on August 12, 2020, 05:12:01 amNo.The critical thing here is that the op-amp will do what it needs to do in order to have its inputs at the same voltage.In other words:- Your 10:1 voltage divider applies a 0V to 500mV voltage (with respect to the ground (i.e. the low side of the sense resistor)) to the (+) input.- The op-amp will drive the MOSFET until the (-) input matches the (+) input. Essentially, making the voltage across the sense resistor equal to the (+) input.- Therefore, controlling the current by maintaining a constant voltage across the constant value sense resistor.The voltage out of the 10:1 voltage divider is maintained across the sense resistor. This sets the current.

author: MarkF
author_rank: Super Contributor
timestamp: August 12, 2020, 08:23:58 am
content: Quote from: bsodmike on August 12, 2020, 06:10:13 amThis is only a partial fix.That 1KΩ resistor ONLY allows you to turn 'OFF' the MOSFET by setting the current (10-turn pot) to zero.If you don't turn the level down, the MOSFET will still be driven to 'full ON' until some current is available from the DUT.I often wondered if Jay_Diddy_B's circuit, where the set value is summed into the (-) input of the op-amp, doesn't suffer the same sight voltage reference issue.But, I opted for a single power supply over a complete fix. Frankly, it's not a big deal to me to turn the level down until the DUT is powered up.Or just let the DUT see the brief short while it's powering up.

author: bsodmike
author_rank: Regular Contributor
timestamp: August 12, 2020, 10:04:54 am
content: Quote from: MarkF on August 12, 2020, 08:08:09 amGreat, that was my initial understanding. Having checked the datasheet again, I noticed we are in fact applying a charge to the gate (hence the Opamp provides a current to the gate). I only asked this as on my bench I placed the Ammeter at the Drain connection with the assumption that the I(d) set via the 10:1 divider (or Vcontrol) stayed within the V(ds) to GND path through the Rsense shunt.With regards to the current applied by the Opamp to the MOSFET gate, typically how many mA is this (~30mA?), I suppose it can be inferred via I = dQ/dt (I digress!).

author: bsodmike
author_rank: Regular Contributor
timestamp: August 12, 2020, 01:36:33 pm
content: Here's my modified sketch of the bench setup.ERRATA:IC1 & IC2: LM324N

author: MarkF
author_rank: Super Contributor
timestamp: August 12, 2020, 05:27:55 pm
content: Quote from: bsodmike on August 12, 2020, 10:04:54 amA BJT transistor is a 'current' controlled device. The current from 'collector' is controlled by the 'base' current and transistor gain.A MOSFET transistor is a 'voltage' controlled device. The 'drain' current is controlled by the 'gate' voltage. The 'gate' of a MOSFET applies a field which allows the 'drain' current. The 'gate' essentially looks like a capacitor.Some visual aids:

author: Vovk_Z
author_rank: Super Contributor
timestamp: August 12, 2020, 09:13:28 pm
content: Quote from: bsodmike on August 11, 2020, 04:25:59 pmOk.Here is an one more example how to do el. load:

author: Jay_Diddy_B
author_rank: Super Contributor
timestamp: August 12, 2020, 10:14:06 pm
content: Hi,I have been half following this thread. Can the OP give a summary of the requirements:1) Maximum Voltage?2) Maximum Current?3) Maximum Power?, not necessarily Imax x Vmax4) Do you need a static load or do you want to be able to step the load current?5) Do you have any MOSFETs that you would like to use?6) Would you prefer to work a single supply or are you open to using +/- power supplies?7) Are you concerned about the transient that is caused if the load is turned on before the power supply (or hot plugging)?I measured the hot-plugging event on my dynamic load project. The results are in this message:https://www.eevblog.com/forum/projects/dynamic-electronic-load-project/msg2725634/#msg2725634Regards,Jay_Diddy_B

author: bsodmike
author_rank: Regular Contributor
timestamp: August 13, 2020, 09:29:52 am
content: Quote from: Jay_Diddy_B on August 12, 2020, 10:14:06 pm1) Maximum Voltage?15Vdc2) Maximum Current?2.5A. I will be using a 0R1 sense resistor in the PCB version.3) Maximum Power?, not necessarily Imax x VmaxThe 0R1 is a TO-247AC package although I've designed around the 65W limitation of TO-220.4) Do you need a static load or do you want to be able to step the load current?Load current can be stepped using the Ext. input using a Microcontroller.5) Do you have any MOSFETs that you would like to use?Right now I'm aiming at the IRL640A, but later I may design a separate version for higher current handling around the Infineon HEXFET series, i.e IRFP250MPBF6) Would you prefer to work a single supply or are you open to using +/- power supplies?Single supply.I added a 1uF across the MOSFET gate and ground, this seems excessive. I will later test my breadboarded version with a smaller cap.Quote from: Jay_Diddy_B on August 12, 2020, 10:14:06 pmIn your thread you've said it backwards "This is 'hot plugging'. Turning the supply on first before connecting the load."??.Hot-plugging should be the scenario where the DC Load is turned on with the MOSFET allowing I(d) to flow and the DUT "Hot-plugged", turned on after the fact. As Mark described, this presents a short to the DUT?I'm ok with Mark's approach of adding the Rmin_resist 1k Ohm and dialing the input pot down to 0V.Attached my current breadboard version for testing.

author: bsodmike
author_rank: Regular Contributor
timestamp: August 13, 2020, 09:42:43 am
content: Quote from: MarkF on August 12, 2020, 05:27:55 pmYes, this brings back memories. The canal is also called the channel at times. Given that the gate acts like a capacitor means it is being charged, which means current must flow into the gate (for it to charge). Hence a small current must be drawn from the Opamp.If I have time I'll try and measure this with a DMM.

author: Jay_Diddy_B
author_rank: Super Contributor
timestamp: August 13, 2020, 11:57:31 am
content: Hi,Based on your input I have built a quick LTspice for your load using the IRL640A MOSFET.I got the Crss, Ciss and Gm numbers from the On-Semi datasheet. I could not find a SPICE model for the IRL640A.The LT1014 is essentially 'a better' LM324. It has similar characteristics but lower offset voltages. The GBW and all other parameters are similar.I have included the lead inductance between the load and the power supply under test. I have included the damping network that is needed to 'tame' the lead inductance. You will find this network in all commercial loads.The lead inductance, you get for 'free', the damping circuit you have to add.Loop gainThe loop should be stable with a 1nF capacitor between the op-amp output and the inverting input.The loop bandwidth is a very respectable 90kHz.Start up transientIf you hot-plug this load there will be a transient lasting about 15 to 20us while the op-amp slews from the positive rail to the operating point.Start up transient suppressionThe start up transient can be suppressed by adding a circuit that turns the load off if the input voltage is less than a certain value. I have selected 1V.With this circuit added the load starts at zero current and ramps up when the voltage is applied:Regards,Jay_Diddy_B

author: bsodmike
author_rank: Regular Contributor
timestamp: August 13, 2020, 12:10:21 pm
content: Quote from: Jay_Diddy_B on August 13, 2020, 11:57:31 amNice, can you share the plot of the startup-transient without the suppression circuitry?Been a while since I read Bode plots: how do you read the phase margin, given -180deg is off the chart? What's considered a respectable phase-margin?

author: Jay_Diddy_B
author_rank: Super Contributor
timestamp: August 13, 2020, 02:06:22 pm
content: Quote from: bsodmike on August 13, 2020, 12:10:21 pmFor two different values of feedback capacitor on the opamp.Quote from: bsodmike on August 13, 2020, 12:10:21 pmThe way I drew the Bode plot and the way I measured loop gain, the phase axis is phase margin. This is how you read it:You have to allow some extra margins, because the transconductance of the MOSFET and the MOSFET capacitances change with the operating point.Regards,Jay_Diddy_B

author: bsodmike
author_rank: Regular Contributor
timestamp: October 20, 2020, 02:27:54 pm
content: Hi all,With limited time available, I have been able to test the first run of prototype PCBs with a design based on a 40x100mm PCB, so that it can slide into the Hammond box.Decided to make some improvements and changes and v0.5 of the PCB is now being made and that's the version I will be using in my build.I'm still working out some changes on the Hammond box design and will post an update once further progress is made with the CAD modelling.