Author Topic: DC Load  (Read 4382 times)

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Offline paulca

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DC Load
« on: November 24, 2017, 02:14:54 pm »
So I'm thinking my next project would be a DC load.

I love the look of this:
https://www.instructables.com/id/Arduino-Programmable-Constant-Current-Power-Resist/

However I think it's a little beyond me.  To build it I would need to order the PCB and it's all surface mount, some of the components are harder to find in the UK so I am likely to make many mistakes.

What I need is something a little lower level, but can be extended to have PWM control from an arduinno or PI later and ADCs for sense and display.

As I want to test LiPo charger circuits etc.  I need something with a decent bit of load, something like 100W.

What about the one Dave outlined in his video, would I be better off trying that one out?

Should I just start at the business end with a chunky mosfet and sense resistor set and work back?

Any defacto threads on here for similar projects?
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Offline IonizedGears

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Re: DC Load
« Reply #1 on: November 24, 2017, 02:20:08 pm »
Look into the YouTube channel ScullCom. He does a bunch of wonderful videos and has a pretty good series on designing and building a DC load.

Sent from my Pixel XL using Tapatalk

I am an EE student with interests in Embedded, RF, Control Systems, and Nanotech.
 

Offline paulca

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Re: DC Load
« Reply #2 on: November 24, 2017, 03:52:20 pm »
That looks good, thanks.  It looks like he does the development iteratively which means I can get something running fairly quickly and then follow his later videos to improve upon it.

Thanks.
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Offline Avacee

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Re: DC Load
« Reply #3 on: November 24, 2017, 04:02:54 pm »
Peter Oakes has a couple of Youtube videos showing similar:
The second video shows some performance improvements regarding response times.

There's also 2 Blog posts on Element 14 with comments and schematic for your viewing pleasure
https://www.element14.com/community/groups/test-and-measurement/blog/2015/06/02/electronic-dc-load--design-and-build-to-test-psu-project
https://www.element14.com/community/groups/test-and-measurement/blog/2015/06/09/electronic-dc-load--performance-improvements




 

Offline MarkF

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Re: DC Load
« Reply #4 on: November 24, 2017, 05:29:26 pm »
Here is my version of Peter's design.  I choose components to limit the load to 2A.

EDIT:  It definitely needs a fan for currents over 0.5A
« Last Edit: November 24, 2017, 05:37:00 pm by MarkF »
 

Offline paulca

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Re: DC Load
« Reply #5 on: November 24, 2017, 06:06:57 pm »
Basic question time. 

Load sensing.  I get the gist.  Ohms law.  If a resistor of a given value has a given voltage drop across it then the current can be calculated.  Put that voltage into an op amp of some form and provide it was a reference voltage and it will alter the control output (to the MosFet) effectively limiting the current to the set value (the reference voltage on the opamp).

However, does this not mean you are always putting the full current load through the sense resistor?  So if you are trying to limit to something like 20A at, say, 15V you need a resistor capable of carrying 15*20=300W?

Is there not a way to decouple the sense circuit somehow, by, say, dividing off 1/100th of the current through a 1K or 10K resistor and then multiplying it with a gain of 100 on the opamp?

The other thing I get a little confused about is, does the current sense resistor not then alter the actual output load current/voltage? (Assuming for this it's not a dummy load, but limiting current to an actual load)

EDIT: I realise I'm confusing two different things.  In a dummy load I *must* consume all the current, that's the point.  In a current limit power supply I instead want to alter the load current and voltage as little as possible so I would sense differently.
« Last Edit: November 24, 2017, 06:27:14 pm by paulca »
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Offline paulca

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Re: DC Load
« Reply #6 on: November 24, 2017, 07:14:42 pm »
In Peter Oaks circuit.  Between the highside + and - there are only the mosfet and the sense resistor.

If I plug in some numbers at the sense resistor:
r=0.1(ohms)
i=5(amps)
v = i*r
v = .5
p = iv
p = 2.5w

But... if I connect a 12V supply (that I want to load) to V+ and V- there is 12V to drop, the sense resistor (if we have 5A) is only dropping 0.5V, so the mosfet must be dropping 11.5V.

The mosfet therefore is taking 11.5V * 5 Amps = 57.5W

Now I understand why the heat sink is on the mosfet.  What I don't understand is how the mosfet voltage drop can be changed.

If I think of the MosFet as a resistance then at 5Amp, 12V it is presenting a resistance found by:

r=i/v
r=5/11.5
r=0.43Ohms

If I run those with 12V and 1Amp

Sense resistor drop:
r=0.1 Ohm
i=1 Amp
v = i*r
v = 0.1V
p = v*i
p = 0.1 Watts

Mosfet:
v = 11.9
i = 1
r = v/i
r = 11.9 Ohms
p = v*i
p = 11.9 watts

So if I wanted to run it at 20A I would see:

rSense = 0.1 Ohms
iSense = 20
v = i*r
v = 2V
p = i*v
p = 20*2
p = 40W

vMos = 10V
iMos = 20A
rMos = 0.5 Ohms
pMos = 200W

I think I see now.  Still need to working on exactly how the voltage drop changes across the mosfet, expect the whole op amp thing that says "It must" for the maths to work.
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Offline MarkF

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Re: DC Load
« Reply #7 on: November 24, 2017, 07:21:48 pm »
The idea in Peter Oakes design is that he applies a 0-500mV voltage to the positive op-amp input. This corresponds to a 0 to max current through the load and sense resistor.  The op-amp supplies the necessary voltage to the MOSFET in order to have 500mV across the sense resistor at max load. 

In your example for a 0-20A load at 15V, the value for the sense resistor is 500mV / 20A = 0.025 ohms with a power rating of at least 10W.  The power dissipated by the MOSFET is (15V - 500mV) * 20A = 290W.
 

Offline MarkF

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Re: DC Load
« Reply #8 on: November 24, 2017, 07:29:40 pm »
Dave did a dummy load along the same design.  A little different perspective my help.


 

Offline paulca

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Re: DC Load
« Reply #9 on: November 24, 2017, 07:44:27 pm »
I think I can set something up to play with what I have now.  Just at mA current. 

Using a basic NPN transistor instead of a mosfet and aiming for 200mA (the limit of my transistor), I could in theory use 5x10R resistors in parallel to create a 2 Ohm sense resistor.  The sense voltage would then be 400mV.  I have an LM741 op amp.

The PSU will limit me to 200mA when (not if), when I mess it up, but I might have a play while I watch the rest of the videos and order some mosfets and sense resistors.
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Offline paulca

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Re: DC Load
« Reply #10 on: November 24, 2017, 07:46:23 pm »
Dave did a dummy load along the same design.  A little different perspective my help.

I watched this at least twice, but the bit I was always missing is my epiphany above.  I was always doing the sums as if the full voltage drop was across the sense resistor and wondering why it wasn't on fire.  When run Ohms law from the "known current" perspective on the sense resistor it starts to make more sense.
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Offline David Hess

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Re: DC Load
« Reply #11 on: November 25, 2017, 04:53:43 am »
The sense resistor sees all of the current but only a fraction of the voltage which is just as well because heating of the sense resistor and its temperature coefficient of resistance is what ultimately limits accuracy.
 

Offline paulca

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Re: DC Load
« Reply #12 on: November 25, 2017, 06:45:41 pm »
Out of interest the attempt at using NPNs and a load of 10R resistors ended badly with one dead transistor and some funky smells :)

Using the circuit from Wikipedia.
https://en.wikipedia.org/wiki/Current_limiting

It's possibly because I didn't have an actual load and was just trying to use a 5k pot as the load.  So the resolution around the "couple of ohms" load was too small and if it went to sub ohm the circuit stopped working and collapsed to a short circuit heating the transistors up. 

Anyway it was incredible hard to balance things with such dicky little transistors and using the 0.6V base to emitter voltage across the sense resistor.

I did get it to limit a couple of times, but at around 250mA but then it quickly collapsed to a short as the NPNs over heated.

A 2n2222 bought the farm in my last attempt.  I don't think it liked 1 amp.  I wasn't paying attention, didn't notice the CC limit light on the PSU, forgot I had dialled it up to 1 amp, but I noticed the smell.  Poor thing.  It now identifies as a couple of resistors or a diode and a resistor in the component identifier gizmo.
« Last Edit: November 25, 2017, 06:47:35 pm by paulca »
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Offline paulca

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Re: DC Load
« Reply #13 on: November 29, 2017, 08:06:14 am »
So I got most of my bits in the post for the dummy load.  No enclosure yet, but I want to breadboard it for a while, then decide on perf board and fan/heatsink or what-not and only then will I know it's size.  Ordered to Peter Oakes spec.  At some point I'll need to consider a meter on it too.

I also got a much smaller 12A mosfet and I thought I would try a baby 1A load first.

So I did my sums.  I need 4V to turn the mosfet on.  I tested that and it starts to conduct a bit lower than that, but is more or less fully on at 4V.  So I need 4V out of my op amp max.

The 0.1Ohm resistor at 1A will only have a 0.1V drop and 0.1W of heat.  Therefore I need a gain of 40 on the op amp.

I only have one of the nice opamps that Peter uses at they were a few quid, not pence, so I don't want to cook it accidentally until I have experimented.  So if I want to use a cheapo LM741, it's minimum output for a 0-9V rail is about 1.3V and it's maximum around 8V.  So it should be fine without splitting the rails.  Assuming the mosfet doesn't start to conduct at 1.3V.  I have a hard time trying to read the various log graphs on the mosfet datasheet.

Anyway.  If I connect a 3S lipo to the mosfet the voltage (from storage charge) will be 11.3V (ish).  Meaning a 11.2V ish drop across the Mosfet.  11.2*1A = 11.2Watts.  I have the a small PC chipset heatsink I ripped out of an old server that I'm sure will take 12 watts if I bolt the mosfet to it.

Can anyone see why this won't work?

I'll test the opamp gain first for 0V to 0.1V input and make sure the LM741 will respond to 0.1V properly and give 4V out.  Then I'll connect it to the gate of the mosfet.  I'll keep the DMM measuring current out of the LiPo so I can pull the plug if it goes run away.  The LiPo is question shouldn't bat an eye lid at delivering 50A if I let it.

Could I use a LM393 comparator instead if the LM741 doesn't work or would that make things more complicated?
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Offline paulca

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Re: DC Load
« Reply #14 on: November 29, 2017, 01:43:47 pm »
I believe this makes more sense:

http://tinyurl.com/yd74fhye

Adjustable between 9mA and 2A.
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Offline MarkF

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Re: DC Load
« Reply #15 on: November 29, 2017, 02:51:18 pm »
Here is the circuit and actual parts I used for my version of Peters circuit:
I did not choose exact value and small tolerances for the resistors because I don't care about precision settings. My capacitor values are a little larger than Peters because I don't need a high frequency response.

With the heat sink I used and NO fan, I reached the maximum power of the MOSFET with a 1A 15V load for a short duration. I believe with the small fan listed above, you could reach a 2.5A load. Peter's 5A load will require a much larger heat sink.

Also, I believe Peter made a mistake with the 5V reference adjustment pot (R2). According to the datasheet, it should be tied to the output pin instead of the input pin.

   
« Last Edit: June 05, 2019, 09:42:13 pm by MarkF »
 

Offline paulca

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Re: DC Load
« Reply #16 on: November 29, 2017, 03:17:22 pm »
I am planing on using a CPU heatsink with it's fan for the mosfet.  I'm not sure of the power rating.  It was originally sinking an AthlonXP 2.4GHz a quick google suggests that CPU was designed for 60-70W dissipation. 

Given a use case of a 3S LiPo capacity tester at 12.6V that gives me a maximum theoretical load of 70/12.6 = 5.5A  (or 70/(12.6-0.5)=5.7A)  So if I follow Peter's circuit directly (which I believe is for 5A) I should be fine.  But will need to keep an eye on the mosfet temp.

I even ordered drill bits to drill a hole in the heat sink for the Mosfet bolt.

Next thing I want to research is thermal monitoring and possibly limiting.  I have worked with Dallas 1-wire sensors and the raspberry PI and if I implement the control signal via it's DAC I could thermal limit based on a 1-wire, but I'm sure there are other options.

I doubt I will run it at 5A for very long.  To test a 5Ah LiPo it would be "nice" to run it at 5A, but I would settle for half that.
« Last Edit: November 29, 2017, 03:20:24 pm by paulca »
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Offline MarkF

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Re: DC Load
« Reply #17 on: November 29, 2017, 03:28:49 pm »
Given a use case of a 3S LiPo capacity tester at 12.6V that gives me a maximum theoretical load of 70/12.6 = 5.5A  (or 70/(12.6-0.5)=5.7A)  So if I follow Peter's circuit directly (which I believe is for 5A) I should be fine.  But will need to keep an eye on the mosfet temp.
If your target max load is 5A, that is fine.
If your target max is eventually something smaller, you will have finer control if you use a larger sense resistor.
Testing with the sense resistor value you will use in your final design will give a feel on how your load is going to respond in general use at varying currents and voltages.
 

Offline MarkF

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Re: DC Load
« Reply #18 on: November 29, 2017, 03:49:37 pm »
In looking at the ref02C spec, it indicates that the min sink current is 8mA. The 10 turn adjust pot should be something like 500 ohms. However, my load is working okay with a 10K ohm pot. Maybe a smaller pot then Peter and I used would be better.
 

Offline paulca

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Re: DC Load
« Reply #19 on: November 29, 2017, 04:16:33 pm »
In looking at the ref02C spec, it indicates that the min sink current is 8mA. The 10 turn adjust pot should be something like 500 ohms. However, my load is working okay with a 10K ohm pot. Maybe a smaller pot then Peter and I used would be better.

I have to watch his videos again, but there are two pots aren't there?  The main control pot and the fine adjustment on the vRef?

I bought a large wire wound pot from ebay for the main adjust pot (it's 50K) and I have a few different 10 turn trim pots, I think I have 1K and 10K.

I'll have to rewatch his video later to revise, I'm in work so it's awkward to watch youtube atm.

I hope to create the smaller <1A load this evening or tomorrow evening to get a feel for things.

EDIT: I can see in the video preview his schematic shows both trim and adjust as 50K pots.  I can make the trim a 1k, 10k, or 50K is I need to.
« Last Edit: November 29, 2017, 04:18:57 pm by paulca »
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Offline MarkF

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Re: DC Load
« Reply #20 on: November 29, 2017, 04:44:49 pm »
I have left the trim pot unpopulated on my PCB. Depending on the precision you are trying to achieve, you may not need the trim pot. I just dial in the current I want by watching the current meter.

I would start with a 1K ohm 10 turn pot for the main adjustment.

Attached is Peter's first design and his new design that I haven't seen him finish. Remember, Peter burnt up his original design.
 

Offline paulca

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Re: DC Load
« Reply #21 on: November 29, 2017, 08:47:45 pm »
I let some smoke out.  Only a little.  My breadboard wires didn't like 4A.

I've been gotten by the b****rd LM741 again.  The thing is so annoying.  No matter how well you plan a circuit out, no matter how much maths and simulation you do, the LM741 has some form of limitation that makes it do the exact opposite of what you wanted. 

After the 4 amp incident I powered the circuit from a 9V battery and used the current limited PSU to feed the hot side at 1A limit.

So I probed while the limiter kept things fairly sane.

Non-Inverting input:  65mV - Set voltage for 650mA.
Inverting input (from the sense resistor): 100mV (1A sensed)
LM741 output: 8.08V

WTF?  Have I jumped into an alternative reality or did I completely miss something?  The inverting input is higher than it's non-inverting it should be driving the output to the lower rail.  Why it's pinned to the upper rail?

I put the lm741 into voltage follower mode, putting it's output directly back onto it's inverting input.  1.9V was the lowest it would go on the output.  I tested by winding up the non-inverting input until it finally unpegged and started climbing once I passed 1.9V

So not only is it's output limited to something like 1/4 Vcc above the lower rail, it's inputs are also limited so something considerably higher than 100mV.  In my tinkering with different sense resistors and cooking a few 10 Ohm resistors till they burnt my fingers, I could not get the op-amp to respond, even at 1.6V inputs, the two inputs stayed apart and the output stayed pinned at 8V

Do you think microwaving an LM741 will help?  By that I mean, do you think it will make me feel better?

On the brighter side the LiPo should have been capable of 10A+, which might have cooked the 12A mosfet, my DMM fuse and melted my jumper leads and possibly smoked the LiPo itself.  So there are positives that it only put out 4A (in fairness it's only at storage voltage and it's only a doddy wee 500mAh 3S pack.

Do I really want to start splitting the rails, coupling caps and that lark or finding a better op-amp?
« Last Edit: November 29, 2017, 08:55:33 pm by paulca »
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Offline MarkF

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Re: DC Load
« Reply #22 on: November 30, 2017, 12:40:06 am »
The LM741 is NOT a single supply op-amp. Nor is it a rail-to-rail op-amp. Its output will not be able to get low enough to control the MOSFET. You will need to bodge together a dual supply for the LM741 or use a single supply op-amp. Use a TLC272 or Dave's LM324 or a LM358 for testing.
 

Offline paulca

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Re: DC Load
« Reply #23 on: November 30, 2017, 09:28:14 am »
The LM741 is NOT a single supply op-amp. Nor is it a rail-to-rail op-amp. Its output will not be able to get low enough to control the MOSFET. You will need to bodge together a dual supply for the LM741 or use a single supply op-amp. Use a TLC272 or Dave's LM324 or a LM358 for testing.

Thanks.

I had realised after much frustration on previous experiments that the LM741 was not rail to rail on it's outputs, but it could go as low as 1.3V, so I figured it's under 2V it might do.  What I did not expect is that it's inputs pretty are far off the rails too.  Nor did I expect it would slam to the top rail when it's inputs where out of range.

I've ordered a few more wider range opamps, the LM324 and a few LM358s.  I also have the TLE2142 from Peter's design which has rail to rail inputs and a Vcc- +0.1V minimum output, which is well under 2V obviously.  I'll play with the 2142 tonight and see if I can keep the smoke in.

I also realise that I was lucky testing it with such a small weedy little LiPo, if I had of wired in a much more beefy LiPo the wires would probably have went red hot, let much more smoke out and probably set the building fire alarm off to my great embarrassment.  I'll test with the current limited PSU until I'm comfortable it's tamed.
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Offline paulca

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Re: DC Load
« Reply #24 on: November 30, 2017, 06:15:22 pm »
It works!  :o

The baby load V1 works.  Minimal circuit, just the opamp, sense resistor, mosfet and a voltage divider pot to set the input.  The key was using the TLE2142 which has rail to rail inputs and close to rail to rail outputs.

Mosfet on a PC chipset heatsink (about 1.5"x1.5"x1.5") attached with one the those little plastic spring clips.  The only high side stuff that went through the bread board was the sense resistor and the ground return. 

I fired it up at 1A, 12V and it got too hot to touch the mosfet after about 30 seconds, a few minutes longer and the heatsink was too hot to hold onto for long.  The sense resistor was cool enough to hold onto.  The breadboard jumper wires groaned and smelt funny, but I could still hold them and they weren't melting.

I put a cooking probe on the heatsink and it got up to about 67*C.  I used an IR temperature probe to see what the mosfet case was and the maximum reading I could get was 80*C.

However it burped and cut out a few times then stabilised again, I'm not sure if I tripped the mosfet thermal protection or breadboard shenanigans but I decided it was probably running too hard without a fan and backed it down to 0.5A and noted that the heatsink temp fell slowly to about 57*C and stayed there for another 5 minutes.

So I recon it can handle 0.5A with 1A short bursts.  With a fan on the heatsink it might do 1A sustained.

This version won't give me an ability to cycle a LiPo in a sensible time, my Accucel6 can sustain 0.5A.  I want something a bit more beefy.  However this one might still be useful from time to time for loading electronics circuits up.

For the LiPo capacity tester I will need to build the full Peter Oakes 5A version with a CPU heatsink and fan with thermal grease and a proper bolt on the heatsink.  Then I'll see how many amps it can sustain.

Thanks to everyone who helped and put up with my frustrations and stupid questions.  Couldn't have done it without you.
« Last Edit: November 30, 2017, 06:19:53 pm by paulca »
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