Electronic load circuit (Re: Episode #102)...  Make it dual rail capable??? - Page 1
Forum section: electronics / beginners

author: Skimask
author_rank: Super Contributor
timestamp: January 26, 2013, 01:17:37 am
content: Watched Episode #102 again yesterday, whipped up an example circuit, more or less the same as the circuit that Dave did up... LM324, 9.6v for the rails, 2N7000's for the MOSFETs (16 of them paralleled), and 33 each 33 ohm resistors for the 1 ohm load.Works like a champ. Semi limited range, just like Dave had. Added a PIC, some PWM for voltage input, an LCD, and I got me a handy electronic load.Now for the tough part...I'm going to use it to load down my LM2588 +12v/-12v power supply I just built. Positive rail load...done as described (weakly) above.Question is how to do the negative side of things? Can't very well feed the load in reverse polarity!I think I have to swap the N-channel for a P-channel, and "flip it over" (swap drain and source), along with moving the grounds around, and so on.Anybody have a simpler solution?

author: c4757p
author_rank: Super Contributor
timestamp: January 26, 2013, 01:26:58 am
content: Connect the power supply ground to the load's high input and the power supply negative to the load's low input. It just needs the voltage on the high input to be positiverelative tothe voltage on the low input, and 0V is certainly more positive than -12V.

author: Skimask
author_rank: Super Contributor
timestamp: January 26, 2013, 01:41:46 am
content: Confused by the symanticsIt almost sounds like your saying just swap the leads on the load input itself. e.g. -12v goes to the original load ground, 0v goes to the positive load input.I understand higher potential on the drain relative to the source, and -12v is lower than 0v (or 0v is higher than -12v), but wouldn't that mess with the voltage on the opamp driving the MOSFET? e.g. drive it negative? If the opamp driving the MOSFET goes negative on the output, wouldn't it turn the MOSFET off harder?ugh...so simple, yet, so confusing...(BTW - I'm doing this mainly as an exercise in opamps. I've been following your build. You kit it, I'll buy it if it meets my needs. One sold.)

author: c4757p
author_rank: Super Contributor
timestamp: January 26, 2013, 01:48:10 am
content: You are correct if the load's own internal power supply's ground is connected to the external power supply's ground, but it shouldn't be. They should be floating. Think about it: if you connect two 12V power supplies in series, you can call the center point 0 and the bottom -12. You can just as well call the bottom 0, the middle 12 and the top 24, or the top 0, middle -12, and bottom -24. As long as they can't see each other except at the one point where they join, they are only relative voltages. The load will see what you call "-12/0" as 0/+12 because it is designed to call whatever goes on the low input zero by definition.QuoteYup, that's exactly what I'm saying

author: Mechatrommer
author_rank: Super Contributor
timestamp: January 26, 2013, 04:36:59 am
content: if the psu design is floating, you can build two similar and connect one's ground to another's positive terminal. that positive terminal will become your circuit's ground. i havent checked dave's psu closely whether its floating or not. or if your load is not significant, you can build virtual ground (middle potential of the psu), you can google or search here its been discussed.

author: Skimask
author_rank: Super Contributor
timestamp: January 26, 2013, 06:05:18 am
content: oooooooofffff.....I hope I didn't throw anybody off track there....back in the first post.When I said +12v/-12v power supply, what I actually meant was a +12v/0v/-12v power supply; 2 opposing 12v taps with a common in the middle. (built the supply to run opamps)Hook that all together in the above described configuration, and I'd end up with a dead short!However, with all of that being said, and I haven't tried it yet, I don't see why I can't hook it all up as a +24v/0v power supply. Not sure if the LM2588 circuit requires all of the current going out of the +12 and -12 rails to go back in thru the ground terminal. I wouldn't think so, but...funnier things have happened.

author: Skimask
author_rank: Super Contributor
timestamp: January 26, 2013, 07:12:08 am
content: Ok...for fun...What do you get when you hook up the above mentioned circuit...to a 24 volt bench supply, and attempt to run...say...3 amps thru it?Well...In a bank of 16 2N7000 MOSFETs, after running for about a minute or so,First one MOSFET lets out all of it's magic smoke gracefully...Then about 3 seconds later, a 2nd MOSFET lets out it's magic smoke with a bit of a sizzle...About a second later, a 3rd MOSFET blows it's top...saw that one go off to the right...Finally, about a second after that, the remaining 13 MOSFETs all blow their tops in rapid succession, like a roll of fireworks!Good times!Luckily, I have a couple hundred of those in the bins...Worked fine at 2 amps for the previous 5 minutes or so, and ran it for about 10-15 minutes at 12v @ 3 amps before that.A little bit of math says I pushed those TO92 case 2N7000s far beyond the thermal limit.Moral of that story? Lighten up a bit...meh...I got the result I was looking for. Test to destruction...

author: c4757p
author_rank: Super Contributor
timestamp: January 26, 2013, 02:44:41 pm
content: If you had them connected the way I think you did, that was thermal runaway. Many transistors have a positive temperature coefficient - for the same gate voltage, they take more current if they are warmer. One transistor took more current due to natural variation of parameters. This current made him heat up and draw even more, until he popped. Then the next-hottest followed, until they were all gone. Parallel transistors sharing a driver is a recipe for mass transistor suicide, unless you have a load-sharing mechanism in place. That op amp just wants the set current to flow, it doesn't care if it all goes through one transistor.

author: Skimask
author_rank: Super Contributor
timestamp: January 26, 2013, 07:09:30 pm
content: Nope, 2N7000MOSFETs.But the same thing applies... One got a little hotter, took itself out, that current increased the load/heat on the other 15, one more of those went out, increased the load/heat on the remaining 14, and like a zipper, the rest of them went out.I tell ya...sometimes you just gotta have destructive fun like this!On the other hand, MOSFETs are generally supposed to 'thermal runaway resistant', in that their internal resistance increases with temperature, thereby limiting their maximum Ids.But, at that same time, that only works if you within a certain envelope. Go outside of that (as I did by FAR!!!), and anything goes...and anything went!!!I put new parts in. Good to go with 12v @ 3amps for an extended run. Let the 2N7000's get up to about 100C and that's about where they stabilized.Like I said before, this is mainly an exercise in understanding how op-amps work, howstuffties together. And good for a few laughs.

author: Christe4nM
author_rank: Unknown
timestamp: January 26, 2013, 07:20:48 pm
content: MetalOxideSemiconductorFieldEffectTRANSISTORA question though, did you put a small 0.01....1 Ohm resistor in the line of each source? That would help balancing/equalizing the current through each MOSFET.Also, why did you use TO-92 packaged FETs? Why not go for TO-220 ones attached to a nice heat sink? Just wondering.As far as your question regarding how to hook +12/0/-12 to your load. Use one load connected to +12/0 and another connected to 0/-12. If the loads are both floating you could even hook the 0V terminal of one to the 0V of the other, even though one is the -input of the first load and the other is the +input of the second load.Most important thing to remember is that Voltage is always relative from one point in a circuit to another. For example we choose to name one node in the circuit "ground". Actually it's just a reference point we take, calling it 0V. As far as the rest of the world is concerned it might actually be at +1000V measured to the earth. But since you measure other circuit nodes relative to that point their voltage is either positive or negative as measured to that point. Thus the earth would be at -1000V relative to that point if it is at +1000V relative to earth.

author: Skimask
author_rank: Super Contributor
timestamp: January 26, 2013, 07:37:57 pm
content: meh...just using parts I had laying around. For some odd reason, I've got a pile of P-channel MOSFETs in TO-220 cases, but the only N-channel I've got are either SOT-23's or TO-92's.And, yes, I know I should've put 'balancing resistors' on the sources for each one...again...meh...it was fun blowing them up. As well, I put this together on a piece of old prototyping board. I almost think this stuff is old enough to have a 'built in' .01 ohms between each MOSFET.With that being said, I looked thru my "scavanged parts" bins earlier and found a handful of heavier duty parts in there in TO-247 cases.2x K2698, 2x K3911, and 1 K2749.When I get to it, I'll bolt them all up to a big heat sink and wire them up in place of the 2N7000's.According to the datasheets, they take more Vgsto turn them on, and I think I've got enough headroom in the opamp to make that happen.Next in line to burn will be the resistor bank. 33 each 33ohm 1/2 watt resistors in parallel to make 1 ohm, and according to my meters, I've got 1.01 ohms.meh...next in line to burn up will probably be one of the traces on that cheap prototyping PCB.It's all good. Sometimes ya gotta make smoke to be entertained! Kinda like that guy on the youtube's lately that likes to short himself out in EVERY video...

author: Mechatrommer
author_rank: Super Contributor
timestamp: January 27, 2013, 07:54:10 am
content: Quote from: Skimask on January 26, 2013, 06:05:18 amthen i guess your built psu is not a floating type.Quote from: Skimask on January 26, 2013, 06:05:18 amlike transistor, an opamp doesnt have such terminology as solid "ground". you hook anything to the Vss-GND terminal (Vce in case of transistor) and all that matter is the "potential difference" between the two terminals. current will come and go out and in from these terminals doesnt matter (V+ to GND) or (V+ to V-). i stand to be corrected.

author: Skimask
author_rank: Super Contributor
timestamp: January 27, 2013, 04:13:46 pm
content: Quote from: Mechatrommer on January 27, 2013, 07:54:10 amYa, it is. Mastech HY3005-D.I think the bigger issue here is wrapping my head around everything.Once it clicks, it'll click good.

author: codeboy2k
author_rank: Super Contributor
timestamp: January 28, 2013, 05:49:24 am
content: Here's a few comments regarding paralleling mosfets1) first as you know, to92 is not a very good package for this application. Even if you could heatsink them, the thermal resistance of the package is too high to make it efficient. I realize you were using parts-on-hand.2) all mosfet sources should have small milliohm resistors in series.3) all mosfet gates should also have low valued resistors, one in each gate. Don't just tie the gates together. I use 10 to 100 ohms it works fine. This will help prevent oscillations due to the gate to drain and gate to source capacitances.4) you should try to put all the parallel mosfets on the same heatsink. This is hard to do with TO-92 but easier with the other power packages (TO-220, D-PAK, D2-PAK, etc). Putting them all on the same heatsink tends to keep all junctions at the same temperature, and helps to prevent any one mosfet from having such a difference in temperature that it takes significantly more current than any other mosfet.5) in an ideal world, each mosfet has it's own opamp driver, not one opamp driving N gates. The world isn't always ideal.

author: Skimask
author_rank: Super Contributor
timestamp: January 28, 2013, 06:09:03 am
content: 1) Yep, parts on hand, junk bin parts, parts I had a big ol' box full, parts that were practically disposable.2) That old prototyping PCB has milliohms worth of resistance all over the place3) So, maybe that's some of the noise I'm seeing on the 'scope when I turn the pot to vary the gate voltage (e.g. current load). I know the pot isn't perfect by any stretch, but maybe between the dirt on the pot itself and not having any sort of gate resistance, both may be contributing to the noise I see in the voltage across the load resistance.4) Next time around, if I decide to rebuild it in a more "proper" manner, I'll do that. I've got a few TO-247 types and a big ol' heatsink with predrilled holes set up just right for them.5) This project isn't ideal!Just an exercise.But at the same time, I'm fairly surprised at how stable and accurate it is.....for basically a thrown together piece of crap such that it is.The 1ohm resistor (34 x 33ohm resistors in parallel) is made up of 10% resistors, old ones at that, with an unknown tempco. I don't have a good enough ohmmeter that goes into sub-/milli- ohm readings. At 1 amp, I'm reading 1.002 V. Whether or not the ammeter I'm using is loading down the circuit, I don't know. Don't really care at this point. The main purpose of this, besides the exercise in opamps, etc., was to find a better option than trying to use various combination of power resistors to get X amount of load.If I thought I could get away with it, I'd build up a dozen or so of these things and sell them at cost...if anything just to clear out some of the pieces/parts laying around.