Help with this dynamic test load

imix500 · 14th December 2007, 02:09 AM

Hello all, found this and it seems to fit the bill for a good adjustable test load. I plan on doubling the output section at first and might try it with a couple large surplus transistors like those used in motor drives.

So a couple questions, the 2N6053 are hard to come by after some quick searching. Do I just need to look for a darlington with similar HFE in this case? (matching current and voltage ratings obviously)

Are there any glaring problems with it?

Thanks!

Conrad Hoffman · 15th December 2007, 04:04 AM

The big feature of that circuit seems to be that the DUT powers it, thus the use of the LM10. It would appear to be unipolar, so it's great for testing power supplies, but not AC devices. If you're willing to include a single 9V battery, you can accomplish the exact same thing with a single op-amp and one or more power MOSFETs, probably with better performance. I put one in a nice box for testing batteries and supplies under an amp, but there's almost no limit on current capacity. Using the MOSFETs keeps the current draw of the circuit near zero, so the battery lasts near to forever. In my case, most of the draw is the large panel meter I used. A high power version will need a huge heat sink, unless you fan cool it, so you might want to power it with a 12V wall wart. Self power doesn't seem to buy much in that case.

imix500 · 15th December 2007, 02:17 PM

Hi Conrad, DC is what I will be testing. I'm looking to scale it up to about 2.4kW of capability. I am looking at some darlington modules that can dissapate about 1.3kW each.

I have the water cooling equip to do that, so it doesn't need to be self powered for fans etc. But, I might make a smaller version that can be air cooled with similar devices like those mentioned in the schem.

So you're saying a regular pass regulator- turned into variable resistor would be simpler. Hmm, that's a good idea.

Conrad Hoffman · 15th December 2007, 05:12 PM

No, not a pass regulator, but the old Siliconix circuit that uses an op-amp to feed back a signal from a very low value sense resistor. I'll dig it up for you and post it.

Conrad Hoffman · 15th December 2007, 08:22 PM

Ok, here's the circuit published by Siliconix back in Jan '83, in their MOSPOWER Design Catalog. I use a better op-amp, a much lower sense resistor, like .1 ohms, and thus a lower control voltage. This can be regulated with a 431 or zener or whatever. Shouldn't be a problem to parallel more FETs for higher power.
Siliconix Current Sink

imix500 · 15th December 2007, 11:05 PM

Excellent. Should I use a 5534 or the like instead? Actually, are there any unipolar op amps you would recommend? Were you saying the control voltage to the mosfet needs to be regulated or the V+ to the op amp needs to be? Thanks.

imix500 · 16th December 2007, 03:16 AM

Also, would an IGBT work well here? Very high current modules are quite cheap on the surplus market. I ask because I mainly see them in switching applications.

Conrad Hoffman · 16th December 2007, 03:53 AM

It's been a few years since I built mine, but I used an AD820 for the op-amp, and a LM385 to give me 2.5V on the adjustment pot. The adjustment voltage will equal the sense voltage so for a 0.1 ohm sense resistor, you get 0.1V/amp. I should probably trace the thing out so I have a schematic. I've never used an IGBT, so somebody else will have to chime in on that one.

edit- digital folks, don't forget you can just use a DAC to control this.

imix500 · 25th February 2008, 01:30 AM

So, I bought this assembly off fleabay. I couldn't pass it up. I thought it was just a simple variable load or pass bank, but upon closer inspection it seems it is something else. I have attached the schematic I reverse engineered. I'm no ace at eagle so please forgive me.

I originally thought applying control voltage at TP4 varied the base current of the 12 output devices. However when I applied voltage it drew far more current than it should have with a pre-driver and driver. So my guess is a self running regulator once the dc rails are above a certain voltage (I only tested it up to 20v). I may only use it for parts, but it would be nice to know what it is. Thanks!

Conrad Hoffman · 25th February 2008, 02:49 AM

Schematic seems a bit strange. That pnp looks like the base is pulled down, turning everything full on. Driving the first npn will turn everything off. Kind of a risky way to do it. Not sure it's capable of any kind of self regulation, more like self ignition.

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15th December 2007, 04:04 AM	#2
Conrad Hoffman diyAudio Member Join Date: Mar 2007 Location: Canandaigua, NY USA	The big feature of that circuit seems to be that the DUT powers it, thus the use of the LM10. It would appear to be unipolar, so it's great for testing power supplies, but not AC devices. If you're willing to include a single 9V battery, you can accomplish the exact same thing with a single op-amp and one or more power MOSFETs, probably with better performance. I put one in a nice box for testing batteries and supplies under an amp, but there's almost no limit on current capacity. Using the MOSFETs keeps the current draw of the circuit near zero, so the battery lasts near to forever. In my case, most of the draw is the large panel meter I used. A high power version will need a huge heat sink, unless you fan cool it, so you might want to power it with a 12V wall wart. Self power doesn't seem to buy much in that case.

15th December 2007, 02:17 PM	#3
imix500 Proud Union Member diyAudio Member Join Date: Mar 2004 Location: Nashville, TN	Hi Conrad, DC is what I will be testing. I'm looking to scale it up to about 2.4kW of capability. I am looking at some darlington modules that can dissapate about 1.3kW each. I have the water cooling equip to do that, so it doesn't need to be self powered for fans etc. But, I might make a smaller version that can be air cooled with similar devices like those mentioned in the schem. So you're saying a regular pass regulator- turned into variable resistor would be simpler. Hmm, that's a good idea.

15th December 2007, 05:12 PM	#4
Conrad Hoffman diyAudio Member Join Date: Mar 2007 Location: Canandaigua, NY USA	No, not a pass regulator, but the old Siliconix circuit that uses an op-amp to feed back a signal from a very low value sense resistor. I'll dig it up for you and post it.

15th December 2007, 08:22 PM	#5
Conrad Hoffman diyAudio Member Join Date: Mar 2007 Location: Canandaigua, NY USA	Ok, here's the circuit published by Siliconix back in Jan '83, in their MOSPOWER Design Catalog. I use a better op-amp, a much lower sense resistor, like .1 ohms, and thus a lower control voltage. This can be regulated with a 431 or zener or whatever. Shouldn't be a problem to parallel more FETs for higher power. Siliconix Current Sink

15th December 2007, 11:05 PM	#6
imix500 Proud Union Member diyAudio Member Join Date: Mar 2004 Location: Nashville, TN	Excellent. Should I use a 5534 or the like instead? Actually, are there any unipolar op amps you would recommend? Were you saying the control voltage to the mosfet needs to be regulated or the V+ to the op amp needs to be? Thanks.

16th December 2007, 03:16 AM	#7
imix500 Proud Union Member diyAudio Member Join Date: Mar 2004 Location: Nashville, TN	Also, would an IGBT work well here? Very high current modules are quite cheap on the surplus market. I ask because I mainly see them in switching applications.

16th December 2007, 03:53 AM	#8
Conrad Hoffman diyAudio Member Join Date: Mar 2007 Location: Canandaigua, NY USA	It's been a few years since I built mine, but I used an AD820 for the op-amp, and a LM385 to give me 2.5V on the adjustment pot. The adjustment voltage will equal the sense voltage so for a 0.1 ohm sense resistor, you get 0.1V/amp. I should probably trace the thing out so I have a schematic. I've never used an IGBT, so somebody else will have to chime in on that one. edit- digital folks, don't forget you can just use a DAC to control this.

25th February 2008, 02:49 AM	#10
Conrad Hoffman diyAudio Member Join Date: Mar 2007 Location: Canandaigua, NY USA	Schematic seems a bit strange. That pnp looks like the base is pulled down, turning everything full on. Driving the first npn will turn everything off. Kind of a risky way to do it. Not sure it's capable of any kind of self regulation, more like self ignition.