Constant current load for testing power supplies (guidelines)

[eccerr0r]

Re: Constant current load for testing power supplies (guidelines)

If you want accuracy and still want to use a light bulb or unstable resistance load (instead of letting the transistor be the power sink), you could still cheese it and use a lower wattage low resistance resistor in series as part of the stack, and using just the bona fide low drift current measurement resistor to measure current (and by choosing the topology carefully, the current through the lamp and measurement resistor are the same). Unfortunately this means the voltage across the resistor will be lower and you'll need a higher quality, lower noise, lower offset op amp to complete the feedback loop.

For me I don't particularly care how accurate the load is; I accept that on startup the light bulb's resistance will be lower and thus more current will draw upon first connection (though the cold resistance will need to be measured carefully to know how much current it will draw when cold). On the positive side, it will also tell me if the transient will cause the PSU to oscillate or shutdown...

I recall some type of nichrome changed around 1.5% or so every hundred degrees Celsius ...

[Dannyx]

Re: Constant current load for testing power supplies (guidelines)

That was my original thought, but I figured I'd compensate for large "jumps" in resistance electronically by building a circuit that can crank higher or lower depending on the resistance. A CV AND CC supply IMO....at the end of the day, isn't a CV supply something thar maintains a constant voltage across a load regardless of the load itself ? I'd need to couple that with a CC section so the current doesn't drift down as resistance goes up....pretty complex

[momaka]

Re: Constant current load for testing power supplies (guidelines)

The reason I'm asking is because I was thinking of making my own resistor out of nichrome wire or to use a lightbulb as one (given the high power it can), though that's silly really because once that thing starts glowing red, the resistance is all over the place so anything BUT constant.....scrap the idea - stupid.

Don't scrap the idea yet.

Yes, an incandescent or halogen light bulb will NOT work due to too much change in resistance. I've actually tried it before (used a low-resistance halogen bulb in place for a low-resistance resistor in a self-built amplifier) and got wonderful oscillations.

The Nichrome wire *could* work, though. But in order for it to work, you will likely have to keep the Nichrome wire at lower temperatures - i.e. not glowing. The less of a temperature variation you put on the Nichrome wire, the more stable its resistance will be.

Of course, you will have to experiment with it to determine if the resistance variation meets the criteria of your circuit. If it doesn't then you may need to put several more series and parallel strands of Nichrome wire together to make each Nichrome wire strand run cooler (and thus exhibit less of a resistance variation).

The best way to do that is to connect your Nichrome "resistor" to a stable voltage source and measure the current draw (with a multimeter) when its cold and when it's warm. By knowing the voltage and the current, you can determine the cold and hot resistances. For example, if your Nichrome "resistor" has a cold resistance of 2.5 Ohms and you connect it to a 5V power supply, then the cold current will be about 2 Amps. Leave it connected to the 5V supply for a few minutes and note the current again. If the current has gone down to let's say 1.5 Amps, then that means the resistance has gone up to 5V/1.5A = 3.3333 Ohms... i.e. that's a 33.3333% increase in the resistance from the original 2.5 Ohms.... i.e. the current in your circuit could vary as much as 33% from what you set it to when it's cold... i.e. not very good. However, if the current only goes down to 1.8 Amps, then that means the resistance has gone up to about 5V/1.8A = 2.778 Ohms... i.e. a 11% increase in the resistance, which is not that bad.

Thus with Nichrome wire used as a resistor, you will find that the higher the current you put through it, the more of a variation you're going to get between cold and hot resistances (and thus currents). So inevitably, you will have to correct for that on your home-made CC source until the Nichrome wire reaches a more stable temperature.

[Dannyx]

Re: Constant current load for testing power supplies (guidelines)

So what you're saying is I shouldn't bother to create some super complex setup to compensate for the resistance because it's not critical.

The reason I'm asking is because I was thinking of making my own resistor out of nichrome wire or to use a lightbulb as one (given the high power it can), though that's silly really because once that thing starts glowing red, the resistance is all over the place so anything BUT constant.....scrap the idea - stupid.

[momaka]

Re: Constant current load for testing power supplies (guidelines)

Sorry for digging up such an old thread, but I just thought of something: all this time we've been discussing how to maintain a constant voltage drop across a sense resistor - which is how this whole project got started - but what if the resistance of the resistor drifts during operation ? Resistors DO change their value with heat I reckon. The op-amp circuit I originally came up with would not be able to compensate.

I thought of this: say I have a 1ohm sense resistor and set the reference at the NI input at 1v. The op-amp will maintain 1v across that resistor so I'd get a 1a current draw. 1v/1ohm=1a.

Now imagine the resistor heats up a bit and its resistance goes up to 2 ohms (ok, let's keep it round). The op-amp won't know to crank the voltage to 2v to still get 1a, since it's still set at 1v, despite the current now being only 0.5a. 1v/2ohm=0.5a. Is this assumption right ?

Yes, resistor resistance can drift with heat, but it's never going to be that bad. Why? Because of tolerance. As long as you are using a resistor within its rated power level and rated operating temperature, its resistance will never drift below or above its tolerance.

So for a 1 Ohm 5% resistor, it's resistance may go up to 1.05 Ohms or dip to 0.95 Ohms when really cold. With these resistances and your example above, your current may go as low as approximately 0.9524 Amps or as high as 1.0526 Amps.... which as you can see, is also only about 5% off (and remember, that is worst case). If you use a resistor with 1% tolerance, then your current will vary accordingly (even less).

If the above wasn't true, then none of the electronics we have today would work at all.

[redwire]

Re: Constant current load for testing power supplies (guidelines)

Temperature Coefficient of Resistance (TCR), you want very low for precision current sense.
Current sense resistors are low TCR parts, special wire or metal film, foil construction.

If you used a plain 1R 5W wirewound resistor, like Yageo SQP500, temperature coefficient is high ±300ppm/°C.

So the 1R resistor at 20°C heated to 70°C (shift of 50°C), gives 1,500ppm to give 0.015 ohms shift or a change of 1.5% just due to temperature.

In reality, power resistors have hot spots and their temperature drift is hard to know exactly.

Also, you need 4-wire Kelvin-sense to prevent voltage drops at the resistor measuring point (leads connection) from giving wrong values to your op-amp.

[Dannyx]

Re: Constant current load for testing power supplies (guidelines)

Sorry for digging up such an old thread, but I just thought of something: all this time we've been discussing how to maintain a constant voltage drop across a sense resistor - which is how this whole project got started - but what if the resistance of the resistor drifts during operation ? Resistors DO change their value with heat I reckon. The op-amp circuit I originally came up with would not be able to compensate.

I thought of this: say I have a 1ohm sense resistor and set the reference at the NI input at 1v. The op-amp will maintain 1v across that resistor so I'd get a 1a current draw. 1v/1ohm=1a.

Now imagine the resistor heats up a bit and its resistance goes up to 2 ohms (ok, let's keep it round). The op-amp won't know to crank the voltage to 2v to still get 1a, since it's still set at 1v, despite the current now being only 0.5a. 1v/2ohm=0.5a. Is this assumption right ?

[momaka]

Re: Constant current load for testing power supplies (guidelines)

That is the Voltage follower circuit. not Constant Current SOURCE that AUTOMATICALLY maintain the output current to be constant with the variable load, the key word is 'CURRENT SOURCE'
https://www.allaboutcircuits.com/tex...tage-follower/

Well, that's if you connect your load, Rl, between the transistor's Emitter and ground, as is shown in the above link.

However, if you connect you load between the power supply positive bus and the transistor's Collector in my schematic, and the resistor Emitter Re is a constant value, then the current going through Rl would be dependent on the voltage you set across Re... so in other words, the circuit *will* work as a constant current source.

Now here you consider a slightly different different topology, I had not considered this topology, but this topology still needs some reference whether a CVVS or a diode/zener drop. This definitely requires a Darlington, the base current would otherwise be a problem.

Yes, a diode or Zener reference would definitely be required for accurate voltage/current regulation.

You don't need a Darlington transistor for that circuit if doing it for low current. That said, even for high current, if you don't have a Darlington, you can build your own with two regular transistors.

[budm]

Re: Constant current load for testing power supplies (guidelines)

That is the Voltage follower circuit. not Constant Current SOURCE that AUTOMATICALLY maintain the output current to be constant with the variable load, the key word is 'CURRENT SOURCE'
https://www.allaboutcircuits.com/tex...tage-follower/
If you add zener to the circuit then it become Voltage regulator.
http://www.circuitstoday.com/voltage-regulators
http://www.radio-electronics.com/inf...ent-source.php
http://www.learningaboutelectronics....ent-source.php

[eccerr0r]

Re: Constant current load for testing power supplies (guidelines)

I've already mentioned that as long as you don't exceed the SOA of the LM317, it will work and work very well. And you still have LM338 and LM350 if needed. Simplicity/component count is the reason for using the LM317, not flexibility. However it is very stable and only two components to worry about variations.

The two transistor topology I was talking about normally does not use a Darlington - the common 2-transistor current sink uses two standard transistors, where one of them is the feedback amplifier. The 2-transistor sink circuit uses the lower current secondary transistor's VBE as the reference voltage and as it uses standard transistors, gain is an issue. I suspect converting it to a 3-transistor (or using a Darlington for the pass transistor) then the gain won't be as big of an issue anymore.

Now here you consider a slightly different different topology, I had not considered this topology, but this topology still needs some reference whether a CVVS or a diode/zener drop. This definitely requires a Darlington, the base current would otherwise be a problem.

[momaka]

Re: Constant current load for testing power supplies (guidelines)

I was not talking about specifically the two transistor or transistor-diode solutions, but also covering the initial op amp solutions - after all, the LM317 contains an op amp. However it still stands, two components (IC regulator + resistor) is less than 4 components (two transistors and two resistors) and building a device with point to point construction with 4 components is more difficult than with 2...

Okay it is... but LM317 can pass only 1.5 Amps of current, and that is only if operating it up to a certain input voltage (in other words, observe the SOA / power dissipation limits).

With transistors, it's much easier to build a high-current CC source.

In fact, most of these "2 transistor" current sinks were designed for the milliamp range where transistor gains are high and heat sinking isn't a problem. I do wonder what happens when things get scaled up into the ampere range and once again the base drive becomes significant with respect to the current sink needed, and consistency will start to falter with transistor/diode variation.

That's not how it works. See attached sample constant current circuit below.

In this circuit, you can set up the current flowing through the transistor to be pretty much independent of its current gain (beta / HFE), provided of course, that you are not pushing the transistor near its maximum specs (particularly near maximum current, as the HFE will start to drop significantly).

Basically, this circuit relies on the fact that transistors have a fairly constant Base-Emitter voltage drop, Vbe. For normal transistors it is about 0.7V. And for Darlington it is usually twice that (i.e. 1.4V, as there are two transistors inside it, hence two Vbe drops).

Therefore, if we set the voltage going to the transistor's Base to be 2V (Vb = 2v), then the Emitter will be at approximately Ve = 2 - 1.4 = 0.6V above ground (note that I am doing the calculations here for the circuit with the TIP120 Darlington transistor). This will hold true regardless of the resistance of Re, provided of course that it is not so low to cause excessive current draw - in which case, the current gain will start to become the limiting factor of how much current will flow through the circuit (and we want to avoid that). That means if Re = 1 Ohm, then the transistor will pass about 0.6 Amps (Ie = Ve / Re = 0.6V / 1 Ohms = 0.6 Amps). And if Re = 0.3 Ohms, then Ie = 2 Amps. Alternatively, we could set the voltage at the Base to be 3.4V. Then the Emitter voltage will be Ve = 3.4 - 1.4 = 2V. If we use the 1 Ohm Emitter resistor, Ib = 2 Amps again.

So essentially, you can control the current by changing Vb and/or choosing appropriate values for Re. But generally, the lower the resistance of Re, the more sensitive the circuit will become to Base voltage changes of the transistor. On that note, I have drawn R1/R2 resistor divider network connected to the same voltage source as the transistor for simplicity. But in reality, you don't want to do that, especially if V1's voltage tends to vary with the load. If you want very good current regulation, then R1 should be connected to a separate voltage source with tight voltage regulation.

Another note: I put resistor Rb there simply to separate the transistor's base voltage from the voltage divider network. But in reality, you don't need Rb at all. However, you could replace Rb with a potentiometer if you want to fine-tune the current. Though, if you want to do that, it is probably better to connect the potentiometer in series either with Ra or Rb - depending on what values you picked for Ra and Rb.

[eccerr0r]

Re: Constant current load for testing power supplies (guidelines)

I was not talking about specifically the two transistor or transistor-diode solutions, but also covering the initial op amp solutions - after all, the LM317 contains an op amp. However it still stands, two components (IC regulator + resistor) is less than 4 components (two transistors and two resistors) and building a device with point to point construction with 4 components is more difficult than with 2...

In fact, most of these "2 transistor" current sinks were designed for the milliamp range where transistor gains are high and heat sinking isn't a problem. I do wonder what happens when things get scaled up into the ampere range and once again the base drive becomes significant with respect to the current sink needed, and consistency will start to falter with transistor/diode variation.

[momaka]

Re: Constant current load for testing power supplies (guidelines)

Except when you're trying to build one without a PCB

Meh. Even then you could get creative and use that hardened cardboard material that is used as back support on a lot of furniture (the technical name for it escapes me for the moment). Then drill holes and connect stuff with wire. Actually, that's how the entire filter board is done for the Sansui SP-7500X speakers I found recently.

Copper clad board is another alternative. Or even breadboard - heck, everyone who's considering playing with electronics more seriously should own one - it's such a useful prototyping tool!

There are so many things that the LM317 solution abstracts away: no power source for op amp, no external voltage reference, no biasing needed for feedback resistor, don't have to worry about gain, it's all done inside the IC as long as you don't fry it or the sense resistor...

Uhmm, the CC source I was talking about in the last several posts does not use any op-amps. Only 1-2 transistors, 1-2 diodes (or transistors), and 2-3 resistors. That includes the shunt resistor and power transistor. In fact, if you wanted to go very simple, you could probably skip the diodes completely. It's just the circuit will be a bit more "unstable" when trying to set the current to the desired value.

That said, the headphone amplifier I build for a university project had its output transistors biased exactly like that - no diodes/transistors. Instead, each transistor was biased separately by a network of resistors and a pot. I didn't put any Emitter feedback resistors either, so the amp could get into thermal runaway. However, I countered that with two massive Xbox 360 CPU heatsinks - one for each upper and lower transistor set (so I could also get away without using silicone pads and washers to isolate the tabs of the transistors from the heatsink - i.e. the heatsinks were "live"...). Didn't have Darlington transistors either, so I build my own pair from discrete components, lol.

[eccerr0r]

Re: Constant current load for testing power supplies (guidelines)

You are looking at two small-signal BJTs (or diodes if you like), a Darlington (or build your own Darlington pair), a pot, and maybe two more resistors max. Not to mention the versatility of this circuit. Beats the LM317 constant current circuits any day.

Except when you're trying to build one without a PCB There are so many things that the LM317 solution abstracts away: no power source for op amp, no external voltage reference, no biasing needed for feedback resistor, don't have to worry about gain, it's all done inside the IC as long as you don't fry it or the sense resistor...

(the one transistor HFE*Ib current sink)'s a bad idea.
As the BJT becomes warmer, its Vbe voltage drop will become lower. If you are using a pot or resistor to control the current going into the Base, that current will increase due to the now-lower Vbe drop. This will make the transistor pass even more current and get even warmer, further dropping the Vbe and biasing itself to death - a phenomenon commonly known as "Thermal Runaway" (if you've dabbled with discrete transistor audio amplifiers, you've probably read about this). Hence why you always need an Emitter sense resistor. The only way you can avoid it is if you have a heatsink large enough that won't allow the transistor to get significantly hotter past a certain point, so the thermal runaway will eventually tapper off. But it is still risky and you will get a lot of drift.

Agreed, it's not a good idea, however it still would be "minimal components" solution when it comes down to it if you can deal with the temperature nonlinearity (of course design such that the expected heat dissipation is handled by the heatsink, and the limited base current would limit conduction). However I'd think this is the only other way to build a stop gap current sink with only 2 components like the LM317 solution - but if you don't count the voltage regulator...

[momaka]

Re: Constant current load for testing power supplies (guidelines)

whereas with a FET, it would max out at around 1.8a while the op-amp's output goes all the way up to the preset "rail" (15v).

That might be bug in the software or something to do with the MOSFET model in the software. MOSFETs are voltage-controlled devices - i.e. the current they pass depends on their Gate voltage. So at 15V at the Gate, that MOSFET should be fully conducting.

The BJT's beta was set to 100 in case you're curious

For large power transistors passing lots of current, your beta / current gain will be much lower - probably in the range of 10-50, and in some cases, even single-digit numbers.

I wouldn't call it simple if you need a bunch of other components to bias and amplify the voltage back - a lot of components are needed.

You are looking at two small-signal BJTs (or diodes if you like), a Darlington (or build your own Darlington pair), a pot, and maybe two more resistors max. Not to mention the versatility of this circuit. Beats the LM317 constant current circuits any day.

Or perhaps just do away with the sense resistor and just rely on the fact BJT collector current in common emitter is completely a function of the base drive.

That's a bad idea.
As the BJT becomes warmer, its Vbe voltage drop will become lower. If you are using a pot or resistor to control the current going into the Base, that current will increase due to the now-lower Vbe drop. This will make the transistor pass even more current and get even warmer, further dropping the Vbe and biasing itself to death - a phenomenon commonly known as "Thermal Runaway" (if you've dabbled with discrete transistor audio amplifiers, you've probably read about this). Hence why you always need an Emitter sense resistor. The only way you can avoid it is if you have a heatsink large enough that won't allow the transistor to get significantly hotter past a certain point, so the thermal runaway will eventually tapper off. But it is still risky and you will get a lot of drift.

[and as an edit response to Dannyx: yes. Darlington is necessary. The base drive is a significant portion of the power that will be dissipated in the resistor that will give you error. With the currents we're talking about here, the gain of a non-Darlington BJT will likely be significantly less than 50, which will cause error, not to mention most op amps probably can't supply enough current to allow a straight BJT to pass many amperes.]

Exactly!

[budm]

Re: Constant current load for testing power supplies (guidelines)

"whereas with a FET, it would max out at around 1.8a while the op-amp's output goes all the way up to the preset "rail" (15v)." May be your simulator has bad MOSFET modeling. You can just set up a another simulator and remove the OP Amp and replace the Gate drive with variable Voltage source to see what the simulator will show.
https://cdn.badcaps-static.com/pdfs/...3b3483e353.pdf

[eccerr0r]

Re: Constant current load for testing power supplies (guidelines)

That's another reason I switched out the one in my circuit for a 358. But again, you will find that LM741 is just soooooo slow and at certain loads, you may get some pretty nasty oscillation. With 358, my circuit was much more stable and regulating much better.

I thought the LM741 was comparable to the LM358... GBW (0.5MHz min, 1.5MHz typ) is not great, but a lot of op amps of the time period aren't that high, the LM358 is also only 1MHz.

Not saying the LM358 is bad or LM741 is good by any means, the LM741 IS horrible by today's standards. But it's not an unworkable solution, hence that's why the LM741 got so popular. It works for so many things that everyone knows what a 741 is! (Ask a random person what LF355, LM358, LM741, LM1458, NE5532, LM6142, LM324, TL062, TLV2372 are and see which ones are most recognizable.)

My problems with LM741:
- power rails that go beyond output voltage swing necessary is by far the worst problem why I don't want to design with them. LM358 only the positive rail needs to go beyond output range. RRIO op amps are better than LM358.
- not perfectly linear operation. It's not unworkably bad but there are a lot better ones out there.
- It's slow and noisy
- Eats significantly more power than LM358.
- input impedance is not as good as JFET or MOSFET. However the LM358 is about the same as the LM741, which is better than some bipolar RRIO op amps.

BTW probably a lot of the music heads will instantly recognize the NE5532 most likely...

[eccerr0r]

Re: Constant current load for testing power supplies (guidelines)

I wouldn't call it simple if you need a bunch of other components to bias and amplify the voltage back - a lot of components are needed. A very common current control is use the sense resistor tuned at the voltage of the base-emitter junction of another transistor, but this requires another package to pull down the base drive of the Darlington, and itself needs biasing.

Or perhaps just do away with the sense resistor and just rely on the fact BJT collector current in common emitter is completely a function of the base drive. However, the pot would vary widely depending on luck of the draw part you got - the gain of the Darlington, though high, is not consistent between parts and negative feedback is necessary.

[and as an edit response to Dannyx: yes. Darlington is necessary. The base drive is a significant portion of the power that will be dissipated in the resistor that will give you error. With the currents we're talking about here, the gain of a non-Darlington BJT will likely be significantly less than 50, which will cause error, not to mention most op amps probably can't supply enough current to allow a straight BJT to pass many amperes.]

At least the LM317 would be fairly consistent, 1.25V±40mV or so, it's got some internal temperature compensation, so just this and one external part (the resistor) is luck of the draw. Plus only two components and only 3 nodes (in, GND, and resistor to out pin on LM317) - which can even be assembled without a PCB, how's that for quick and simple?

However there is one major drawback of the LM317 - you can't easily pot tune a LM317 circuit. You'd potentially need a fairly hefty watt-rated pot that can deal with a significant amount of current to adjust the current. The Darlington and op amp solutions at least have high gain and a 1/4W pot is usable.

[Dannyx]

Re: Constant current load for testing power supplies (guidelines)

I messed around with a simulator I've used before and came up with this. It seems to work as intended with a BJT rather than a FET for some reason: with a BJT connected the way Momaka suggested, the current follows the control voltage all the way up to the calculated 10a (with a 0.1 ohm resistor) as it should, whereas with a FET, it would max out at around 1.8a while the op-amp's output goes all the way up to the preset "rail" (15v).

This is just a simulation done with a tool that's far from professional and uses rather generic components, so it could just be due to plain buginess or it could indeed be accurate and perform similarly with real life components as well. The BJT's beta was set to 100 in case you're curious, so at least it's got some parameters you can tweak for some of the components. It also means I was able to dial in a very precise control voltage down to the millivolts range, which may not be so easy to achieve...it's a good start though

[momaka]

Re: Constant current load for testing power supplies (guidelines)

I don't think the LM741 won't work at all however. The key thing is that it requires the usual bipolar supply in order for it to work in this situation. You'll need the V- of the LM741 to go to at least -3V relative to GND, and the V+ supply probably +12V or so relative to GND. Yes the supplies are not equal to each other, but who's checking? Op amp won't care...

That's another reason I switched out the one in my circuit for a 358. But again, you will find that LM741 is just soooooo slow and at certain loads, you may get some pretty nasty oscillation. With 358, my circuit was much more stable and regulating much better.

If one can get away with just using a LM317, it's a lot easier to just implement it this way as everything but the sense resistor is inside the LM317 (op amp and dissipation transistor). Using the LM317 means you have to get all the way to 1.25V, so even in this prepackaged solution it's a fairly high sense voltage.

About the largest problem with the LM317 is that it's only 1.5A max. A LM350 will get you to 3A and LM338 to 5A, though each of these would require beefier and beefier sense resistors. A 10W resistor should handle all cases (5A * 1.25V = 6.25W, which is within 10W).

Actually, if you want to build a really simple constant current source, your best bet is with a BJT, preferably a Darlington (or if you don't have one, use a small BJT to drive the BJT just like Darlington does, and you will achieve the same thing). Simply wire the Emitter to one end of your sense resistor and the other end of the sense resistor to ground. Then connect the Collector to whatever it is you want to sink constant current from.
And the base? Drive it on with a simple diode biasing network and a pot to fine-tune the biasing of the Darlington BJT. For better thermal stability of the circuit, use small signal transistors instead of the diodes and thermally couple to the same heatsink as the Darlington BJT.

This is by far the simplest circuit and doesn't require any ICs. It's actually almost the same thing as what stereo/music amplifiers use at their output stages.