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analog circuit to mimic constant resistance load? Answered

I unfortunately do not have many big resistors that can dissipate lots of power, and would like to make a dummy load to test and parameterize batteries and power supplies. I want constant power, constant current, and constant resistance load, I do have lots of opamps and a few N and P channel enhancement mode MOSFETs as well as many 2N3055s and a couple MJE2955's.

I think constant current is the easiest, since all I need to do is make a closed loop controller that will turn off the transistor as the current is exceeded, and turn it on as the current falls. Constant power might be more tricky, as I need a device that controls the bias on a transistor as a function of the product of the voltage and current, since P=IE or W=AV, and similarly, constant resistance, I need a thing that will divide voltage across the pass element by the current through it.

I don't want to use a "slow" microcontroller for this, I like nice continuous analog control system, if that is possible.


I unfortunately do not have many big resistors that can dissipate lots
of power,

Yes you do, they are called light bulbs....

I have used the christmas tree lamps as small loads, they are convenient to use since once pulled out of the plastic case, they can plug directly into the breadboards. They tend to burn out over 6V though, also, I am not sure how the resistance changes with voltage or current. I know the resistance will be a function of the tempurature of the tungstan filliment. I also have a large DC drill motor that draws significant current.

Car lamps.

Car lamps in parallel/series combinations.

I actually used to use one a long time ago, though I caused the quartz to fog up, I guess I broke the seal on it and it slowly let air in causing some reaction! I might try to build up a little board of them and see if it works.

What about constant power though?

Use an analog multiplier circuit, multiplying instantaneous voltage and current, then applying the result to a feedback loop around the fet.

There's no real benefit to using a FET here

That's precisely what I lack the knowledge of. It never occurred that such a device would be called 'analog multiplier' I was trying to look for things that are along the lines of 'adjustable gain amps' and stuff, never found one that could be easily made with a few op amps and interfaced.

Whats a good opamp circuit that will do that? I see tons of complicated looking ones on google.

There's a reason they're complicated....

Try an off the shelf chip like the AD632

I have come to the same conclusion. :( I learned a bit about multiplying DACs though :)

Good point, I do have a lot of Xmas tree lamps & some household lamps, but they are too high in resistance. I need lamps that would be drawing 1A-10A range at <30V. I thought maybe using a MOSFET on a CPU heatsink would be a more convenient solution.

If you drive the FET fully on it will be almost a short circuit and draw a LOT of current (for a short time)

You need a load. Car lamps can pass 80 watts at 12 volts that's 6 amps.

3 in series will need 36 volts to drive them you may be able to pass even more current or you can put some in parallel to share the load.

An old type electric fire where the element is an open resistance wire or an old toaster is full of resistance wire. Time to hit the charity shops!

Which is why I was thinking about driving the FET in the linear region almost exclusively, and just using a bigass heatsink to keep them cool. I know from experience that FETs do act somewhat like resistors, although they don't have very linear relationships because the depletion region is affected by the electric field from the source, which is why I think I need a control loop to precisely control the current through it and voltage across it.

True, the best type of resistive load is either lamps or resistive wire (though honestly I didn't want to go through all that effort to obtain more junk to repurpose. Really I just thought I could make a small compact and convenient module.)

I think he's shooting for operating the FET in its linear mode.

You can get H4 lamps for boating use with 66/100W power.
For over 12V either select the 24V variety or put two in series.
Add a few switches and you you can select the load ;)

For constant power you could use something like a MAX4210 in the feedback path of the circuit you proposed for constant current. Then the reference voltage to the comparator will represent target power rather than target current. I'd like to find something like the MAX4210 to do constant resistance but so far I haven't found it yet.

Constant current is really easy, I can do that with nothing more than a single NPN transistor and 3 resistors. (or 2 resistors and a zener.) Simply by connecting one small resistor between the emitter and ground, and using the leftover resistors or resistor and zener to apply a bias voltage to the transistor, which the transistor will try and regulate that voltage - 0.6v across the emitter-ground resistor, and a constant current is pulled from the collector.

Are you thinking of a circuit like this only veritable controlled with a pot?

Sorry I trying to limit the number of up loaded images and I picked this one out of the more than 1700 images and files i have already up loaded.

78xx Current Regulating.bmp

That looks like a basic schematic for a linear regulator (and my god with that many pass elements, looks like it is some crazy 100A linear reg!)

It should be important to note however that the output voltage of that regulator is not tightly controlled, and will be about 1.2-2V lower than the output of the voltage regulator. Since it is effectively a giant darlington transistor in emitter follower configuration following the 78xx.

Yea it is a basic schematic.

For a finished veritable circuit you would add a pot on the regulator and load limiting resistors to the transistors.

But you can control the voltage and the current up to 35 volts and 100 amps.

IDK where I would even find heatsinks for like 10 transistors like that! :O I am actually working on a linear power supply design, I am planning on using a TIP35C and TIP36C for the positive and negative voltage rail respectively.

However, what I am looking for in this question is a good dummy load. Linear power supplies tend to make decent constant current loads, because voltage maintained across a resistive load means that the current through that resistive load will be relatively constant, and kirchhoff's current law states that the current entering the regulator has to be the sum of the quiescent current (negligible) and current to the load. Therefore, beyond the dropout voltage of the regulator, a constant current source can be achieved.

That will work, the pot will need to be installed the same way you would for a LM317 adjustable voltage regulator. However do not go above maximum voltage for the regulator and say under the ripple voltage by at least 2 to 5 volts.

Now for the heat sink, do you have a scrap aluminum screen door?

But like I said in my original question, I can make a constant current source easily, making one that works down to 0V is harder, obviously requiring external voltage, but also making a constant resistance and constant power load. Those ones are proving to be much more tricky!

well no

0 volts 0 current they are related you need some of one to have the other.

There are times where it appears the rules have been broken like in my Instructable Circuit Testing,


In the one step if you look at the oscilloscope it said 65 volts on a five volt circuit.

Sometimes your equipment deceives you.

Constant current easy even to large currents.

Constant voltage easy even to large voltages.

The two together can be challenging but they are the same circuits and components just put together in different ways, here is one for controlling motor speed where the voltage never changes but the current does changing the speed of the motor.


I looked at the instructable where you measured 65V across a 5V circuit, I think that can be explained with inductor action. My (basic) understanding is that inductors are sort of like the water-hammer effect if you are into plumbing.

Because the voltage across an inductor is proportional to the rate of change of current through it, if you abruptly stop the current, then you will get a nasty voltage spike as the inductor "tries" to maintain that current for some time. The voltage will instantaneously up until the current finds a path. This effect is generally known as back-EMF, EMF kickback, or the flyback effect.

I'm glad you didn't damage that little scope when messing around with that! that voltage could have been much, much higher, tens of hundreds of volts!

Yea the point of it was to look a little harder and just don't take the readings for granted. you got to tweek.

Yeah, absolutly. In the old days those cool looking analog voltmeters were completly passive devices, and the needle movement was achieved via small current through the coil, so they would have some significant loading effects. Luckily modern Meters have input impedance like 10 Mohms minimum, some with input impedance in the Gohms!!!

"0 volts 0 current they are related you need some of one to have the other." Precisely! Which is why I mentioned an external supply! Adding a battery in series with a CC dummy load should allow it to pull the fixed current even if the voltage across the device under test drops to zero volts. The addition of the voltage from that battery should supply the necessary EMF, as long as it is great enough to allow the constant current load to function properly. I think a 4.5V battery put in series with a LM317 based constant current dummy load should do the trick. However, if the device becomes high impedance (as if it was disconnected or not present) then I would expect the voltage across it to become reverse biased to some degree.

I have a heatsink from a xbox CPU, some 12V PC fans, and some large LED heatsinks. However none of those will work that well with a stupid TO-3 package. The TO-247 and TO220 are much nicer packages to deal with.

I am not sure how important it really is, but I think there should also be some balancing resistors following the emitters of the paralleled 3055's.