"DIY Constant Current Dummy Load" [EEVblog #102]

[scrat]

The only question I had is this.  The resistor(s) isn't there to dissipate the energy?  That's weird to me, that - well that and the I'm not use to seeing anything between the "source" pin and ground.  I'm use to seeing the "load" on the drain pin.  What's up with that?  Is there an easy explanation?

That resistor acts as a sensing resistor. Dave put it between source and ground because it is far easier to pick up a voltage referred to ground, to use it for feedback (it goes to the opamp). It has to be choosen so that its voltage is small (to avoid resistance variation due to heating), but sufficiently large to be measured. If you use an opamp in a differential configuration, you could pick up current proportional voltage from any other place in the circuit, but in this case it isn't much worth.

I think that a good tool could be an MCU-controlled PWM load, that you could control from PC or from a simple LCD+buttons. Feedback could pass (after a little amplification, if needed) through MCU's ADC, since a large bandwidth is not required. Then you directly generate PWM signal for the MOSFET. To verify control precision you still can measure the resistor's voltage with a DMM.

[Simon]

sounds like a good idea

[scrat]

express pcb is very sinmple and easy to use and create custom parts for, as for adding sketches that's down to your image editing skills, expresspcb can also save schematics as any number of image formats

Thanks! I tried the schematic tool inside  ExpressPCB. It's quite good, but I will take another look around, since there are some annoying features (grid and wire connection behaviour, for example). Of course, there is no perfect tool...

[Simon]

its chaep (free) and nasty but can get a job done quick i'm trying to migrate to KiCAD

[george graves]

What do you think would be a good way to expand dave's dummy load to handle more current?

I was thinking of just adding more 3055's and run them in parallel with more sense resistors not connected to the opmap?  Or is there a better way?

[scrat]

Sense resistors, by definition, must sense current for feedback. If you don't connect them to the opamp, they just become dissipating loads, which could be good, but you have to sense ALL of the current and send it as a feedback to the opamp.
Putting BJTs in parallel can be a problem if they do not have each a resistor in series. Otherwise, the one which conducts slightly more current will heat the more and so increase its conductivity. Then it will burn, and the other transistors will do the same one at a time.
MOSFETs are good for paralleling.

[george graves]

Interesting.  So how would you expand upon Dave's design handle more current?

[scrat]

I'll use many parallel resistors, with a BJT or MOSFET in each leg. Then all of these must be in series with a single sense resistor, which voltage will be the feedback for the opamp. So you can split power dissipation among many resistors and transistors.
If you need a very high max.current/min.voltage ratio (= a very low min. resistance) then paralleled MOSFETs are the only solution, IMHO.

There is at least another recent (the last posts were about a month ago, I think) thread on this topic, a good search should point it out. There was a big dicussion, and some schematics. If I find it, I will post the link..

[scrat]

I found the thread I mentioned above: https://www.eevblog.com/forum/index.php?topic=1049.0

[scrat]

I'd try with paralleled MOSFET+resistor followed by sense, with a single controller.
Maybe power resistors can help lowering the cost (smaller MOSFETs and wider power distribution).

I think multiple controller won't be necessary, unless you need a very low resistance and thus you have to eliminate the power resistors.

[scrat]

@AcHmed99: Pleased to learn from who has much more experience than me...
I didn't consider a very wide output range.

@george: The point is (as usual) that specs are the base for almost any design option, it is not possible to give generic answers.