Go Down

Topic: Dummy load (Read 8 times) previous topic - next topic

Nick Gammon

If you wanted to have a computer-controlled dummy load you could simplify the circuit like this (untested):



Now you just use analogWrite to set up the required voltage (and thus, load current). Since analogWrite (255) outputs 5V roughly then you would need to scale it. For example, to have a 2.5 A load you would analogWrite (127).

retrolefty

#6
Feb 05, 2012, 10:18 am Last Edit: Feb 05, 2012, 10:32 am by retrolefty Reason: 1

If you wanted to have a computer-controlled dummy load you could simplify the circuit like this (untested):



Now you just use analogWrite to set up the required voltage (and thus, load current). Since analogWrite (255) outputs 5V roughly then you would need to scale it. For example, to have a 2.5 A load you would analogWrite (127).


Not going to work with that wire going from pin 1 to pin 2 of that op-amp. Pin 2 should wire to top of the 1R resistor. I think the main disadvantage of this circuit is the inefficiency due to the 1 ohm feedback resistor which leads to lots of wasted power. I would redesign it using a .1 ohm resistor and scale the feedback op-amp to compensate.

Lefty

Nick Gammon


Not going to work with that wire going from pin 1 to pin 2 of that op-amp. Pin 2 should wire to top of the 1R resistor.


Oops. Major blunder there with reworking the circuit. Thanks! Fixed schematic now.

Nick Gammon


I think the main disadvantage of this circuit is the inefficiency due to the 1 ohm feedback resistor which leads to lots of wasted power. I would redesign it using a .1 ohm resistor and scale the feedback op-amp to compensate.


Thanks for the feedback. You are probably right in terms of efficiency, weighed against the difficulty of finding a 0.1 ohm resistor with high accuracy. I think Dave did his 1 ohm resistor by putting 10 x 10 ohm in parallel, which tends to indicate that he found the likelihood of finding a 0.1 ohm resistor rather low.

I've got one here, I wonder if the heat would make 0.1 ohm drift faster than 1 ohm? I suppose there are always trade-offs in this sort of thing. For situations where you might just want to test a "wall-wart" to see if it delivers the promised voltage at the promised rating, a few seconds' test would probably suffice, in which case you don't really care if you waste power or not.

retrolefty

#9
Feb 05, 2012, 03:30 pm Last Edit: Feb 05, 2012, 10:50 pm by retrolefty Reason: 1
Certainly if the circuit's function is for just using as a programmable 'dummy load' then worrying about efficiency is silly, it's all going to be converted to heat anyway. However what you have drawn is really a programmable constant current source useful for continuous operation for say driving high power LEDs or other constant current applications where increased efficiency is always welcomed. I guess these days one could look at high current hall effect current shunt sensors, their costs are higher but they would result in the most efficient operation.

I recent completed building a 100+ watt oil cooled 50 ohm dummy load used in ham radio transmitter testing. I used 20 1,000 1% 2 watt resistors ordered on E-bay for just a couple of bucks. I wired them in a circular parallel manner and soldered them to a BNC connector mounted on the lid of a 1 quart paint can, filled the can with mineral oil and have been able to pump 100+ watts of RF into it for many mins at a time. After about tens mins the can is warm to the touch but handles the load nicely.

http://k4eaa.com/dummy.html

Lefty

Go Up