Electronics > Projects, Designs, and Technical Stuff
MOSFET and OPAMP Load tester / constant current load
lordvader88:
I have soldered some power resistors to a PCB for load testing, I need to put in in a box with wires/knobs/etc....never ending parts list. Now I what to make something like this
, but I've barely used mosfets, and I don't have any like Dave uses in the video, that work at 5V logic and predictably near zero, or whatever he means.
So I have LM741/358, TL072/082, LT1013, and some IRF540 that should be fine, but how would I go about biasing the fet(s) ? I have a huge 1 Ohm 300W resistor, that will do perfect.
Thanks
MarkF:
Here you go: https://www.eevblog.com/forum/beginners/a-load-off-my-mnd/msg2151265/#msg2151265
lordvader88:
Great, watch that Sullcom guy's video too. Now I'm even more intrigued, at the possibliity of powering this from some analog PWM circuits I could make, or with a PWM chip I have (for SMPS) or whatever else is allowable.
Someday I'll have to get my Adriano working and learn the coding (not not forget it) for going digital too
Is the only real reason to use a fet, is for it's higher effeciency ? I have tons of power BJTs, I should look up old test gear circuits.
lordvader88:
Ok so if a mosfet is used the opamp is not driving a base current, so thats a big difference.
I have +10V/-5V regulators hooked up to a small centertapped transformer, and IRF540N. LTspice thinks the circuit works well enough, I'm not worried if it doesn't behave at the milli-amp level.
I better be careful if I hook this up to the PSU I built.
MarkF:
Assuming 20V @ 2.5A load target requirements.
Parts and sources:
* If you follow along with my circuit and use the LM358 or LT1013 op-amp, you will NOT need a negative power supply.
The +10V will be okay. A +12V supply would be better since you will need a fan.
* Use the IRF540. If you buy something else, get a MOSFET in a bigger TO-247 package. Better heat dissipation.
A IRFP250 or IRFP064 for example.
* A Bourns 10K ohm 10-turn potentiometer.
* A 1 ohm 300W is over the top. A 5W should be big enough. If you use a 0.200 ohm resistor, you will have a 0 to 2.5A range.
Vishay LVR05 Series or Vishay AC05 Series.
* Heatsink I used is a RA-T2X-64E. You WILL need a fan.
* Example 40x40x10mm 12VDC Fan.
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