ELECTRONIC LOAD

[jlaf14]

Hi
I would like some help regarding the construction (or schematics) of a electronic load using transistor is for test DC power supplies.

Thanks in advance !!

 

[panic mode]

psu? what range? what application?

you can use LM317 or something similar as constant current load.
add potentiometer and you can change the current. run it through transistor and you can
get bigger current. you can try to use big Ohmite rheostats as well.

at home or work i like to use piece of wire and short the psu output for a while.
it should not overheat and it should resume with operation once the short is removed. the only things i didn't test this way are power supplies in my PCs and little wallwarts.

no, i don't consider it abuse, this is just simulation of real life conditions psu will evetually have to endure
sooner or later and i don't want to be the poor soul stranded on site with a dead machine
because psu puked... heck, our electricians are making shorts on purpose to identify wires
(no spark = wrong one, spark = that's the one) :lol:

when Omron launched S8VS model, nobody didn't test it until equipment using it was
delivered and installed. everything was ok and ready for production when someone working
inside panel lost one wire which then touched cabinet frame. everything went dead - and stayed dead
even after he picked up the wire. everyone held the breath and scratched their head until
guy cycled main power and everything came back.

the S8VS-24024 wasn't damaged off course but this was unexpected behavior. while this is still ok
for many applications, it is not something to recommend for critical or high demand industrial equipment.
you just don't walk in and turn the power off to reset faulted power supply because this means
additional downtime due rebooting of some of equipment etc. (PLC boot in a second or two but computers
and robots are much slower, any axis with incremental encoders need to be homed etc.).

 

[k7elp60]

I'll describe a circuit I have been using for a number of years to act as a variable current load on batteries and power supplies. It has a range of about 4 volts to 24 volts and an adjustable current from less than 50mA to about 10 amps. If you need an exact circuit diagram it may take me several days to generate, but here is the description.
It uses a darlington NPN, or a N-channel Mosfet. The emitter or the source is connected to ground thru a resistor of less than 1 ohm. The base or the gate goes directly to the output of a 741 opamp. The inverting input goes to the junction of the resistor and the emitter or source previously mentioned. The noninverting input of the opamp is connected to the wiper of a 10 turn pot. This pot has a resistor in series withs it's high side to a 5 volt regulator. The low side of the pot is connected to ground. The collector or drain of the power transistor is connected to the + of the voltage source, and the ground is connected to the - of the same source.
Adjusting the pot determines the constant current.
This circuit is like a variable resistor, except when testing batteries or power supplies the current remains constant as the battery voltage decreases or the power supply is varied in voltage.
If you need the schematic please let me know.