Dummy load with constant voltage

[rezaf]

Hi,
I want to make a dummy load circuit with this specifications : for testing purpose of a power supply, this circuit should have about 10 different current channels, the input of each channel is fixed 12v from power supply and every dummy load channel must take a specified current for testing the power supply at the status. for example the first channel be 12v 7A the next channel be 12v 13A and etc. also I change the channel manually or automatically.
now I'm confused that what method is suitable for making this circuit, is it better to use resistor network or use mosfets or transistors or PWM ...
please help me to choose the right and good design method.
Thanks

 

[kanni1303]

For testing power supply boards you can prefer high watt resistors, where you can easily note the voltage drop with simple calculation and ADC if automatic. I prefer Resistor network using relays where relays won't drop the voltages as other switches...

 

[rezaf]

For testing power supply boards you can prefer high watt resistors, where you can easily note the voltage drop with simple calculation and ADC if automatic. I prefer Resistor network using relays where relays won't drop the voltages as other switches...

thanks kanni1303, I tested resistor network in parallel with 10w ceramic power resistors. with my calculations the resistors operate in desired current but the main problem is that the resistors go very hot after a few seconds even with use in 50% of it's nominal power. is this temperature normal ? (I can't near my hand from it's 2 centimeter) . of course this temperature is without any fan or cooling method. another problem is that how I attach this type of resistor to heatsink with screw ? (because it don't have hole for clamping screw)
Thanks and Regards

 

[kanni1303]

this is not normal, as the temperature can rise but in your case it is abnormal, what is the total power and you mentioned that the resistors are operated in it 50% nominal... so can I assume that to be 5W...
Try these steps
* Don't switch on for long time
* Try to increase the load one by one(that is adding resistors one by one to load instead of switch all at a time) with this you can also get the behavior of the PSU by 25% load, 50% load and full load operations
* Use low resistance which reduces the heat

 

[rezaf]

according to your comments, I get connect one resistor to power supply with this spec : 0.15 ohm/ 10w
and connect a 1.3v power only for test and seen the resistors current flow is 3.5A and it's heat is touchable for a few seconds and is better than last test. now I'm confused that how this can occur ? (where I=U/R and I must be about 1.3/0.15~=8.6A and the power is 1.3x3.5 = 4.55 !!!!!)
in my main calculations for one channel with 6x18ohm 10W resistor I can make a 4A load but in real test all resistors get very hot.
Thanks kanni1303.

 

[rezaf]

any new answer? Please help me to solve the problem
Thanks