Dummy load

[ronv]

Good job Doc. Can you post a picture?

 

[Doctore]

Ok, here are two pictures(I have more):






Beeper starts beeping after exceeding 89C, how to adjust this function to 75C??

EDIT: I replace R20 from 4k3 to 3k3 and I have 70C and stop on 63.

 

[ronv]

Looking good.

 

[Doctore]

Yes, but this is not end of problems, fk.
Already I have soldered all fets, when I get this to test and connect multimeter for AMPS I get 3A from multimeter and 0,0 from display, not 0,3.
What's going on?
When I turned to 8A display show 0,1......

 

[ronv]

Check the values of R11, 12 and 13. You might also measure the voltage with your multimeter sometimes those little meters don't like to measure using their own ground.

 

[Doctore]

R13 unsoldered on one leg...
Now circuit measures AMPS properly.
I'm after tests with 15V/17A and temp. of fets in this situation is 42C(fet on end of flow) with slow flow of water from tap.
Here is photo(photo is not good, I will take new):


Now only I must make case

 

[ronv]

Good. The trick may be to have the pipe full of water, so if the flow is small you may want to restrict the outlet tube so the pipe fills with water.

 

[Doctore]

Meybe, but I have small flow because I set small flow on tap(20%)

 

[Doctore]

In tests I check voltage betwen ground of circuit and ground in wall outlet, what I see on multimeter?
15V from PS :O
So I must disconnect ground from ps's(where this ps's have connected -12v to ground)?
In this situation I have ps unconnected to ground(plastic case - xbox ps) so I don't have any problems.

 

[ronv]

I don't understand. What power supply has -12 connected to ground?

 

[Doctore]

95% of server PS have connected negative in output to the ground(case) of the AC in.
jocannon don't tell this you?

 

[ronv]

Shouldn't matter as long as it is the ground ties to AC ground.

 

[Doctore]

After about two months I still have to make case of this dummy load, no time...
But now I want get real summary of specifications of this DL.
Max safe:
-voltage is 30V?
-amperage is 50A?
-wattage is 1500W?
-temp. of fets?

Thanks so much Ronv, you are the best!!!

 

[ronv]

It is hard to say for sure without knowing the temperature. If you have one of those non contact thermal probes we could measure a fet at various loads to determine better the temperature rise per watt.
At 1500 watts each FET is 150 watts which means the chip itself will be about 100C if the temperature of the case is 25C. The absolute maximum temperature is 175C.

The best spec is watts. The current can be higher if the voltage is lower and vice versa.

Very professional job!

 

[Doctore]

Very professional job!

Thanks but I'm only kid, and you are real professionalist!
So, I can get 200VDC 57A(here is my FET https://www.tme.eu/en/details/sup57n20-33/tht-n-channel-transistors/vishay/sup57n20-33-e3/ ) but I cant exceed 1500W yes?
About "non contact thermal probes" you mean IR temp. meters(pirometers?)?
Because this I don't have.

maximum temperature is 175C.

This is internal temp. of fet OR temp of fet case, which is readed by LM35?

EDIT: about max voltage, you tell me previously, that max safe voltage is 30V

 

[ronv]

I would stay within about 1/2 of the DC curve.
I would not exceed 1500 watts unless we can figure out the FET temperature. They smell so bad when they burn.
I was thinking IR probe. To can look down at the black case and come close to the FETs internal temperature, then look at the tab and come close to the case temperature.
The 175C is the internal temperature it will be much higher than the case.
Before we didn't talk about lower currents we were always looking for more amps.

 

[Doctore]

I'm after tests with 13,1V 45A, power supply can give 72A.
But problem is that I can get only 45-47A(potentiometer is on max.), on "Gate" of fets is 4,9-5,2V, where is problem???
Temperature on fets is 49C, what temp. is inside fets in this situation?

 

[ronv]

You can increase the current that can be set by making R26 smaller. The current should be the voltage on the + pin of the op amp divided by the .13 ohm resistor. But be careful if you test a power supply with a higher voltage you will still be able to set the higher current so the power will be higher.
If the temperature on the fet is 49C and ambient temperature is 25C and voltage is 13.1 volts and 45 amps the power is about 590 watts in the fets. So the temperature rise is about .042C per watt in your heat-sink. From the fet data-sheet the temperature rise from case to junction is .5C per watt or another 30C. So the junction will be 49 + 30 or 79C. About 1/2 of the absolute maximum of 175C. This would make it good for about 1500 watts.

 

[Doctore]

Very thanks!
How to calculate R26 to max. 50A?

 

[ronv]

Hmm, It should already be ok for more than 50 amps.
R28=2.7k, R26=68K, R29=5K pot???? If so from ohms law the voltage at the top of the 5K pot should be .79 volts. To drop .79 volts across the .13 ohm would take 6 amps. Times 10 fets is 60 amps.
Maybe you can measure the top of the 5k pot?