losses of mosfet transistor

Thread Starter

mah

Joined Mar 15, 2010
393
i made an electronic load for 150 watt photovoltaic panel . i can control the circuit but the measured voltage by the voltmeter isn't the real volt relevant to the current , for example i got 3 amp it should give 30 volt on the drain ( where the +ve terminal of the panel is connected ) but the voltmeter reads 27 volt , where did this loss go ? , i used four IRF530 but they are from the same manufacturer ( they have different numbers on it) but i don't think that this is the problem . this is the circuit without using 10 KΩ in feedbackhttp://forum.allaboutcircuits.com/attachment.php?attachmentid=62147&d=1385954290
 

crutschow

Joined Mar 14, 2008
34,285
Sounds like you have some resistance between the panel and the transistor drains.
What is the voltage directly at the panel output?
How are you measuring the current?
 

Thread Starter

mah

Joined Mar 15, 2010
393
Sounds like you have some resistance between the panel and the transistor drains.
What is the voltage directly at the panel output?
How are you measuring the current?
no, there is no resistance between panel and drain.
the voltage of the panel is ranging from 0 to 40 volt.
i measure the current using clamp meter.
 

Thread Starter

mah

Joined Mar 15, 2010
393
i measured voltage on the resistors connected to source , they aren't equal . the time i measured voltages on them were ranging from 0.5 to 0.8 v. two of them give .8 and the others .5 volt
 

tubeguy

Joined Nov 3, 2012
1,157
How much heat ?
Are Mosfets and/or source resistors warm, or too hot to touch?

0.5 to 0.8 volts across 3.3 ohms doesn't make sense if you are drawing 3 amps split between the 4 devices.

3 amps/4 = 0.75 amps per source resistor.
0.75 amps * 3.3 ohms = 2.48 volts drop.
 

inwo

Joined Nov 7, 2013
2,419
i measure the current using clamp meter.

Trust the voltmeter!;)

The current will be the same as voltmeter with the same precision as resistance and voltmeter. Meter load should be negligible.

Unless you have some high precision clamp on ammeter, much better than mine, I wouldn't bet on it.
 

Thread Starter

mah

Joined Mar 15, 2010
393
Trust the voltmeter!;)

The current will be the same as voltmeter with the same precision as resistance and voltmeter. Meter load should be negligible.

Unless you have some high precision clamp on ammeter, much better than mine, I wouldn't bet on it.
i can't trust the voltmeter because i put the panel on 50 watt potentiometer and it gave 30 volt and 3 amp with the same voltmeter so that mean the circuit has issue .
 

tubeguy

Joined Nov 3, 2012
1,157
It's always best to post your actual circuit values. ;)

The 1 ohm resistors equate to 1 amp current per volt drop.
Makes it simple to measure the current using your voltmeter across these.
I agree with inwo. Use the voltmeter to measure the current!

Add the currents through each leg (calculated using the voltage drops). The accuracy depends on the voltmeter and the accuracy of the 1 ohm resistors.
 
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Thread Starter

mah

Joined Mar 15, 2010
393
what is the relation between this talk and the drop in voltage problem, i measured the voltage directly on the drain
 

tubeguy

Joined Nov 3, 2012
1,157
i measured voltage on the resistors connected to source , they aren't equal . the time i measured voltages on them were ranging from 0.5 to 0.8 v. two of them give .8 and the others .5 volt
With exact 1 ohm resistors the total current will then be:
1 amp + 1.6 amps = 2.6 amps

EDIT: When you used the 50 watt pot, did you set the current using the clamp meter or by setting the resistance to 10 ohms?
 
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inwo

Joined Nov 7, 2013
2,419
what is the relation between this talk and the drop in voltage problem, i measured the voltage directly on the drain
If you are just asking about voltage drop from panel to circuit board (drain), then see post #2.

Voltage drop= resistance.
 

MikeML

Joined Oct 2, 2009
5,444
Ω
i made an electronic load for 150 watt photovoltaic panel . i can control the circuit but the measured voltage by the voltmeter isn't the real volt relevant to the current , for example i got 3 amp it should give 30 volt on the drain ( where the +ve terminal of the panel is connected ) but the voltmeter reads 27 volt , where did this loss go ? , i used four IRF530 but they are from the same manufacturer ( they have different numbers on it) but i don't think that this is the problem . this is the circuit without using 10 KΩ in feedbackhttp://forum.allaboutcircuits.com/attachment.php?attachmentid=62147&d=1385954290
I am fed up with mah. This is the fifth thread he has started about this same topic. The threads are so fragmented and disjoint, that no one can follow them.

I have posted several iterations of schematics for his electronic load for him to follow. Each time, he comes back, starts a new thread, complains that the circuit doesn't work, but each time he has made modifications which he doesn't tell us about which are likely the root cause of his problems...

This time, by reading through this thread, I find that instead of the 3.3Ω source resistors (which are there to equalize the currents between the FETs), he has subsituted 1Ω resistors instead. One of his early problems was mismatched threshold voltages Fet to Fet so that the FET with the lowest threshold voltage turned on first and hogged all the current, causing a FET to burn up. Most likely caused by the substitution of the 1Ω for the 3.3Ω which he never mentioned till now...

While we where trying to get to the bottom of his early FET failures, it was suggested that the circuit might be unstable, and is oscillating. To prevent that, I added frequency compensation to the feedback network to make the circuit unconditionally stable. Now by reading this thread, I find that he left out the 10K resistor specifically added to slow the step response of the circuit to make it overdamped.

I'm finished. I have better things to do with my time...
 
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Thread Starter

mah

Joined Mar 15, 2010
393
Ω



This time, by reading through this thread, I find that instead of the 3.3Ω source resistors (which are there to equalize the currents between the FETs), he has subsituted 1Ω resistors instead. One of his early problems was mismatched threshold voltages Fet to Fet so that the FET with the lowest threshold voltage turned on first and hogged all the current, causing a FET to burn up. Most likely caused by the substitution of the 1Ω for the 3.3Ω which he never mentioned till now...
i changed al the resistors with 1 Ω so they are all equal
 

Thread Starter

mah

Joined Mar 15, 2010
393
EDIT: When you used the 50 watt pot, did you set the current using the clamp meter or by setting the resistance to 10 ohms?
by setting the resistance to 10 Ω . i noticed that there is a difference between the potentiometer and the circuit by .4 v in creased to 2 volt when i increased current to 2.5 A before this value it was small i can ignore it.
 
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