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Old 3rd March 2011, 02:35 AM   #11
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Changed the resistors over the pots to 3.3k and now it works like a charm!

Pictures tomorrow!
Thanks
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Old 3rd March 2011, 02:40 AM   #12
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Little higher Vgs figure for your Mosfets.

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Old 4th March 2011, 09:03 PM   #13
ENSen is offline ENSen  Sweden
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I have a similar problem.

I can not get Vgs over 750mV on any of the mosfets. When I start turning P1 and P2 Vgs starts increasing and goes up to 750mV pretty quickly where it stops. It does not matter how much I turn the pots after that, Vgs does not change.

The voltage over R122 and R12 is 0V all the time.

I have exactly the same problem on both channels.

I use the Fairchild MOSFETs.

Any suggestions on what it could be?
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Old 4th March 2011, 09:17 PM   #14
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Same solution.

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Old 5th March 2011, 07:20 AM   #15
ENSen is offline ENSen  Sweden
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Ok, I will change the resistors.

I did not think my problem was exactly the same since my max Vgs of 750mV is very far from the~3-3,5V needed for the fets to switch on.
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Old 5th March 2011, 10:50 AM   #16
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Have you installed the correct pot?
Or have you been supplied the correct pot? Did you measure it before installing it?
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Old 5th March 2011, 11:21 AM   #17
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Just one more possibility. What is the Idss value of the Jfets you used?
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Old 5th March 2011, 12:18 PM   #18
ENSen is offline ENSen  Sweden
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Quote:
Originally Posted by Melon Head View Post
Have you installed the correct pot?
Or have you been supplied the correct pot? Did you measure it before installing it?
I usually measure all components before installing them but I am not 100% since I do not have any memory of measuring the pots. I have some unplanned hours this afternoon, I will spend them checking all components, starting with the pots.
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Old 5th March 2011, 12:19 PM   #19
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Quote:
Originally Posted by Melon Head View Post
Just one more possibility. What is the Idss value of the Jfets you used?
They are all ~6,5mA.
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Old 5th March 2011, 01:02 PM   #20
ENSen is offline ENSen  Sweden
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I measured the resistance from the positive rail to the gate of the mosfet when turning the pot. It goes from 0 to 1.2 kohm nice and smooth. It is the same on the positive and negative sides. I think 1.2kohm is the value to be expected with a 5k pot?

These measurements where done with the PSU disconnected. First I measured the resistance with the PSU still connected, then I got ~140ohm max between the rails and the gates of the mosfets. Could that be the problem or is it just that the capacitors in the PSU are still charged and thus interfering with my resistance measurement?
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