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Old 13th May 2011, 06:02 PM   #1
neskor is offline neskor  Serbia
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Default DHT for 300B problem

Hello
I just finished DHT for 300B, but when I start on output I have 8.75V for 10 sec. and than goes to desired 5V.
Can I leave like this or it will damage 300B?
Any one have idea how to solve this problem?
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Old 13th May 2011, 06:27 PM   #2
tomchr is offline tomchr  United States
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That's about the opposite of what you want to do. Applying 8.75 V to a dead cold filament will cause a lot of current to run in the filament, thus, causing a thermal chock. That's not nice...

I wonder if it has to do with the fact that you have two control loops in your circuit. One that controls the voltage across the filament, the other controls the current. I would eliminate the voltage control loop and just program the current to 1.4 A. You could make it adjustable so that you can adjust for 5.0 V across a hot filament.

~Tom
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Old 13th May 2011, 06:51 PM   #3
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The schematic says a maximum of 7 volts input and you get 8.75 out? Perhaps you are overloading it?
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Old 13th May 2011, 06:53 PM   #4
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It looks very strange. You should have slowly increased voltage between + and - signs for filament connection, as drawn.
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Old 13th May 2011, 07:01 PM   #5
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Why not taking a dead simple regulator/pass transistor ? The PSU in the pic is over engineered in many ways. There are so many parts, I could build another amplifier from that
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Old 13th May 2011, 07:24 PM   #6
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Here is my example of more than adequate voltage regulator:
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Old 13th May 2011, 09:09 PM   #7
neskor is offline neskor  Serbia
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I found what was a problem.
Value of capacitor C8 determine time to reach desired voltage.
So I put 10uF and now is working like a charm
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