Simplistic Mosfet HV Shunt Regs - Page 21 - diyAudio
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Old 28th March 2009, 11:34 AM   #201
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Upon startup it is drawing 74mA , then a long wheeeeeee.... sets in and then the chirping. Voltage and current are all over the place then....
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Old 28th March 2009, 11:47 AM   #202
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Old 28th March 2009, 11:49 AM   #203
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Go back to 33R. Its 65mA, more than enough. If it still oscillates see for 1k R7 since the Fairchild has much more input capacitance than the 840, and maybe it oscillates. See if it took out some driver MPSA or BC from the ring of two CCS. Keep leads short and twisted to the Aikido. Also use a local 0.1uF decoupler to ground directly from where the B+ is on Aikido.
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Old 28th March 2009, 11:53 AM   #204
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Thanks Salas for the quick reply...

This reg still has the IRF840 but I will do the rest as suggested.
Keeps me busy on those rainy and cold saturdays...
(though I can hear my daughter clearly from the background suggesting to play with her (4y's ;-)).
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Old 28th March 2009, 11:56 AM   #205
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If there is some interaction (smells like a typical oscillation) 1k gate stopper R7 will not hurt the 840 either. The weather is sunny here so I am off for a stroll. See you later to know if you tamed it.
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Old 28th March 2009, 03:29 PM   #206
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A few notes.

When making some HV shunt, there are practical matters to consider for its layout. Gary Pimm's earlier contribution in this thread about making a PCB has invaluable information about nodes practice, and is applicable in P2P construction style too. Gate stopper resistors for the Mosfets must be connected directly to the gate legs in minimal distance (very short leg) and better be carbon composition. This is very important.
Also when connected to the audio circuitry, the leads inductance must be low. This means twisted pair. Some circuits may react to the very low output impedance of the shunt in very high frequencies. Layout and local EMI decoupling by means of a fast 0,1uF PP cap or stacked ceramic with very short leads directly on the B+ receiving end to ground, may prove invaluable on those audio circuits
Avoid trimming Vout and probing on the shunt when there is some oscillation going on. It can easily take out the driving MPSA mainly, but possibly a BC560 on the Vref ring of two CCS. Making R7 IRF840 gate stopper bigger in value can help in the event that the layout or some other interaction brings possible oscillation. Also a 47R base stopper for the MPSA can help its protection.
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Old 28th March 2009, 04:00 PM   #207
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Excellent advice!

Stoppers are CC but only one really close to the pin (IRFP9240).
I don't have 1K CC resistors so metal film will have to do, and I will move it closer to the gate pin of IRF840.
Wires are short from transformer to the psu but inevitably longer to the Aikido (mere physical size of my breadboard) so I will try to improve on that.
And I will add the decoupling resistor (s)...

Will get back to you tomorrow.
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Old 28th March 2009, 04:02 PM   #208
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The longer I think about it the more I am sure that the problem is caused by my temporary wiring...
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Old 28th March 2009, 06:37 PM   #209
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Quote:
And I will add the decoupling resistor (s)...
Err, I meant to say decoupling cap(s)...

Can I check the BC560 in the circuit? With my mods and against a load resistor (4K) I have output voltage but no response to the trimmer (carefully turning it... )
Replaced the MPSA and added a 33R base stopper (didn't have any 47R).


Will continue tomorrow...
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Old 28th March 2009, 07:16 PM   #210
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Yes check the BCs. If you have an hfe socket on your DVM see if they are OK. If you want on circuit, I don't advise probing them, but you can look for voltage drops across resistors around them. What 4k resistor? If you don't use the original values around the ring of two, they are going to burn out at those voltages. Just change Q5 as a prediction, and see if it was alive or not off circuit IMO.
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