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Old 9th January 2013, 07:38 PM   #1
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Default Nmos 200 problem

Well, sometimes it blows 4A fuses when turned on... I think it has to be with the voltage multiplier, its transistor probalby can't turn on as fast as it should, so as it's opened current through power transistors (IRF640) is too high, could be that?

Any ideas will help, of course, and I'm thinking on removing voltage multiplier and leave a single preset.
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Old 9th January 2013, 07:41 PM   #2
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Btw, voltage multiplier transistor is a BD139
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Old 10th January 2013, 03:06 AM   #3
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But does not the Nmos200 use BC546 as Vbe multiplier?
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Old 10th January 2013, 11:45 AM   #4
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Should be the same... but maybe not.
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Old 10th January 2013, 08:01 PM   #5
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Change to BC546, even with bias at minimum, still blows both fuses (used to be only the -40V rail). Measured output mosfets with diode tester and are not shorted, all of them measured 502 mV. also measured other transistors and none of them are shorted. Don't know really what the problem is...
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Old 10th January 2013, 09:05 PM   #6
AKSA is offline AKSA  Australia
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Could you give us the schematic, if possible?
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Old 10th January 2013, 09:26 PM   #7
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Mine is running perfectly with MJE340 instead of BC546 and no other changes.

Hugh here is Quasi Nmos200 Quasi's web page : https://sites.google.com/site/quasis...ries/nmos200-1

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Old 10th January 2013, 11:19 PM   #8
AKSA is offline AKSA  Australia
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Thanks Marc,

I will examine it, might be able to help.

Hugh
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Old 11th January 2013, 03:33 PM   #9
JMFahey is offline JMFahey  Argentina
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Quote:
all of them measured 502 mV.
That's shorted .
They should measure open with red to drain, black to source.
The opposite way you will find a parasite diode (built-in) which will give around 650mV.
To be certain, pull them from the PCB and measure them "outside".
The Vbe multiplier is never "too slow".
Before pulling them, check that drains are not shorted to the heatsink.

PS: you *did* use nipples on those exposed IRF640 metal tabs, didn't you?
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Old 11th January 2013, 10:14 PM   #10
AKSA is offline AKSA  Australia
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Nmosquasi200.gif

SLD82,

Here is the schematic (I understand this is OK to publish since it's on the public domain).

If the Vbe multiplier BC546 is substituted BD139, the bias voltage at the output gates will be slightly increased because the BD139 has MUCH less beta than the BC device. This would be usually 80 compared with 300, increasing the voltage dropped across the base and collector. This increases the bias voltage considerably which drives the output very hard.

The reason is that the bias generator has around 5.3 volts as designed, and the source resistors are very small at 0.22R. If a small increase from 5.3V to say 5.5V the additional bias will be impressed - anything up to 10A - upon the source resistors, hugely increasing the quiescent, in your case, blowing the fuses.

Vbe multipliers are extremely tetchy, and should has be built as designed to deliver the intended quiescent. You should be able to correct this using a BD139 by reducing the base/collector resistor from 470R to 390R. This is a guestimate, you will need to experiment with the pot at half set, ie 100R.

Cheers,

Hugh
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Last edited by AKSA; 11th January 2013 at 10:22 PM.
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