source resistor

svemir · 6th February 2007, 09:52 AM

I have built Aleph 5 before 6 months and it works fine. But i notice that one heatsink heats a litle bit more than the other so I measure voltage accros source resistors and all vlotages are around 0.6 and 0.65V but on one source res. is 0.8 V. So what could be the problem mosfet Off-toleranc of source resistor or something else?
Sorry obout my bad english.

wuffwaff · 6th February 2007, 10:09 AM

Hi,

if it´s just one than probably the fet wasn´t matched good enough.

William

svemir · 6th February 2007, 11:21 AM

Could it be off tolerance of source resistor rather than the unmatched irfp240. Thats the first thing that came on my mind?

sek · 6th February 2007, 12:11 PM

That would indicate a resistance value about 33% off. Unlikely, but possible. You can actually measure this in-circuit.

Indeed, a FET which has it's channel "opened wider" (!?) due to unmatched gate characteristics is much more likely, as just a little deviation from the rest of it's bank would make this FET conduct more of the total current than each of the other FETs in this bank. We're talking 33% here...

Cheers.

svemir · 6th February 2007, 12:22 PM

On R42

svemir · 6th February 2007, 12:22 PM

ok thank for the advice FET is going out.

6th February 2007, 09:52 AM	#1
svemir diyAudio Member Join Date: Dec 2005 Location: zagreb	source resistor I have built Aleph 5 before 6 months and it works fine. But i notice that one heatsink heats a litle bit more than the other so I measure voltage accros source resistors and all vlotages are around 0.6 and 0.65V but on one source res. is 0.8 V. So what could be the problem mosfet Off-toleranc of source resistor or something else? Sorry obout my bad english.

6th February 2007, 10:09 AM	#2
wuffwaff diyAudio Member Join Date: Aug 2001 Location: Ingolstadt Germany	Hi, if it´s just one than probably the fet wasn´t matched good enough. William __________________ een ooievaar is geen konijn want zijn oren zijn te klein!

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6th February 2007, 11:21 AM	#3
svemir diyAudio Member Join Date: Dec 2005 Location: zagreb	Could it be off tolerance of source resistor rather than the unmatched irfp240. Thats the first thing that came on my mind?

6th February 2007, 12:11 PM	#4
sek diyAudio Member Join Date: May 2003 Location: Berlin	That would indicate a resistance value about 33% off. Unlikely, but possible. You can actually measure this in-circuit. Indeed, a FET which has it's channel "opened wider" (!?) due to unmatched gate characteristics is much more likely, as just a little deviation from the rest of it's bank would make this FET conduct more of the total current than each of the other FETs in this bank. We're talking 33% here... Cheers.

6th February 2007, 12:22 PM	#5
svemir diyAudio Member Join Date: Dec 2005 Location: zagreb	On R42