First channel of Alpeh 4 is working, however... - diyAudio
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Old 7th June 2002, 05:02 PM   #1
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Default First channel of Alpeh 4 is working, however...

Hello,

my first channel of my Aleph 4 (soon when all is working converted to a Aleph 2) is finished, however...

It plays, It sings etc. It runs great in a 8 ohm resistor at full power etc. But when I feel temperature of the mosfets (IRFP240) some are colder than others. When I measure voltage over the 1.5 ohm resistors, they are different, the colder ones are lower.

I have a difference of about 5.78 V tot 6.01 V in one sixpack and 5.74V to 5.94 in the other sixpack...

I did the 10 second 'Nelson Pass' way of testing the mosfets and they were all within 0.01V ...

I use 221 1% metalfilm and 1.5 ohm 2% 4W metal film resistors.

After about 15 minutes of running the coolelements are about 44 degrees celcius. Not very high! Measured with a thermocouple placed next to one of the mosfets.

Should I worry, and do my matching again? All mosfets were from the same lotcode...

Let me know,

Edwin
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Old 7th June 2002, 07:09 PM   #2
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Default Re: First channel of Alpeh 4 is working, however...

By the way, my DC offset is between 0.08 to 0.055 (cold to not so cold) about 0.066 volt (about 44 degrees celcius)...

Edwin
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Old 7th June 2002, 07:43 PM   #3
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Smile Mosfet diff. temp. Aleph4 ?

Check mosfet matching again, there is nothing else except that you may want to check your mica isolators (or whatever you using)and the way you have fixed your mosfets on the heatsinks.

I am collecting parts as well for my Aleph 4 and mostly have everything except that I am waiting for my toroids from Plitron.
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Old 7th June 2002, 08:33 PM   #4
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Maybe you should check source resistors as well?
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Old 7th June 2002, 09:21 PM   #5
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Quote:
Originally posted by HPotter
Maybe you should check source resistors as well?

I am using Intertechniek MOX4 2% metalfilm resistors. I do not have a device capable of measure with more precision than .1 ohm. So 1.5 ohm is what I see...

Edwin
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Old 7th June 2002, 09:26 PM   #6
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Default Re: Mosfet diff. temp. Aleph4 ?

Quote:
Originally posted by trigon
Check mosfet matching again, there is nothing else except that you may want to check your mica isolators (or whatever you using)and the way you have fixed your mosfets on the heatsinks.

I am collecting parts as well for my Aleph 4 and mostly have everything except that I am waiting for my toroids from Plitron.
Have someone else used the 'Nelson Pass 10 seconds is enough' for one to settle way? Or do you all go for the 5 minute wait till it settles way?

Edwin
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Old 7th June 2002, 10:52 PM   #7
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I used 5 seconds.
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Old 7th June 2002, 11:07 PM   #8
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I will use 6 seconds

Just kidding, I think I will go with 5 sec. as well. But just to
confirm what HPotter has mentioned and I agree with him and
that is that you have to be sure that you match the resistors as
well.
Good point HPotter
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Old 8th June 2002, 04:15 AM   #9
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Given my experience with matching various IRF and Motorola (aka OnSemi) MOSFETs, I'd have to say give it until it settles down. Try a few in your test rig, watching until they give fairly consistent readings. Note the time. Then test all others for that same length of time.
I ran IRF644s for ten minutes each, which seemed to amuse/horrify some people. The IRFP044s I got in for the Aleph-X project settled down in less than a minute. If memory serves, the Motorola parts were more like 2 or 3 minutes each. My assumption is that it's a thermal issue, but I still haven't decided for sure.
This is all a long-winded way of saying that I vote for going back through and matching the output devices again.
By the way, I'm assuming that you read the current across all the Source resistors. You should have noted a fair difference in current draw from the hot devices to the cold ones. If you're reading zero, you've got a bad part or a bad solder joint.

Grey
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Old 8th June 2002, 07:43 AM   #10
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Quote:
Originally posted by GRollins
This is all a long-winded way of saying that I vote for going back through and matching the output devices again.
By the way, I'm assuming that you read the current across all the Source resistors. You should have noted a fair difference in current draw from the hot devices to the cold ones. If you're reading zero, you've got a bad part or a bad solder joint.

Grey
Oops, major typo, My current draws (drop in voltage) are:

I have a difference of about 0.578 V tot 0.601 V in one sixpack and 0.574V to 0.594 in the other sixpack...

This makes more sense!

Hmm... one camp says; 5 seconds is enough, and the other go until it settles... what to do!

I can not match the source 1.5 resistors, as I do not have the gear precise enough to do it. Come to think of it, perhaps put it between a precise power supply and measure the voltage drop instead of using the ohm-meter. What do you think?

Edwin
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