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Old 30th August 2013, 05:08 AM   #1021
Salas is offline Salas  Greece
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Quote:
Originally Posted by JOHNNYCR100 View Post

cheers Gents
Johnny
Post a close enough clear photo so we may spot what's wrong. You probably shorted them MOSFETs backs.
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Old 30th August 2013, 09:54 AM   #1022
AndrewT is online now AndrewT  Scotland
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The two main "pass" mosFETs do not dissipate maximum power at the same time.
This makes it economic for the two to share a heatsink. see below.
But both must be electrically isolated form the Chassis.

Worst case 1.
If the output is shorted, the CCS FET passes normal current but drops the whole supply voltage. CCS is at max dissipation and the Shunt FET has zero dissipation.

Worst case 2.
When output is open circuit, the Shunt FET is at max dissipation. Now the TOTAL dissipation of both FETs is the same as the worst case in the shorted condition. The same size heatsink will have the same power input.

Nortmal case.
When operating normally the total heatsink dissipation is less than the two worst cases above and the FETs will run cooler.
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Old 30th August 2013, 10:08 AM   #1023
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Default my Hot Mosfets

Thanks gents,

I will post a picture when i can get my hands on my camera - strange to relate all the voltages are " ball park" and the DC offset is also OK (low- 5 mv / 1 mv) but its to hot (well one side is) , I will read posts and digest- get back soon, thanks boys.

Johnny
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Old 30th August 2013, 10:12 AM   #1024
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Default and

PS The mosfets that get hot each have there own heat sink and not joined to anything else.
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Old 30th August 2013, 10:35 AM   #1025
AndrewT is online now AndrewT  Scotland
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The two main "pass" mosFETs do not dissipate maximum power at the same time.
This makes it economic for the two to share a heatsink. See below.
But both must be electrically isolated from the Chassis.

Worst case 1.
If the output is shorted, the CCS mosFET passes normal current, but drops the whole supply voltage. CCS is at max dissipation and the Shunt mosFET has zero dissipation.

Worst case 2.
When output is open circuit, the Shunt mosFET is at max dissipation. Now the TOTAL dissipation of both mosFETs is the same as in the worst case 1 (the shorted condition). The same size heatsink will have the same power input.

Normal case.
When operating normally the total heatsink dissipation is less than the two worst cases above and the FETs will run cooler.

repost with errors corrected.
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Old 30th August 2013, 12:23 PM   #1026
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Mine is cool to the touch as well. Sounds like there is a problem somewhere.
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Old 13th September 2013, 01:48 PM   #1027
Gooch is offline Gooch  United States
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I am about finished with making my chassis and ready to start stuffing the PCB
I only need 2 inputs so will there be a problem with using only two relays and leaving the other spots just empty. It looks like the single path tones out with my DMM with out the use of jumpers is this correct.

Thanks Dave
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Old 13th September 2013, 01:53 PM   #1028
Eric is offline Eric  United States
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If you only need two inputs, then only use two relays... No harm in leaving the rest empty. I have need for 2-3 inputs, so I installed only 4 relays and diodes.

I'm not exactly clear what you asking in your second question.
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Old 13th September 2013, 02:06 PM   #1029
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I used three inputs and left off the relays and diodes for the other three. No jumpers are needed.
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Old 13th September 2013, 03:57 PM   #1030
Gooch is offline Gooch  United States
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Default Mezmerize DCB1 Building Thread

Thanks guys appreciate it hope to have sweet music soon


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