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Old 3rd December 2008, 06:37 PM   #1
NV&H is offline NV&H  United States
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Default Thermistor mounting shown in 6moons

I was recently reading the excellent F5 article in 6moons. A couple of the hi-res pictures show the thermistors clearly NOT mounted to anything. Just sitting in mid-air supported by their leads.

Shouldn't they be mounted, alongside their corresponding power transitors, on a sink to thermally track together?

What's going on?
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Old 3rd December 2008, 07:26 PM   #2
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Hello NV&H, The thermistors are installed correctly in the F5. If you look at the schematic for the F5 (www.firstwatt.com), you will see that the thermistors are used for limiting the inrush surge current when the amp is first turned on. They aren't being used as any sort of temp. compensation for the output stage.

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Old 3rd December 2008, 07:32 PM   #3
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Silly me.

Thanks.
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Old 3rd December 2008, 07:34 PM   #4
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I understand that the 4,7k-themistors are mounted near to the place where the temperature rise is produced by the output MOSFETs for the thermistors to sense the temperature rise. Then, the thermistors sense the temp rise and reduce the bias of the MOSFETs. Finally, the temperature and the bias level would reach to the steady-state equilibrium.


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Old 3rd December 2008, 07:43 PM   #5
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Actually they work both ways. Obviously the response time is
quicker if you glue them to the top of the devices.

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Old 3rd December 2008, 07:45 PM   #6
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Please visit here. And, read page 11 about TH1 and TH2.


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Opps . . . Papa was already here.
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Old 3rd December 2008, 07:52 PM   #7
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Whoops, my mistake, I didn't see that there is a set of thermistors in the actual amp schematic. I was only thinking of the thermistors in the schematic of the power supply.

Sorry about that.

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Dave
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Old 3rd December 2008, 08:05 PM   #8
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Quote:
Originally posted by Nelson Pass


... the response time is
quicker if you glue them to the top of the devices.

. . . M . . . ! I was wasting my time to keep the distance of the precision-2mm from the heat sinks for my F5 . . .



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Old 3rd December 2008, 09:37 PM   #9
juma is offline juma  Germany
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It's already noted in the F5 thread. The said picture from 6moons site clearly shows that NTC is mounted close to middle pin of MOSFET, the DRAIN, which is also the back of the MOSFET and bolted to the heatsink. So, you got NTC very close to hot pin - that's the thermal coupling you were looking for.

The pic, once again:
Attached Images
File Type: gif ntc.gif (45.9 KB, 707 views)
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Old 26th June 2012, 04:25 PM   #10
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I had a problematic F5 which I've just gotten around to repairing. I had hooked it up to my FFT analyzer set on "repeat mode" -- and you could see the harmonics change over time as the thermistor was moved the tiniest bit. I just glued the thermistors to the drain with 3M "3779" Hot Melt adhesive. As Elwood said in The Blues Brothers "It's glue, it's strong stuff".
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