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?
Shouldn't they be mounted, alongside their corresponding power transitors, on a sink to thermally track together?
What's going on?
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.
Peace,
Dave
Peace,
Dave
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.
Cheers,
Cheers,
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:
The pic, once again:
Attachments
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".
Jack,
Thank you for the adhesive recommendation. That looks like good stuff.
I'm building an F5 turbo at the moment and am going to try these under the head of the transistor mounting bolt.
NTCALUG02A502G Vishay BC Components | BC2740-ND | DigiKey
Graeme
Thank you for the adhesive recommendation. That looks like good stuff.
I'm building an F5 turbo at the moment and am going to try these under the head of the transistor mounting bolt.
NTCALUG02A502G Vishay BC Components | BC2740-ND | DigiKey
Graeme
I think it just needs to be good enough to keep the output transistor temps reasonable. It's an F5 turbo and I'm changing a number of resistor values per the article. So I've bought some 3K and 2k thermistors as well, and am preparing myself for some serious tweeking.
I can't recall that a specific thermistor was called out by Nelson. Was it?
gl
I can't recall that a specific thermistor was called out by Nelson. Was it?
gl
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