Eeyore wrote in message
...


N Cook wrote:

http://home.graffiti.net/diverse:graffiti.net/prot.jpg
The thermal switch monitors the heatsink but what is the function of the
depletion type FET, monitoring the negative half cycle of the the AC3 ac
line, via D3 ? What happens in a fault mode to that line that can be
monitored and could be of use for protection.operation?
What does D1 do ?
The relay contact cuts in serious attenuation of the signal going to the
pa
in fault mode

For starters it's not a 'depletion type FET'. It's a plain old JFET
(junction
fet). JFETs are on with zero gate volts so it conducts when AC is not
present
holding the relay off by diverting base current from TR10. When AC is
present,
it turns off due to the negative gate voltage - energising the relay.

Graham



I should retain this J112 as a collecor's item then.
Circuit operates with the relay energised in normal audio operation , after
power-on hold-off, relay contacts are o/c in audio use

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Diverse Devices, Southampton, England
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