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'[EE]: Analyzing Audio Amp thermal protection circu'
2016\09\04@083611 by Justin Richards

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part 1 708 bytes content-type:text/plain; charset="utf-8" (decoded base64)

Trying to repair Amp and understand how the thermal cutout works.

The circuit employs a thermal switch that is N.O.

It is connected to the base of an NPN and PNP in series.

Attached is image of a circuit that hopefully captures the essence of the
real circuit. I hope that I have not missed anything too important.

I have tried to analyse in LTspice but the voltage levels around the
circuit remain unchanged regardless of the state of the switch.

I assume the idea is that the balanced input signal is clamped when the
thermal switch closes.

Hoping someone may be familiar with this approach and help explain how it
is intended to work.

Justin

part 2 9687 bytes content-type:image/jpeg; name="clamp1.JPG" (decode)


part 3 197 bytes content-type:text/plain; name="ATT00001.txt"
(decoded base64)

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2016\09\04@090542 by Justin Richards

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I think I have drawn it wrong.  Bases should be connected to collectors in
both NPN and PNP.  re-analysing ....



On 4 September 2016 at 20:36, Justin Richards <spam_OUTjustin.richardsTakeThisOuTspamgmail.com>
wrote:

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2016\09\04@092217 by Justin Richards

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part 1 1162 bytes content-type:text/plain; charset="utf-8" (decoded base64)

and upper pnp is upside down

redrawn attached, hopefully correct this time.

Justin

On 4 September 2016 at 21:05, Justin Richards <.....justin.richardsKILLspamspam@spam@gmail.com>
wrote:

> I think I have drawn it wrong.  Bases should be connected to collectors in
> both NPN and PNP.  re-analysing ....
>
>
>
> On 4 September 2016 at 20:36, Justin Richards <justin.richardsspamKILLspamgmail.com>
> wrote:
>
>> Trying to repair Amp and understand how the thermal cutout works.
>>
>> The circuit employs a thermal switch that is N.O.
>>
>> It is connected to the base of an NPN and PNP in series.
>>
>> Attached is image of a circuit that hopefully captures the essence of the
>> real circuit. I hope that I have not missed anything too important.
>>
>> I have tried to analyse in LTspice but the voltage levels around the
>> circuit remain unchanged regardless of the state of the switch.
>>
>> I assume the idea is that the balanced input signal is clamped when the
>> thermal switch closes.
>>
>> Hoping someone may be familiar with this approach and help explain how it
>> is intended to work.
>>
>> Justin
>>
>
>

part 2 9528 bytes content-type:image/jpeg; name="clamp2.JPG" (decode)


part 3 197 bytes content-type:text/plain; name="ATT00001.txt"
(decoded base64)

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2016\09\04@230940 by RussellMc

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Are collectors really joined to bases, as shown?
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2016\09\05@034233 by Justin Richards

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I have confirmed with both the schematic and the actual amp that the
collectors are tied to the bases and both transistors bases are tied to
each other,.

It remains a mystery and considering that they are used in some way to
provide diode volt drops that match another pair of the same transistors
that are closely bonded on the same heat sink.

I would like to make the manufacturers schematic available but I have been
asked not to do so.

Justin

On 5 September 2016 at 11:08, RussellMc <.....apptechnzKILLspamspam.....gmail.com> wrote:

> Are collectors really joined to bases, as shown?
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2016\09\05@053609 by Justin Richards

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in previous post

"..and considering ..".

should read

"...and I am considering ..."


Is it possible that they are temperature compensated biasing.  Still not
100% happy as does not explain how the thermal shutdown works.


Justin

On 5 September 2016 at 15:42, Justin Richards <EraseMEjustin.richardsspam_OUTspamTakeThisOuTgmail.com>
wrote:

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2016\09\05@063623 by Justin Richards

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Solved.

I am now reasonably confident it is used for both temp compensation and
clamps the input to the next stage at one BE voltage drop above and below
GND at thermal shutdown.

Justin

On 5 September 2016 at 17:36, Justin Richards <@spam@justin.richardsKILLspamspamgmail.com>
wrote:

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