🚀 go-pugleaf

On Sun, 04 Jan 2009 14:41:04 +1100, Sylvia Else
<sylvia@not.at.this.address> wrote:

>legg wrote:
>> On Sun, 04 Jan 2009 13:32:44 +1100, Sylvia Else
>> <sylvia@not.at.this.address> wrote:
>> 
>>> After my airconditioner failed the other day, and being reluctant to pay 
>>> a technician to come and fix it, I've been taking a look at its 
>>> electronics board.
>>>
>>> It's clear that its power supply circuit has failed. From the board 
>>> itself I've inferred this partial circuit:
>>>
>>> http://members.optusnet.com.au/sylviae/smps.jpg
>>>
>>> I am pretty sure there are no other components connected to the 
>>> transistor labelled Q1, and it is this transistor that has failed. The 
>>> failure mode is a short (a few ohms, polarity insensitive) from base to 
>>> emitter. The collector is open circuit. The transistor is thus unable to 
>>> sink enough current to prevent the switching transistor from turning on, 
>>> and as a result the 8.2 Ohm fusible resistor has also failed.
>>>
>>> It seems moderately likely that by replacing these two components I can 
>>> get the board working again.
>>>
>>> The circuit nevertheless puzzles me. The function of Q1 appears to be to 
>>>  bias the switching transistor. But this seems to rely somewhat on the 
>>> characteristics of the two transistors, which I would have thought was 
>>> asking for trouble. In particular, it looks to me as if Q1 could simply 
>>> prevent the switching transistor from ever conducting, and nothing would 
>>> happen.
>>>
>>> Is this an accepted technique? Or have I misunderstood the purpose of Q1?
>>>
>>> BTW, this is from the external unit of a nine year old Daikin split system.
>>>
>>> Sylvia.
>> 
>> As drawn, Q1 is intended to limit current in the 8R2 emitter resistor,
>> when the main switch is functional. The event that killed the main
>> switch and fused the emitter resistor, could have fried Q1 in the
>> process. I think you'll find that Q1 is intentionally inverted
>> (collector and emitter swapped). When it functions, it resembles a
>> temperature-dependant voltage reference of ~ 1V4.
>> 
>> This is one of the simpler self-oscillating flyback circuits popular
>> below 25W (in switch-mode power supplies) since about 1970.
>> 
>> Without the appropriate tools, training and test procedures, you
>> cannot ensure that this unit, even restored to an apparently
>> functional condition, is safe to use.
>> 
>> If you know the supply's output requirements, you would probably be
>> better off replacing the whole assembly.
>> 
>> RL
>
>The main switching transistor appears to be intact. I haven't removed it 
>from the circuit, but the voltage/current plot given by a component 
>tester feature on my oscilloscope gives the expected traces for the base 
>collector and base emitter junctions, allowing for the presence of the 
>diode across the base emitter junction.
>
>Sylvia.
You seem to have access to some tools and a fair understanding of
basic electronics. There's nothing preventing you from undertaking a
repair by component substitution, for your own interest's sake,
providing you are aware of, and use, use safe procedures around
powered circuitry.

This assembly was designed to fail in a safe manner - no fire,
explosion or shock hazard was intended to result from a single fault
resulting in the fusing of the emitter resistor. However, this does
not mean that the repaired unit will meet the same standards. The
events that occurred may have affected the safety isolation of
magnetic isolators in such a way that a second similar event may not
be as benign.

If the main switch is not damaged (a big if), damage to the magnetic
component's internal insulation is also unlikely. With the
introduction of foreign particles, anything is possible. It should at
least be subjected to a basic hipot stress test procedure, after the
repaired and cleaned assembly is burned in and still warm.

RL