🚀 go-pugleaf

Arfa Daily wrote:
> "legg" <legg@nospam.magma.ca> wrote in message 
> news:jud2m4l2afaog9p6hghechvbuu317gftb6@4ax.com...
>> On Mon, 05 Jan 2009 09:21:54 +1100, Sylvia Else
>> <sylvia@not.at.this.address> wrote:
>>
>>> Jamie wrote:
>>>> Sylvia Else 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.
>>>> How about Q1 being a thyristor as a crow bar on the bias of that second
>>>> transistor?
>>>>
>>>>    In which case, your symbol is incorrect. And would show the reason
>>>> why you're getting low ohm reading is my guess on what you call the
>>>>   base-emitter and Collector being opened which is actually the M1
>>>>  terminal etc..
>>>>
>>>>    That's just a guess of course.
>>> The small transistor is a C1815 - an NPN, with an annoyingly difficult
>>> to match pin-out.
>>>
>>> Sylvia.
>> Looking at the package's flat surface, with leads pointing down, the
>> pin-out of 2SC1815 is ECB.
>>
>> RL
> 
> i.e. absolutely standard pinning for just about any and all Japanese TO92 
> transistors starting 2SA, B, C, or D ...

Well, to avoid P&P costs, I need to use what I already have, or what I 
can buy from DSE or Jaycar.
> 
> FWIW, the '1815 is about the commonest general purpose small signal NPN 
> Jappo transistor to be found anywhere, and anything similarly general 
> purpose will sub for it. It's generally not hard to rearrange the leads of a 
> differently pinned device, with a bit of sleeving on one or two legs to stop 
> them touching.

Yes, I'd already concluded that given its position in the circuit, any 
vaguely similar type will suffice, and that rearranging the leads is 
practical, if not very elegant.

I've obtained a suitable resistor (not fuisible, but OK to test the 
solution) and will give that a go later. Shame it's getting so hot outside.

Sylvia.