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Subject: Flash Inverter Analysis - Help!
X-Newsreader: Microsoft Outlook Express 6.00.2720.3000
Date: Mon, 13 Jan 2003 12:22:08 -0000
NNTP-Posting-Date: Mon, 13 Jan 2003 12:22:09 GMT
Organization: ntlworld News Service
I took apart a disposable camera to study the flash circuit, after 1.5 days
i still can't convince myself exactly how this circuit functions. I pretty
peeved as i have a degree in electronics and feel i should be able to do
So could you guys take a look at http://homepage.ntlworld.com/guy.stanley
and tell me whats going on?
I realise that C1 is part of an oscillator used to generate AC and thus
allow TR1 to generate 300V from the 1.5V battery. Also once the voltage
across C2 reaches approx -300V (with respect to GND) the MOV begins to
conduct and switches on Q1 thus disabling the inverter.
The part i'm struggling with is exactly how the oscillator functions. I see
that C1 can only charge to a max of 0.6V due to Q2 turning on and clamping
the voltage. Q2 on makes Q3 switch on and current will flow through TR1's
primary. Due to the high turns ratio a large voltage will appear across the
secondary, since node P1 is 0.6V node P3 must go negative to maintain the
voltage across TR1's secondary.
This is where i get stuck. D1 is now forward biased and this suggests
current being drawn from C2, But where does it go then? I can't see the
current path back to GND and thus the other plate of C2. The only route is
through the secondary coil but how can this be as the current would be
flowing from negative to positive then.
I have other queries as well but if someone can explain the circuit
operation, especially with regard to the above then i might be able to work
those answers out for myself.
Thanks in advance
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