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I need help with transistors and a power supply, posted schm

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darkbrace

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I have the following circuit connected, but when power is applied the transistors become excessively hot, in under 15 seconds. The 12V power source consists of two 6V lantern batteries in series. I belive that the voltage/current I'm using would be within the SOA for a single transistor, but I have two connected in parallel. I think that it may be a problem of the transistors not being turned completely on/off with the pulses of the 555, and instead only mostly off, leaving some resistance, which in turn leads to the heating. If anyone here could help me figure this out, I'd be very greatful.
 

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mechie

New Member
Add a driver stage ?

As you don't mention the transistors being used or the impedance of the transformer (or its current)...

I think your assumtion is probably correct that the transistors are always biassed on,
I looked at the datasheet for a 555 and it suggests that the output could be 1.2v below the positive rail at 1mA :?:
I think the easiest 'fix' to the above would be adding an NPN driver transistor as below.
Without knowing more info it is difficult to calculate resistor values but I would suggest ...
Tr1 = BC109 (hfe ~ 500)
R1 = 22k (allows full 100mA Ic to be used)
R2 & R3 = higher than 240R (allows max of 50mA to each of Q1 and Q2)

I notice in your drawing that you have no base resistor for Q1 and Q2.
Also I see no reason for having a 9v supply to the 555, why not run everything on 12 V :?:
Also you run the 555 at 48kHz, isn't this a bit fast for an inverter? (depends on design frequency of transformer).
 

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darkbrace

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I didn't realize I hadn't posted the transistors. My computer died after writing that post the first time, so I had to redo it and must have forgotten to include it the second time 'round.

They are TIP 42.
I haven't much time right now, but I'll definately try that out with an NPN driver transistor.
Thanks
 

darkbrace

New Member
Alright, thx.
As soon as I can dig around here and find an NPN transistor (just rearranged house) I'll give it a go, and post the results.

While I'm posting this:

In a voltage multiplier circuit, does the capacitance of the capacitors have much effect on the output?

I have several 2kV 0.01uF ceramic disk capacitors, but I also have 10 1.75kV 1uF and 10 2.5kV 1uF capacitors.
 

mechie

New Member
Why ?

Sebi
What is that extra resistor for ?
Why do you think it is definitely required ?

Surely if Tr1 tuns off (due to having insufficient base current) there will be
no base current path for the two output transistors (Q1 & Q2)
If this is the case they will also be turned off ?

I see this additional resistor as just additional load for TR1, it adds nothing to its function :?:
 
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