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Old 10th January 2013, 05:39 PM   #1
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Default In a series pass voltage regulator, where should the fuse be placed?

I already have a fuse on the mains line, but I want an additional fuse after the transformer.

Do is it better to be a fuse at the output of the regulator, or at the output of the transformer?
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Old 10th January 2013, 05:45 PM   #2
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If you have chosen an appropriate fuse value, the primary fuse will be enough. I'll guess you PS has some current limitation built-in?
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Old 10th January 2013, 05:51 PM   #3
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Originally Posted by peranders View Post
If you have chosen an appropriate fuse value, the primary fuse will be enough. I'll guess you PS has some current limitation built-in?
No current limiting circuit, thus the fuse.
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Old 10th January 2013, 06:02 PM   #4
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I would put it between the main regulator and the bulk capacitor.
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Old 10th January 2013, 06:13 PM   #5
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Originally Posted by Osvaldo de Banfield View Post
I would put it between the main regulator and the bulk capacitor.
Theres no main regular, just a series pass pnp, a opamp, and a voltage reference.

So the places I can put the fuses are the following:

(1) At the output of the PNP
(2) At the output of the mains transformer
(3) At the input to the PNP (emitter)
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Old 10th January 2013, 06:21 PM   #6
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A lot of unknowns.

Put a short across the bridge rectifier input and see if the mains fuse blows within say 5 seconds or, more scientifically, monitor the primary current and see what it is with a short.

Your assuming the fuse will blow... you have to prove it ! Very small transformers can even be inherently short circuit proof... and we don't know what you have.

As to a secondary fuse. It depends on the result of the first test. If the primary one doesn't blow then you need one to protect the transformer (and that will have to be rated to stand the reservoir cap charging current and perhaps another at the input to the regulator. Fusing the output of the reg could cause problems if the fuse isn't incorporated in the feedback loop and even when it is, its not always the best of solutions.
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Old 10th January 2013, 06:26 PM   #7
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I'd place it between the bulk capacitor and the regulator, because, is the capacitor who can give lots of surge current in case of a sudden short circuit at the load or regulator.
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Old 10th January 2013, 06:29 PM   #8
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The fuse won't save the transistor if the regulated o/p gets shorted - no chance. A 0R1 resistor (assuming your PSU is a couple of amps or so o/p) in the emitter with a couple of series diodes base to the non emitter end of the resistor, plus a fuse, may save it though.
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Old 10th January 2013, 06:45 PM   #9
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Originally Posted by sbrads View Post
The fuse won't save the transistor if the regulated o/p gets shorted - no chance. A 0R1 resistor (assuming your PSU is a couple of amps or so o/p) in the emitter with a couple of series diodes base to the non emitter end of the resistor, plus a fuse, may save it though.
Can you post a schematic of this?
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