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Old 20th July 2013, 12:36 PM   #1
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Default Automatic Power Cap Discharger

Dear Member,
I am studying electronics and require your help and support.
I have a fairly good experience in making/cloning equipment. I often have a tendency to forget discharging capacitors when I am working on the unit.
In case of tube amps its even troublesome when it sparks or injurious perhaps.

I have designed this basic diagram using a relay, some fuses and drain resistors.
The moment I swich off the mains, the relay will flip position and the cap will be discharged automatically. The moment I switch on the mains, the relay again flips and the drain resistors are not any more in circuit.


There will be voltmeters across the drain resistors and rectifiers have been used mainly to maintain the polarity received by the volt meter so that there are no negative voltage readings. I have kept the capacitors in the diagram to be ceramic (non polar) intending that drain probes can be connected without any polarity concern.

I just have one worry and that is in case of a relay failure, if I switch on the unit and the relay fails to flip which would mean that the drain resistors are in circuit, I will be severely in trouble.

I am whole heatedly game for some belting and leg pulling on this design but look out for the valuable inputs.

Simran
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Old 20th July 2013, 01:32 PM   #2
DUG is offline DUG  Canada
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Consider using a "safety relay" and monitor one of the contacts with an LED circuit.

A "safety relay" is designed such that all of the contacts are mechanically ganged together so that if one contact is stuck then they will all be in that position.

Is that what you are thinking of?

Or you could put in a thermal cutout switch on each resistor leg like they do in hairdryers or some plugin transformers if that is what you require.

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Old 20th July 2013, 03:40 PM   #3
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Hello,
Thankyou for the reply. I shall read on Safety Relays more and try to incorporate.
Second thing is a wide range of voltages will be neutralized
Like from 400 volts (B+) to +-20v of a bipolar supply.

What should I read more about that will give me insight on how a LED can be incorporated in this circuit so that it is lit when the cap is fully discharged
or is lit with a charged cap and gets off when the cap is discharged?
Perhaps if the volt meter can be replaced with a simple LED, my budget will be minimized.

Please provide me with pointers so that I can work on this project and self learn in the process.

Sincerely
Simran
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Old 20th July 2013, 06:50 PM   #4
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The volt meters which I plan to incorporate is something like this.
Led display digital Voltmeter
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Old 20th July 2013, 07:04 PM   #5
Mooly is offline Mooly  United Kingdom
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You could use an opamp or comparator to light an LED when the cap voltage falls below some preset value.
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Old 20th July 2013, 07:46 PM   #6
DUG is offline DUG  Canada
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If you use an active voltmeter circuit or an LED that is ON when power is OFF then you still need a power supply for those indicators.

For the High Voltage use a small 50uA Analog panel meter scaled to 500VFS.

For lower voltage just use an LED circuit to indicate power.

Simple.

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Old 20th July 2013, 09:58 PM   #7
Enzo is offline Enzo  United States
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I don't mean to sound unkind, but are we really trying to invent something so we can be less cautious? If you have safety so far from your thoughts that you forget to discharge your caps, adding a circuit like this will only reinforce the feeling you need not be concerned. But you DO need to be concerned.
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Old 21st July 2013, 12:13 AM   #8
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Thanks for the active support Dug and Mooly. OP amps and Comparators are in syllabus of this semester and they have to be studied for the December Exams.
Enzo, you are right. One has to be really cautious but I some felt the urge to have this with me. Its not a replacement of course to manual draining and if it works correctly, it will definitely exploit my habits. This project is nothing more than a learning curve to exploit electronics so to reinforce the laziness in me.

Let me try a few more things and incorporate it in the schematics and get back.

Sincerely
Simran
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Old 21st July 2013, 02:50 AM   #9
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Enzo: +1!
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Old 21st July 2013, 11:50 AM   #10
DUG is offline DUG  Canada
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"...This project is nothing more than a learning curve to exploit electronics..."

Then definitely incorporate everything suggested.

Happy learning.

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