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hameeds01
Joined: 23 Aug 2005
Posts: 308
Helped: 15
Location: NUST Islamabad
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 31 Oct 2009 6:40 invisible Resistance value |
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i have a transformerless suppply composed of following components capacitor,Resistance, Bridige and a 5 volt zener diode, input voltage 110 volt output 5 volts but the current is unknown, the problem is that the resistance tha is connected in series with non polar capacitor has burnt away now how i can read the resistance value as there is nothing left behind??
this supply is used to run a small contolling circuit having 10 LED + a 5 volt small Relay
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IanP
Joined: 05 Oct 2004
Posts: 6492
Helped: 1542
Location: West Coast
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31 Oct 2009 11:54 Re: invisible Resistance value |
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In circuits like yours this resistor mainly works like a thermal fuse.
It’s value is usually 1/20>to>1/10 of the impedance of the bipolar capacitor at 50(or 60) Hz.
For example, if the capacitance is 330nF – that’s an equivalent of 9.5kohm (at 50Hz), you may use a 1kohm resistor rated for, I’d say, 1W ..
Rgds,
IanP
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hameeds01
Joined: 23 Aug 2005
Posts: 308
Helped: 15
Location: NUST Islamabad
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31 Oct 2009 13:16 Re: invisible Resistance value |
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IanP
Joined: 05 Oct 2004
Posts: 6492
Helped: 1542
Location: West Coast
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01 Nov 2009 2:09 Re: invisible Resistance value |
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The 1.5µF capacitor limits the current in the loop to roughly 50mA, so it should be something like 150Ω to 220Ω rated for 1W and the value is not critical ..
Rgds,
IanP
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