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Old 1st Sep 2009, 5:15 pm   #1
Mister C
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Default (Possibly stupid) question about resistors

Hi

I've been buying sets of resistors and rectifiers for converting 700 series GPO phones, but in some cases a rectifier is already fitted....so over time I need a few more resistors than rectifiers, and was about to order some.

I know that it's a 3k3 resistor which is needed, however I've read in various places that a 0.5W resisitor should be used, whereas elsewhere I've seen it suggested that it's 0.25W.

Does anyone know which one would be correct?

Thanks in advance for your help, always much appreciated!
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Old 1st Sep 2009, 5:41 pm   #2
ppppenguin
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Default Re: (Possibly stupid) question about resistors

Without doing the sums accurately, the ringing voltage can be up to 75VAC, much of which will appear across the resistor during ringing. 75^2/3300 gives 1.7W. In practice it's never going to be anything like this bad. The bell coil is in series and the voltage is unlikely to be that high. I'd probably use a 1W resistor to be safe. Small, readily available and cheap.

PS: I've assumed 75V RMS. If it's actually 75V peak, or even peak-peak, the dissipation will be much lower.
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Old 1st Sep 2009, 7:54 pm   #3
Nickthedentist
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Default Re: (Possibly stupid) question about resistors

I too would use 1W resistors, simply because they're usually physically bigger and thus easier to handle.

However, I'm sure I've used 0.25W parts in the past and had no problems, but that would be an unneccesary risk.

Nick.
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Old 1st Sep 2009, 9:21 pm   #4
Mister C
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Default Re: (Possibly stupid) question about resistors

Thanks for the replies - much appreciated!
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