Resistor Wattage for Cornet2

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Minorityof1

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Resistor Wattage for Cornet2
« on: 27 Mar 2013, 05:55 pm »
Buying resistors for my Cornet2 build. I would like to use PRP resistors, they look like a good place between price and quality, but they don't carry the 910R in the 1W rating, only the 1/2 W rating. In scouring this forum, I found one guy who said he used these, but I was wondering if anyone else had a comment?

I could also go with Kiwame at these points, though it irks my perfectionist/OCD tendencies to have carbon at some points and metal film at others. I'm only a little crazy.

Thoughts?

mgalusha

Re: Resistor Wattage for Cornet2
« Reply #1 on: 27 Mar 2013, 06:10 pm »
Looking at the schematic I'd say they should be fine but Jim would be authority and likely has a very good reason for specifying 1W. I used the specified sizes when I built mine (original) long ago.

mike

el34

Re: Resistor Wattage for Cornet2
« Reply #2 on: 27 Mar 2013, 10:36 pm »
Vishay/Dale RN series are metal film mil-spec and widely available in many values and wattages.

poty

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Re: Resistor Wattage for Cornet2
« Reply #3 on: 28 Mar 2013, 07:52 pm »
Buying resistors for my Cornet2 build. I would like to use PRP resistors, they look like a good place between price and quality, but they don't carry the 910R in the 1W rating, only the 1/2 W rating. In scouring this forum, I found one guy who said he used these, but I was wondering if anyone else had a comment?
Way 1: Use two PRP 1.8k resistors in parallel (it can be 1/2W or 1W rating).
Way 2: Use 1/2W rating for 910 Ohm. It can be used here definitely (low voltage, very low currents).
Way 3: Use some other metal film manufacturer (something like http://www.digikey.com/product-detail/en/PR01000109100JR500/PPC910W-1CT-ND/597305).
Way 4: Really you could use 1k resistors - you won't lose anything. Some voltages will have more than 10% deviations from marked values in the case, but nothing serious. For "purist" way - R203, R210 should save their values.

hagtech

Re: Resistor Wattage for Cornet2
« Reply #4 on: 29 Mar 2013, 04:30 am »
Yes, a 1/2W or 1/4W rating for the 910 ohm resistors is fine. 

jh

avahifi

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Re: Resistor Wattage for Cornet2
« Reply #5 on: 29 Mar 2013, 02:54 pm »
The way to determine what value resistors you should use in doing replacements requires that you very carefully measure the DC voltage at each end of the resistor in question in the working circuit.  Determine the difference in voltage between each end, multiple that number by itself, and then divide by the value of the resistor in ohms.  That will tell you the power being dissipated by that resistor in watts.  (V squared divided by R).

Then select a resistor with at least three times the power rating you came up with and if the part is going to put out 1 watt or more, mount it clear of the PC card to provide additional cooling space.

Note that at turn on, some resistors may see substantially higher surge voltage then in steady state operation.  Make sure your choice of power ratings takes this into consideration.  The reason resistors fail in a circuit is almost always that some active device (transistor, tube or IC) failed and this caused the voltage across some resistors to be MUCH higher than their design ratings and thus overheat and fail too.  The circuit damage hardly ever is caused by a failed resistor (if it was properly specified in the original design) but is collateral damage.

Example:  a 5000 ohm resistor with 50V at one end, 10V at the other end.  Voltage drop is 40V. 40 x 40 = 1600.  1600 divided by 5000 = 0.32 watts.  You would need a 1W rated resistor in this case (2W would even be a better long term choice)

Regards,

Frank Van Alstine