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Resistors Volatges - plymouth53 - 12-16-2013

Hi to all,

I have a basic question on working voltages of resistors?

When I used to know tube circuit like the back of my hand, gees the knowledge has kind of flowed out of my brain. Just an couple of days ago, more like 45 years +, I never was concerned about resistor working voltages in most circuits. Today it looks like that could be a problem with most circuits running at a lot less voltage.

I have been noticing that most 1/2 W resistors have a working voltage of between 300 to 350 Volts that are on ebay. I have tried to look at Mouser catalog, but that one is very confusing with so many variables to wade through.

I have noticed either film or carbon has the same working voltage so that does not seem to be a problem. My question is what do you do when a set has a higher working voltage? Do you use the ebay ones? Or were do you get the higher rated voltages.

Or do you even worry about running these resistors at maximum working voltage or a little over. (I have a Philco set that runs around 400 volts on the plates by the schematic but the secondary on the transformer is around 700 volts, have not been able to figured out were the rest is going to).

I hope I have not confused any one to much as I kind of ramble on. At least that is what my wife says.



Thanks Ed.

This is my therapy for keeping my brain from turning into mush in my younger years. I hope.

RE: Resistors Volatges - tom.seeger - 12-16-2013

If you calculate that the resistor will actually see more than its rated voltage across it, you can use 2 resistors of half the value each. In that way you double the voltage rating. Make sure the resistors are not touching any metal chassis parts or other wiring, as this is a place they may arc over.

RE: Resistors Volatges - morzh - 12-16-2013

Ed

The voltage on the plates of the rectifier is
1) AC voltage
2) The rectifier is centertapped meaning that the actually you only get half of each winding working at the time. This is the same as non-center-tapped with a bridge rectifier. That is your 700VAC rectify as 350VAC would in full bridge.

This said, 1W film resistors from say Vishay-Dale have WV up to 500V. I use 1W.
However 1/2W WV is typical 350V. In most cases that is quite enough.

Now you have to realize what WV is, it is pretty much the breakdown voltage where after crossing it (well, there will be a good margin) the spark occurs across the part.
Which does not mean you can apply the WV or even below.

Say, ,1W 10K resistor has WV=700V but 700V will produce 49W dissipation so leave it on for 1s and the resistor will go poof!
It still makes sense as you can apply duty cycle, say, 2% and still not exceed the power rating.

In the radios there will rarely be 350V voltage across any one resistor; if it is use 1W, or as Tom suggested, two in series.

RE: Resistors Volatges - Bradley Rush - 12-16-2013

If you want to know several places to get higher wattage for older radio's pm me i'll link you some.

RE: Resistors Volatges - plymouth53 - 12-17-2013

Thanks for all your answers. All makes sense. At one time before I had a brain leak, due to age-----,
I hope in time I can reverse leakage.
Ed.

RE: Resistors Volatges - exray - 12-17-2013

As I understand it, the breakdown voltage is rated from one side to the other, or to the external insulation resistance which tends to be higher. As mentioned, rare is the case where you have 350 volts of potential across a resistor in an old radio.