| Electronics > Beginners |
| Voltage rating of 1W 10M carbon film resistors ? |
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| lordvader88:
I've been looking around today and so far I only really found 1 datasheet, for 1/4W and 1/2W CF- resistors, and they are listing 250V, 300V for max continuous operation. I have 100 of the really light blue ceramic coated type, and want to make some high voltage probes for DMM use as a start. Later I could buy proper HV rated resistors, at the time I thought that these were maybe 3.2kV just from sqrt (1W* 10M), but highly doubt thats right. |
| Tomorokoshi:
Resistor ratings are more than just the active electrical element. Power dissipation is based on the package independent of resistance in some specified mounting condition with specified lead lengths, air flow, track sizes, etc. Voltage rating is often listed as the minimum of what it takes to achieve either the derated power or the dielectric strength of the package. The dielectric strength can breakdown either across the body of the resistor, from lead to lead, or through the package of the resistor to a track from a different circuit. Therefore, in your case, the 250V or 300V rating applies. Watch out for pulse ratings as well. A big ceramic might have a 5x pulse power rating, while a 1206 might have a 2.5x pulse power rating. Take into account safety margins knowing that your installation and use conditions may not be the ideal used to characterize the parts. Perhaps 50%. |
| lordvader88:
Yup, For now I'm just like to measure my little plasma ball, not TV CRTs or anything. |
| JS:
As you already considered, high value resistors are usually not rated for it's full power. Also true for some low value resistors. Usually resistor ratings show power, voltage and some current ratings, mainly packaged limited. A certain package has some power dissipation capability considering it's size, which limits the power, also a certain distance between the pins, limiting the voltage and a certain lead thickness limiting the current to maximum rates. After that, to stay within specs you want a maximum temperature rise, limiting the power even further, a certain linearity, limiting voltage as well and the internal elements have a current rating that could also be a limiting factor. Current is the one that could not be present as the range of resistors goes so much low and power might hit the limit first for the lowest value of the product. Power and voltage are usually there, certainly for a 10M resistor voltage is the limiting factor so don't trust the power to have any significance there. JS |
| Wolfgang:
Hi, you could consult H&H Art of electronics for the "Voltage Effect" in resistors. For normal high-value resistors, the voltage limit from the datasheets is normally a lot lower that sqrt(P*R). Play safe ! |
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