# Resistor rating

Discussion in 'Electronic Design' started by Tom Sanders, Feb 12, 2004.

1. ### Tom SandersGuest

Hello,

I am trying to put together a circuit on a breadboard and am not sure about the
resistor ratings. I am using a two 10Mohm resistors in series with a 500V
supply. The power ratings of the resistors are 0.25V. Is this circuit safe
to build? The circuit would be drawing approximately 25uA of current.

What does the voltage rating on a resistor mean?

In sci.electronics.design, (Tom Sanders)
wrote:
That would probably be 0.25W, or 0.25 watts.
As long as you don't touch it...

Seriously (not that I'm not serious above!), it appears you want to
calculate the power dissipated in the resistors and compare it with
their rated value.
Let's see, E=I*R, 25uA * 10megohm is 250V, so two such in series is
500V. You got the current right.
Power (in watts) is current (in amperes) times voltage (in volts,
of course) or P=I*E. For one resistor, that's 250V*25uA = 0.00625
watts, which is well below the rated 0.25 watts.
That's generally a maximum safe voltage across a resistor without
the voltage arcing through. This is a maximum voltage even if the
resistor is well within its power rating at that voltage.

3. ### Jeffrey TurnerGuest

The power dissipated in a resistor is V^2/R. A 10 Mohm resistor
across a 500V supply would give 500 * 500 / 10,000,000 or 0.025W. So
a 1/4 W resistor is perfectly fine. Resistors are typically rated in
Watts, not Volts. Maybe you're misreading them.

--Jeff

4. ### CFoley1064Guest

From: (Tom Sanders)
Hi, Tom. If you're still with us (respond at your convenience ;-) you're over
voltage on the breadboard, and you're probably right on the edge with the
resistors. I wasn't able to find any information on 3M or Global protoboards.
Try calling or emailing them tomorrow. 300V peak sounds like a maximum -- it
might be less. I've never trusted solderless breadboards with more than 120
VAC or VDC myself.

Resistors have maximum voltage ratings, which usually refer to how much voltage
can be applied across a resistor before it arcs over the body of the resistor.
Some 1/4W are rated for 250V, some less.

If I were putting together a 500VDC prototype circuit, I'd use a bare perfboard
with point-to-point wiring. I'd also use 3 ea. 6.8 Meg resistors or 4 ea 4.7
Meg in series.

Good luck
Chris

5. ### Spehro PefhanyGuest

At 250V/resistor that's within the voltage rating of *some* 1/4-W
resistors. I've seen some major long-term drift in precision
metal-film resistors exposed to high DC voltage, so if this is a
serious application, care is called for- study the manufacturer's data
carefully.

Best regards,
Spehro Pefhany

6. ### John WoodgateGuest

I read in sci.electronics.design that Jeffrey Turner
No, you are misleading. Resistors do indeed have a voltage rating, which
is the maximum voltage that should be applied, **irrespective of power
dissipation**. Non-SMD resistors have a voltage rating of 250 V,
typically. SOME higher-power resistors have a higher rating, such as 350
V, and special (small, not costly) resistors can be obtained with
voltage ratings of several kilovolts

7. ### Robert BaerGuest

A 1206 SMD resistor has a 200V rating, the smaller sizes are less.
1% metal film resistors have a 300V rating, and 5% metal oxide film
(power) resistors have a 350V rating.
Resistors have a voltage rating, a current rating, and a power rating;
usually the current rating is not specified, except that a temperature
derating curve (for current) is given.

8. ### Robert BaerGuest

*WRONG*
Resistors *do* have a voltage rating; look in a DigiKey catalog as a
quick refeernce; else look at the manufacturer's data sheets.

9. ### Robert BaerGuest

Solderless breadboards should bwe banned; they are utter crap: too
much capacitive coupling between buses, too much inductance, solder and
other bits of metal gets into the works and can cause nasty shorts, etc.
A "jungle jim" can be superior in a number of ways; using proto boards
with power buses and lands for parts is better, and wire-wrap (if DIPs
are used) can be better yet, as X and Y busbars can be used along with
appropiately placed bypass caps - giving results close to good PCB
layout practice.

10. ### John WoodgateGuest

I read in sci.electronics.design that Spehro Pefhany <[email protected]?>
It's a very common problem with 'start-up' resistors in SMPS, which
typically have 330 - 340 V DC applied. Even two 250 V rated resistors in
series are not reliable. It's an application where a special part is
needed, like a metal glaze resistor of lower value than those normally

11. ### John WoodgateGuest

I read in sci.electronics.design that Robert Baer
Not all of them. Check data sheets.

12. ### JeffGuest

Scraping off the paint/ceramic coating on such failed carbon or metal film
resistors shows the resistor is in good shape, except for a small section of
the resistive coating which looks like it corroded away. It seems like what
happens is a small section overheats, and looses some material, creating an
even hotter hot spot, which gets hotter, evaporating more conductive
coating, getting hotter, etc until the resistor fails.

13. ### Jim MeyerGuest

Exactly! Resistors are not conductors like wires and they are
not insulators like ceramic. Resistors are semiconductors and like
all semiconductors they have voltage ratings because their
characteristic resistance WILL become nonlinear when their voltage
rating is exceeded.

Jim

14. ### Jeffrey TurnerGuest

Resistors' "power ratings" are in Watts, not Volts. If you have a
resistor which is no good above 0.25 Volts then, unless it is a very
low resistance value, you should get rid of it and replace it with a
good resistor.

--Jeff

15. ### Tom SandersGuest

I have certain followup questions:

1. Somebody mentioned that a solderless breadboard may not work above
120V.