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Resistor rating

T

Tom Sanders

Jan 1, 1970
0
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?
 
B

Ben Bradley

Jan 1, 1970
0
In sci.electronics.design, [email protected] (Tom Sanders)
wrote:
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.

That would probably be 0.25W, or 0.25 watts.
Is this circuit safe
to build?

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.
The circuit would be drawing approximately 25uA of current.

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.
What does the voltage rating on a resistor mean?

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.
 
J

Jeffrey Turner

Jan 1, 1970
0
Tom said:
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?

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
 
C

CFoley1064

Jan 1, 1970
0
From: [email protected] (Tom Sanders)
Date: 2/11/2004 9:39 PM Central Standard Time
Message-id: <[email protected]>

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?

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
 
S

Spehro Pefhany

Jan 1, 1970
0
In sci.electronics.design, [email protected] (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.

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
 
J

John Woodgate

Jan 1, 1970
0
I read in sci.electronics.design that Jeffrey Turner
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.

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
 
R

Robert Baer

Jan 1, 1970
0
Ben said:
In sci.electronics.design, [email protected] (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.

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.
 
R

Robert Baer

Jan 1, 1970
0
Jeffrey said:
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

--
Ho, ho, ho, hee, hee, hee
and a couple of ha, ha, has;
That's how we pass the day away,
in the merry old land of Oz.

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

Robert Baer

Jan 1, 1970
0
CFoley1064 said:
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

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.
 
J

John Woodgate

Jan 1, 1970
0
I read in sci.electronics.design that Spehro Pefhany <[email protected]?>
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,

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
made, with kilovolt ratings.
 
J

John Woodgate

Jan 1, 1970
0
I read in sci.electronics.design that Robert Baer
1% metal film resistors have a 300V rating, and 5% metal oxide film
(power) resistors have a 350V rating.

Not all of them. Check data sheets.
 
J

Jeff

Jan 1, 1970
0
John Woodgate said:
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
made, with kilovolt ratings.

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.
 
J

Jim Meyer

Jan 1, 1970
0
John Woodgate said:
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

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
 
J

Jeffrey Turner

Jan 1, 1970
0
Robert said:
*WRONG*
Resistors *do* have a voltage rating; look in a DigiKey catalog as a
quick refeernce; else look at the manufacturer's data sheets.

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
 
T

Tom Sanders

Jan 1, 1970
0
Thank you everybody for your great advice.

I have certain followup questions:

1. Somebody mentioned that a solderless breadboard may not work above
120V.
I contacted 3M but received no response about their breadboard specs.

If I have a high voltage dc supply of 500V but two resistors of 10Mohm
in
series, the current being drawn would be low. Does the low current
make it safe and prevent the metallic surface of the board from
melting?
 
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