resistor voltage rating AC vs DC

[surfsup]

If a resistor is rated at 400VDC max working voltage, could i assume the max AC voltage would be 400*1.414 = 565VAC?

I assume the AC should be higher because AC is peak to peak. Or am I assuming incorrectly?

[cbass]

I think you've got it backwards the working DC voltage will be 1.4 * the AC voltage.

I could be wrong though

[Firestorm]

If the DC working voltage is 400, I think the AC rating would be 282.8 (RMS) since that corresponds to 400V peak. The issue is internal arcing, which is about potential. The limit is pretty real with metal and carbon films (and wirewounds, I think) because of how they're made; once they arc, they're pretty much toast. Carbon comps and metal oxide are a little more forgiving, but using them overvoltage makes them noisier, and eventually causes them to fail from overdisspation as they draw more current. Bear in mind, it's the voltage across the resistor that matters, not the absolute voltage applied. If you have a situation where you need a higher working voltage, put two in series.

[tubeswell]

what the others said already. You can think about it like this:

1VAC = 1.4 VDC

1VDC = 0.7VAC

But this is not strictly 'true' of course. What is does show is the relative relationship of the peak value to the average value when looking at a sine wave, and this is useful for comparing AC and DC voltage ratings.

[surfsup]

Tubeswell, your equations agree with me. If

1 vac = 1.4*vdc then 1.4*400vdc is 560 vac

So now i am more confused. I thought dc was the average (0.7) of alternating vac. So vdc would be less. So a resistor rated at 400 vdc (average) would handle more ac. Firestorm seems to say the peak is what gets you.

[diagrammatiks]

you are doing the equation backwards.

1vdc = .7vac


if a resistor is rated at 500 vdc then

500 vdc =.7xvac

350 vac.

However that's not how the actual rating works.

It's combined peak voltage.

Any AC component rides on top of the DC component.

The combined instantaneous voltage cannot be more then 500.

For example if you have 400 vdc across a resistor with a 100vac signal on top of it...it will alternate between 500v and 300v as far as the resistor is concerned.

[surfsup]

with all due respect. starting with your equation:

1vdc =.7vac = 0.7*vac, then divide both sides by 0.7...

(1/0.7)vdc = (0.7/0.7)vac, then do the fractions...

1.4vdc=1vac

subbing 400 for vdc =

1.4*400 = VAC = 560VAC

[diagrammatiks]

Right but that only matters if the resistor is rated in max VAC working voltage.

Or if you start with a VAC rating.

If you start with a VDC rating then you work the other way.

the way that you did the translation is wrong.

if

1vac = 1.4vdc then you get 1 vac for every 1.4vdc units you count.

if you have 400vdc overall that divides into 285 1.4 size units which is the VAC.

[surfsup]

Working one way or the other doesn't matter in mathematics. You will still get the same result, so I'm not sure what you're trying to say.

I found the answer and it was what I was thinking originally, just wanted to be sure:

VDC = RMS voltage
RMS voltage is 0.7*peak voltage (AC)

Therefore 400VDC = 400 RMS which is 400/0.7 = 560 VAC

[diagrammatiks]

Working one way or the other doesn't matter in mathematics. You will still get the same result, so I'm not sure what you're trying to say.

I found the answer and it was what I was thinking originally, just wanted to be sure:

VDC = RMS voltage
RMS voltage is 0.7*peak voltage (AC)

Therefore 400VDC = 400 RMS which is 400/0.7 = 560 VAC

hang on.

[surfsup]

How can RMS voltage be greater than the peak voltage?

[diagrammatiks]

ah here we go.

The AC voltage can be give in RMS or Peak.

Vac(peak) is effectively equal to VDC or

V(peak) = 1.4 *rms = VDC

V(rms) = .7V(peak) =.7vdc.

Maximum working voltage is given in V(peak)

A 300Vac(rms) signal will have an effective V(peak) of 420.

[John_P_WI]

First of all, it has been nearly 30 years since I studied this in college, I'll try to do the best I can to remember a few things. I hope someone like Vacuum Voodoo will chime in and set me straight - I'm a mechanical guy.

This is also difficult to discuss without pictures or graphics so I'll try my best.

When discussing voltage in AC, it is mostly discussed in RMS values unless measured on a scope where peak to peak voltage is measured. RMS, root mean squared, is the "average" voltage, or .707 of the peak. Therefore as previously mentioned 1 vac (rms) has a peak value of 1.4 v = (1 / .7) or (1 x 1.4)

As an example, think of the sine wave being flipped into a series of humps or mountains next to each other with no downward troughs-like a row of over turned bowls sitting edge to edge. If you cut the tops off the mountains and filled in the valleys, this would be the average or .707 of the mountain height, or rms value.

In the question of the resistor rating, one could think that the potential, or difference from the maximum peak to ground is what matters. 400 v ac peak to ground has the same voltage potential as 400 dc to ground, but it does not have the same potential power (area under the curve) as dc. One also has to consider if the resistor is a component in circuit with a high dc offset and also has a large ac component "riding" on the dc, adding to the potential difference from "ground".

Now all of this being said, carbon comp resistors are used in the medical field as de-fibrillation protection resistors in patient lead cables (the resistors are used to protect the front end pre amplifiers of monitoring devices in the event that a patient is zapped in an attempt to return their heart to a normal rhythm). 1/2 watt 10K ohm carbon comp is a common value, and these resistors are routinely tested for compliance to various regulatory agency standards. It is not uncommon to discharge a capacitor charged to 4000 v or 5000 v across the 1/2 watt carbon comp resistor!

Sorry for the dissertation, guess I was feeling a little pedantic.

[tubeswell]

Tubeswell, your equations agree with me. If

1 vac = 1.4*vdc then 1.4*400vdc is 560 vac

So now i am more confused. I thought dc was the average (0.7) of alternating vac. So vdc would be less. So a resistor rated at 400 vdc (average) would handle more ac. Firestorm seems to say the peak is what gets you.

When you start from DC you have to do the equation the other way

1.4VDC = 1VAC, so

VDC/1.4 = 1VAC, or rearranging

VAC x 1.4 = VDC

560VAC x 1.4 = 784VDC

But if you are starting with 400VDC, then

400VDC x 0.7 = 280VAC.

[diagrammatiks]

First of all, it has been nearly 30 years since I studied this in college, I'll try to do the best I can to remember a few things. I hope someone like Vacuum Voodoo will chime in and set me straight - I'm a mechanical guy.

This is also difficult to discuss without pictures or graphics so I'll try my best.

When discussing voltage in AC, it is mostly discussed in RMS values unless measured on a scope where peak to peak voltage is measured. RMS, root mean squared, is the "average" voltage, or .707 of the peak. Therefore as previously mentioned 1 vac (rms) has a peak value of 1.4 v = (1 / .7) or (1 x 1.4)

As an example, think of the sine wave being flipped into a series of humps or mountains next to each other with no downward troughs-like a row of over turned bowls sitting edge to edge. If you cut the tops off the mountains and filled in the valleys, this would be the average or .707 of the mountain height, or rms value.

In the question of the resistor rating, one could think that the potential, or difference from the maximum peak to ground is what matters. 400 v ac peak to ground has the same voltage potential as 400 dc to ground, but it does not have the same potential power (area under the curve) as dc. One also has to consider if the resistor is a component in circuit with a high dc offset and also has a large ac component "riding" on the dc, adding to the potential difference from "ground".

Now all of this being said, carbon comp resistors are used in the medical field as de-fibrillation protection resistors in patient lead cables (the resistors are used to protect the front end pre amplifiers of monitoring devices in the event that a patient is zapped in an attempt to return their heart to a normal rhythm). 1/2 watt 10K ohm carbon comp is a common value, and these resistors are routinely tested for compliance to various regulatory agency standards. It is not uncommon to discharge a capacitor charged to 4000 v or 5000 v across the 1/2 watt carbon comp resistor!

Sorry for the dissertation, guess I was feeling a little pedantic.

Ya most of the regular resistors, carbon/metal film, top out at 1k in the higher wattages.

thick film and wirewounds easily do 1k+