Author Topic: Exceeding a resistors rated voltage  (Read 6730 times)

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Offline Simon

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Exceeding a resistors rated voltage
« on: June 04, 2016, 09:40:22 am »
I have something bugging me with surface mount resistors in this case. Obviously the maximum working voltage of a resistor is actually defined by its wattage and resistance as it will be the voltage at which the maximum power dissipation you are happy with or that the device can sustain has been reached. But what about transients and spikes?

I ask because I quite often design very low power devices for automotive and I do include a TVS diode but in order to make spiky world of automotive electrics a bit easier I tend to put a resistor in series with the power input as even a couple of hundred ohms is acceptable. Obviously at 12 or 24 V this is not a problem but what happens if I get a 250 V spike coming into a resistor that is spec for 150 V? And of course I have to assume that the host vehicle might not be perfect and might be worse than that and have higher voltage spikes. Will the resistor be damaged or does the maximum working voltage mean that beyond this the current would just arc between the caps of the resistor? If arcing occurs then frankly I'm not too concerned as there is a TVS diode right after it to take the voltage causing that arc and clamp it. I also tend to pot my products which I'm assuming would prevent voltages arcing through the air from one end of the resistor to the other.
« Last Edit: June 04, 2016, 09:42:22 am by Simon »
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Re: Exceeding a resistors rated voltage
« Reply #1 on: June 04, 2016, 09:48:26 am »
It comes down to the package size your using, an 0402 and your out of luck, a generic 1/4W through hole and you should be fine, based on most guidelines a separation distance of almost 2 mm is required for "functional isolation" for 250V,

Now i do think the resistor will drift from these encounters, but it would take a fair few to start causing worry. (large negative spikes at low duty cycle greatly reduced the value on some foil type resistors i have used)

Again that drift will depend on the materials chosen, and if they spec for low duty cycle currents,
 

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Re: Exceeding a resistors rated voltage
« Reply #2 on: June 04, 2016, 10:10:24 am »
Thick film resistors will reduce their resistance if the arching occurs within them.
http://www.rohm.com/web/in/r_what10
Quote
I'm assuming would prevent voltages arcing through the air from one end of the resistor to the other.
Does your potting flow under the part?
 

Offline Simon

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Re: Exceeding a resistors rated voltage
« Reply #3 on: June 04, 2016, 10:17:55 am »
Thick film resistors will reduce their resistance if the arching occurs within them.
http://www.rohm.com/web/in/r_what10
Quote
I'm assuming would prevent voltages arcing through the air from one end of the resistor to the other.
Does your potting flow under the part?


Well at least the resistor would continue to conduct which is better than filing open circuit. No I could not guarantee that the potting compound flows under the resistor.

I have to look into resistors with bigger voltage ratings. Or just put a small but higher voltage TVS diode right at the input.
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Offline coppice

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Re: Exceeding a resistors rated voltage
« Reply #4 on: June 04, 2016, 11:09:12 am »
The specifications for SMD resistors usually specify the maximum working voltage, and its quite low for the smaller ones. It has little to do with resistance and power. If you applied a large pulsed voltage you would stay within the power rating of the resistor, but go way over the rated working voltage. The voltage rating is mostly about breakdown.
 

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Re: Exceeding a resistors rated voltage
« Reply #5 on: June 04, 2016, 11:10:49 am »
Usually not an issue if you pot the board or use thick conforming coating. In free air and high pollution level, the clearance will be an issue.
 

Offline coppice

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Re: Exceeding a resistors rated voltage
« Reply #6 on: June 04, 2016, 11:20:43 am »
Usually not an issue if you pot the board or use thick conforming coating. In free air and high pollution level, the clearance will be an issue.
If you pot properly, in a vacuum, that might be true (I'm not sure. There might still be issues with the case of the resistor for all I know). I you pot the crude way most electronics is potted there will be so much gas present that you still have discharge issues.
 

Offline Simon

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Re: Exceeding a resistors rated voltage
« Reply #7 on: June 04, 2016, 11:21:07 am »
The specifications for SMD resistors usually specify the maximum working voltage, and its quite low for the smaller ones. It has little to do with resistance and power. If you applied a large pulsed voltage you would stay within the power rating of the resistor, but go way over the rated working voltage. The voltage rating is mostly about breakdown.

yes it is the break down voltage I am lookng at
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Re: Exceeding a resistors rated voltage
« Reply #8 on: June 04, 2016, 11:33:23 am »
If you pot properly, in a vacuum, that might be true (I'm not sure. There might still be issues with the case of the resistor for all I know). I you pot the crude way most electronics is potted there will be so much gas present that you still have discharge issues.

Depends on the material used. Some potting materials designed for IP protection are not suitable for this application. Some HV potting materials are designed with low viscosity and high wettability in mind.
Also, it's standard protocol, as least for an ISO9001 company, to clean, pot and ultrasonic degas before curing.
 

Offline Simon

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Re: Exceeding a resistors rated voltage
« Reply #9 on: June 04, 2016, 11:38:40 am »
the potting is not the issue, the resistor itself will break down anyway.
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Offline KJDS

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Re: Exceeding a resistors rated voltage
« Reply #10 on: June 04, 2016, 11:41:14 am »
I've seen a few SMD resistor data sheets with detailed voltage spike characteristics specified.

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Re: Exceeding a resistors rated voltage
« Reply #11 on: June 04, 2016, 11:44:36 am »
the potting is not the issue, the resistor itself will break down anyway.

Any reason why evenly distributed carbon film on ceramic substrate will breakdown?
 

Offline Zero999

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Re: Exceeding a resistors rated voltage
« Reply #12 on: June 04, 2016, 11:53:31 am »
Unless the overvoltage is extreme, the resistor will not normally breakdown and become a conductor. What will happen is the resistance will drop below the nominal value. Whether it's reversible or not depends on the extent of the surge. I suspect twice the rated voltage, for short periods, is unlikely to do any real damage.
 

Offline Simon

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Re: Exceeding a resistors rated voltage
« Reply #13 on: June 04, 2016, 11:56:00 am »
judjing from the link posted earlier it causes damage to gthe material and starts to make links that should not be there gradually reducing the resistance.
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Re: Exceeding a resistors rated voltage
« Reply #14 on: June 04, 2016, 11:59:03 am »
judjing from the link posted earlier it causes damage to gthe material and starts to make links that should not be there gradually reducing the resistance.

That's usually caused by carbon deposition. How can ceramic turn into carbon without having a nuclear reaction? The carbon trace usually happens to heavily organic coated resistors, or resistors installed on FR4 with continuous over power stress that the resistor fried the FR4 and turned it into carbon.
 

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Re: Exceeding a resistors rated voltage
« Reply #15 on: June 04, 2016, 12:42:09 pm »
That's usually caused by carbon deposition. How can ceramic turn into carbon without having a nuclear reaction? The carbon trace usually happens to heavily organic coated resistors, or resistors installed on FR4 with continuous over power stress that the resistor fried the FR4 and turned it into carbon.
:palm: Why do you think there is any carbon involved?
 

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Re: Exceeding a resistors rated voltage
« Reply #16 on: June 04, 2016, 12:48:15 pm »
Quote
Thick Film resistors are comprised of a random dispersion of conducting metal particles within a non-conducting particulate medium, usually ceramic; hence they are also known as “cermet” resistors. Current through the resistor follows along the random contacts formed among the metal particles. Power surges cause breakdowns in some of the inter-particulate isolation, thereby reducing resistance by establishing new additional current paths. Thus, ESD surges almost always cause a reduction in resistance. This fact is so well established that Thick Film manufacturers use controlled power surges to tune the resistors to the required resistance and tolerance, which typically ranges from 5% to 20%. The susceptibility to change does not stop at manufacture and the resistor is subject to similar changes every time the resistor experiences an ESD event. ESD-induced changes while in service can cause resistance changes up to 50%, which is easily sufficient to cause a malfunction.
 

Offline Jeroen3

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Re: Exceeding a resistors rated voltage
« Reply #17 on: June 04, 2016, 03:04:58 pm »
Use MELF resistors. They have better surge capability. You're going to need the larger power anyway for a load dump scenario.
 

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Re: Exceeding a resistors rated voltage
« Reply #18 on: June 04, 2016, 03:13:38 pm »
Quote
Thick Film resistors are comprised of a random dispersion of conducting metal particles within a non-conducting particulate medium, usually ceramic; hence they are also known as “cermet” resistors. Current through the resistor follows along the random contacts formed among the metal particles. Power surges cause breakdowns in some of the inter-particulate isolation, thereby reducing resistance by establishing new additional current paths. Thus, ESD surges almost always cause a reduction in resistance. This fact is so well established that Thick Film manufacturers use controlled power surges to tune the resistors to the required resistance and tolerance, which typically ranges from 5% to 20%. The susceptibility to change does not stop at manufacture and the resistor is subject to similar changes every time the resistor experiences an ESD event. ESD-induced changes while in service can cause resistance changes up to 50%, which is easily sufficient to cause a malfunction.

I see. Thanks. I thought it was classic organic-carbon current path.
 

Offline madires

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Re: Exceeding a resistors rated voltage
« Reply #19 on: June 04, 2016, 03:22:28 pm »
Multiple resistors in series to match the expected voltage spike?
 

Offline coppice

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Re: Exceeding a resistors rated voltage
« Reply #20 on: June 04, 2016, 04:13:32 pm »
Multiple resistors in series to match the expected voltage spike?
That's a very common and effective technique, but its prohibited by regulations in a few applications, so take care.
 

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Re: Exceeding a resistors rated voltage
« Reply #21 on: June 04, 2016, 06:34:57 pm »
Multiple resistors in series to match the expected voltage spike?
That's a very common and effective technique, but its prohibited by regulations in a few applications, so take care.

Huh, got any cites?

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Offline MK14

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Re: Exceeding a resistors rated voltage
« Reply #22 on: June 04, 2016, 08:03:51 pm »
Multiple resistors in series to match the expected voltage spike?

The problem with that method, is that if the voltage spikes, are very high frequency, it is more the stray capacitance (and inductance etc), and specific PCB layout etc, which effect how the voltage spike is transiently split amongst the resistors.
So very briefly, one (or more) of the resistors, may see MOST of the voltage transient, EXCEEDING the datasheet specification, and hence potentially gradually breaking down the insulating properties of the resistor.
There are some very good article(s) about this, on the internet. Offhand, I've forgotten where a really good one was, that I read many years ago.
tl;dr
Best to use a properly voltage rated resistor, rather than try to share the voltage amongst multiple resistors which may be unreliable or even unsafe in the longer term.

EDIT:
For similar reasons. This is why on some relatively high voltage PCB circuits, slots are cut out in the PCB, in appropriate places. To stop the high voltages/transients gradually breaking down the insulating properties of the PCB.
« Last Edit: June 04, 2016, 08:15:44 pm by MK14 »
 

Offline Neilm

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Re: Exceeding a resistors rated voltage
« Reply #23 on: June 04, 2016, 09:08:13 pm »
Use MELF resistors. They have better surge capability. You're going to need the larger power anyway for a load dump scenario.
I have used Vishay cmb0207 MELF resistors in pulse applications. Normally good for 1W rated for almost 500W pulses at voltages up to 10kV.
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Offline Simon

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Re: Exceeding a resistors rated voltage
« Reply #24 on: June 04, 2016, 09:52:31 pm »
I would have thought that if you are pushing resistors in series to increase their voltage rating would need to make sure they are placed in a straight line end to end increasing the distance between 2 points where the potential is applied. If for example you put the resistor's head to tail next to each other this could mean a potential spike would jump from one resistor to another. I don't think a suitable layout is too difficult to do it just requires a little bit of common sense and the acceptance that you cannot place components any old how to save space and make things the way you want them. Dave has often pointed to examples of resistors in series to increase the rated voltage in his videos. In my case I am not trying to make up massive voltage ratings with multiple smaller resistors. Officially a 24 V system should not have a spike higher than 250 V which is what a standard 1206 resistor package is rated for so it would be prudent to put 2 of the 1206 packages in series just to ensure I have 500 V for safety sake bearing in mind that the system should only see half of that which is what each resistor is rated for anyway. I also would not see a problem putting to 0805 packages in series which are rated to 150 V each as this would get you 300 V total.

Looking on Farnell I would have to buy at least 150 resistors and a resistor rated for 500 V costs between 13 and 30p versus the 1 or 2p of a standard 250 V resistor and these resistors are much larger packages than a 1206 so they are not exactly anything special they are just bigger. I'm going to guess that if I get a high voltage rated 1206 package it will cost the earth whereas there is not really anything against me putting to 1206 resistors end to end to give myself plenty of margin.
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