Resistor voltage rating

Not safe. The AC line will have spikes well above the expected RMS voltage. Use a bunch in series.

--

John Larkin         Highland Technology, Inc 
picosecond timing   laser drivers and controllers 

jlarkin att highlandtechnology dott com 
http://www.highlandtechnology.com

^^^

Not only do you get the voltage rating (1W resistors are usually ~350V), you get the added benefit of redundant insulation layers. Even if the remainder doesn't handle the total (i.e., (N-1) * Vrated < Voperating), you're better off than one alone.

In my opinion, 2 or 3 would be fine.

I've seen 1206 chip resistors used three at a time for across-the-line bleeders.

Tim

--
Seven Transistor Labs 
Electrical Engineering Consultation 
Website: http://seventransistorlabs.com

Rohm (Japan) has/had 1206 resistors with a 500v rating. Digikey recently discontinued them for undisclosed reasons.

Hul

Tim Williams wrote:

Through hole Rs are normally rated 200v or more. Peak mains voltage from sw itching transients really depends on the nature of the supply. In India I'd expect it to be higher impedance than here, so would allow a kV for 240, a couple kV for 440 to be on the safe side.

NT

Yes... just hope you dont get the same fault manufacturing on all of them :)

NT

Most resistors are rated between 150v and 250V provided power rating is observed.

Maybe even more in Addis Ababa where they are upgrading the infrastructure.

Not safe. Carbon resistors have a positive temperature coefficient of resis tance, and can form "hot channels" at high voltage levels - the curren thro ugh the resistor concentrates on a single path, which gots hotter, thus low ering its resistance, concentrating the current further.

I've seen a nominally 10k 0.5W carbon film resistor carrying about 1A at a

0.5V drop. The paint over the hot channel was slighly discoloured, but the resistor was fine afterwards and still measured 10k.

Metal film and metal-oxide resistors don't have this failure mode. 0.5W par ts are too small to keep the high-voltage ends far enough apart, so take Jo hn Larkin's advice and put a few in series.

--
Bill Sloman, Sydney

** Carbon composition type have positive tempcos, but carbon film ones are normally negative. About -0.1% per degree C, or a 10% reduction in value for temp rises near the max rating. ** Carbon comp - right? ** That is impossible for a carbon film type.

** A miracle.

** The failure mode seen most often with carbon and metal film resistors when subjected to high voltages is they go high in value or open circuit. Part of the spiral burns away over time due to tracking across the adjacent gap.

DC voltages as low as 70V can do this or somewhat higher AC voltages - although the part is well within maker's power and voltage ratings.

Most examples seen are with 0.25W and 0.5W watt types, rarely with 1W types unless the voltage is near the max rating.

My advice to the OP is to use 1W MF resistors and put 3 in series to get the value, but even better is to use a part made for the job - eg:

.... Phil

Sorry about that. It should have been negative. What followed doesn't make sense if the resistance doesn't decrease as the temperature of the hot channel increases.

The example I saw was carbon film.

It was certainly unexpected, and the rigmarole of charging the right value capacitor to the right voltage to provide enough energy to create the hot channel in the first place without doing any permanent damage apparently took some working out.

It was a party trick to make a point in the setting up of the intrinsically safe process control standard. I wasn't involved, but the guy who showed us the trick had been,

No. A rather clever trick.

I wouldn't argue with that.

--
Bill Sloman, Sydney

Equipment intended for 230/400 V mains should withstand 1500 V peaks (or 2500 V in distribution panels). Definitively multiple resistors in series should be used.

It also appears that the manufacturer have dropped the voltage ratings (to 200-350 V based on other posts in this thread) while previously single ordinary resistor were used in tube amplifiers with at least

400 V anode voltage.

One other aspect with waveform with a high peak to average ratio is

peak on an average 400 V waveform, the peak power is 14 times the average power dissipation. With repetitive peaks, the average power dissipation is doubled, if the peak duty cycle is more than 7 %.

** The actual DC voltage across individual resistors rarely exceeds 250V - cos if it does many of them will not last. I find myself regularly replacin g small 100k, 150k and 220k carbon film types that have gone open when used as plate loads for 12AX7s etc.

The voltage figures quoted on maker's data sheets are usable maximums, not lifetime exposure ratings.

I normally use 0.75W Welwyn MRF5 series MF resistors for replacements - nev er had one fail.

... Phil

BTW, what kind of anode voltage rectifiers did those devices use, tube rectifiers or solid state (selenium/silicon) rectifiers ?

In a tube amplifier with semiconductor rectifiers, the full anode voltage becomes active immediately after switch on, while it takes

15-60 seconds before the output filament becomes hot enough so that the full output tube anode/cathode current will flow, finally stabilizing the voltages at various stages. During this warmup period, unexpected voltages can occur, which can be hatefully for both resistors and capacitors.

If the amplifier was originally designed assuming tube rectifier (even direct heated double rectifiers in the AXnn series), the anode voltage build up was much slower and the risk for extreme voltages during startup was not so bad.

snipped-for-privacy@downunder.com wrote: Phil Allison

- cos if it does many of them will not last. I find myself regularly repla cing small 100k, 150k and 220k carbon film types that have gone open when u sed as plate loads for 12AX7s etc.

ot lifetime exposure ratings.

never had one fail.

** The voltage drop seen across plate resistors starts out at zero and incr ease to a maximum when the tubes warm fully.

The open circuit failures I see are due only to LONG exposure to high DC vo ltages - thousands of hours.

PSU electros sometimes go a bit over their max rating during the first few seconds if the rectifiers are silicon, as they mostly are. Never seems to w orry them much.

... Phil

Where do you get them?

--
 Thanks, 
    - Win

Caddock makes nice high-voltage axial resistors. Some are good to many GHz.

--

John Larkin         Highland Technology, Inc 
picosecond timing   laser drivers and controllers 

jlarkin att highlandtechnology dott com 
http://www.highlandtechnology.com

Excellent advice, 0.25W HVR2500, 15 cents ea qty 100.

--
 Thanks, 
    - Win

.......................

Thanks for the inputs, everyone. The concensus is clearly that single 1W resistors are not good enough. So multiple resistors it is.

I'm not completely ignorant about the technical aspects and would probably dispense the same advice in a clinically objective situation. But while stressing that I am *not* arguing against the points raised in the replies, I feel that I probably should have elaborated on the circumstances and the spirit in which I asked the question, which is essentially "Can I get away with it?" Here are some factors that made me consider using single resistors:

1. The tool is to be a one-off for my own use, perhaps no more than a couple of times a year and certainly no more than half a dozen times. And for short periods at each session. So I was hoping to keep it simple and compact while maintaining adequate clearances/creepages.
2. Most of the datasheets I've seen for carbon resistors give a maximum rating of 500V for 1W GP types with 1000V as the "intermittent" or "overload" rating.
3. I cannot easily procure special high-voltage resistors.

1. I've seen numerous crude products by Chinese and small Indian companies where they make 1206 - even 0805 - SMD and 0.25W TH resistors bear the full 230VAC. I've seen a few failures that were most likely to be due to over-dissipation (e.g. a 100k 0.25W TH resistor in series with an LED across 230VAC), but never one I could clearly attribute to voltage breakdown. But I'm not inferring that it won't ever happen.

Now, about creepage. I'm more concerned with sensible practice than about conforming to any official standard. Is 200 mil (5mm) good enough?

To anticipate some questions - I can use phenolic or FR4. The unit will be kept in a non-airtight box most of the time. Atmospheric pollution is still light, but it can get rather dusty in the dry season and damp in the monsoon season.

Farnell/Element 14 stocks them here, so presumably you and John could get them from Newark.

--
Bill Sloman, Sydney