High Voltage Resistors - UK Vintage Radio Repair and Restoration Discussion Forum

SatCure · 19th Dec 2012, 1:43 pm

I've repaired a lot of modern equipment, which had failed because a resistor had burnt out. This often looks as if a resistor of too low power dissipation had been used but fitting a higher power resistor doesn't solve it - it burns out again. The real reason is that the voltage rating of the resistor is too low and arcing occurs.

In modern circuits, there's usually one or two bleed resistors feeding a 100uF capacitor in a switch-mode PSU. The total voltage across the resistors can reach 400v DC so "ordinary" resistors won't do.

Obviously, this can also apply to resistors used in valve circuits but the uninitiated might not realise it. (They don't teach stuff like this at college.)

The resistor type that I prefer is the MFR5 resistor by Welwyn. It's rated at 0.75 Watt, 1% tolerance and, importantly, 350v DC. You can order it from Farnell Electronics. If you need a higher voltage or power rating, use two in series.

GrimJosef · 19th Dec 2012, 4:17 pm

The Vishay PR02 range is rated to 500V and available from RS e.g. http://uk.rs-online.com/web/p/throug...stors/6835805/. The Neohm ROX3S range, also available from RS http://uk.rs-online.com/web/p/throug...stors/2142572/, are only rated to 350V but have an 'overload' capacity, whatever that means, of 600V.

Cheers,

GJ

paulsherwin · 19th Dec 2012, 4:54 pm

It's important to keep a sense of proportion about this though. Most resistors in vintage domestic equipment don't have anything like the full HT voltage across them, even under a fault condition. Typically they'll be exposed to less than 100V.

There are exceptions of course, and it's certainly important to bear this issue in mind when replacing resistors.

19th Dec 2012, 6:01 pm

Well done for pointing out one of the pits that can be fallen down by the uninitiated. Simmilar things are, not setting a suitable current limit on a power supply, not checking power dissipation (of resistors especially), logic levels for different types CMOS/TTL 3.3V/5V etc..

Ed_Dinning · 19th Dec 2012, 8:38 pm

Hi Gents, this type of resistor can often be identified by a yellow band after the tolerance band.

Ed

19th Dec 2012, 1:43 pm	#1
SatCure Retired Dormant Member Join Date: Dec 2012 Location: Chania, Crete Posts: 7	High Voltage Resistors I've repaired a lot of modern equipment, which had failed because a resistor had burnt out. This often looks as if a resistor of too low power dissipation had been used but fitting a higher power resistor doesn't solve it - it burns out again. The real reason is that the voltage rating of the resistor is too low and arcing occurs. In modern circuits, there's usually one or two bleed resistors feeding a 100uF capacitor in a switch-mode PSU. The total voltage across the resistors can reach 400v DC so "ordinary" resistors won't do. Obviously, this can also apply to resistors used in valve circuits but the uninitiated might not realise it. (They don't teach stuff like this at college.) The resistor type that I prefer is the MFR5 resistor by Welwyn. It's rated at 0.75 Watt, 1% tolerance and, importantly, 350v DC. You can order it from Farnell Electronics. If you need a higher voltage or power rating, use two in series.

19th Dec 2012, 4:17 pm	#2
GrimJosef Dekatron Join Date: Sep 2007 Location: Oxfordshire, UK. Posts: 4,311	Re: High Voltage Resistors The Vishay PR02 range is rated to 500V and available from RS e.g. http://uk.rs-online.com/web/p/throug...stors/6835805/. The Neohm ROX3S range, also available from RS http://uk.rs-online.com/web/p/throug...stors/2142572/, are only rated to 350V but have an 'overload' capacity, whatever that means, of 600V. Cheers, GJ

19th Dec 2012, 4:54 pm	#3
paulsherwin Moderator Join Date: Jun 2003 Location: Oxford, UK Posts: 27,929	Re: High Voltage Resistors It's important to keep a sense of proportion about this though. Most resistors in vintage domestic equipment don't have anything like the full HT voltage across them, even under a fault condition. Typically they'll be exposed to less than 100V. There are exceptions of course, and it's certainly important to bear this issue in mind when replacing resistors.

19th Dec 2012, 6:01 pm	#4
Guest Guest Posts: n/a	Re: High Voltage Resistors Well done for pointing out one of the pits that can be fallen down by the uninitiated. Simmilar things are, not setting a suitable current limit on a power supply, not checking power dissipation (of resistors especially), logic levels for different types CMOS/TTL 3.3V/5V etc..

19th Dec 2012, 8:38 pm	#5
Ed_Dinning Dekatron Join Date: Jan 2004 Location: Newcastle upon Tyne, Tyne & Wear, UK. Posts: 8,194	Re: High Voltage Resistors Hi Gents, this type of resistor can often be identified by a yellow band after the tolerance band. Ed