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Components and Circuits For discussions about component types, alternatives and availability, circuit configurations and modifications etc. Discussions here should be of a general nature and not about specific sets. |
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19th Dec 2012, 1:43 pm | #1 |
Retired Dormant Member
Join Date: Dec 2012
Location: Chania, Crete
Posts: 7
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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 |
Dekatron
Join Date: Sep 2007
Location: Oxfordshire, UK.
Posts: 4,311
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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 |
Moderator
Join Date: Jun 2003
Location: Oxford, UK
Posts: 27,967
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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
Posts: n/a
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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..
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19th Dec 2012, 8:38 pm | #5 |
Dekatron
Join Date: Jan 2004
Location: Newcastle upon Tyne, Tyne & Wear, UK.
Posts: 8,195
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Re: High Voltage Resistors
Hi Gents, this type of resistor can often be identified by a yellow band after the tolerance band.
Ed |