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Old 20th September 2012, 08:09 AM   #1
Stuey is offline Stuey  Australia
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Default What are 'safe' resistors?

Hi guys,

I'm looking at a transistor preamp service manual on which all the resistors in the signal parts of the circuit are 0.25W rated. However, a few are marked as 'Safe 0.25W'. See pic - an example are the 39 ohm ones near the output relay.

Can someone advise what makes these different from standard resistors?

Thanks

Stuey
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Old 20th September 2012, 08:49 AM   #2
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Default Overload safe.

If the output transistors fail as short circuit, as they usually do, then the 39R resistors will be overloaded. A "safe" resistor is tested to fail as open circuit when overloaded and without any flames or nasties. Other resistors may not need this safety because their value or circuit position is such that even a short circuit failure elsewhere will not overload them.

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David
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Old 20th September 2012, 08:58 AM   #3
Stuey is offline Stuey  Australia
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Thanks David. I guess that's what I'd surmised anyway, but I've never seen these listed for sale anywhere. What are they marketed as - do you know? I've had a quick look round, but can't see any. Are they just those claimed to be flame-proof, or is there more to it re. failing open?

Last edited by Stuey; 20th September 2012 at 09:11 AM.
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Old 20th September 2012, 10:59 AM   #4
jaycee is offline jaycee  United Kingdom
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fusible resistors
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Old 20th September 2012, 11:32 AM   #5
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A further question, if it's OK.
We often see metal oxide types rather than metal film used as emitter resistors in power amplifiers and supplies. I've seen various claims of superior qualitities but do these also have a better failure characteristic?
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Old 20th September 2012, 11:38 AM   #6
Mooly is offline Mooly  United Kingdom
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Safety or fusible resistors are common in the service industry. They are designed to fail in a safe and predictable and repeatable way without letting off noxious fumes or smoke and also the consequential damage of charred PCB's and so on.

Heres a small selection if the link works,
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Old 20th September 2012, 11:48 AM   #7
PHEONIX is offline PHEONIX  Australia
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Hello Ian

Metal Oxide resistors fail open circuit. Metal film resistors unless stated in their respective data sheet that they fusible types fail giving off lots of smoke and heat and damage the PCB.
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Old 20th September 2012, 12:03 PM   #8
Stuey is offline Stuey  Australia
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Thanks gentlemen.
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Old 20th September 2012, 03:21 PM   #9
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Good thread for info.

Worth adding that in analog TV days, with a lot of power dissipation in the circuits driving the CRT, certain resistors were mounted "upside-down" such that if they became extremely hot and melted their solder they would drop out and fall to the cabinet base!
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Old 22nd September 2012, 08:45 AM   #10
Stuey is offline Stuey  Australia
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Hmmm, I hadn't noticed before, but some of the resistors in the parts list are marked as 'Fusible', so I assume those marked 'Safe' are different in some way...

The Fusible-marked resistors are only found in the power supply.
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