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Question: Fusible resistors

M

Martin.Ca

Super Member
I've seen varying advice (or maybe I missed the specific contexts) regarding fusistors/fusible resistors.

When dealing with old fusistors, what to do? Replace them when out of spec? Replace them regardless of whether they're in or out of spec? Replace with new fusistors? Replace them with a specific other type of resistor?
 
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Don't replace them unless they aren't any good any more. If the resistance is close, leave them alone. If they open, replace them with the same value if you can get it. If not, evaluate the circuit and see if you can use a standard resistor plus a fuse, or whatever.
 
I use metal film of the same wattage and resistance value.
 
Fusable Resistor

I find these fusable resistors tend to increase in value or they open completely based on my experience with pioneer amps.

RS components has a reasonable selection of fusable resistors. I'd replace them with metal films only when fusables are not available.
 
Don't have RS anymore up here, and the fusibles on Mouser don't cover all the possible requirements on the amp I'm looking at (Yamaha p2200, evidence of past fusible sacrifice, and one side is getting warmer despite idle adjust).

What's the fail mode of metal film resistors?

On fusibles, in the p2200 parts list, there are currents mentioned (eg. 22 Ohm 150mA) but how do I equate that to power dissipation rating? I^2*R ? Would that simply be a 1/2 watt resistor?

I'm asking what people with more electronics repair experience than I have, would do.
 
avionic said:
I use metal film of the same wattage and resistance value.
Click to expand...

This is what I do - but only because ^he^ does - and he knows what he is doing :yes:

All the fusibles I have removed seem to have gone high in value for no particular reason - so I don't trust them anymore. The bit about replacing with same wattage equivalents is important - you want the replacement resistor to open if excessive current flows.
 
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Like avionic, I use metal film or a non inductive wire wound resistor where its required. I will use a fusible resistor in lighting circuits or critical applications in electronics, that's about it. That's actually their designated purpose, ie protection.
 
Often fuse resistors were used because they were flameproof, not to limit or fuse the current. Yamaha power amps, for example, blew everything up, despite having fuse resistors...they offer no protection when the output stage shorts.
I would recommend using fuse resistors in motor drive and power supply applications. If you can compute the fuse current, then you can install a metal film resistor in series with a "pico fuse" of the proper current value.
I doubt all your fuse resistors increased in value. More likely you meter has poor accuracy at low resistance scales.
 
Nakdoc said:
Often fuse resistors were used because they were flameproof, not to limit or fuse the current. Yamaha power amps, for example, blew everything up, despite having fuse resistors...they offer no protection when the output stage shorts.
I would recommend using fuse resistors in motor drive and power supply applications. If you can compute the fuse current, then you can install a metal film resistor in series with a "pico fuse" of the proper current value.
I doubt all your fuse resistors increased in value. More likely you meter has poor accuracy at low resistance scales.
Click to expand...

I haven't checked them yet. I was was asking what were considered the appropriate conditions and circumstances for changing/not-changing them.
 
Nakdoc said:
<snip...>I doubt all your fuse resistors increased in value. More likely you meter has poor accuracy at low resistance scales.
Click to expand...

As an example of some I have dealt with, four 82Ω fuse resistors increased in value in varying amounts up to 10KΩ.

Oh, and I have a digital ohmeter - 2 actually :smoke:
 
Nakdoc said:
Often fuse resistors were used because they were flameproof, not to limit or fuse the current. Yamaha power amps, for example, blew everything up, despite having fuse resistors...they offer no protection when the output stage shorts.
Click to expand...
Where a flameproof resistor is required, we use a flameproof. Where a fused resistor is used, we use a fused resistor. There are no fused resistors in series with the output stages, protection in this context is to be covered by an active relay circuit, if employed.
 
I don't see what kind of protection this (150mA) fusistor can provide in this circuit. Normal current in them (R132 and R133) is ~14mA. If the current in the fusistors exceeds 150mA, the driver transistors are already blown.
 
ecluser said:
I don't see what kind of protection this (150mA) fusistor can provide in this circuit. Normal current in them (R132 and R133) is ~14mA. If the current in the fusistors exceeds 150mA, the driver transistors are already blown.
Click to expand...

Well, in my P2200, the fusible in line (R139,R140) with the (output?) drivers seems to be a 390 Ohm 1/4Watt, which I believe works out to a max current of ~25mA.

As for TR111,TR112, I'm not well versed enough in amplifier design to know their purpose , so 150mA is considered large?
 
R139 and R140 are parts of the protection circuit, and if there is resistors that must never become open, they are. Replace them for 1/2W to play safe.

TR111 and TR112 are the driver transistors, they drive the bases of the (6) power transistors. The DC voltage at the emitter of TR112 is ~-0.6V, and the DC voltage at the emitter of TR111 is ~+0.6V (there is an error on the schematic). So, the current in R132 and R133 is approximately 0.6V / (R132 + R133), or ~14mA

In order to have a current in excess of 150mA in R132, the voltage drop in R132 would need to be in excess of 3.3V. It is possible only if the power transistors are blasted AND there is a malfunction in the bias voltage source circuit (TR110 and TR115) or the driver transistors. In other words, if the fusistors R132 and R133 become open, they will not be the first components to blast (in hope of providing some kind of protection for the rest of the circuit), they will be the last components to blast, after everything else is blasted.
 
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