Hot speaker relay

[c.coyle]

I have a Nikko TRM-750 integrated amp. There is a 560/2W (R761) resistor right next to the speaker relay that gets too hot to touch, and the relay itself ends up very warm, but not hot, after 30 minutes. It looks like this resistor brute force drops the B+ down to 24V for the coil. It is only dissipating 800mw.

I installed a new relay yesterday, and never noticed whether the original was also getting warm, but I suspect it was. Now that I think about it, the original relay's clear housing was discolored yellow, suggesting heat.

I am going to check all resistors in the relay circuit and change all associated caps, transistors, and diodes. Maybe up the wattage of the hot resistor. But I'm betting that resistor still gets hot, and the only thing to do, other than moving the resistor "further up the line" or adding some sort of regulator, will be raising it further off the board and away from the relay.

This seems like something that I shouldn't lose sleep over. A relay is just a bunch of metal, and unless the heat is so severe as to warp the contacts, a little heat shouldn't affect operation.

Anybody disagree?

 

[JoeESP9]

I've always been more than a little leery of any resister that gets to hot to touch under normal operation. I would investigate further.

 

[highvoltage_]

I agree. It's only supposed to be dissipating 800mW. Sounds like it's trying to dissipate much more than that.

 

[c.coyle]

I agree. It's only supposed to be dissipating 800mW. Sounds like it's trying to dissipate much more than that.

No, it really is dissipating 800 mw. 21 volt drop across 560 ohms.

P=E[squared]/R

P=21*21/560

P=441/560

P=0.7875 watts

Right?

Even if all associated components check out OK, I think this resistor will still run hot.

I'm thinking of moving the resistor off the board, if practical, and just running a lead to the board. This is probably overkill, but if you can make a circuit run cooler, economically and without compromising performance, why not?

 

[highvoltage_]

It's a 2 watt resistor. 800 mW should not make it too hot to touch. I suspect there's something else going on in that part of the circuit that's pulling more current than designed.

 

[TerryS]

My experience is that running close to a watt in a 2 watt resistor will make it too hot to touch. Get it up off the board if it is discoloring the pcb, but I doubt that it is hurting a relay, unless it is actually melting the plastic.

Terry

 

[c.coyle]

My experience is that running close to a watt in a 2 watt resistor will make it too hot to touch. Get it up off the board if it is discoloring the pcb, but I doubt that it is hurting a relay, unless it is actually melting the plastic.

Terry

It's going way off the board and getting upped to 3 watts. Going to mount it on a terminal strip screwed to a conveniently pre-drilled hole on the main heatsink, then run 3 1/2 inch leads to the board. Easily reversible if I don't like it.

 

[JoeESP9]

This is what I would do.

 

[whoaru99]

It's going way off the board and getting upped to 3 watts. Going to mount it on a terminal strip screwed to a conveniently pre-drilled hole on the main heatsink, then run 3 1/2 inch leads to the board. Easily reversible if I don't like it.

Unless the 3W resistor is appreciably larger it won't run much cooler. It still has to dissipate 800mW so the only reason it would run cooler is if has more surface area to dissipate heat faster.

Considering the original parts are/were, what, roughly 40 years old? And, still working fine in terms of relay control, it would seem, to me anyway, the original design probably doesn't need much help.

 

[c.coyle]

Unless the 3W resistor is appreciably larger it won't run much cooler. It still has to dissipate 800mW so the only reason it would run cooler is if has more surface area to dissipate heat faster.

Considering the original parts are/were, what, roughly 40 years old? And, still working fine in terms of relay control, it would seem, to me anyway, the original design probably doesn't need much help.

All true. But resistors are smaller nowadays, so I bump them all up one step unless there's a fit issue. I just have this thing with hot components near other components. No downside to making things run cooler if you can.

 

[whoaru99]

No downside to making things run cooler if you can.

No, not really other than time and money. Granted in this case rather minimal. It's really just me inasmuch as I have no special attachment to this stuff so I don't really understand the point/value in that sort of "fixing".