PHIHONG PSM-5341: blown (?) resistor R35

[wolfman]

Hi @all,

just to doublecheck:

I have a PSU (PHIHONG PSM-5341) working absolutely fine.
Voltages are OK, PC111 is in place - so far so good.
I decided to open the case (metal shielding) to inspect the PCB and found a burned resistor R35.

burned R35

IMG_20230410_124209.jpg (77.07 KiB) Viewed 1852 times

According to the schematics this is a 10 Ohms resistor - and the funny thing is: my multimeter shows 10 Ohms for this resistor.
I can't read the colorings on the resistor anymore...

My plan is to replace the existing fan with a Noctua NF-A6x25 FLX.
The previous owner has soldered a 280 Ohms resistor to the fan-header...

[exxos]

Can't even see R35 ?

https://exxosforum.co.uk/atari/last/psu ... 30_PSU.pdf

[exxos]

I found it..

Capture.PNG (13.16 KiB) Viewed 1839 times

Probably better to replace it with a slightly higher wattage value as it is only driving the fan anyway.

[wolfman]

Probably better to replace it with a slightly higher wattage value as it is only driving the fan anyway.

I assume the existing one is a 1/4 Watt?
What wattage would you suggest?

[exxos]

I assume the existing one is a 1/4 Watt?

I would assume so yes.

What wattage would you suggest?

It really depends what fan you are putting in there.

The Noctua NF-A6x25 FLX fan you suggest pulls 1.44watts max according to the specification. So you will probably need a 2 W resistor in that respect otherwise it will burn out just the same.

[wolfman]

It really depends what fan you are putting in there.

The Noctua NF-A6x25 FLX fan you suggest pulls 1.44watts max according to the specification. So you will probably need a 2 W resistor in that respect otherwise it will burn out just the same.

Thanks for the infos!
The existing fan pulls 12V/0.16A and has soldered a 280 Ohms resistor in serial.
No wonder that a 1/4W resistor can't handle that load...

[exxos]

The existing fan pulls 12V/0.16A and has soldered a 280 Ohms resistor in serial.

12V/0.16A is almost 2 watts.

280R resistor will limited the current to about 54mA to the fan and "burn up" the other power in the resistor. Or about 0.650 watts.

I assume whoever fitted the 280R resistor wanted to half the power to the fan. But even that would probably end up being like 1 watt.

Either way, A 1/4watt resistor being 0.250watts isn't going to like it. But I think you already figured that part out

[Steve]

@wolfman please be careful, I put a Noctua (probably the same model) in my TT, I put it on the quietest setting, shortly after a MOSFET blew in the PSU and sent 7v down the 5v rail in my TT. I was very lucky it didn't do permanent damage.

Recently @mikro I think did some investigation about some of the MOSFET temperatures in the TT PSU and one was regularly running at 89 degrees C, which apparently is normal. But if you lower the cooling it will easily go over its 100 degree maximum.

I have my Noctua in still, but I run it at the high speed setting.

[wolfman]

Either way, A 1/4watt resistor being 0.250watts isn't going to like it. But I think you already figured that part out

yolo?? living on the edge??

[wolfman]

I have my Noctua in still, but I run it at the high speed setting.

Good to know: I planned to put the fan to the "middle" setting - but I think a Noctua fan is quiet enough running full speed.