Blown Resistor - Advice?

I have an older test power supply that I have probably abused 1 too many times, which has a pretty cooked resistor inside (still works but it looks bad). Instead of tossing it, I was hoping to replace the failing resistor. All of the other components appear to be visually ok (ie. not burnt).

Unfortunately, the resistor must have gotten so hot that its insulation began burning off, so I can no longer see any color bands on it and its ceramic coating is flaking off. If I check with my multi meter, it reads right around 7.7 ohms, so that could be either 7.5 or 8 ohms given the +/- 5% tolerance.

So, I am wondering what value I should replace it with? I am not sure how important .5 ohms is but I was trying not to let [the rest] of the smoke out.

Also trying to figure the wattage. Its about 25mm long by about 7mm round. My guess is its at least 5 watt but is there any rule of thumb for these things??

Post the make and model of it and a pic of the board. Maybe someone can double check on another one for you

If its been toasted as much as it sounds like it may not read to the original specifications. Even a small difference in value could make a difference on voltage regulating circuitry.

7.7 ohms is right in the middle of the E12 standard values (6.8 and 8.2), and also between E24 (7.5 and 8.2). If there are no markings, it's a crapshoot, because overheating can change the resistance. A 25mm resistor is very likely to be a 5W resistor though. Depending on where in the circuit the resistor is, you might need a very specific value, or might have a large range of acceptable values; it's hard to tell without having designed the circuit.

Manufacture & Model of the PSU?
Do you have pictures of the resistor and the area around it?

We may be able to do some reversing engineering of sorts from there.

yeah, need more info

Unless you have an oscilloscope to do full testing, I would advise on just replacing it. It can be hard to tell if a component is bad just by looking at it and you could have several components around it that also failed. It isn't worth loosing your entire system over it. A multimeter can only tell you so much.

Resistors in PSU don't normally fail on their own, not to the point of burning off the enamel on them anyway. Odds are you had another component fail which caused excessive current through the resistor, and if you replace it the same thing is going to happen again to the new resistor.

As a couple of people already mentioned, more info including a top-down picture of the resistor and surrounding PCB area might help.

The only type of burned resistors I've seen in a PSU were biasing resistors used on the secondary side power transistors. Cheap IED PSUs are usually heavily overrated and when they're overloaded, but not to the point of exploding, excessive current draw will cause the bias resistors to overheat and burn.

I've come across several Logisys 480W units with this problem. Out of curiosity, I replaced the 1/8W bias resistors with much bigger 1W resistors and the unit never had a problem after that.

^ Can you give an example, schematic or picture, of these secondary side transistor biasing resistors? I'm asking because I can't think of where a junk PSU would even have transistors on the secondary side.

I'd take a picture but I don't have any of those Logisys units anymore, they were either used in 10 year old machines that I donate to low income households or sent to recycling with other old desktops.

While they're obviously shitty units, they hold up on really old machines that don't draw more than 200W, which is the only application I have for them. I've had surprisingly few failures with them, the most common being that the cheap fans seize up from the bearings running dry. The biasing resistors burning up is something I really only see on Athlon XP rigs for whatever reason.

They're fun to tinker with if you're into that sort of thing. The older Logisys 480W units made between 2007 and 2009 used a PCB from a more expensive unit and had open pads for a full transient filtering stage, a proper bridge rectifier, extra capacitors and chokes. It won't fix the awful efficiency and maximum power output, but the unit won't backfeed switching frequency into the mains anymore and the DC output is far better.

I remember going hog wild with one unit since I had a pile of dead PSUs and a whole box of spare parts. I added the transient filtering stage, replaced the diodes with a proper bridge rectifier, replaced the mains caps with better parts, removed and replaced the cheap thin primary and secondary heatsinks with ones from a dead Antec PSU (this was the most annoying since I had to drill new holes), replaced all of the secondary caps with higher rated parts and filled the empty pads with more caps, replaced the toroidal inductors with better parts and replaced all of the 20AWG output wires with 16AWG wires.

I think that was the best polished turd of a Logisys that ever existed.

As everyone else pointed out, just replace the PSU.

You cannot test resistance while the component is in the PSU anyways. It need to be isolated to get an accurate reading. I would just buy a new cheap tester PSU.