I have 2 POLYSWITCH 3.5A/6V 0805 in circuit getting hot. My question is are these reset-able fuses and or they faulty? Could there be short further in the circuit? They're part of a USB circuit if that helps. 1 more question as I am learning Is the 3.5A=3.5amps right? It is hard to find replacements at that at 0805.

 

POLYSWITCHES get hot!! That is how they work. Of course they need to be selected to have their normal (cold) resistance low enough.
So your polyswitches re not deffective, but they might be the wrong value.

 

POLYSWITCHES get hot!! That is how they work. Of course they need to be selected to have their normal (cold) resistance low enough.
So your polyswitches re not deffective, but they might be the wrong value.

Sweet they may have gotten defective, it's on a b550 motherboard. I am getting the part type from a boardview file and can not track down exact part that is getting too hot that's causing a usb over voltage error in post. I was checking surrounding components in that circuit and none of the others are getting overly hot of testing faulty.

 

Sweet they may have gotten defective, it's on a b550 motherboard. I am getting the part type from a boardview file and can not track down exact part that is getting too hot that's causing a usb over voltage error in post. I was checking surrounding components in that circuit and none of the others are getting overly hot of testing faulty.

or could it be the wrong bios flash for the board asus tuf has 3 different types.that would account fro the wrong values set by bios??? I will try another flash with theb550 tuf... actually I have enough of the heat sinks off to probably get the actual model number.

 

The fact is that selection of the correct value "PolySwitch" is not so very simple, and that their rather non-linear resistance versus temperature curve really makes knowing the exact current quite important. Besides that, it seems that there may possibly be some tolerance issues as well. It might be useful to install the POLYSWITCH components with longer leads so that they could get better air circulation. Certainly it could help them run a bit cooler.

 

Bill, I don't think an 0805 device has leads.

 

I was reverse tracing the power back to maybe a different component supplying too much voltage,and yes they're smd devices?
I have not checked voltages just resistances, as i have 15 sec of power to check until bios shuts the board down. Could be a faulty Usb port, maybe a 5v pin is shorting in there ? I can not get a good angle with my scope to see. I am going to first remove the component, loose usb c and another usb port and see how it reacts. I did find replacement parts I just want to fix the cause and not the symptoms lol.

 

you have been very helpful btw, on other forums no one answers or they do not want to give the bad news eitherway thanks all.

 

OK, 0805bSMD parts to not come with leads, but with only a moderate amount of effort and a bit of skill one can certainly wrap a couple of turns of #28 solid wire around the device so that it can have leads soldered on. That trick also worked with those round resistors used for a while before flat SMD resistors became available. (Does anybody even remember them???

 

The problem with soldering leads to SMD's is that when you solder it to the board you inadvertently melt the solder holding the lead to the component. It's tricky as heck. Certainly not something for a beginner to attempt.

Yes, I remember those round SM resistors with leads. They weren't welded to the component with ordinary solder. That gave the installer a chance to make the joint on the board before the solder on the resistor liquified. And a big reason why those went away was because of assembly problems. Flat SMD's will center themselves up on the pads - provided the pads are properly sized and positioned. Tombstoneing is when one solder pad goes molten before the other. The surface tension of the solder will pull the solder up on one end (tombstone). Billboarding is even weirder. Not sure how that occurs. Possibly an installation problem before going through the oven. But I don't have any information on that phenomena.

If worse comes to worst one can take a leaded device and bend a stress relief after flattening the ends of the lead. Make a foot out of the lead to hold the component and you are in business.

 

The problem with soldering leads to SMD's is that when you solder it to the board you inadvertently melt the solder holding the lead to the component. It's tricky as heck. Certainly not something for a beginner to attempt.

Yes, I remember those round SM resistors with leads. They weren't welded to the component with ordinary solder. That gave the installer a chance to make the joint on the board before the solder on the resistor liquified. And a big reason why those went away was because of assembly problems. Flat SMD's will center themselves up on the pads - provided the pads are properly sized and positioned. Tombstoneing is when one solder pad goes molten before the other. The surface tension of the solder will pull the solder up on one end (tombstone). Billboarding is even weirder. Not sure how that occurs. Possibly an installation problem before going through the oven. But I don't have any information on that phenomena.

If worse comes to worst one can take a leaded device and bend a stress relief after flattening the ends of the lead. Make a foot out of the lead to hold the component and you are in business.

The quick fix to the solder melting would be the same as was used to heat-sink transistor and diode leads when that was required. Quick soldering and longer leads are the other tricks that avoid the problem.