Resistors meter cards get very hot (BIG 44)

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Resistors meter cards get very hot (BIG 44)

Postby Sirbastian » Sun Oct 29, 2017 6:55 am

Hello!
Yesterday I removed my metercards for recap and noticed that around the 3k3 resistors the pcb is discolored/baked and the resistors look like they have fumed/smoked. The pcb is also warped at the places where the resistors are (near edge of board) and the caps placed directly next to the resistors had very high ESR and/or abnormal values.
Under a blacklight you can clearly see the burned spots.

So the resistors get really hot but why and whats going on here? And what is the best solution for this "problem"? Does the lm339 introduce peaks of high current?
When looking at the schematic I think that 17,5V runs over the 3k3(1%) resistor so..
17,5x17,5=306,25/3300=0,0928W to be safe you double the needed wattage so 0,185W resistor needed.
Looking at the schematic a higher wattage resistor is not needed but I think the pcb would benefit these.
I am about to order some 3k3 0.5W resistors which I would mount raised from the pcb in order to dissipate the heat by air and not by the pcb board.


If someone has ideas or tips about this please comment, thanks!

Best,
Sebastiaan
Attachments
image.jpeg
Schematic
image.jpeg
Fumed resistor
image.jpeg
Using blacklight to see the burned spots
Sirbastian
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Re: Resistors meter cards get very hot (BIG 44)

Postby Jim Williams » Sun Oct 29, 2017 10:49 am

BC makes smaller case sized metal film resistors. Digi-Key sells them. The 1/2 watt is the same case size as a 1/8th watt. The one watt is 1/4 watt sized, use those and raise them off the pcb.
Jim Williams
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Re: Resistors meter cards get very hot (BIG 44)

Postby Sirbastian » Sun Oct 29, 2017 12:20 pm

Hello Jim,

Thanks for your reply, great.
I could not find better than .5W in a normal housing at Farnell.
I will order some from digi-key then :)

Cheers,
Sebastiaan
Sirbastian
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