Resistor Issue

kt88seamp

For my holiday lights animator, I built a circuit similar to this (see zero cross) only the 15K ohm resistors are 22K ohm 3/4 watt. The second pic I took with my phone (its blurry) shows a picture of the SMT board I made for it. The 22K ohm resistors get hot. When I feel them with my·pinkey finger knuckle it burns it. When I feel it·with the base of my pinkey·it·only feels warm. Guess the knuckle is more sensitive. When·I remove power it·almost immediately cools off.··There is:

60.2 volts across the first 22K ohm resistor·········· V = 60.2

the first 22K ohm resistor measures 22.04K ohms··· R = 22.04K

P = V^2 / R = 166.2mW

There is no where near the 750mW limit so why is it so hot? Should I up the wattage or just let it go?

Peter Jakacki

The resistors will get hot because they are dissipating the excess power. In a very small package 160mw is noticeable but that doesn't mean it's not handling it. I prefer to use through-hole parts for this sort of thing and space them off the pcb for some extra cooling and it will be less likely to char the pcb over time. Make sure the resistors are rated for well above the operating voltage and taking into account that you have them in series. Remember the peak voltage is 1.414 times the RMS mains voltage.

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*Peter*

Toby Seckshund

Peter is right. with high volts it is better to use many smaller value ones in series. One reason is that the power dissipated is shared out and the other thing is that there is a "stress" created with volts that will in time degrade the solitary version. All comercial units would use three or four in series, as you have effectivly got two already perhaps four (2 lots of 2) 10Ks would be better.

Of course this comes from the UK where it is 240 Volts.

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Style and grace : Nil point

kwinn

Peter and Toby both have good points regarding board charring and stress due to high voltages on single chips. In your case though having 0.75W resistors that only dissipates 164mW should not cause any problems. The reason it feels so much warmer when you use the knuckle compared to the base of your pinkey is that there is much better blood circulation in the base to conduct the heat away.

Toby Seckshund

And, another thing ....

Back when I were a lad ... 1 Watt of dissipation would be given a wholesome area to radiate it from, much as Peter was pointing to, nowerdays the miniturisation has left everything with no surface area. It might be able to withstand 1 Watt, without vapourizing, but nature still needs that Watt to wander off.

(and electrolytic caps are too small for their own good ...)

Digital probing is ok until you find that your fingerprints have been left behind, and there's a burning flesh aroama, again.

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Style and grace : Nil point