Current Limiting Resistor Power Rating

[Merlinb]

I've added a new page to my site that should address the power-rating problem of limiting resistors, if you like doing things by hand! :
The Valve Wizard

[R.G.]

It occurs to me that since RMS current is a mathematical calculation used to reflect the heating value of non-DC currents, and since resistors are fairly cheap, one way to measure an RMS current is to put in a resistor of some kind and measure the temperature of the resistor. Most power resistor datasheets specify the derating factor and surface temperature versus power dissipated curves in some way. Since the whole point is to keep it cool enough, one could use one "victim" resistor, then from the temperature rise calculate a big enough power rating for the next shot.

Clumsy? Sure. But good for the math-impaired.

[R.G.]

It occurs to me that since RMS current is a mathematical calculation used to reflect the heating value of non-DC currents, and since resistors are fairly cheap, one way to measure an RMS current is to put in a resistor of some kind and measure the temperature of the resistor. Most power resistor datasheets specify the derating factor and surface temperature versus power dissipated curves in some way. Since the whole point is to keep it cool enough, one could use one "victim" resistor, then from the temperature rise calculate a big enough power rating for the next shot.

Clumsy? Sure. But good for the math-impaired.

[trobbins]

rms current in a resistor is directly related to rms voltage across the resistor - which is eminently more measureable than surface temp :-)

[Steve Conner]

Not without a true RMS meter, and even then, the high crest factor of the currents in a rectifier could cause problems. If you smell burning, use a bigger resistor.

[R.G.]

Harbor Freight and other places sell a US$30.00 infra red temperature meter. You point it at the object of temperature and it gives you the temperature in F or C. I bought one of these the first time I saw it for sale.

True, it makes assumptions about the surface emissivity in calculating temps, but in checking with thermometers and thermistors, I've never found the difference between the meter reading and true contact temps to be more than about 1C. Very, very handy indeed if you do anything related to power electronics. Or heating and air conditioning.

True RMS meters are a bit more expensive, at least from what I've run into so far. I always intend to buy one when a good opportunity presents itself, but that hasn't happened yet.

I realize that I didn't finish the thought on the resistor temps. There is a reasonably accurate temperature gauge (not meter...) on the end of your index finger. Your internal reflex sensory neural wiring will pull your index finger off a hot metal surface automagically at about 130F/54C for most people's wiring. If it's much hotter than that, it's a burn hazard. Index fingers are expensive, but tend to be widely available.

Obviously, smoke, charring circuit boards, melting solder, etc. are other temperature indicators one could use, but if the resistor is under 54C, it's going to live a long time in the application. If a damp index finger sizzles on it (the test used for clothes ironing in a simpler time), it's probably too hot.

But I'm rambling again.

[Merlinb]

Analog Devices released a new RMS-to-DC converter IC in July, the AD8436ARQZ, and you can get free samples off their website!

[Enzo]

My memory could be faulty, but I seem to recall additionally that 158 degrees is the upper limit to heat discrimination. In other words 140 degrees might feel hotter than 130 to your finger, but 175 won;t FEEL any hotter than 158. Both will be painful of course, and one might cause more damage, but the sensation won;t discriminate between them.

[R.G.]

Interesting!

The thing that makes 130 a temperature of note is that this is the temp where the vast majority of people will not keep their skin in contact with the heat - they'll jerk back.

I guess it makes sense that at some point all the heat/pain sensors are yelling bloody murder, so you can't get more heat/pain sensation. Never thought about it.

[R.G.]

Interesting!

The thing that makes 130 a temperature of note is that this is the temp where the vast majority of people will not keep their skin in contact with the heat - they'll jerk back.

I guess it makes sense that at some point all the heat/pain sensors are yelling bloody murder, so you can't get more heat/pain sensation. Never thought about it.

[Chuck H]

I guess it makes sense that at some point all the heat/pain sensors are yelling bloody murder, so you can't get more heat/pain sensation. Never thought about it.

Certainly nothing I'm going to test personally. How DID results for this sort of research come about??? "OK Mr. Homeless Guy... Thank you for signing up with the Pain Tolerance study group. The straps are for your own protection."

[trobbins]

Yes, most commercial meter specs I've come across only state a level up to about 3 for crest factor, even for standard lab grade DVMs. I'm not sure if the increase in inaccuracy (for higher crest factors) beyond the spec level for rms voltage is substantial, or influenced by the peak level vs full-scale level, or influenced by the frequency response spec for Vrms.

But I'd be anticipating that Vrms inaccuracy would be much less than a temperature related measurement technique, and well within the normal margin applied when choosing a resistor power rating.