RMS or average current to determine resistoring heating?

n

RMS is correct. In your case (and assuming a 1 Ohm load for simplicity), you have 25W generated for 50us, and 0 for the remaining

450us, giving an average over 500us of 2.5W. The current corresponding to 2.5W is 1.58A (your 1.6A figure).

That was simple enough.

thanks.

Think of it this way.

Heat generated is proportional to power (I*I*R). The average is then the average of the I*I*R terms. Therefore you are looking at a RMS term.

Obviously cooling figures in here so frequency matters but over a relatively broad range of frequencies RMS will give you a good place to start and will probably be sufficient.

Also take a look at your resistor's power derating curves. They will sometimes provide some additional information that will help.

If the resistor will dissipate the heat between pulses then your calculation may rely more on the pulse rating of the resistor than the power rating and of course there is some current for which the resistor is underated no matter how short the pulse but only a few parts have that value fully specified that I have seen.

Robert

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"mook Johnson"

** No matter what the wave shape - the rms value is the one for heat dissipation in a resistor. ** The rms one.

** Correct.

BTW:

The most dramatic example of this I have seen in a commercial product is with " Glow Plug " drivers for model size methanol engines ( 2 or 4 stroke). Model size glow plugs are made with platinum wire and draw up to 5 amps at

1.2 volts - so the best way to power them is from a single Ni-Cd or NIMH cell.

However, some modellers like to use their 12 volt lead-acid starter motor battery to drive the glow plug too.

But as 5 amps at 12 volts is 60 watts, a 1.2 volt linear regulator is not practical - a switching regulator would be ideal but there is a much cheaper ( and nastier) solution that the Asian makers of cheap glow driver use. Simple PWM.

Yep - the 12 volt battery is switched ( with a FET ) straight onto the 1.2 volt glow plug at a high (but audible ) frequency with a very low duty cycle - about 1%.

This means the current pulses are in the order of 50 amps !!!!

The average current draw from the 12 volt battery is then 50/100 = 0.5 mp - while the glow plug has 5 amps rms flowing through it.

....... Phil

--
If your pulse is rectangular, then the current won\'t be RMS, it\'ll
be peak, and you should use average.

John Fields is LOSING IT BIG TIME

** Wow - now THAT is really crazy bullshit.

Rectangular pulses have no RMS value ??

The average value of a rectangular pulse stream gives the heating effect voltage ?

The song for John:

----------------------

They're coming to take me away, HA HA

They're coming to take me away, HO HO HEE HEE HA HA

To the funny farm

Where life is beautiful all the time

And I'll be happy to see

Those nice, young men

In their clean, white coats

And they're coming to take me away, Ha-haaa!

...... Phil

Any waveform has an RMS value, including rectangular=20 pulses. Of course, for rectangular pulses that are always greater than or equal to zero the average=20 will work out to be the same as the RMS value. For waveforms that traverse zero, the average can be=20 any value between the peaks including zero (consider a=20 square wave with equal positive and negative peaks).

[snip]

The RMS value for your 505s/505s waveform is about 3.536 A. With a one ohm load the power dissipated is then (3.536 A)^2 * 1 Ohm =3D 12.5 W, same as using the average.

"Greg Neill" "John Fields"

Any waveform has an RMS value, including rectangular pulses. Of course, for rectangular pulses that are always greater than or equal to zero the average will work out to be the same as the RMS value.

** WHAAAATTTT ???????????????

For single polarity, rectangular pulses, the average and rms values are NOT the same !!!!!!

I av = I pk times duty cycle.

I rms = Ipk times sq.rt duty cycle.

Get real, d*****ad.

...... Phil

--
So I was wrong?

Thanks

I too run RC boats (IMPBA) and have one of that hobbico power panels that make the glow plugs queel. :) I never considered the heating affects of the

50Amp pulses through the batteries internal impedance. Good thing we only need them for a few seconds during startup.

"Mook Johnson"

** How did you know I was ( once) a model boat racer ?

It's the AMPBA here.

I ran a couple of monos and a tunnel hull (aka catamaran), 3.5cc and 7.5cc.

** Those "power panels" must be the WORST possible way to light up a glow plug.

Some of the evil problems include:

1. The 12 volt battery's voltage sags and hence plug wire temp DROPS when you use the electric starter, often so much so that the engine will not start. Particularly true if the engine is a little flooded.

1. Accidental contact between the starter motor frame and the engine sends

12 volts straight to the plug = instant plug death.

1. When the engine starts and you throttle it up briefly, the plug wire becomes white hot and may fail. This is because the power delivered to the plug by the panel increases with resistance, which is suddenly raised when the engine runs.

None of this happens with a Ni-Cd cell - even a 1400mAH sub C size is fine.

Waaaaaay better than any stupid MOSFET power panel.

...... Phil

Your method works fine for a postive-going waveform (as in this particular case), but would=20 not be correct for a general waveform. If the OP's pulses were rectangular but not anchored=20 on zero volts, the average and RMS currents would be different, and the RMS value would have to be used to find the power dissipated in a resistive=20 load.

7.5 and 15CC outriggers here.

Yup had the same problems you discribe. Was using 4AH gell cells and the 90 boat would start relaibly. Switched to low impedance 7.5Ah gell cells (low internal resistiance) and that went away but the field pack got heavy.

We run 50% - 60% nitro and the plugs are COLD and we run rich. So we need the 4 - 5A get them glowing. I had a setup that used a Alkaline D cell for lighter. Worked well but then I "graduated" to a power panel. :(

Might be time to flunk back a grade. That D cell seemed to last FOREVER. (my boats start pretty easy)

[snip]

Phil,

Everyone's allowed one mistake in their life. Take your mother, for example.

Bob

You shouldn't be concerned with either one. Power dissipation is the rate at which the resistor is converting electrical to thermal energy in joules per unit of time. When the periodicity is small compared to the thermal time constant of the heat sink, then you can you can use the average value of joule dissipation per cycle, because much like a low pass filter, the heat sink temperature will not respond to the high frequency component of the thermal dissipation profile, it will only respond to its DC value, and another way of saying 'DC value' is 'average'. You might notice that computing average power dissipation per cycle is the exact same thing as computing RMS current and multiplying by R. Forget about the formulas and consider what it is you really want to know.

--
Did I say that?

" John Fields is LOSING IT BIG TIME "

** Completely insane.

They're coming to take him away, HA HA

They're coming to take him away, HO HO HEE HEE HA HA

To the funny farm

Where life is beautiful all the time

And he'll be happy to see

Those nice, young men

In their clean, white coats

And they're coming to take him away, Ha-haaa!

...... Phil

"Greg Neill" "John Fields"

Your method works fine for a postive-going waveform (as in this particular case), but would not be correct for a general waveform. If the OP's pulses were rectangular but not anchored on zero volts, the average and RMS currents would be different, and the RMS value would have to be used to find the power dissipated in a resistive load.

** WHAAAATTTT ???????????????

For single polarity, rectangular pulses, the average and rms values are NOT the same !!!!!!

I av = I pk times duty cycle.

I rms = Ipk times sq.rt duty cycle.

Get real, d*****ad.

...... Phil

She made more than one:

1. She let his dad get her pregnant.
2. She didn't have an abortion.
3. She didn't flush Phil down he toilet they day he clawed his way out of her body.
4. She didn't have him committed, for life.

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