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UL Watt ratings rms or peak?

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Village_Idiot

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I am in a pinch and need a crude shunt but I have no multimeter. I would like to use a toaster in conjunction with a microcontroller. The toaster is rated 750 watts at 120v/60hz ac. Are these RMS watt ratings (120v) or peak (177v)?

Do you think these ratings are for a cold nichrome wire or for one at operating temperature (I could use a fudge factor).

Thanks.
 
Hi,

That 750 would be average power at the operating line voltage. However, i would not expect it to be very accurate, so think about 650 to 850 and it would be in there somewhere. And that would be 'hot' not 'cold'. Cold would be lower resistance so higher wattage for a time as it heats up.
 
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Thanks MrAl. I am having a problem with my circuit; let me give you a background.

I have a LiIon battery, which has discharged very low; the charge chip will not charge it due to it's voltage. I am trying to make a variable current source so that I may bypass the battery's charge circuitry and charge the battery (no lectures on LiIon charging please, I am familiar with their charge curves/dangers).

My circuit is a pwm output --> low pass filter --> darlington pair [control charge current] --> Shunt --> ground.

I am testing the circuit with an automotive light bulb for now. When I have the shunt (toaster) in place the circuit does not work. When I remove the toaster, I can dim the bulb at will using the transistor (the max intensity is about the same as without the transistor). When I remove the transistor from the circuit, the bulb illuminates (albeit at a lower intensity due to the resistance of toaster).

I have tried this so many times to make sure I'm not making a mistake, what gives? Why doesn't the bulb illuminate with the toaster AND transistor in the circuit but does illuminate with one or the other in place?

Stumped.

EDIT: DOH! I just figured it out, so simple but it stumped me. The toaster is acting like a divider, the voltage drop at the toaster must be greater than 5-0.7-0.7. I can't believe I didn't realize this problem. I don't know a simple way to get around it, I could put the toaster before the transistor (which would take out the need for a Ibe current correction). The problem is then there could be modes of operation where the voltage across the shut would be >>5v which will fry my ADC.
 
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Hello,

Oh ok. BTW the cold resistance could be 10 to 20 percent less than the hot resistance. You can look up NiChrome wire on the web for the more exact specs though. There are a few different alloys, but that would still give you some idea what is going on.
 
I have a LiIon battery, which has discharged very low; the charge chip will not charge it due to it's voltage.
You better be very sure you know what you're doing, knowing how to properly charge a Lithium cell isn't that complicated at all, however knowing when NOT to try to recharge a Lithium cell is important, discharging bellow the cells cutoff voltage and allowing it to set for any length of time (more than a couple days) will alter the chemistry of the cell permanently, after that has occurred there is no safe method to charge the battery and there is no way to reverse the condition.

Your charge schematic seems to be massively over complicated for simply trying to nudge a Lithium sell back to a safe charge state, it can be done easily with a fixed voltage supply and a current limit resistor. Exactly how big is the battery you're trying to recover how many cells in series/parallel are there?
 
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