Code:
O--|<----/\/\/\/\/\-----
Diode \
12KΩ /
\
12KΩ /
\
/
|
|
120v |---------(Gnd of Mcu)
60Hz | |
\ |
/ _
\ | |
1KΩ / | |
\ | |
/ | |
| _
| |
| |
O-----------------------|--------|(Analog Pin of MCU)
I have the above circuit set up and for some reason the 1000 ohm resistor blew, along with my arduino board. Why did the 1K resistor blow since most of the power should have been disipated in the larger 12K resistors. I am using 1/4 watt resistors. I'm trying to use my arduino to plot the voltage of an AC signal, eventually I want to use it to trouble shoot AC-DC converters to determine the correct size smoothing capacitor to use. The voltage divider is to drop the 120 down to 4.8 so my arduino can read it.
Hi there,
You calculated the rms voltage at the AD pin to be 4.8vrms but that is rms, not peak. The peak will be about 1.4 times greater which is 6.72v peak which is too much for the uC chip.
That still doesnt explain why the 1k resistor blew out however, as with the values you have shown that should not have happened. With the 12k and 12k resistors in series with the 1k the max power the 1k would have seen would have been about 12mw, which is well under the 250mw rating of the resistor.
This means of course that something else must be wrong with the actual real life circuit. Perhaps the two 12k resistors are not really 12k but are really much lower in value. That's about the only way that 1k can blow unless the input voltage is much higher than you think it is.
This means you should check the resistance of the two 12k resistors with an ohm meter and might as well check the (next) 1k you plan to use.
You can test this resistor circuit out first by connecting only the resistors and diode and dont connect the uC chip yet. Measure the voltage across the 1k and make sure it is really what you think it should be.
Also, because the peak is really 170v and at high line it may goes as high as 200v, you should use different resistors in place of the two 12k resistors. Two 20k resistors should do the trick, with a max output across the 1k resistor of about 4.9v even at high line. You might also incorporate a zener or some other protection against high voltage spikes on the line.
Another item you should add is a reverse connected diode across the 1k to clamp short lived but significant reverse voltages. It may seem strange why we would want to add this diode, but during turn on (and other times too) a high voltage negative input with a fast rise time could cause the input diode to conduct for a short time due to its capacitance. This voltage would appear to the uC chip as a reverse polarity voltage spike which of course could kill the chip. Alternately, a small value capacitor or depending on the required response time of your application an even larger value cap to smooth out the pulsing dc as well as shunt these spikes to ground.