I'm having trouble measuring the output power of a coil in a pulse-motor.
I'm rectifying the output, which is 282 V.
I am measuring the voltage using a DMM, with a 10 Ohm resistor in series with the positive output of the coil and the positive input of the DMM.
I am trying to measure current and i can't, i have tried using a second DMM across the resistor and still get a zero reading.
I know there is current there because little 25mA light bulbs are lit brightly from the output.
Also, using Ohm's Law, I = V/R, I = 282/10, I=28.2, obviously i'm not getting 28 Amps from this little thing, what am i doing wrong !
Any help greatly appreciated.
DC.
You are likely reading the peak voltage of a ... pulse. Or rather, an inductive spike caused by the collapse of the coil's magnetic field when it's power is removed. The pulse is probably very short. So it could indeed be momentarily reaching 28 amps for a very short time, and thus lighting up your bulbs.
You can't measure current in this system the way you have described. About the only easy way to measure current in a system with rapid pulses is to use an oscilloscope and monitor the voltage drop across a low-value resistor in series with your circuit. Don't you remember this from the Ainslie affair?
You can, however, measure the power output of your pulses relatively easily, by timing how long it takes to charge a known capacitance to a known voltage, through a diode. Again, this was covered to exhaustion during the Ainslie days.
Let's be clear about the use of the DMM: to measure current directly, it has to be placed in SERIES with your circuit, set to the appropriate range, and even using the correct holes for the probes. Most DMMs have a 200 mA setting and the fuse for this is often blown without the user being aware of it. To measure indirectly, the DMM can be used in the "Voltmeter" mode to monitor a voltage drop across a known, low value, resistor. But this method only will work with a DC voltage; fast spiky signals will confuse the meter and are best measured with an oscilloscope.
OK thanks TK :)