OF
Well-Known Member
Sorry done editing I promise! Just straight dmm (10A limit) from negative battery to the inside threading of the top, while the postive of the batteries made contact with the head. Could very well be a loose connection or the switch limiting the current, the imr's can put out 10a+ I think
Cool. Thanks for the additional details. I still think your number is artificially low, here's why:
To measure current, the DMM (which in fact can only measure voltage) has to use a shunt (low value resistor) in series with the current. The meter then measures the voltage drop across it (a measure of current via Ohm's law) and reports that value as a correctly scaled current. Changing ranges changes these resistors. The top range one is probably a piece of heavy wire inside the front panel or on the PCB between common and the special 'high current' jacks. Typically they are 'trimmed' (calibrated) with some solder on the side as the exact value is hard to control. The size is chosen so at full range the full range of the basic meter is met, 200mV. At ten amps, 100 mV is 'lost' on this resistor. BTW, the 10 Amp limit is time related, you can go to 20 for a second or two without issues.
There's also more resistance added through the leads and connections. All this adds up to more resistance (most likely) than normal, therefore lower current. Typical numbers for the leads are hundreds of miliohms, most of an ohm happening sometimes. Does your lowest range ohms scale say 00.0 when the leads are shorted? Doubtful. This last is why.
If you have another meter handy (or can borrow one) there are two easy ways to back this out and get a solid reading. One is to measure the voltage drop on the meter by probing the minus terminal of the battery and the shell of the battery, position not important since there will be trivial currents in this circuit. Then use that number to back 8.4 Volts off. The other way is to put a second current meter in the circuit, note the new current, then remove the first and scale using the difference in readings. I suspect this effect is important and might explain some of the loss. Some of it is, of course, the internal resistance of the battery in play causing it to 'sag' under load, but we can't get to the positive side to make this reading easily.
Fun stuff. Thanks again.
OF