Hello,
I am writing about the PMLCDL Panel Meter, also called PMLCD/L some places. I don’t know where to post this, so I’m putting it here.
The Panel Meter is being used as both a Voltmeter and an Ohmmeter, but we are having trouble configuring it for the decimal point and readout that we want for the Ohmmeter Scale. The Panel Meter is used as an Ohmmeter by using an LM317LZ to supply Power for the Resistance testing, when a multi-pole switch is set to Ohms. The LM317LZ is Powered by a 9V Battery with its (+) at V[size=85]in[/size]. A 200Ω Multi-Turn Potentiometer is between V[size=85]out[/size] and its ADJ pin, to set the 100mA current for the Meter (I presume). That 9V Battery (-) goes to the Meter’s Measurement (-) and to the (-) Test Lead for reading Ohms, and to the Capacitor’s (-) because of reading Volts, too. The ADJ pin of the LM317LZ (and the Pot Wiper arm) goes to the PMLCDL Measurement (+) and the (+) Test Lead when reading Ohms. Only two connections are made to the Pot, with one on the Wiper and one on a Leg.
The PMLCDL has its own 9V Battery, with all the Negatives Isolated from each other. All 3 Batteries’ (-) terminals are Isolated from each other. The Caps’ (-) side does not connect to 1.5V (-) through the Flash Board.
Here’s a schematic.
It is configured as in the schematic, with a 100K across R[size=85]A[/size] for reading Ohms, and a 9.99M has replaced R[size=85]B[/size] (all Resistors are from the Meter Resistor Kit), with P1 shorted to P0, for an XXX.X Scale on the Meter.
This Scale is fine for the Voltmeter, as we need to be able to read over 300V DC. However, for the Ohmmeter, I want to be able to see an X.XX Scale, and really I want an X.XXX Scale, for greater accuracy. The Meter will not be used for measuring over 6 Ohms, so the X.XXX Scale would be perfect.
With P1 tied to P0, this is an XXX.X Scale, which works OK. I can test 1.5Ω and see 1.5. But as configured, except for having P3 tied to P0 instead, when testing a 1.5Ω Resistor, the Meter reads .015, which is not helpful at all. All that happens is that the decimal point moves.
Maybe a different Resistor is needed at R[size=85]B[/size] for the Ohms Scale that I want. Maybe something other than the 100K that is across R[size=85]A[/size] now. Maybe both. If so, I could change those Scale Resistors with a Switch when changing from measuring Volts to Ohms. I don’t know, so I’m writing to you about it, to ask how this X.XXX Scale that I want could be accomplished for something like a 1.458Ω reading. Some of the Resistance Coils I’ll be making are less than 1 Ohm, so I want more decimal places, and hopefully better accuracy, like being able to see 0.753Ω, and even 0.051Ω.
Do you think this Panel Meter could be configured for these kinds of low Ohm readings and have this X.XXX Scale? Can the X.XXX Scale be attained? What kind of Accuracy could be expected at a Resistance of 0.05X Ohms, or even 0.5XX Ohms? I think the Meter is rated at 5% accuracy, but I’m asking if this Scale would have an impact on that.
Thank you in advance.