Using a Multi-meter

[Greg@mnrr]

I have a Radio Shack electrical multi-meter that's about 10 years old. Please tell what I already suspect, but would like to confirm.

I have a circuit for the layout lighting that's supplied from a 12 volt DC transformer. I run the load through a potentiometer to drop the voltage down to 10 volts and use a panel mounted volt meter to confirm the voltage drop. Also, the fixtures on this circuit are dimmer which indicates operating at a lower voltage.

When I check the circuit at a terminal strip with my multi-meter, the voltage always exceeds 12 volts.

Bad meter?

Thanks.

Greg

 

[montanan]

It's hard to say what's going on. I also have an ancient Rat Shack digital multimeter and haven't had any problems with it. I would say check the battery, but on a voltage reading, the only thing the battery is used for is the readout. I would check it out on a known voltage output and see what it reads. Those old meters are pretty well bullet proof.

 

[Iron Horseman]

Probably a stupid question, but have you double checked to be certain the meter is set for the right range?

 

[CBCNSfan]

Multiply the R.S. multi meter reading by 0.707 and let us know what you get for an answer

Willis

 

[Greg@mnrr]

Willis:

The first reading I took was 15.50 X 0.707 = 10.9, then I took a reading on the same circuit at a different terminal strip and the reading was 16.12 X 0.707 = 11.48.

The reading on my panel mounted meter was at 10 volts.

Each time the meter slowly worked up to the reading on the digital dial.

Thanks.

Greg

 

[Greg@mnrr]

Engineer:

The range was set at 20 volts.

Thanks.

Greg

 

[Greg@mnrr]

I tested the meter on a 1.5 battery and the reading at 1.87 volts.

Greg

 

[montanan]

Sounds like the merer is off Greg. Is it a digital or analog?

 

[Greg@mnrr]

Chet:

The meter is digital. I'm off to buy another meter.

Thanks.

Greg

 

[CBCNSfan]

I tested the meter on a 1.5 battery and the reading at 1.87 volts.

Greg

To me that would indicate that the meter was reading a little high. The point I was going to make was that voltage from a throttle pack is actually pulsating DC of which the peaks are the value AC multiplied by 1.414. If the DC voltage is filtered with a capacitor and if the load draw was slight then the filtered DC value (RMS) could be nearly as high as the peak value The multi meters ON DC read RMS voltage which is .707 of the peak AC voltage so when comparing the voltage reading on a panel meter to a reading on a multi meter, this would have to be considered in case the panel meter was calibrated to read RMS value with a predetermined current draw.

I blabber too much hope it's not confusing
Willis

 

[Greg@mnrr]

Forum:

I purchased another digital meter from Sears, a real bargain at $9.99, normally $23.99.

I checked the circuits and they read closer to what the panel meter indicated. I use a high quality wall wart with to power my Tortoise switch machines and at the 9 volt setting, the meter showed 9 volts exactly.

Testing batteries, the meter showed the correct range of voltage.

Thanks everyone for your input.

Greg

 

[jdetray]

Many wall warts are unregulated. They are designed to provide their listed voltage only when connected to the load for which they were designed. So ... If you measure the voltage of an unregulated wall wart without a load, you will usually see a voltage that is higher than the listed voltage.

It is just something you need to consider when using unregulated wall warts.

- Jeff

 

[Greg@mnrr]

Jeff:

Good point on wall warts and being a regulated or unregulated power source. I use separate wall warts for powering my switch machines, lighting and signal power. Each are regulated power sources. I checked the warts with my new meter, without a load, and they are providing the output as indicated on their labels.

Thanks.

Greg

 

[whowey]

My 20 year old Cell Shack DMM will start reading everything high when the battery is starting to go bad. It will slowly drop back down then read low as the battery voltage drops off further. My Harbor Freight one will do the same thing.