Troubleshooting Digital Voltmeters

[servo]

Hello, everyone. I am trying to add a digital voltmeter to a control box that I built. I cannot get it to respond to the inputs. It reads zero volts at all times. I have the supply regulated and isolated, so that is not the issue. I am reading the proper voltage from the test points I had installed, but I continue to get no response from the meter. I had been getting proper readings before, but the decimal point was in the wrong place (e.g. 123.4V instead of 12.34 volts). When I switched the jumper to the other pin, the decimal point was in the right place, but the meter stopped responding. I do not have the schematics for the meter and I cannot figure out what the issue is. This project is for work and my bosses are starting to get anxious. Please, help.

 

[Tea Jay]

Can you jump the jumped pin, so you use the pin that reads on the wrong scale to see if the potentially non-responsive pin causes a reading?

Do you have anything that can indicate volts other than that meter, to test the circuits?

 

[Carl Pugh]

What have you done to get a schematic for the meter?

If you cannot get a schematic for the meter, junk the meter and get a meter with a schematic.

 

[servo]

I have a DMM that I have used all over this circuit. The box operates properly and I'm getting the proper voltage at all points. I tried a second panel meter of the same kind and the same thing happened.

Also, my bad, here is the datasheet with a schematic, but it hasn't done me any good.
http://www.murata-ps.com/data/meters/30lcd.pdf

Edit: And, the reading I'm getting for the supply is about 5.5 volts.

 

[Carl Pugh]

I have had good luck with a similar Datel panel meter. DMS-20LCD-0-9-C. This brand seems OK.
Could the meter be plugged in upside down?
More likely the problem is the 5 volt power supply negative is not connected to the measured voltage common. If a 9 volt battery is used, the battery floats at the correct voltage, but a power supply has too much leakage current.
Let us know what the problem is.

 

[jim hardy]

how are you using it in your box?
these things are picky about common mode voltage.

Datasheet cautions about +-2 volt common mode limit.


make sure you are in conformity with notes 1 & 2 in applications section of meter datasheet,
and tech note 2,, """Pin 10 should not be connected to pin 3 (5V RETURN/–BATTERY) or to your system’s analog ground."""

 

[servo]

It is connected to a +/- 10V control signal and I will have another set up to monitor a 2V - 12V feedback signal. Pin 10 is not connected to anything. The meter is not plugged in upside-down. I've checked and rechecked that. And, like I said, it worked for a bit until I moved the decimal point and it completely stopped responding. I was using a 5V DC/DC converter, but that didn't work, so I even ordered the circuit board converter from Murata (http://www.murata-ps.com/data/meters/eb-dcdc.pdf) but that really hasn't worked either.

 

[jim hardy]

servo wrote: """Edit: And, the reading I'm getting for the supply is about 5.5 volts.""

i'm about out of ideas.

i presume you have the 30LCD-2 or -3, 20 or 200 volt range for the signal levels you mentioned

and i assume everything in your box uses same ground reference

only other thing i notice is datasheet specifies max power supply 5.25 volts not 5.5
near botttom second page:
"
Power Supply Requirements (5V Models)

Supply Voltage +4.75 +5.00 +5.25 Volts
"

let us know what you find

old jim

 

[pantaz]

It would be helpful to see a diagram of how you have connected the meter.

 

[Carl Pugh]

Try meter operation with batteries.
Use three 1 1/2 volt batteries for 5 volt supply. (voltage should be near 5 volt if batteries are new)
Measure the voltage of a battery or use a resistive divider and a battery if meter input is 200 mV.
It is probably advisable to connect negative of 5 volt and negative of battery being measured together.

What is the part number for your meter?

 

[servo]

The part number is DMS-30LCD-2-5B-C. Now, I have the converter board as mentioned above that is supplied by Murata. I have attached the 5v regulator to it as per the datasheet. Why would the voltage be at 5.5V and not at 5.0V like it should be?

Also, I don't have a diagram drawn up, but it's a very simple setup. The voltmeter is hooked up between the output of the potentiometer and the converter ground. Both are on the output side of an isolated dual-output converter. The power comes from the 24VDC ac/dc converter run through a 5V regulator and the isolated converter board I mentioned above. The box functions as intended, but I cannot get a proper reading from the voltmeter.

 

[jim hardy]

wow you're surely almost there. Mother Nature makes us work for our lessons, doesn't she? Ahh well she knows we need to be pushed...
if you said what regulator you are using i missed it.
i noticed the popular LM7805 datasheet guarantees it'll hold voltage only when load is more than 5 milliamps. and meter draws not quite one milliamp with backlight off per its datasheet.

try turning on the backlight? or tack a 1k resistor across 5v rail?

and you did use capacitors as shown in your regulator's datasheet, if any...
http://www.fairchildsemi.com/ds/LM/LM7805.pdf

 

[servo]

The backlight is on which draws about 60mA. The converter board has the capacitors already installed and I'm using an NTE960 which is equivalent to the LM7805. This is getting frustrating.

 

[Carl Pugh]

Check that there is a jumper from pin 7 to pin 8 on meter.

At present you don't know if the problem is the circuit, or the power supply or the meter or even if the circuit is bad and has damaged your meters.

You need to eleminate some of the unknowns. I would recommend checking the meter using batteries or at least a different 5 volt power supply. Forget your circuit and wire the meter up using the most simple circuit possible. Wire circuit up on a test bench using point to point wiring. Don't worry how it looks, just be sure that all connections are made and that there are no shorts.

 

[servo]

Problem solved. After going over the entire box yet again I discovered that the wire going to the + input was loose. I tightened it down again and it works great. Thanks for your help, Jim and Carl!

 

[jim hardy]

bravo for persevering!