opamp analog meter

Hey,I'm wanting to build a multimeter with a 1mA meter,resistors and a lm324 opamp. I think it can source 5-10mA. I'm using the opamp as a voltage follower. I know I will not be able to measure mains power but I think for me a low voltage vom meter is good enough for me. I'm thinking about powering it from 2x 9v batteries, so it gives the opamp 18v so it has a wide range of voltage

Your 1mA meter needs about 100mV to show full scale, it means that the input to the op amp is also 100mV. You get this voltage by using 2 resistors voltage devider. It also means that you are OK with one 9V battery even when you measure up to 1000V.

Welcome to the forum hack.

You can do without the op amp as mentioned, however sensitivity will be reduced, ie the meter will load the circuit being measured.
I spose it depends what you need, an op amp would give you high sensitivity and you'd be able to measure low voltages, a lm324 is a fairly basic chip, a rail to rail cmos version might be better like a lmc660, pinouts are the same.

Years ago in high school we had a project to make a VOM from a small meter like what you have. The goal was to not use another meter to make this meter.
I started out with a new 9V battery as a reference. By adding a resistor of about 10k (in series withe the meter) you can make the meter read 9V when powered from the battery. That makes a 10V meter. Also you can use a new 1.5 battery and about 1.5k or 2k resistor to make a 1.5 or 2V range. We used a rotary switch and did some math to make a meter that will measure 20V, 10, 5V, 2V and 1V. (DC)

We know your meter is 1mA but we don't know its resistance. At 20V you can use a 20k resistor because 20V and 20k ohms allows 1mA of current. The meter resistance has little error. But at 2V you need 2k resistance mostly from the external resistance but some of it is from the meter that might have 100 or 200 ohms internal.

Hope this helps. Ask questions!

there was a time where if you was an amateur you had to build ypur own gear. nice info from ron there. i,m not quite old enough to remember a vtvm, the predecessor to this.

dr pepper said:

there was a time where if you was an amateur you had to build ypur own gear. nice info from ron there. i,m not quite old enough to remember a vtvm, the predecessor to this.

Click to expand...

When measuring an audio signal with a meter, does a VTVM give a "warmer" reading?

ak

Here is a circuit.
It produces 1mA in the meter if the input is 1V
It uses a second opamp as a rail splitter.
It has an input impedance of ~2megΩ
It has a modicum of protection in case you connect it to a high voltage
It can use a nothing-special opamp like a LM358

You can make R2 switchable to change the input scaling.
You can reduce R4 to 50Ω or 5Ω to make the input sensitivity 100mV or 10mV full-scale, respectively.

Hello there,

In any meter design it is also customary to use two back to back diodes to protect against over voltages of either polarity.
Since the voltage divider resistors are there already in most cases, the current to the diodes will already be limited, but if not a series resistor can always be added which will then limit the current to the diodes.
A 1k resistor at 1000 volts will draw 1 amp, so that's probably not good enough. A 10k resistor will draw 0.1 amps at 1000 volts, so that's probably better even if you dont intend to measure 1000 volts, but it would also be a lot of power in the resistor, but then again the voltage divider would have a higher resistor anyway. 100k at 1000 volts would draw 0.01 amps at a power of 10 watts, so that may be doable At 500 volts it's only 2.5 watts, and at 250 volts it's 0.625 watts.
I'll leave all these details to you though.

In current meters it is nice to use as low a resistance as possible to measure current, but there is also the thought about going a little higher for the low current ranges because larger value resistors dont drop much at very low currents. A 1 ohm resistor only drops 1mv measuring a 1ma current level, and even a 10 ohm resistor only drops 10mv. I'll leave the further details to you again.

Nice circuit.
Can I suggest a nice little addition, a 1n4148 across the meter, with its anode to the meter +, this means no more more than around 6x the meters rated voltage will be across it, the op amp might blow up but the meter ought to survive a mishap.
The lm358 and the lm324 are very similar, the former being a dual op amp, if you want current ranges too you could use the spare ones in a lm324 for that.