old school volt meter how to run??

[large_ghostman]

happy new year,
I have got my dad to get me some old volt meters from the bay, theese ones **broken link removed** but i am not sure if i need a resistor (i am pretty sure i do) to make them work without blowing them up. My aim is to have a go at altering the weather thing i made mum and having the output show on the meters and i have seen the clock on here with old meters. so how do i make the meter go from 0-fullscale?? do i put a resistor in parallel with the terminals to limit the current? the voltage i want to use is 5v for max scale. i hoipe i have made it clear??? the output is pwm from micro (sfware pwm). thank you for looking.
p.s tell dad they were a bargain please he thinks they were expensive!

 

[alec_t]

Have a good look for a label/marking on the meter housing or scale to get a clue as to its basic sensitivity. It may be marked as so many μA or mA.
Putting a resistor in series (not parallel) will limit the current. The total circuit resistance (including the meter coil) to get 5V full-scale will be 5/I, where I is the basic sensitivity in Amps.

 

[large_ghostman]

thank you for that mr alec, i think this pic shows the number you mean but i am not sure how it works??? so is it 100 uA for full scale deflection? and a 1k resistor?

 

[chemelec]

I am not sure about there Markins on that meter.

But FIRST I would put a 100K ohm resistor in series with the meter and than see what it reads with a 1.5 Volt Battery.

Than you can determine a more suitable resistor value.

QUOTE: "the output is pwm from micro"
I am Doubtful it is suitable for this purpose.

 

[large_ghostman]

I am not sure about there Markins on that meter.

But FIRST I would put a 100K ohm resistor in series with the meter and than see what it reads with a 1.5 Volt Battery.

Than you can determine a more suitable resistor value.

QUOTE: "the output is pwm from micro"
I am Doubtful it is suitable for this purpose.

thank you for the reply. i was hopeing the pwm could feed into a cap to make it more stable and maybe use a highish frequency so it dosnt move about as much, otherwise i was going to use a shift reg with resistor dividers but that is a bit clunky

 

[large_ghostman]

they havnt got here yet so i cant play with them

 

[chemelec]

they havnt got here yet so i cant play with them

When you get them, Do What I told you first.

THAN, If you have more Question about a correct resistor value, Possibly Email me for help.

RE: PWM
Put a Diode (1N4148) between the PWM Output and the Cap. (possibly a 1uf cap)
Measure across this cap and This should allow you to read the Average Voltage.

 

[Diver300]

I am fairly sure, from the markings, that they read 100 μA forward and 20 μA backwards. The internal resistance is 1000 Ω.

If you wanted them to read -2V to +10 V, you would want at total resistance of 100 kΩ, so you should put a 99 kΩ resistor in series (or use 100 kΩ which is easier to find and gives an error of 1%, which is probably less than the tolerance of the resistor).

 

[Diver300]

If you want to drive the meter from a PWM output from a micro, if you go above about 1kHz (maybe 100 Hz) then the response time of the meter will be sufficient that you don't need any damping. With a 5 V output, a 49k resistor in series will suffice and the the meter will read 0 for 0% duty cycle, 5 for 50% duty cycle, 10 for 100% duty cycle etc.

If you want to damp the signal with a capacitor, split the 49k resistor into two approximately equal series resistors, and put a capacitor with its positive connected to where the two resistors join, and its negative to ground. A 100 uF will give you a time constant of around 1 second. If you split the resistors unevenly, you need to use a bigger capacitor for the same timing constant.

Do not use a diode. It will give you the peak, not the average voltage.

 

[alec_t]

I agree with Diver300. You need 49k (= 47k +2k2) in series with the meter to give a full-scale range of 5V (actually -1V to +5V because of the zero-offset of that particular meter).

 

[large_ghostman]

great information thank you both, i will try it out soon as they get here! and post a update, mum and dad both like old stuff (cos they are old i guess) so i thought theese meters would be great for a few things

 

[KeepItSimpleStupid]

Hey, you forgot the Experiment. You actually know the FS current, but don't know the meters internal resistance. So take a pot of at least V/(FS current), Current in amps. V (whatever you have handy) Set the pot to it's highest resistance. Adjust so the meter reads half the value. Take the pot out of the circuit and measure the resistance. That will be the internal resistance of the meter.

 

[alec_t]

...and it will probably be 1000Ω; the figure shown in the meter pic .

 

[large_ghostman]

i also think it will be the 1k on the meter but i will do the experiment to confirm and also because i like to do experiments!! is a handy tip to put in my book thank you for the tip! we dont have that sort of meter at my school they are all digital