How to have a certain volts range on a meter?

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AlainB

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Hi,

I bought this meter today. It is already displaying 3 volts if 3 volts is applied to it. The resistance of the coil is 3370 Ohms (maybe the coil plus a built in resistor) and the current mesure .000839 Amp. when the needle is on 3 volts. These are my measurements. They are approximative.

I would like to mesure up to 8 volts. So a 6,17K resistor should do it. I will then have a reading from 0 to 8 volts. Right?

But what is interesting me it to be able to monitor the voltage from 6 to 8 volts only. So instead of 0 to 8 volts, I would like the total range of the meter to be from 6 to 8 volts.

How can I possibly be able to do that? If I can do it with a zener diode, please explain how.

Thanks!

Alain
 

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It is called an expanded scale voltmeter.
There are circuits to do this. It might be easier if the actual meter resistance is known. With a high input DVM (digital volt meter) it is possible to measure the actual meter resistance. Do you have DVM you could use to measure the actual meter resistance?
 
You have the resistance of the meter (at the terminals, I would assume), the voltage that the meter is indicating and the current you're measuring through the meter when it is indicating three volts. The E, I and R of this little scenario do not agree. I suggest that you measure the voltage at the meter terminals when it's indicating three volts to get a "good" E value. Voltage applied divided by meter resistance should equal the measured current.

One thing that bothers me about your meter is the non-linear scale toward the full-scale end. That tells me that either it's a weird movement or there's some "electronics" inside to alter the scale and those parts are affecting what's going on here.

Dean
 
Yes I know!

I said "approximative"!

My question is "How can I display a 2 volts range, from 6 to 8 volts on this cheap 2$ meter and that is really what is interesting me. Maybe you have an answer for that? No need to be very accurate in your figure. I will figure it out.

Thanks!

Alain
 
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Just put a 6.2V zener in series, anode to the meter, cathode to +Vin. Your meter will still require that the voltage across it be 3V in order to read full scale. It should expand your scale to approximately 6V to 9V. And it will be somewhat nonlinear, but you did say approximate.
 
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Bingo Ron! Thank you!

I tried with zener diodes before posting but it was not working. Probably a question of polarity.

I want to use this meter to monitor the voltage of the thermistor that is controlling the temperature of the HP laserjet fuser that I am working on. (see my other post).

On my circuit, The differential betwen the cut in and the cut out to the power of the heating lamp is about 0,6 volts.

With the proper zener and resistor, I should now be able to see the needle move a lot between the cut in and cut out. There will not be any indication of the voltage on the meter. Just a green zone for the cut in cut out zone and a red zone after the green zone. With a range of 0 to 8 volts, the movement is too small to draw zones.

Thank you again!

Alain
 
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It will not be easy to make a simple circuit that does what you want. One of the problems is that you want a 2 volt meter while you have a 3 volt meter.

I do not know for certain but I think you meter that now reads 0 to 3 volts can be modified for 0 to 1 volt. Inside the meter there is probably a resistor that can be modified.

Now try the zene. I think you want a 6.5 to 7.5 volt meter.
 
AlainB said:
I want to use this meter to monitor the voltage of the thermistor that is controlling the temperature of the HP laserjet fuser that I am working on. (see my other post).
Click to expand...

Please mark on a circuit diagram the intended meter connection points and upload here. It might not work as you have wanted it to. The meter's impedance is too low.
 
Yes, you are right Ronsimpson. I did not catch that when Ron (Roff) said it.

There is a 2,96 K (+/-) resistor in the meter but it has been easy to bypass it. Now the meter is "clean". His coil resistance being 392 Ohms it will indicate full range, the (not relevant anymore) 3 volts mark on the meter, when 0,33 volts is apply to it.

With the proper zener diode and resistor I should now be able to use any portion of the meter that I want to display as being the green zone.

Alain
 
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Hi L.Chung

Here's where I am with my circuit now.

I tried the meter yesterday, with no zener diode, and it was working. I could see the needle move up and down but just a little bit, the scale being 0 to 8 volts. I dont know if the zener will change something.

I do however need to reajust the VR.

Alain
 

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Hi AlainB.

Wire your meter like the following circuit and you can isolate the effect of the meter on the set point of the circuit. You don't need to readjust the VR every time.

It uses the leftover opamp to form a 1:1 voltage follower.
 

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eblc1388 said:
Hi AlainB.

Wire your meter like the following circuit and you can isolate the effect of the meter on the set point of the circuit. You don't need to readjust the VR every time.

It uses the leftover opamp to form a 1:1 voltage follower.
Click to expand...
Great idea, eblc. Expanding on that, the meter can be made linear (as linear as the movement is), and adjustable, with the circuit below.
 

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michale

hello:
i want to make a project that were covert a 500v pulsating dc into digital and then display the actual value of voltage(e.g 500v) into LCD.In this project i want to use a micro controller.plz help me for my this project with schematic.
 
Hi,

Yes, it is a great idea and it is working very well. Plus, isolating the meter circuit from the fuser lamp controlling circuit add a very good insurance against murphy's law.

Roff, I added your circuit and it is working also very well. I now have my 2 volts range, from 6 to 8 volts and, as a bonus, I can ajust the needle to be in the middle of the meter when 7 volts is applied. The meter cannot be made linear though. I dont think that there is any "electronic" left inside. Just his mechanical properties. But this is not very important for my usage.

What would I have to change to have say a one volt range instead of 2? And what is the purpose of the diode? I suppose it has to do with your circuit being connected to 12 volts somewhere? Or is it the inverse??

Thanks!

Alain
 

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AlainB said:
What would I have to change to have say a one volt range instead of 2? And what is the purpose of the diode? I suppose it has to do with your circuit being connected to 12 volts somewhere? Or is it the inverse??

Thanks!

Alain
Click to expand...
See schematic below for 1V range. If you don't want the full scale adjustment, I can post a schematic which eliminates it, and just leaves the center adjustment.
The diode linearizes the circuit (excluding the nonlinearity of the movement, of course).
EDIT: I erroneously designed this schematic for 7.5V center (I think). See new schematic and design methodology on the next page of this thread.
 

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Hi,

Thanks Roff. For the benefit of my education I would really like you to post the other schematic. And if it is not too complicated, could you also show me how to fish. How do you calculate the resistor values to get the volts range, one volt or two or ... whatever. I don't get it.

Here is a picture of my monster machine in action with the new circuit. Not really transferring toner to copper. Just faking. I have it work at low temperature for now for testing. Maybe around 120 C. The operating temperature will be around 180 C. It is the normal operating temp. of the HP laserjet when in printing mode.

The circuit is working well. As is, the green zone of the meter is big enough. The needle is going nicely back and forth on about one third of the total range.

Alain
 

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Post which other schematic?

Do you understand Ohm's law, voltage dividers and Thevenin equivalences? If not, it might be easier to teach you to actually catch fish than to use the equations. I could post equations, but I think you would still get tripped up.
Going back to the fishing analogy, I have tried teaching people to fish (I'm pretty good at it), and they went through the exact same motions as I did, and I caught fish and they didn't. Do you see what I'm getting at?
 
Well, I understand Ohm's law and I know that a voltage divider is a linear circuit that produces an output voltage that is a fraction of its input voltage and that calculating Thevenin equivalences derive from the Thévenin's theorem, discovered or more accurately rediscovered by Mr Léon Charles Thévenin, a French telegraph engineer.

Just jocking!

I got that from Wikipedia, just a moment ago.

But I understand Ohm's law! At least partially!

Anyway, by pointing me to those concepts, in a way, you already show me how to fish. Thanks. I'll try to undrestand those new things (to me).

About the schematic, It is the one you offer to post eleminating the full scale adjustment.

Alain
 
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I'll think about posting the equations. It will be simpler if we leave out the full scale pot. I'll post the schematic when I get the time.
 
With a 1V range, at what voltage do you want the meter to be centered? I may have screwed up the last schematic.
 
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