I am thinking about creating an analog meter clock. The meters will be driven by PWM from a pic.

I like the retro look of these meters:

http://www.ebay.com/itm/AC-0-300V-R...733319?hash=item27fc6d1b07:g:rL8AAOSwbdpWT~lN

Is any way to know if this meter is actually a 0-300VAC meter? Or is the scale just printed that way?

If it is actually 0-300V can I modify it to work with DC?

 

If it is actually 0-300V can I modify it to work with DC?

Looks like it really is.
You might be able to modify it if you can open it up and get access directly to the meter movement, which likely will require only a small DC voltage to move it full-scale.
(If you do that, be careful you don't overdrive the movement and damage it. Start with a 100k resistor in series when you check it).

 

The scale looks non-linear so it may be a moving iron meter rather than moving coil. This type of meter will work with either AC or DC though the calibration will be different.

 

Looks like it really is.
You might be able to modify it if you can open it up and get access directly to the meter movement, which likely will require only a small DC voltage to move it full-scale.
(If you do that, be careful you don't overdrive the movement and damage it. Start with a 100k resistor in series when you check it).

Ah you answered one of my follow up questions. But what starting voltage? Would 5V do?

I will need two maybe 3. Since they are coming from China I would hate to order 1 and then just find out I was able to make it work then have to order 2 more again. Theh again, I would hate to order 3 then find out I have 3 bricks.

Considering cheap price, time to ship and appearance of the meter would you just order 3?

 

The scale looks non-linear so it may be a moving iron meter rather than moving coil. This type of meter will work with either AC or DC though the calibration will be different.

I think that that might not be an issue for me. I was already considering having a table of "time positions" rather than try to calculate the position for each tick.

Would that resolve the calibration issue?

 

I think that that might not be an issue for me. I was already considering having a table of "time positions" rather than try to calculate the position for each tick.

Would that resolve the calibration issue?

Yes, that would work.

 

The meters should work. They likely have a small selenium rectifier and voltage dropping resistor inside. I would order 4. 3 for the clock and one to mess with (or as a spare).
See also here for designing new dial scales:
http://tonnesoftware.com/meter2.html
and here:
http://alan-parekh.vstore.ca
he built a multimeter clock. It works great, but unfortunately he left out a daylight-savings function. E

 

The meters should work. They likely have a small selenium rectifier and voltage dropping resistor inside. I would order 4. 3 for the clock and one to mess with (or as a spare).
See also here for designing new dial scales:
http://tonnesoftware.com/meter2.html
and here:
http://alan-parekh.vstore.ca
he built a multimeter clock. It works great, but unfortunately he left out a daylight-savings function. E

I was trying to figure if I wanted to use 2 or 3 meters for the project.

I have that same meter face software. It is excellent! I played with it a bit and 60 ticks is a little tight for that size of a meter. I was thinking of doing 2 meters 10s and 1s but that might look odd. Then again, I could use one for seconds. That would be kind of cool.

I was going to add some retro panel lights for AM and PM.

I like is implementation. Kind of cool with the Simpson look alike.

Daylight savings could be problematic. It would mean you would need to have some way of setting the date. I suppose you could have a simple switch and just set it manually. I will need to remember that when I start the design.

 

But what starting voltage? Would 5V do?

Don't you have a variable supply that you can start from near 0V.
Otherwise 5V with 100kΩ should be fine.

 

Most of these old analog meters are actually a current driven device. May have a resistor in series with the meter movement which acts to limit the current. On many of them if we look way down on the meter face they will have the full scale current shown and often the meter movement resistance. As an example, here is both a very basic Ampmeter circuit and a Voltmeter circuit. Take note of how the range resistors are used in each example:

Voltmeter


Note that the meter movement resistance is 500 Ohms and the F/S (Full Scale) current is only 1 mA. On for example the 1 Volt range the series resistance is 500 Ohms so we have 500 Ohms meter movement and we have 500 Ohms series resistance. So we get 500 Ohms + 500 Ohms = 1,000 Ohms. ! Volt / 1,000 Ohms = .001 Amp and there is our 1 mA F/S Current. So a Voltmeter of this type uses a series resistance to limit the current to the meter movement's full scale current.

Here is an example of an Ampmeter circuit:

Notice how with the Ampmeter the resistor is in parallel rather than series with the meter movement. These parallel resistors are frequently called a shunt resistor because they shunt current away from the meter movement in such a way the meter movement will only see 1 mA F/S. For example if we place the R3 resistor 500.5005 milli Ohm in parallel with the 500 Ohm meter movement 999 mA will pass through the shunt resistance and 1 mA through the meter movement with 1.0 Amp applied.

A voltmeter measures across the load and a ammeter measures in series with the load. Even when used as a voltmeter the meter movement is in fact measuring current and has a meter face scaled in voltage.

Ron

 

Don't you have a variable supply that you can start from near 0V.
Otherwise 5V with 100kΩ should be fine.

Yeah and I can limit the current too. I think I will go with that.

 

Yeah and I can limit the current too. I think I will go with that.

You still want to use the resistor, since even a momentary overload can damage the movement.

 

I wouldn't buy one.
I would just buy one, 1 ma meter movement.........and see how it likes pwm first.

And uamp movements are available.

 

I wouldn't buy one.
I would just buy one, 1 ma meter movement.........and see how it likes pwm first.

And uamp movements are available.

Huh?

 

micro amp

 

micro amp

Yes I know what uamp is. First you say you wouldn't buy one. Then you say you would buy one.

Did you mean you would not buy this type but buy one with micro amp movement? Don't know if it is available in this style.

What if I am able to hack the meter? Will it make a difference?

 

There are ma meters available so far not finding uamp. At least not on ebay. Want that retro look plus want cheap.

Darn I had some old meters from an old welder years ago and tossed them

 

Sorry for the confusion. I like the old style too.

The unit you showed, is not a meter movement. It's a AC voltmeter. BIG difference.

And if you are going to pwm dc........you just want a current movement. And most movements aren't expecting high frequency pulses.

And I didn't know your power requirements......so I mentioned the low power movements.

Didn't mean to confuse.

 

And if you are going to pwm dc........you just want a current movement.

A volt meter is just a current meter with a series resistor. The mechanical inertia and damping of the movement will adequately average a PWM signal.

 

That's great.