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Old 16th Jun 2017, 11:12 am   #1
MrBungle
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Default Damping overshoot on cheap panel meters

I have a number of cheap analogue panel meters. A couple of them overshoot and then slowly settle. Is there any way to dampen these so that they settle immediately?
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Old 16th Jun 2017, 12:03 pm   #2
Refugee
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Default Re: Damping overshoot on cheap panel meters

You could add a capacitor across them if they are DC ones.
I have some AC ones that are factory fit to the amps in my vintage stereo.
They have no damping and take about 20-30 seconds to settle even if there is a change of loading on the phase that supplies our side of the street.
The bobbin is normally made of metal and acts as a shorted turn to effect damping.
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Old 16th Jun 2017, 12:12 pm   #3
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Default Re: Damping overshoot on cheap panel meters

Most likely these are moving coil meters. You can think of these as a motor (of sorts) When the coil is rotating in the field, the movement generates an emf. This emf opposes the applied emf, so while the movement is moving at its fastest, in about the middle of its travel, the circuit current is at its lowest.
Since the movement acquires kinetic energy and the moving coil has mass and the hair springs or taught bands have elasticity and are springs with a restoring force, and there are frictional forces including the loading of the coil by its series resistance to the supply driving it, you have the 3 qualities of damped simple harmonic motion.

So, in the case that the proportions of the mass, elasticity and frictional resistance/losses (analogous to L, C and R in an electrical resonant circuit) are under-damped there will be an overshoot and even possibly a small oscillation before the needle position settles to a new equilibrium after any displacement.AS noted the shorted turn of the bobbin helps the damping.

So you can fix this by increasing the damping; putting a resistor either in series or in parallel with the movement connections to force it to a state of critical damping or over damping. But, as you know, this will affect the calibration/sensitivity. So the trick is to use a capacitor with a series resistor and put this in parallel with the movement's connections and have a fairly small series resistance. It might be that the resistance of the coil itself and the supply feeding is adequate and you can just use a parallel capacitor to get sufficient damping, experimentation is required.

You will probably notice this issue with the meters mainly when they are connected across a supply with a low internal resistance and are sent suddenly to full deflection.

If they are moving iron meters, it's a difficult one to solve.
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Old 16th Jun 2017, 12:23 pm   #4
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Default Re: Damping overshoot on cheap panel meters

The primary candidate here is a moving coil meter. I'm driving it from a current source (50uA) which causes the overshoot. I will try damping with an RC circuit and see where I get. Thanks for the tip.

I did try a capacitor on it's own across it with the current source, up to 220uF and it did nothing unfortunately. Perhaps a series R to change the time constant as well will work.

Edit: here's a video of it overshooting: https://www.youtube.com/watch?v=fAIMRgiSMIk

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Old 16th Jun 2017, 6:32 pm   #5
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Default Re: Damping overshoot on cheap panel meters

I guess in theory then just a parallel capacitor of enough capacity should turn the current source drive into a voltage source which becomes a linear ramp. I'm surprised though that 220uF didn't fix it.
I guess one other option could be a tiny drop of viscous oil on the pivots..but that damping would be unstable and unlikely to last in the long run.
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Old 16th Jun 2017, 6:39 pm   #6
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Default Re: Damping overshoot on cheap panel meters

"Classically", damping on a M/C meter was provided by winding the moving-coil on an aluminium former, which acted as a single-shorted-turn. Eddy-currents induced in this by the movement of the former around the fixed magnet provided the damping.

Is your meter perhaps one with the coil wound on something non-conducting?
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Old 16th Jun 2017, 7:04 pm   #7
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Default Re: Damping overshoot on cheap panel meters

I think the coil is wound on the thing that weighed the least to avoid China post cost

The movement was £4.29 if I remember correctly including delivery so I'm not expecting miracles here.
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Old 16th Jun 2017, 7:47 pm   #8
John M0GLN
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Default Re: Damping overshoot on cheap panel meters

I bought a 50μA cheap moving coil meter, outwardly it looks very good but the problem is the needle never returns to exactly zero, close but up to a millimetre plus or minus, my theory is that to produce such a cheap meter they have used a low powered magnet and to get the sensitivity a weak spring and this doesn't have the strength to return the needle to the same position every time, if my thinking is correct perhaps a weak spring could also be the cause of overshoot.

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Old 16th Jun 2017, 7:59 pm   #9
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Default Re: Damping overshoot on cheap panel meters

I noticed that with this one. I think the 1mA movements might be a better bet. This was destined for something with an amplifier so the 50uA movement isn't a design facet fortunately.

I may end up sticking across by bench supply and letting the smoke out yet. It's clearly not fit for purpose and there's no return in posting it because RM costs so much.
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Old 16th Jun 2017, 9:10 pm   #10
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Default Re: Damping overshoot on cheap panel meters

Most likely if you parallel a resistor of about the same value as the resistance of the movement the damping would be enough. It will halve the sensitivity of the meter, but that might not matter if you can adjust the driver circuit.
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Old 16th Jun 2017, 10:27 pm   #11
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Default Re: Damping overshoot on cheap panel meters

If the meter is driven from an amplifier having low Zout, then there should be damping from that. If a sensitive meter is used with a big series resistor, then a capacitor is needed.

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