Electronics > Beginners

Old Analog Ammeter - Minimum Current

(1/2) > >>

luiz_aug:
Hi There.
I have here a bunch of old analog ammeter and its maximum scale value (here in Brazil we call it "fundo de escala", witch term is used for this in english?) is 10 A.
I want to use these analog ammeters as a weather display, connected to some uP over the wifi.
But to do so, as a energy efficiency concern, I need to be able to deflect the pointer over all the scale by using the minimum current possible.
I have already opened one and removed the "shunt" resistance, and by applying a 3.3V signal through a potentiometer, I discovered that the new value of the full deflection current is about 5.18 mA.

Now comes my questions:
(1) - Is there a way to reduce even more this current ? I thought about it for a while and Im guessing that the only way is by reducing the spring resistance that opposes to the pointer movement (aka: changing to another analog ammeter).
(2) - As a magnetic circuit, is there any way to "recycle" some energy?

I know these are very specific (and maybe newbie) questions, so I appreciate a lot your comments here.

Thanks!

Nusa:
Energy (power over time), power (rate of energy use, P=IE) and current (I) aren't the same things. 3.3 Volts * 0.00518 Amps = 0.017 Watts for the circuit. The bulk of that is being converted to heat at the resistor (potentiometer in your case) in the circuit, not the meter itself. If you measure the voltage drop across the meter alone you will discover the voltage at FSD, which will then let you calculate the internal resistance of the meter. Measuring that directly with a DMM is not advised, as the test voltages used could destroy a sensitive meter.

Here's an excellent series of articles on analog meters:
https://www.embedded.com/electronics-blogs/embedded-down-under/4439175/Analog-meters---The-joy-of-movement

Brumby:
@Nusa - while what you say is not wrong, the OP appears to have put together a test setup that would seem to have been effective.  Voltages aren't all that important - but the current IS.  So long as the combination of voltage and resistance allows control of an appropriate current, then all is well.

@luiz_aug - 5mA is fairly high for FSD of a meter movement. A 50uA movement shouldn't be hard to find.

But to answer you questions
(1) - Possibly - but the spring is a very carefully manufactured part.  I would not suggest you go any further than a thought experiment.
(2) - No.  The meter movement consists of a balance of magnetic fields between the permanent magnet and the energised coil against the spring.  You could improve sensitivity by using a stronger permanent magnet - however, the geometry of the magnet and its field is important - and critical for accuracy across the scale.  Again, I would not suggest you step beyond the thought experiment.

I seriously suggest looking for a meter with greater sensitivity.  An affordable 50uA should not be too difficult to find (Hmmm... not sure about Brazil)  but I think the movement out of an old multimeter would be an option.

Edit: Typo

Nusa:
@BrumbyYes, his test circuit works, although there's a risk to the meter being tested if he ever sets the pot to a low or zero value.

He asked two questions. Other than terminology, my explanation was to point out that the power cost of the circuit is negligible at the meter itself, compared to the rest of the circuit. For instance, if one changes the source voltage from 3.3 V to 1 V, while changing the resistance to get the same result, you've cut your power expenditure by over a third. That's the math.

Current, Voltage Drop, and Meter resistance are all related in the usual way. You can't change one without affecting another. The articles I linked really are a good explanation of it all. The last part has some really practical stuff.

Brumby:
I'm not saying what you said was wrong - just irrelevant.

It is perfectly clear to me that the OP was talking about their testing setup to determine the meter's sensitivity.  There has been NO description of the actual circuit into which they want to put such a meter.

Navigation

[0] Message Index

[#] Next page

There was an error while thanking
Thanking...
Go to full version
Powered by SMFPacks Advanced Attachments Uploader Mod