Panel Analogue Meter Circuit - Page 1
Forum section: electronics / beginners

author: alin_im
author_rank: Regular Contributor
timestamp: May 20, 2015, 03:50:13 pm
content: Hi guys,I recently found a panel analogue voltmeter, but i can not find a circuit for calibrating it! can you please help me??? Any good articles / circuits to read about this topic ?

author: Asmyldof
author_rank: Regular Contributor
timestamp: May 20, 2015, 04:15:08 pm
content: You need one or more known quantities.Usually for analogue the calibration procedure is in the range of 0% and 100% scale optimised by turning the little screw until both are satisfactory (depending on the accuracy of the coils and the printer that made the papers it might be a trade off between either one perfect or both sort of near perfect).A known quantity can be a very accurate voltage source, but for analogue mV scale will not be needed.Tuning 0 is easy: Short the two terminals and hold it exactly level, while tuning.Tuning full scale can be done with a power supply and a decent multimeter, or a power supply with a good enough display of its voltage output.Then also see how off it is, and choosing what manner of averaging between the errors will work for you.If it's a spiffy one that's been built for accuracy, you might want a mV scale and a very good eye and might be able to get both ends spot on.EDIT:Ah hang on, it says volts, but has no internal resistor, just a 1mA response coil.So create a known current, by putting a resistor in series, so you can tune the voltage on your power supply to adjust the current, and again use another measurement interface to get to exactly 1mA. If you have some reasonably accurate resistors you could measure voltage across a 1k or 10k type and go for 1V or 10V. Measuring single volts is usually easier on affordable multimeters than measuring tenths of a mA.

author: mikerj
author_rank: Super Contributor
timestamp: May 20, 2015, 04:42:55 pm
content: How you calibrate it depends entirely on what you want to measure with it. Voltage/Current/Power etc.

author: alin_im
author_rank: Regular Contributor
timestamp: May 20, 2015, 04:44:58 pm
content: Quote from: Asmyldof on May 20, 2015, 04:15:08 pmThank you for your response, i tried what you suggested and if I have a 12 kohm resistor in series at 12 volts the a. voltmeter goes to 12 , which means when the current through the meter is 1mA the niddle is fully deflected. The problem is that it is not a linear relationship between the voltage and niddle deflection, because when i put 5 volts in the niddle is at 6V and if the power supply is set to 1.25V the voltage shown is 2.

author: alin_im
author_rank: Regular Contributor
timestamp: May 20, 2015, 04:46:13 pm
content: Quote from: mikerj on May 20, 2015, 04:42:55 pmI would like to measure voltages from 0 - 12 v

author: Asmyldof
author_rank: Regular Contributor
timestamp: May 20, 2015, 04:53:09 pm
content: Don't discount the fact that the meter might have a small voltage drop itself, which is a different kind of linear or near-linear to the resistor itself, that might make it slightly unlinear as a total system.Keep an eye on the voltage across the resistor.If you want perfect precision of the voltage measurement only a resistor will never give you accuracy at both ends because of the not-exactly-perfect behaviour of the meter itself.If you want the best possible precision at full range you will need to convert the input voltage to a forced current (code name: VCCS, for Voltage Controlled Current Source).You can also do some tricks to compensate or the coil's performance, if you want it to run off only the voltage you are measuring, but they will never be fully accurate at the low end.

author: PSR B1257
author_rank: Frequent Contributor
timestamp: May 20, 2015, 05:13:05 pm
content: QuoteMight? It will certainly have a resistance, since I can't see a bottle of liquid nitrogen anywhereQuoteThat's the application for a simple voltage divider

author: jimmc
author_rank: Frequent Contributor
timestamp: May 20, 2015, 08:43:22 pm
content: Since it has 'Volts pk-pk' on the scale-plate it is meant to measure AC voltages in conjunction with an external rectifier.The non-linear scale is designed to compensate for the non linearity of a simple rectifier and series resistor.Single diode or bridge using silicon or germanium diode(s) is difficult to tell, time to experiment.Jim

author: alin_im
author_rank: Regular Contributor
timestamp: May 20, 2015, 11:29:07 pm
content: Quote from: jimmc on May 20, 2015, 08:43:22 pmYeah i was thinking about that but i thought i could work for DC also...how can i make sure i will not burn italways have a current max of 1mA??? and that is it

author: PSR B1257
author_rank: Frequent Contributor
timestamp: May 21, 2015, 03:25:46 pm
content: QuoteOnly the dB-scale is non-linear, as usual.The "peak-to-peak" (rather stupid declaration, since it also shows "Batt" - and as far as I know is there no such thing as a AC battery) scale is linear, as usual for a moving coil movement.QuoteYes, this would be a good ideaDid you meanwhile measure the coil resistance?

author: jimmc
author_rank: Frequent Contributor
timestamp: May 22, 2015, 09:19:45 pm
content: Quote from: PSR B1257 on May 21, 2015, 03:25:46 pmPlease look at the photo in the original post,the pk-pk scale is non linear, the arc from 0 to 1v is much less than that from 11 to 12v.I agree the moving coil meter is linear hence...Quote from: alin_im May 2015, 15:44:58(The presence of a 'Batt' mark is irrelevant, the meter will have been switched from normal use with an EXTERNAL rectifier to 'Battery Check'.)alin_imIf you want to use the meter for DC then you really need to replace the scale with a linear one, however if you want to avoid this then you could try adding a silicon diode (cathode to +ve to the meter. Something like a 1N4148 - not critical) in series with the meter and resistor.The typical coil resistance for a 1mA meter is 75 ohm and the diode drop approximately 700mV so the resistor will need to be (12-0.7)/.001 -75 = 11225 ohms; try 11k.Adding the diode will 'cramp' meter deflection at lower voltages in the same way as the scale markings.The compensation won't be perfect but it will give better results than you measured before.Jim