ALTERING THE FULL SCALE RANGE OF A PANEL VOLT-METER
This method will work in many cases to reduce the maximum voltage value that a panel meter reads. This is mainly for typical modern moving coil meters but will work for some but not all meter types.
If you read this and understand it you will be able to use the results obtained for other purposes.
The method calculates the internal meter resistance and meter full scale current allowing calculation of the correct series resistance for an other voltage (higher or lower). Meter resistance is allowed for.
A 30 V meter is assumed as the original question related to a 30 V meter.
The text has been altered to be more general as Vfs has been used as meter full scale voltage.
In this specific answer only Vfs = 30V.
(1) Apply a test voltage not more than Vfs (here=30V) but as close as possible to Vfs.
eg 30V (= Vfs) = excellent
24 V = Good
12V = acceptable
9V (eg PP3 battery) = OK, higher would be better.
5V = too low really
Read and record voltage shown on meter.
Call this V1.
(2) Connect a resistor in series with the meter and apply V1 again.
Current path is V1+, into resistor, out of resistor, into meter +ve, out of meter -ve to V1-.
Ideally select a resistor such that reading is about half V1 but as long as it is between say 10V and 20V it should be OK.
If reading with resistor is above 20V add more resistance.
If reading with series resistor is below 20 V use a smaller resistor.
When reading is > 10V and < 20V goto next step
Call series resistor R2 (there is NO R1)
Call new voltage reading V2
Meter resistance consists of
- internal series resistor = Rint
- resistance of meter movement = Rmeter.
2(b) Alternative: You can by pass steps 1 & 2 above by measuring (Rint + Rmeter) directly using an ohm meter.
Ideally use a meter with a 9V battery for ohms testing.
Measure on several ohms ranges.
Best value is probably one not at extreme high or low end of a range.
2(c) Semi-expert alternative to (1) + (2) : If you have an external power supply with an accurate current meter or an external current meter of appropriate range you can measure (Rint + Rmeter) by applying
R=V/I = (V_across_meter/I_through_meter)
Note that if u=you us a series current meter it will drop some voltage. Measuring V_across_meter literally across the meter with a voltmeter may be a good idea. If the series current meter changes value more than very very slightly if at all when you apply or remove the voltmeter across the meter on test EITHER deal with it if you understand how OR uses tests above instead.
(3) Calculate (Rint + Rmeter) = R2 x V2 / (V1 - V2)
This is the effective resistance of the meter as seen by the circuit being measured.
Calculate: I1 = V1 / (Rint + Rmeter)
I1 is the current needed to produce the reading V1.
(4) Calculate: Ifs = Vfs/V1 x I1
Ifs is the full scale meter current.
Ifs MAY be 1 mA or 100 uA or in a few cases 25 uA or some other current.
Regardless of what it is, this is what it takes to deflect meter to full scale.
(5) Dismantle meter.
There will PROBABLY be a single resistor in series with the meter movement.
ie Meter +ve terminal via resistor via meter to meter-ve terminal.
OR meter +ve terminal via meter via resistor to meter -ve terminal
6(a) IF the arrangement is NOT as above with meter + a single resistor, take a photo, record any markings on internal components and report back.
6(b) IF arrangement is as above with meter movement + single resistor then:
Remove resistor - this will PROBABLY be best done by cutting the leds near but not at where they are terminated soldered / ... so there is room to solder on a nw resistor. IF the resistors is surface mount either do what seems best to allow a new resistor to be inserted OR report back. DO NO destroy resistor when removing if possible.
Measure resistance of removed resistor.
Call this Rint
(7) To make meter read full scale voltage of your choice = Vnew:
Calculate: R3 = [Vnew / Ifs] - Rint
Install resistor of value R3 in place of old Rint
Note that Vnew may be higher or lower than the original full scale voltage.
Note that the MINIMUM direct voltage that can be set as Vfs is when Inew + 0.
ie Vfs_min = Ifs/Rmeter
Where Rmeter = (Rint + Rm) - Rint