A cheap old multimeter as Figure 1 have very useful. But a its voltmeter range is so long cause see not clear such as the 2.5V range then away to 10V and then go to 50V range, it makes we can not read thoroughly. How can we extend the range of voltmeter?
Figure 1 the cheap old multimeter that still useful for everyone.
If there are the 5-volt range to rises better or 25V range, it may help to measure voltage easily to read and convenient up. For example: measuring the 1.5-voltage battery has 1.0V to 1.6V range, or 12-volts battery which provide voltage output during 10V-15V range etc.
a adding range, we need to know before that, scale on panel the multimeter are proper to application (as Figure 2) such as number: 50 can be also use for 5V or 500V or number 250 can be also for 25V.
We will also have others scale that can roughly read (not well) such as a ICEO scale, normally leak current between C-E lead of transistor will full scale is : 15 can do as 1.5V range.
Which it is good to measuring, because generally rechargeable battery have voltage of 1.2V, when lower than 1.1V should re charge again. (In case the Ni-MH rechargeable battery only) But when voltage over 1.5V is fully can use them.
Understand the voltmeter before!
First of all, we set range of multimeter which called “a voltmeter”. Normally it same the common devices, when current flow through them cause a internal impedance (RM) and voltage across that range.
As principle, the good voltmeter that must have the many more internal resistances. And lowest current to reduce a bad effect to load.
See Figure below
To see picture as natural form. We try to find the resistance meter (RM) better. I select the 0.1V or 50uA range since lowest so low error too.
the many ways for find them such as:
RM = VM/IM
RM = Internal Resistance Meter
VM = Voltage across meter or voltage range
IM = Current in the range
In this case:
VM = 0.1 V, IM = 50 mA
RM = 0.1 V / 50 mA
= 0.002 M or 2 K ohms
Another, see panel will show the resistance per 1V. This meter has value : 20 k/V (20 k ohms per 1V) indicate that if the 0.1 V range has resistance of 2 k ohms there.
Also, if you do not have any way to measure the resistance directly. with the another ohmmeter I often this way such as modify VU-METER to the various tools by it is small and cheaper.
Try adding Measurement range
The modifying that voltmeter, should understand it before that When our main the meter is limited voltage as that ranges. We must rotate to 0.1V range only.
We will see that the main meter can get only voltage 0.1V. But we must to use voltage up to 25V so cause the meter can be damaged with this over voltage.
In this case we use also the resistor to reduce easily voltage from 25V to 0.1V. See picture it same connecting LED with normal resistor just charge LED into meter only.
Something we need is a simple formula which is Rs :
Rs = (Vin – VM)/IM
Try adding a simple Measurement range of 25V.
(How think, take Vin delete VM before, then take IM divided again)
Generally, common meter, always not show voltage value but indicting current (IM) only for example : VU-METER full scale of 250 uA
Thus we calculate voltage of meter (VM) before so easy : VM = IM x RM We can apply to find Rs:
Rs = (25V – 0.1 V) /50 uA
= 0.498m ohms or 498K
In this case we use 500K 0.25W instead which is closest.
For the error or deviation of meter. I do not worry because there are METER analog errors are common. But should not exceed 10%.
The Advantages of the meter needle is more easily able to see the changes digital form.
Hope you that use this idea to application the meter so well.