I have a question about digital multimeters. What happens when I initially set a higher range prior to measuring current/voltage in DC electric circuit? How does setting a higher range prevent burning of the fuse/ internal components?
Figure 1. Current range on a typical basic multimeter. The burden resistor values shown are for ease of calculation and assume that at full scale the burden voltage will be 200 mV.
How it works:
- Most of these meters use an LCD module with a full-scale reading of 200 mV (actually 199.9 mV). If you exceed the full scale reading they typically display '1 '.
- On the 10 A range current flows from the 10 A socket to COM through R5. At 10 A there will be a voltage drop of 100 mV across R5. The meter will read this (and it has a high input impedance so R1, R2, R3 and R4 won't affect that) and will display 1000. The additional contacts of the range select will add the decimal point in the right location so that 10.00 A is read.
- Each of the other ranges selects a total burden set to give 200 mV at full scale current.
- I have shown a 10 A fuse on the 10 A range. Cheap meters usually omit these as the meter is probably cheaper than the fuse. This could be lethal. Good quality meters are expensive for a reason.
Voltmeters have a very high input impedance and so do not draw significant current and do not require a fuse. For more details on auto-ranging see @RusselMcMahon's answer to https://engineering.stackexchange.com/a/560/3505.
If I start with the highest range of measuring, 10A, the current will pass through a 10A-fuse (if present) and then through a 10mΩ-resistor. The fuse and the resistor will together prevent damage to the meter or?
The meter will be able to handle up to 10 A on the 10 A range and the fuse gives protection should you exceed that. It should be clear that if you try to measure 10 A on one of the lower ranges that you will overheat the resistors. Even if you didn't burn out the resistor you would be introducing 10, 100 or 1000 times the voltage drop on the circuit you are measuring depending on which range you selected.
For current measurements, if you start with the highest range (usually 10 amp) and assuming the range is fused, then you would have to apply more than 10 amp for the fuse to blow. If you start with a lower range (for which the fuse is commonly about 300 ma), then the fuse will blow at a much lower value. For example, if you have a 1 amp current, then the fuse will not blow on the l0 amp range but will blow on the lower ranges. If your current is 200 ma, then the fuse will not blow on the 10 amp range but you can safely use a lower current range to get a higher resolution measurement. This is why it is normally recommended to use the highest current range to initially measure an unknown current. Similarly, it is always safer to use the highest voltage range to measure an unknown voltage to avoid any possible damage to your meter. You should consult the manual of your meter to see what the maximum allowed voltage is for each range. Most can handle a reasonable high voltage on the lower ranges.