# Ammeter design

#### fp_geo

Joined Oct 9, 2008
2
Hey there - first post on here from me...

I feel there's a point missing from the chapter about ammeter design. The sentence "The five-position switch makes contact with only one resistor at a time, of course." isn't entirely correct - the chapter fails to mention the 'make-before-break' switch needed for multi-range ammeters, since if you break the connection before making a new connection, you'll end up frying the meter, right?

#### raybo

Joined Oct 18, 2008
22
An amp meter to me means 50ua flow full scale. by adding shunts you can make it reads amps. the switch make or no break is immaterial since excessive current must flow to blow it up. break means no current so where is the danger?

#### fp_geo

Joined Oct 9, 2008
2
What I mean is, if you have the ammeter in a circuit, and it goes off scale, you want to change the scale (switch between shunts) without having to unplug or turn off the circuit, the switch HAS to make a new contact with the new shunt BEFORE it breaks the contact to the old shunt (make before break).

If it breaks the shunt connection before it makes a new one (break before make),the entire flow runs through the 50µA meter, because it is connected parallel to the shunt resistor, therefore the meter is likely to burn up.... right?

#### thingmaker3

Joined May 16, 2005
5,084
What I mean is, if you have the ammeter in a circuit, and it goes off scale, you want to change the scale
Good protocol is to set the meter for the highest scale. One may then lower the scale if more accuracy is needed.

If it breaks the shunt connection before it makes a new one (break before make),the entire flow runs through the 50µA meter, because it is connected parallel to the shunt resistor, therefore the meter is likely to burn up.... right?
If one were to switch slowly enough, or if the current measured were high enough, the meter movement might be endangered. Typically, switching does not provided a lengthy enough current pulse to harm the meter movement.

If one were designing an in-line meter specifically for high amperage, one might be wise to select a make-before-break switch. Most meters won't need one, as most in-line meters are not intended to measure high amperage. Clamp-on types are made for high-amperage measurements.