# How to measure the current need for an unknown device?

#### Kelvin Lee

Joined Oct 22, 2018
111

I know how to use the multimeter to measure the current through a circuit but how can I know the minimum current need for an unknown device? What is the proper way to measure the current need for an unknown device if I don't have the datasheet?

Best regards,

Kelvin.

#### crutschow

Joined Mar 14, 2008
34,845
Why can't you use the multimeter to measure the current?

#### rsjsouza

Joined Apr 21, 2014
383
Kelvin, a golden rule of measurement (mostly applicable to manual range multimeters) is to always start with the largest range available and then decrease it as soon as you get a feel of the levels used by your equipment or system under test.

For measuring current on modern DMMs with auto-range, you always start plugging your test leads to the "A" and "COM" terminals and setting its dial to the "A" range.

An alternative is to insert a small resistor in series with the circuit (also called "shunt") and measure its voltage with the meter.

Obviously that you can't tell from above that I am unaware of your prior experience and skills, thus my answer may be very basic for you.

#### dl324

Joined Mar 30, 2015
17,151
What is the proper way to measure the current need for an unknown device if I don't have the datasheet?
You can't. You'll measure a current for sure, but you have no idea if it represents the maximum current draw. If you wimp out on the current capacity, it will likely result in unreliable operation.

Joined Jul 18, 2013
28,831
What is the nature of the "Unkown Device'"?
If this is a piece of equipment a very Rough idea is the operating voltage and the physical size and what its end function is.
Max.

#### BobaMosfet

Joined Jul 1, 2009
2,123

I know how to use the multimeter to measure the current through a circuit but how can I know the minimum current need for an unknown device? What is the proper way to measure the current need for an unknown device if I don't have the datasheet?

Best regards,

Kelvin.
Unless it is a very simple component where you can test using a voltage and current source and a measuring tool (meter or scope), you can't. If you know what it's for, and the pinout, you might make an educated guess. If it's more than a component, like and actual device comprised of several or many components and component subsystems-- all bets are off.

Every component in electronics is _designed_ to work within a specific or a range of voltages and current levels, and a tolerance. And in some cases, the ability of an item to withstand the electromagnetic forces arrayed against it literally comes down to the materials it's made of and their intrinsic ability to act as a dielectric.