Monday, December 11, 2023

How to measure resistance with an ammeter?

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There are many ways to measure a resistance, the easiest would be with a tester in the ohmmeter function, or in the voltmeter function. But in the next article we will show how to do it using only an ammeter as a measuring instrument. For this you must learn some concepts, first.

What is an Ammeter?

A ammeter It is an instrument to measure the electric current that is passing through a cable or conductor. It can be in the shape of a Truper Hook or it can be integrated into one of the functionalities of a Tester.

In the case of ammeters hook-shaped no need to open the circuit to make a current measurement Since this is responsible for measuring the magnetic field generated by the surrounding current and calculating the value based on the intensity of the field. While if it is to be measured with a tester in mode ammeter open the circuit and insert the instrument probes into the ends of the open circuit.

Hook Ammeter

Range setting

The electric current is measured in Amperes (A) and when a measurement You must have a notion of the range of values ​​that you can, that is, you must know if we are going to measure thousandths of amperes (mA) or if we are going to measure 1 ampere or more.

This is to select the range of measurement more appropriate to achieve a more accurate reading. In some digital ammeters this adjustment is done automatically but in others it has to be done manually.

If the range of the current to be measured is not known, it is adjusted to the average value and if the reading shows 0 it is because the established range is too high and it must be reduced. Otherwise, if the screen indicates an error, it is because the range is too low and it must be raised.

What is a resistor?

The electrical resistance is precisely the resistance that a certain electrical material presents to the passage of current through it, the analogy can be made with a valve inserted in a pipe where a liquid is circulating. Greater resistance would imply that the valve is more closed and therefore reduces the amount of liquid that passes through the pipe, less resistance implies the open valve and greater flow of liquid (electric current) through it.

Cables have a minimum resistance, almost zero, while components such as resistors have a known value of this parameter that can range from thousandths of Ohms (mΩ) to millions of Ohms (MΩ). Knowing these values ​​allows the designer of a circuit to control the current flowing through certain parts of the circuit and / or to set voltage points if required.

Ohm’s law

In mathematical terms, resistance is defined as the relationship between the voltage between two points in the circuit (expressed in Volts) divided by the current through them (expressed in Amps).

R = V / I

In the case of ceramic resistors, like the one in the photo below, they present a series of colored lines. These lines are not to decorate the component but rather they express the electrical resistance value of the component, but to know it you have to know the color code of the resistors. If you don’t know it, or if you are working with resistors that do not show this code on their surface, such as surface-mounted ones, you should use a measuring instrument to find out their value.



Series and parallel connection

It is said that two components are connected in series when they share the current that passes through them, that is, they are on the same cable or conductor.

Serial connection

Serial connection

Instead, it is said that two components are in parallel when they are excited with the same electrical voltage, that is, their terminals are connected.

Parallel connection

Parallel connection

Carrying out the measurement:

The most practical instrument to carry out this measurement would be an ohmmeter, but if it must be done with a ammeter It should be done as follows:

Take the component to be measured and apply a known electrical voltage to it. It can be a 9V or 12V battery, but it is important to know the precise value of this applied voltage. The next step to follow is to find out the value of current that is passing through the resistor once that voltage is applied. To do this, the ammeter in series with the resistor and battery, as shown in the picture.

Measuring a resistance with an ammeter

Measuring a resistance with an ammeter

What happened here?

Applying a voltage to the resistor generated an electric current that flows in the direction established by the power supply. As the ammeter It has almost zero internal resistance, the voltage drop that it is generating in the circuit could be neglected, thus considering that the entire battery voltage falls on the resistance.

Returning to Ohm’s Law, the resistance value can be calculated as the battery voltage over the current that the ammeter is measuring. For example, if we apply a voltage of 12V (conventional battery) and the ammeter is reading 12 mA, then the resistance value is 1kΩ, or 1000 Ω.


Something very important to take into account is that the component to be measured must not be soldered to any plate, that is, it must not be connected to any electrical circuit. If you want to measure a resistance that is connected to other components, it must be desoldered to isolate it from any type of component that may be modifying its intrinsic value. After the measurement has been carried out, the component can be soldered again, taking care not to damage any track or component on the board.

Safety aspects – working with voltage

Whenever you are going to work with voltage, you must be very careful and put certain safety criteria into practice, to avoid damaging any component of the circuit or, in the worst case, suffering a burn to the body (in case of working with large voltage values).

Although the procedure outlined in this text works with a very small voltage, it must be avoided that with the two ends of the ammeter make a short circuit, that is, they both touch the same source terminal. This could demand a current from the source that it cannot deliver and therefore burn it if it is not protected. You could also burn the ammeter, but in most cases they have an internal protection fuse. In case you have had a problem of this type and the instrument stop working, open it and check the status of the protection fuse.

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