Measuring Voltage, Current and Resistance
Clamp multimeters use the Hall effect to measure current.
When you’re installing, commissioning or trouble shooting electronic security systems, measuring voltage, current and resistance are at the heart of your skillset.
Before we look at testing, let’s consider the electrical properties of voltage, current and resistance. Voltage is electrical pressure that occurs between 2 points and is also called potential difference. In essence this potential difference is the result of one point having more electrons than another. An atom with more electrons than protons has a negative charge, while an atom with more protons than electrons has a positive charge.
Think of voltage as the pressure that forces electrons to flow. This flow of electrons is called current. It’s measured in amperes (6.25 x 10 to the power of 18 electrons flowing past a point per second). In a basic circuit when voltage (a power source) is connected to a circuit, current will flow from the negative terminal down wiring to a light bulb, through the bulb generating light, then out of the bulb and into the positive terminal of the battery and around again.
There’s a quirk with current. Our definition of conventional current flow is exactly the opposite of the path electrons take – not long ago no one knew electrons existed and by the time scientists worked things out, it was considered too late to go back and reverse conventional positive to negative thinking. The 2 important things to realise are firstly that there is a flow of electrons and secondly that all equipment, circuit symbols and the rest, register what’s known as conventional current travelling from positive to negative. Regardless, you should remove the negative wire first and connect it last.
Now let’s look at resistance. Think of a hose turned to full pressure. The water pressure is voltage and current the water flow. Now bend the hose tightly so that despite the original pressure (voltage) and water flow (current), only a small amount of water trickles from the nozzle. The bend is resistance. In an electric circuit any opposition to the passage of electricity is called resistance, with the overall resistance of a circuit determined by its components and the resistance of its conductors and connections.
Voltage Measurement Options
Now we’ve got the fundamentals covered it’s time to get our measuring tools out. These will be an ammeter, voltmeter and ohmmeter, or a digital multimeter combining all these testing devices in one. In some ways the latter is the more complex measuring tool because it incorporates multiple setting and display options.
Let’s start with the easiest test of all, voltage. What’s nice about voltage is that because it’s always between 2 points in a functioning circuit, your voltmeter/DMM simply needs to be placed across the points voltage should be present. As a rule, one of these points will be the circuit’s common rail and you’ll measure voltage from that point. A lot of alarm and access control panels have the negative side of the source wired to the common rail, but it’s not always the case.
If you’re measuring a voltage drop across the positive and negative terminals of a panel, or across any component that you believe may be the cause of a voltage drop, there are variations and vagaries depending on the problem and the overall system design. When testing you can think of a voltage drop as loss of pressure caused by too much constriction – too much resistance. You’ll discover this point of higher resistance because the voltage will be higher upstream of the component than downstream. You’ll calculate the voltage drop by subtracting the lesser voltage from the greater.
You can also measure voltage drop using Ohms Law, with the voltage drop equalling current x resistance. Putting your voltmeter across a component’s resistor will allow you to measure any drop in voltage directly. Just remember to put the positive side of the voltmeter on the positive side of the resistor and the negative side downstream to ensure the voltmeter shows the proper polarity for a digital meter (upscale). In the event that current isn’t flowing, things are going to be a bit more complicated – if the switch in an electrical circuit is open, testing either side of a component will reveal battery voltage.
Current and Resistance
When measuring current you actually need to get into the wiring by cutting or breaching in order to insert the test device into the circuit. In short, current must go into the ammeter or DMM at the positive lead and go out of the negative – further, the current leaving the test device must be virtually identical to the current that went in – resistance should be limited to less than 1 ohm per Amp of current.
When using an analogue multimeter connect the probes and set the meter switch to current, ensuring you are testing the right range while giving some headroom for unexpected variations. It’s better the meter be set too high as a low setting might damage the test device. You can adjust range downwards later to ensure maximum deflection for more accurate measurements.
When using a digital multimeter turn on the device, connect the probes – black to common and red to current. Next set the selection switch to measure for current in high or low ranges – for maximum accuracy tweak range to ensure none of the first couple of digits reads 0.
Hall effect clamp meters are another option – most incorporate a DMM, though the pro versions are expensive, and the affordable versions suffer measurement drift. A Hall effect meter can measure AC and DC flowing in a conductor. The meter works because as current flows through a conductor, the iron jaws of the meter form a core that facilitates easier passage of the conductor’s magnetic field than surrounding air. When the magnetic field reaches the air gap at the tip of the jaw it must jump the gap, allowing the Hall Effect sensor to measure a voltage proportional to the magnetic flux in the core, which it converts into a current reading. If you use a Hall Effect meter, be sure to zero it before making a measurement.
If you have no current meter at all you can also use a series resistor to make a calculation – you place a small resistor into the circuit with an end at ground potential to avoid ground shorts during the test. Next measure the voltage across the resistor – if it’s a 10Ω resistor and 100mV is measured then you can calculate current is V/R = 0.1/10 = 10mA using Ohms Law. Such a measurement will not be perfectly accurate but if your measurement can tolerate variation, it will get you out of trouble.
Resistance is measured with an ohmmeter – a unit that is essentially a meter complete with an inbuilt battery and circuitry. When you’re using an ohmmeter remember that you need to ensure the resistor being measured has at least one end disconnected from the circuit and you must only touch one resistor lead when taking a measurement. If the resistor remains connected to other parts of the circuit it will impact on the reading you get from your ohmmeter, while a voltage in the circuit could damage the instrument.
Another issue is that an installer testing resistance also contains an electrical charge and can inadvertently connect themselves across the resistor. In this case the ohmmeter will measure the resistance of the circuit and the installer’s body. The reading from the body will be a parallel path, resulting in a lower reading than would otherwise be the case.
Definitions to remember:
* Current is the flow of electrons
* Voltage is pressure or potential difference
* Resistance is anything that opposes electron flow
* Ohm’s Law states that Current equals Voltage x Resistance
* Atoms with more electrons than protons are negative
* Atoms with more protons than electrons are positive.