How Do I Use a Voltmeter to Troubleshoot My Alarm System?

You can use a voltmeter to troubleshoot your alarm system by checking to make sure the panel is getting enough battery and transformer power. A voltmeter can really come in handy if your system is experiencing reoccurring AC loss or low-battery conditions that you cannot seem to clear.


A voltmeter, or multimeter, is a device used to measure electrical power. These devices are frequently used with alarm systems to make sure the system is getting enough power. A lack of power could result in various trouble conditions, most notably AC loss and low-battery. Although you can often fix these problems without a voltmeter through trial and error, using a voltmeter provides a more accurate diagnosis of the problem.

First, it is important to understand how a voltmeter works. The typical voltmeter will have multiple options for the power it can measure. Some of the most common options you will see on a voltmeter are for AC, DC, Amps, and Ohms. For an alarm system, you will typically be working with AC power and DC power. The AC option is usually designated by a capital V with a ~ symbol next to it or on top of it. The DC option is usually designated by a capital V next to a symbol consisting of a solid line on top of a dotted line. That symbol looks like this: .

When choosing the power option on a voltmeter with ranges, you want to select the next scale up from where the voltage should be. For example, if you are measuring for a transformer rated at 9VDC, 2.5A, then you might select the 20 Volts DC option. This is because 20 Volts DC is the next highest option for what you would expect to see in the reading. Remember to always scale up, never scale down. The red lead should be connected to the port for the type of power you are measuring. The black lead should be connected to the COM port. You can see an example in the following picture. Note that the black lead on the left voltmeter is connected to the incorrect port.


Remember that polarity matters when working with DC power. If you are measuring DC voltage, you must put the leads from the voltmeter on the correct terminals. Black is used for negative (-), and red is used for positive (+). Sometimes the terminals for the DC device you are measuring will be color-coded. In that case, you can often just match the colors. If not, just remember that black goes to negative (-), and red goes to positive (+). AC power does not observe polarity, so you can use either lead at either terminal.

Before you begin using a voltmeter to check your system's battery and transformer power, you should understand why the transformer and battery specifications are important. The supplied transformer power will vary between different systems. You should check the specifications for the transformer to see what kind of power is being supplied. The purpose of an AC transformer is to simply restrict the amount of power that is supplied to a device so that the device is not overloaded with too much current. A standard power outlet in the United States provides around 110 volts of AC power. If you are ever in doubt that an outlet is working, you can use a voltmeter to check. You should expect the reading to be between 110 Volts AC and 122 Volts AC. Meanwhile, a DC transformer is not only restricting power, but also converting the AC power from the outlet to DC power. Never use an AC transformer on a DC powered device, or vice-versa.

If you have a transformer rated at 16.5VAC, 25VA, then the transformer is actually bringing down the supplied voltage to a little higher than 16.5VAC. On this transformer, you would expect to see a voltage reading of somewhere between 17 Volts AC and 20 Volts AC. If you get a reading of 16.5 Volts AC or less, and the outlet is good, then it is possible that you have a bad transformer, though further testing is required to make this determination. If you are curious about the other power specification for that transformer, the 25VA, that is the power rating. This determines how large of a load the transformer can support.

When determining what transformer to use with a device, you want to make sure that the VAC or VDC specification matches, and that the required powered rating for the device is equal to or less than what the transformer supplies. If you have a device that needs 16.5VAC, 25VA of power, then you can use the transformer we mentioned earlier. But you could actually also use a transformer rated at 16.5VAC, 40VA if you wanted to. This is because the VAC specification matches, and the power rating for the transformer is greater than the power requirement for the device. This is good to know if you ever need to get a new transformer for your security system.

As for batteries, you will see specifications that measure Volts (V) and Amps (Ah). For example, you might see a battery with the specifications 12V and 4.5Ah. When determining what battery to use, you want the Volt measurement to match exactly, and you want the Amp rating for the battery to be equal to or greater than what the device requires. If you have a device that needs 12V and 4.5Ah of power, then you could use the battery we mentioned before. But you could also use a battery rated at 12V and 7Ah. This is because the Volt reading matches, and the Amp reading for the battery is greater than what the device requires. Don't go crazy though. Most devices also have a maximum recommended Amp-Hour rating for the battery. Do not exceed this limit.

Once you understand the panel and battery specifications for your system, you can begin using a voltmeter to troubleshoot your system. To do this, complete the following steps:

1. Check transformer voltage. Making sure your panel is getting enough power from the transformer is extremely important. Your panel not only uses transformer power as its main power source, but it also uses this power to charge the backup battery. Your panel needs enough transformer power to not only keep itself running, but to also share some with the backup battery. Even if your panel is getting enough power to prevent an AC loss trouble condition, it might not be getting enough power to charge the backup battery. This can explain why you might get a reoccurring low-battery trouble condition.

We will assume that the outlet is supplying the proper 110VAC to 122VAC of power. Check the specifications for your transformer. Set your voltmeter to the proper AC or DC power setting. If your voltmeter uses ranges, set it to the next scale up. For example, on a transformer rated at 16.5VAC, you might select the 200 Volts AC option on the voltmeter. When taking the reading, you should expect to see a result a little bit higher than what the transformer is rated. 17VAC to 20VAC is an expected reading here.

You can take the reading by putting the leads for the voltmeter against the ends of the wire that is connected to the transformer. Make sure the transformer is plugged in when you do this. If you are getting a low reading, then it's possible that the wire might be bad. You can rule this out by checking the reading at the transformer screw terminals. Again, the transformer has to be plugged in when you do this. If you are having trouble accessing the screw terminals while the transformer is plugged in, then try doing this with the transformer plugged into a power strip. If you get the same low reading, then the transformer is bad.

If you get a good reading at the ends of the wire, then it indicates that the wire and transformer and power outlet are all good. You need to make sure the wires are connected with the panel properly. If they are, then the cause could be either a bad battery or a bad panel. How you will proceed here depends on what type of panel you have.

If you have a wireless panel, and you are getting an "AC loss" trouble, then you should check to see if the battery is stealing too much power from the transformer. If the battery takes too much power, then you might get an AC loss condition because the panel is not getting enough power for itself. This is sometimes called a "parasitic current draw". You should try checking the reading without the battery connected. If it improves, then it is a bad battery. If not, then something is wrong with the panel. Remember to make sure the transformer cable is securely connected with the panel.

If you have a wireless panel, and you are getting a "low battery" trouble, then try giving the backup battery some time to charge, or replace it with a new one. It will take about 24 hours of continuous running on transformer power for a backup battery to charge before you can clear the trouble condition. Keep in mind that you must acknowledge the trouble at the panel before the condition will clear. Remember that most backup batteries last about three (3) to five (5) years. If it has been that long since the last replacement, then a dead battery is the likely problem. If a brand-new battery doesn't charge, then there is probably something wrong with the charging circuit, and you will most likely need to replace the panel. Please note that there are sometimes new batteries that are faulty, though this is quite rare. You might want to try a second battery before you give up and assume that the panel is bad.

If you have a hardwired panel, then continue to Step 2, as you need to do further testing to determine if the cause is a bad panel or a bad battery.

2. Check auxiliary power. Auxiliary power refers to the power that your panel uses to power other devices. This is commonplace for hardwired systems, where various sensors and other devices get their power from the panel. Wireless systems do not rely on auxiliary power as much, as wireless sensors have their own power supply. You really only need to check auxiliary power on a hardwired system.

Your hardwired panel should have some terminals that provide auxiliary power. For example, on a VISTA P-Series System, these are terminals 4 and 5. Auxiliary power is measured in Volts DC, so set your voltmeter as though you were measuring the power for your backup battery. Also remember to scale up if necessary. If your backup battery is rated at 12V, 7Ah, then you would set the voltmeter to 20 Volts DC.

Put the black lead on the negative (-) auxiliary power output. For a VISTA P-Series System, this is terminal 4. Put the red lead on the positive auxiliary power output. For a P-Series System, this is terminal 5. You should see a reading that is slightly higher than the volt rating of your battery. If your battery is rated at 12V, 7Ah, then a reading of 13 Volts DC or higher is fine. Continue on to Step 3 if you get a good reading here.

3. Check battery voltage. Now measure the battery itself. You can keep the voltmeter on the same setting that you used when checking auxiliary power. Put the black voltmeter lead on the black battery tab (negative). Put the red voltmeter lead on the red battery tab (positive). The battery should be disconnected when doing this. You should get about the same reading as the auxiliary power reading. If this voltage is low, below 13 Volts DC, the problem could be the charging circuit. Continue to step 4.

4. Check the charging circuit. With the battery still disconnected, connect the black voltmeter lead on or inside the connector for the black battery wire. Connect the red voltmeter lead in the same manner to the connector for the red battery wire. This voltage should be almost exactly the same as the auxiliary power voltage metered in Step 2 above.

If incoming panel power is good, but charging voltage is low or nonexistent then proceed. Power the panel completely down by unplugging the transformer. The battery should already be disconnected. Remove the panel from its metal enclosure, and place it upon a non-conductive surface, such as some cardboard. Power the panel back on with AC power only, without the battery connected. Then check the leads on the panel where the battery would connect. If you still get a low or nonexistent reading, then the charging circuit is bad. You will need to replace the panel in that case.

If you get a good reading here, and all of your previous readings have been good, then you likely have a bad battery. The battery should charge after 24 hours with continuous incoming transformer power available. This will allow you to clear the trouble. If it doesn't, and you've given the system ample time for the battery to charge, you need to replace the battery with a new one. Most batteries last between three (3) and five (5) years depending upon usage. If it has been that long since your last replacement, then it is probably time to get a new one.

If after all this you are still having problems, then you should note all your readings and contact your alarm monitoring company for additional assistance. Alarm Grid offers free technical support for all of its monitored customers. If you are monitored by Alarm Grid, we recommend contacting us at support@alarmgrid.com for help troubleshooting your panel.

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