Low Voltage Transformer Troubleshooting (2023)

This article explains how to trouble shoot and repair an outdoor lighting system. Outdoor lighting provides illumination to draw attention and attraction, to homes, buildings and premises, for cosmetic and security benefits.

Check Power To Transformer

Most transformers plugs into an outlet, usually 120 AC. Some transformers are wired directly from the wall mount.
Verify the transformer is plugged or wired to a working power source of correct voltage.
If you don't have any electrical sensor, plug a light or power tool into the outlet. Make sure it works.
If the transformer has an ON/OFF switch, turn it on. (you may need to come back to this later to make sure it works. Switches get dirty, corroded, loose wires etc.)
Low Voltage Transformer Troubleshooting (1)

If the transformer uses a photocell, timer, switch.
Photo cell, timer, switch failure is a common cause of transformer malfunction.
You should have an option to turn the transformer on manually. For now select manual operation, which means not to use photo cell or timer.
If no photocell, timer or switches;
Verify AC power to primary winding of transformer (primary winding is where plug wires connect to transformer), then go to step two.
This means you need to open up the cover, access the wire that feeds the transformer.
Note: At any time along the way, if you are apprehensive, a new transformer is the way to go. You get better design, clean contacts, and save time.
Verify that the Photocell, Timer, Switch Operate Properly

What we are looking for is switches in the circuit pass AC power to the primary winding of the transformer when they are supposed to.

If you do not have a photocell but switch or timer, wiring will be nearly the same. You will be looking to see AC power to the primary winding of the transformer.

  1. See if power is to the photocell. Check the wiring to and from from the photocell. Most transformers in place today get power to the primary winding through the photocell. This will be 120v AC.
  2. 2 wires attached to the photocell are power in and power out. The third wire is the ground. Disconnect the wires. Remove it for testing. Pay attention to condition of wires.
  3. Cover over the lens on the photocell with black tape. Photo cell lens Must be dark. A photocell is a light sensitive switch, and can take seconds to minutes to switch on.
  4. A multimeter can be used to ohm out the two power wires. The power wires will show no resistance when the photocell lens is taped over. The power wires will show High resistance when the photocell lens is NOT taped over.
  5. If the photocell is exposed to enough light at night, street lights etc. The black tape will reveal this.
With a multimeter you can ohm around the switches. UN-PLUG FIRST! If hard wired, MAKE CERTAIN POWER IS OFF!
Verify power supply is off to transformer. Non Contact Tester is handy here.
Photocell, Timer, Switch, are all switches.

If you are not able to ohm the circuit, You can jumper the power supply around the switches.

Transformers can also use a fuse and or a reset (circuit breaker) switch.
You can open any transformer and measure where voltage is or is not; and debug the circuit. All most never will the coils (the heavy part you can't fix) be the problem. In wet conditions with older installs, you may be better off buying a new transformer.

A Non Contact Tester saves lots of time.
You will still need a multimeter To accurately measure voltage, or ohm out a circuit, like a photocell.

Verify AC power to the primary winding of the transformer before moving to step 2.

Check Power To Transformer
Check Power Out From Transformer
Get Power From Secondary To Landscape Wire
About Wire Connections

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(Video) Low Voltage Electronic Transformer Troubleshooting Guide by Total Transformers

Check Power Out From Transformer
If you have power to the transformer

  1. Verify power on the secondary side of the transformer. Looking for 12+ DC volts here.
  2. Use a voltage tester and check for power on the landscape wire. 12 - 15 volts DC is typical and what to look for.

If power is not detected on the landscape wire;

  • Open the transformer for access to where the landscape wire is attached to the transformer output terminals.
  • Check the connections. Check where the landscape wire connects to the transformer. Make sure the wire is strong and clean. If connections are old and or feeble, disconnect the landscape wire, cut off to wire looks new. Then strip enough to make a strong connection and re connect.
  • Check for power anywhere on the secondary side of the transformer. This will be where wires from the transformer are connected to the output block where the landscape wire connects. You want to check for voltage at the transformer itself. Not farther down a wire. Could be a broken connection here.

Low Voltage Transformer Troubleshooting (2)

If you DO NOT HAVE power at the secondary side of the transformer;

  • Look for a Reset Switch or Circuit Breaker
  • Look for a Fuse

If you still cannot detect power on the secondary side of the transformer, consider a new transformer.

If you consider a new modern transformer;

Also consider it is a good time to switch to LEDs. Most older fixtures can be equipped with retrofit LED bulbs. 1/5th wattage over halogen. There is an LED retrofit for almost all popular fixture and bulb types.

If you have power out the secondary side of the transformer, follow wires to the wire tap block.
When the wire tap block gets 12+ DC volts, we know the transformer works.

Verify DC power to the secondary side of the transformer before moving to step 3.

Newer Transformers like the one below might pay for themselves in the long run. Prices are right, parts are available. Usually just the photocell. The Malibu, replacement parts (photocells) are available. This model the photocell just plugs in. Another feature with this model transformer, the photocell is on a long cord. This allows the option mounting the transformer in locations where an attached photocell would not provide the desired results for dusk and dawn. The photocell on a cord, allows better placement for light detection.
Low Voltage Transformer Troubleshooting (3)Low Voltage Transformer Troubleshooting (4)
Malibu 120 Watt Power Pack with Sensor and Weather Shield for Low Voltage Landscape Lighting Spotlight Outdoor Transformer 120V Input 12V Output 8100-9120-01Low Voltage Transformer Troubleshooting (5)

(Video) How to check your outside low voltage lighting transformer

Check Power To Transformer
Check Power Out From Transformer
Get Power From Secondary To Landscape Wire
About Wire Connections

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Get Power From Secondary To Landscape Wire

If you DO HAVE DC POWER on the secondary side of the transformer and no power to the landscape wire;

  • Circuit to landscape wire connection block may be fused.
  • Check wiring from secondary to landscape wire block.
  • Some higher end transformers feature timer and photocell on secondary side multi tap. Check the Photo Cell is working, may be fused also.

Verify DC power landscape wire block. 12+ volts is what to look for.

When DC 12+ volts is verified on the output block and wire, check for voltage at the nearest light. Keep in mind bulbs and fixtures fail. Most common problem is poor wire connections.

Verify DC power on the output block and wire before moving to step 4.

Check Power To Transformer
Check Power Out From Transformer
Get Power From Secondary To Landscape Wire
About Wire Connections

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(Video) Finding Low Voltage Shorts Fast! HVAC Troubleshooting!

About Wire Connections
The quick connectors which accompany most outdoor low voltage fixtures are designed such that moisture gets in the wire. First thing we do on a repair is remove the connectors, strip the wire back 1 1/2 inches. Twist a solid connection and use outdoor wire nuts. If the wire is in an area where connections are challenged, then take time to tape it well. You will be surprised how a really good connections can affect bulb life. Even LEDs.

Wire Connections Make the Difference
Landscape light wire sits in moisture. Quick connects do not last. The connection area receives moisture, connection becomes poor. The new mechanical wire strippers allow the following connection be easy and fast. If it seems like over kill; at least you know your connections are strong and will last.

Low Voltage Transformer Troubleshooting (6)

Cutting off the crimp connector.

This connector came on the fixture. It uses piercing the landscape wire to hit the coper braid in the wire. it relies on a thin tooth to enter the wire where the copper connector is. Overall a connection is made.

The hand strip and hand twist is many times more robust and reliable.

Strip twist, and tape or water proof wire nut connectors. Best to strip at least 1 1/2 inch. Twist the wires neat and tight as possible. Tape real well. With wire nuts, snip the twisted copper to length.

New style wire stripper makes the work of stripping heavy wire easier.
The unit works well on larger wire, but presents issues when attempting to strip stranded wire 22 gauge and smaller.
Most fixtures connection wire is a little thin for this size stripper.
Low Voltage Transformer Troubleshooting (7)Low Voltage Transformer Troubleshooting (8)
IRWIN VISE-GRIP 2078300 Self-Adjusting Wire Stripper, 8"Low Voltage Transformer Troubleshooting (9)

Low Voltage Transformer Troubleshooting (10)

(Video) Landscape lighting - what most people get WRONG about landscape lighting transformers

Landscape wire is stranded

Twist wire clockwise.

Twist individually before connecting with another wire.

Twisted together, this make for a tough connection. Wire laid out in garden areas gets some abuse from general landscape maintenance.

Using wire nuts, snip twisted connection to length for wire nut. Use the hand stripped, hand twisted connections for a robust install. Wire nuts don't twist the wire properly by them self. The procedure shown here will save lots of maintenance time in future.

Low Voltage Transformer Troubleshooting (11)

Snip the twisted pair to length for wire nuts.

Best light output from your installation of outdoor lighting, depends on strong and reliable connections.

Low Voltage Transformer Troubleshooting (12)
Your low voltage landscape wire lays in the ground, where moisture collects.
Far better than the quick connects; better connection that will hold up over time.

Low Voltage Transformer Troubleshooting (13)Low Voltage Transformer Troubleshooting (14)
King Innovation 62225 DryConn Outdoor Electrical Wire Connector 20/Bag, Aqua/RedLow Voltage Transformer Troubleshooting (15)

(Video) How to troubleshoot the transformer

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What causes low voltage on a transformer? ›

Old wires and corrosion are two of the common causes of low voltage. Another reason can be dirty connections and weak insulation. Apart from that, if the population increases the demand increases and this leaves an impact on the distribution of voltage.

What happens if you overload a low voltage transformer? ›

If you exceed the max amps the fuse will blow before the transformer, but don't panic, you can easily buy these fuses at home improvement stores, just make sure you buy the same size that came with the transformer and insure that it is a slow burst fuse. Slow burst fuses have a coil around the outside.

Why would a transformer stop working? ›

Transformer windings can go open circuit for various reasons. The transformer may have a thermal fuse in the primary to stop current if the transformer gets too hot. This not the case if you have a connection through the primary. Corrosion can affect either winding.

How do you check if a transformer is working? ›

To test your transformer, simply touch the red and black pins of your ohmmeter to the opposite ends of the transformer's wiring. Read the display and compare the resistance on your ohmmeter to the resistance stated on the transformer's data sheet. This is sometimes listed on the transformer's enclosure.

How many ohms should a transformer read? ›

Look for a reading of somewhere between one and about 10 ohms. If any winding reads higher than 10 ohms you have probably found a bad transformer. Unless you didn't get a good connection to the coil leads with your test leads.

How do you tell if a transformer is going bad? ›

Symptoms of power quality issues include vibration, excessive buzzing or humming and overheating. Technicians should occasionally check the power of transformers that supply nonlinear loads, such as variable frequency drives (VFDs) or switching power supplies.

Can a bad transformer cause low voltage? ›

From my understanding, a bad transformer may have high internal resistance. This causes a lower output voltage than approx. 120V.

Should a transformer have continuity? ›

Transformer Continuity - YouTube

How many lights can you put on a low voltage transformer? ›

There's no absolute maximum limit on how many lights you can add to a low voltage transformer. You can add as many as you like, providing the total Wattage doesn't exceed 80% of the capacity of the transformer. Use this table as a guide – the lights will vary.

How long do low voltage transformers last? ›

How long do low voltage transformers last? 20 to 25 years. They operate at about 90 to 95 percent efficiency and can run for a very long time, anywhere from 20 to 25 years.

Do low voltage transformers have fuses? ›

To protect the LX from ground surges caused by lightning or damaged connections, there is a buss fuse contained in the top of the transformer. This fuse does not protect the incoming voltage, just the low voltage side of the transformer.

How do you test a low voltage lighting transformer? ›

To test the voltage delivered to a light, remove the bulb and apply the multimeter's leads to the inside of the bulb's socket. The optimal voltage for this type of outdoor lighting is usually between 12 and 10.8 volts. Test the current at the lead going into the fixture.

How do you tell if a transformer is shorted? ›

A suspected short can be checked with an ohmmeter across the secondary winding with the source and load disconnected. If there is a short, the meter will read 0 Ω. Again the meter should be on a low scale.

How do I test a transformer with a multimeter? ›

To do this, first set your meter to read AC voltage (not DC). Then connect the black lead from your meter to one of the output terminals on the transformer and use an alligator clip or other similar device to connect it securely. Next, touch both probes together and set your meter to read resistance (ohms).

How do you test a 24v transformer with a multimeter? ›

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