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HotWired, Master Electrician
Category: Electrical
Satisfied Customers: 156
Experience:  30 years of wiring, from Air Force power plants to basement remodels and anything in-between.
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I have an outside lamp post light. It doesnt work. I took

Customer Question

I have an outside lamp post light. It doesn't work. I took it apart and there is a transformer and light sensor. How can I check to see if the bulb, sensor, or transformer is working?
Submitted: 6 years ago.
Category: Electrical
Expert:  HotWired replied 6 years ago.

HotWired :

Do you have a voltage meter?

JACUSTOMER-6nt45egm- :


HotWired :

No reading out the ohm meter for the bulb tells me the bulb is bad. were is the transformer and do you have output voltage from it?

JACUSTOMER-6nt45egm- :

The transformer is in the top of the post. I didn't check voltage yet because I didn't understand the wiring. There is a red, black, and white wire to the sensor. What is the voltage output of the transformer suppose to be and what is the purpose of the transformer?

HotWired :

take volt reading in and out of the transformer and let me know what you get. Does the red wire from the sensor go to the transformer?

JACUSTOMER-6nt45egm- :

I get 120V to the transformer. There are 4 wires to the transformer. 1 goes to the white on the bulb and sensor. 1 goes to the black on the bulb. 1 goes to the red on the sensor.1 goes to the white on the white on the main neutral. The sensor is labeled black is line, red is load, white is neutral

Expert:  Rich replied 6 years ago.

Hello .. the proper wiring for the power would be power white to the white sensor and one wire on the high side of the transformer

black power wire to the black sensor wire

red sensor wire to the other input wire , high side , on the transformer

the other two wires on the transformer, low voltage side go to the lamp holder of the post light


Any thoughts ,, concerns ??

Customer: replied 6 years ago.
The lamp worked before as wired. I just wanted to know if I had to replace the bulb, sensor, or both. Can I just hook up 120V to the light and test it? What is the transformer for?
Expert:  Rich replied 6 years ago.

I am assuming that the lamp is low voltage .. that is the purpose of a transformer .. unless it is a flourescent lamp then it would be a higher voltage

either way the lamp is not designed for 120 volts

The designed voltage should be stamped on the lamp .. sometimes on the screw shell .. sometimes on the glass of the lamp


The way to determine what part is at fault is to test with a meter .. a two wire probe meter ..

testing the input power for 120 volts ..

testing the sensor power out red / white for 120 volts when activated

testing the power out of the transformer .. ?? voltage , may be stamped on the side of the transformer

testing the lamp for continuity .. ohms .. to see that there is a reading thru the lamp


Any thoughts , concerns ??

Expert:  HotWired replied 6 years ago.
Thanks Rich for entertaining my client while I was out doing electrical work. The fixture is obviously a HID fixture most likely HPS Thats is the one that has a kind of orange glow. If this is a yard post (less than 10 Feet) I would guess the lamp to be 70 watt HPS. It sound like the Ballast or transformer is bad. This size fixture is cheaper to replace than to fix. If you can find a replacement ballast and replace it your self, you will save a little. I would just look for a new fixture to install on the post. High-Intensity Discharge lamps use a surge from the ballast to ignite or excite a gas and then normal voltage to keep the gas glowing. They give you more light with less power. These lamps have no continuity before the are excited so you can't check them with an ohm meter. I suggest replacing the whole fixture. The lamp an the ballast must be the same size and with a new fixture they will be. If you want to fix the old one I will need more info? Photo’s would give me some clues, and any numbers or names you can find!