Replacing Low Voltage Halogen Lamps

 I puzzled over changing my campervan halogen lamps after reading descriptions of replacement LED lamps. It just wasn't clear whether the new types were AC, DC or suitable for both. It was only when I read the box of one type on which I'd gambled some cash and, I must admit, after having looked at an enlarged view of a lamp on CPCs website (and spotting some chips) that I now understood their characteristics.

In fact the usual type (shown below) includes a full wave bridge designed to power a DC regulator suitable for the LEDs fitted in the lamp. It so happens that a full wave bridge has a rather useful feature.. being able to provide a guaranteed positive and negative connection to the internal DC regulator. This means an AC LED lamp can be connected either way round in a campervan lamp socket. This is obviously not the case where a "DC" LED lamp is plugged in.


 I'm guessing about the 9 volt regulator but, it's a fair assumption to make given say 2 volts forward drop per LED and a test using my variable output PSU.. with a current limiter! The LEDs suddenly assumed full brightness after something a little greater than 9 volts input was noted.


 Above is a picture of the smaller of the two LED lamps I purchased. You'll note the claim about the physical height. In fact this is entirely misleading because its the way the base is fitted that's the important feature. If the halogen lamp is fitted hanging downwards then the replacement LED lamp will occupy more space. If the lamp is fitted horizontally then there's a fair chance the extra height will not matter. In my case I unscrewed the holders and twisted them to be horizontal and with the LEDs pointing to the sides. In two other light fittings the halogens were mounted at 45 degrees in a horizontal position making the new LED lamps a plug-in swap.

The rating for this lamp is given as 12V 2watt. A quick check revealed it drew 105mA at precisely 12VDC which represents 1.26 watts so somewhat under the expected value. It replaces 10 watt G4 halogens.

Below is a reflector type of halogen bulb, a type GU4.


 And here is a suitable LED replacement. The rating on the box is 2.4watt at 12V. I powered it from 12VDC and it drew exactly 194mA giving a power input of 12 x 0.194 = 2.328 watts. This replaces my 15 watt GU4 halogens.


 I'm unsure of the meaning of the various claims. For example the quoted brightness of 200 lumens.

As far as instructions are concerned there's a warning on the larger type that a transformer must be used ie. the lamp must not be plugged directly into a mains supply. A second warning states that if a low voltage halogen power supply is used then the LED lamp won't work properly. I can surmise that this is because a mains transformer is normally specified to provide a given output voltage at a specified output current. Disconnect say 10 halogen lamps each rated at 12VAC and 20 watts that consume 200 watts represents over 16 Amps. Such a transformer might have an open circuit output voltage of say 18 volts. Fit 10 LED replacements drawing say 1.2 Amps and you'll be applying maybe 17 volts across the LEDs. The specifications remain quiet on the max-min voltages but I'd imagine overheating and either sporadically cutting out or just going bang would be the result.

No real problem using a 12 volt battery. Maybe 14 volts might be its maximum if it's being charged and this should be within a decent replacement LED spec.

In my campervan the halogen lamp consumption = 90 watts or 7.5 Amps

Replacement LEDs = 12.2 watts or 0.98 Amps, representing 13% of the original maximum drain.

Read customer comments on the Amazon website for example for any low voltage LED lamp and it's pretty clear that there are real problems in the marketplace.

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