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 plugin 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 maxmin 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. 
