On Saturday, 24 September 2016 19:52:17 UTC+1, The Natural Philosopher wrote:
On 24/09/16 19:02, tabbypurr wrote:
On Saturday, 24 September 2016 18:18:37 UTC+1, The Natural Philosopher wrote:
On 24/09/16 18:04, Andrew Gabriel wrote:
In article ,
Scott writes:

I've got 12 volt halogen spots run from a torriodal transformer - five
units from each transformer. Can I replace these with LCDs or would
this cause problems with under-loading the transformer? I have read
something about having to have one conventional lamp in the circuit
but I don't fancy this as I would rather have them all the same.

Electronic transformers have a minimum load, but a toroidal
transformer probably doesn't. The voltage will be higher, but
an LED probably has a switched-mode PSU inside which will
compensate for this (although there are some cheap ones which
just use a resistor).

Actually the cheap way to drop LED voltage for mains uses is a series
capacitor. Then FW bridge rect and if you want smoothing caps, and put
the LED chips in series.

for e.g a 7W LED you need 30mA or about 7.5k impedance at 50hz.

About 0.47uF.

need to add a resistor too

No, you don't.
Especially with a smoothing cap.

Without one the only Rs in circuit are the dropper cap, the diodes and the reservoir cap. The LEDs would then be hit with brief massive overpower at times, and the current limit of the dropper & diodes exceeded. You can build it like that if you want, but it's not a smart choice.


NT