Electronics problem

t_naki, Apr 21, 6:59am
Does any one know approximately what DC voltage I would get if I fit a bridge rectifier onto a 12VAC power supply.
shall, Apr 21, 7:07am
0.6 - 07 volts drop per diode,and you will have two diodes per cycle,making1.2 - 1.4 volts droppedgiving you approx as low as10.6 Volts
custom2, Apr 21, 7:07am
Its about 1.4 times the AC voltage
kamitchell, Apr 21, 7:22am
+
about 15Vdc no load.
t_naki, Apr 21, 7:34am
That's awesome I have some LED lamps that will run from 11VDC to 21VDC so that should work fine. Thanks heaps
kamitchell, Apr 21, 5:03pm
If the leds flicker a bit put a suitable cap across the dc supply. something rated 25v or more around 100uF.
t_naki, Apr 21, 5:40pm
Thanks, great advice
trade4us2, Apr 21, 6:40pm
You say LED lamps. Are they just bare LEDs or is there some circuitry with them! There should be a resistor in series with an LED to limit the current. My LEDs burnt out because of a slight overvoltage.
t_naki, Apr 21, 6:54pm
It says 11VDC - 21VDC so it should be fine with 15
gj, Apr 21, 7:47pm
An in-line resistor is needed primarily to limit the current across the LED. If you have a part number for the LEDsyou should be able to look up the characteristics online. Here's a good resource which has plenty of info plus an online calculator if you want to wire up several LEDs in an array. http://led.linear1.org/category/led-basics/
If you are using pre-built lamps they may have a resistor already fitted.
t_naki, Apr 22, 5:25am
They are lamps made up from a number of LED's. I am trying to alter existing light fittings to run on LED's but the only lamps I have found small enough are DC. I thought an easy way of converting would be to fit a bridge rectifier onto the existing transformer and pop in the new lamps.

Hopefully it works.
russ18, Apr 22, 5:58am
As you say they are ratedfor 11 - 21VDC which means they have built in regulators, bridge rectifier on a wire-wound transformer would work fine but not 100% sure about an electronic lighting transformer! and many have a minimum and maximum load e.g. 20 - 60VA

Might be better to count up your total wattage and replace the transformer with a suitably sized constant voltage LED driver.
tmenz, Apr 22, 7:50am
Several factors to consider:
Nominal unloaded voltge from transformer,
Inherent regulation of the transformer,
How much the transformer is being loaded by the LEDs (%),
What sort of rectifier is being used (Schottky, standard!),
Whether reservoir capacitors are fitted, and how big,
Actual DC voltage could be anywhere from about 11~19!
t_naki, Apr 22, 5:18pm
Funnily enough one of the transformers is a wire wound one so I am using that to feed the lights off as the electronic ones need 20W triggering load and I have 4 LED's at 1.9W each giving 7.6W total. The only thing I am really worried about is the volt drop on the cable from the tranny to light but with the large operating voltage range I am going to give it a go and if they don't work I will rig them up on a battery and solar panel and use them when we go camping.
aredwood, May 12, 4:56am
Just install a large smoothing capacitor on the output of the bridge rectifier. 1,000uF should be fine for the load you will have. (but bigger is always better) Current will be less than 700mA so I doubt that voltage drop will be an issue. Definitely don't try to use an electronic halogen transformer. They output approx 40Khz AC with the voltage varying between 0V and 12V at 100Hz. This will play havoc with anything that is not a filament lamp.

Do the LEDs need to be dimmable!