Using LEDs

[Canetoad]

This may be a bit of a silly question but can an LED run off AC voltage?

I have a toroidal transformer (12v) I want to put in a case with power LED to replace the standard SMPS PSU used on my CA 640P phono stage. I want to get rid of the last SMPS PSU in my system.

[bobbasrah]

This may be a bit of a silly question but can an LED run off AC voltage?

I have a toroidal transformer (12v) I want to put in a case with power LED to replace the standard SMPS PSU used on my CA 640P phono stage. I want to get rid of the last SMPS PSU in my system.

Hi Bernie, LED is a low voltage diode which won't like high reverse voltages, so not on it's own even through a resistor. I presume you are thinking to do this on the secondary 12v rather than 250? :eek:
It should be possible to put it across the secondary with another diode and resistor to get choppy DC, to suit it's working parameters. Usually about 3v and 20mA from memory.:rolleyes: No idea what noise you would get with that in circuit on a phono stage though....:scratch:

PS - Can you pull a small DC voltage off the phono stage someplace instead ? If it proves noisy you can remove it

[MartinT]

Bernie - SMPS don't usually output AC, so I think the Cambridge PSU must be driving the phono stage with DC. In which case you will need more than a transformer in a case; you will need rectification, smoothing and possibly regulation, too. Once you have a DC output, running the LED through a resistor is trivial!

[Canetoad]

No the PSU for the phono stage is definitely AC. So it's not an SMPS PSU then. :doh:

I just want to use a better quality supply for it. I have an 80VA 2 x 12v Tx not doing anything and thought I'd use it for the CA 640P. Apparently, a better supply does make a difference. It doesn't have to have an LED. I just would like to know if it's on or off.

... and yes, I was thinking of running it off the 12v side. ;)

[Reid Malenfant]

No the PSU for the phono stage is definitely AC. So it's not an SMPS PSU then. :doh:
It could be, there are shed loads of AC output SMPS out there, used as lighting transformers ;)

[MartinT]

In that case, you do only need the transformer in a case. Do you know what voltage AC the 640P expects?

You could run an LED in series with a protection diode and current limit resistor from the secondary AC. There would be some noise, though, so I don't really recommend it for a phono PSU.

[Reid Malenfant]

I'd recommend something like that battery pack that you just linked to in the BM thread Martin :eyebrows:

It's obvious that the phono stage must have some form of rectification if fed an AC supply, so feeding it DC won't make one jot of difference, except to the noise floor :idea:

[MartinT]

Yes indeed, Mark. We still don't know what voltage the phono stage needs, though.

[Mark Grant]

Looking here:

http://www.cambridgeaudio.com/summary.php?PID=29

it needs 12V AC 0.5 amps.

http://www.cambridgeaudio.com/gallery/rear/640p-rear.jpg

Take the cover off and post a detailed picture of the PCB and Reid Malenfant Mark will have some ideas to mod it :)

[NRG]

This may be a bit of a silly question but can an LED run off AC voltage?

I have a toroidal transformer (12v) I want to put in a case with power LED to replace the standard SMPS PSU used on my CA 640P phono stage. I want to get rid of the last SMPS PSU in my system.

Yes, but you'll need to half wave rectify the AC voltage first with a suitable diode.

Wire the diode in parallel with the LED but head to toe way around IE Cathode to Anode...

You then need a suitable current limit resistor, for 12VAC a 1K resistor will give you approx 4ma

[walpurgis]

Use LEDs to make the rectifier?

[MartinT]

You can't. Reverse voltage breakdown would destroy them.

[walpurgis]

I'm no electronics buff at all, but I remember sucessfully rebuilding the rectifier in my Quad electrostatics with diodes. Don't LEDs function similarly?

[MartinT]

No.

[Welder]

http://uk.farnell.com/marl/512-532-76/led-indicator-240vac-green/dp/9737650

[MartinT]

Perfect, John.

[NRG]

Perfect, John.


Except for the price! :doh: