LEDS on heater supply?

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untune

Well-known member
Joined
Oct 28, 2016
Messages
228
Location
Manchester, UK
Hi all,

I've searched around but found various conflicting info so Im wondering if anyone can help with this particular application.

I have a small Sifam VU meter, the newer kinds with the 'retro' look, which also has 12v LED illumination. I've had to do some modification to get it to sit into the frame of an old VU meter to go in a preamp - I nearly lopped the two wires off the back to get them out of the way but instead cut a channel for them to sit into as I didn't originally intend to use them.  Last night, since I thought I had damaged the wires, I quickly connected it up to a 9v battery to check if they were still working and they lit up nicely - and I actually like the look.

So I'd like to hook them up but the problem is that in terms of power, all I really have to work with is a spare 5.7v, 300mA tap on the transformer that was originally for a preamp valve heater (I moved it to a 6.3v with some others)

Would it be possible to utilise that tap to power the LEDs or am I just going to run into issues? I have come across various sources saying I can run LEDS on the AC, and some saying it needs to be rectified, but the difference in voltage makes me wonder whether it would work at all. Has anyone done it before?

Cheers!
 
You could take a parallel from the heater supply and rectify it with a diode bridge and then apply a resistor to get proper voltage for the diode.  Personally I have tapped my DC heater supply to do this on various occasions.

Best regards

/John
 
You should be OK with just a series resistor to limit the current and possibly a reverse diode to make sure the reverse breakdown of the LED is not exceeded.

Cheers

Ian
 
Thankyou both; Ian - I had seen that solution elsewhere and it seemed the simplest option.

I'll give it a go - my worry was killing or reducing the lifespan of the LEDs. Unfortunately the sticker with the LED info on the meter has come off, so it's guesswork - some snooping has revealed that it's marked 12v, 20mA and has a 330R 1/8W resistor between the two surface mount LEDS inside the meter case.

I'm not sure what the voltage drop on those surface mount LEDs is but some more snooping suggests approx. 2.6v, which gives a CLR of 27R. Assuming 2v gives a CLR of 100R. I'll experiment, cheers
 

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