Quick Q about CT transformers

[lordvader]

Hey all.

I'm building an amp, and have decided to add a small transformer to power an LED (don't want the LED to tap into the amps power supply).

However, the small transformers are usually center tapped (the one I'm looking at is 6.3 - 0 - 6.3). Since I only want 6.3V, is it OK to have the other 6.3V "flying" ?

[Geek]

Hi,

Since it's to power a LED, no problem!

But a seperate transformer for one LED is kind of silly because....

Originally posted by lordvader
(don't want the LED to tap into the amps power supply).

You can use a superbright LED, that way you only need to draw 300-500uA from the supply to get the same brightness as a standard LED @ 10mA

Cheers!

[lordvader]

True, but I didn't like the idea of a massive power resistor to drop 50V down to 3V ...

I'll work something out
(not at the build stage yet, and I think the board I'm using may have an LED tap anyway ...)

Thanks

[TheMG]

Massive resistor?

Not really. Say your LED needs 10mA at 3 volts, a 1/2W 4.7K resistor will do just fine.

Even though you're "wasting" 1/2W of power, that's probably less than that extra transformer would use. In the grand scheme of things, half a watt is nothing unless it's battery powered.

[zix]

Feel free to correct me if I am wrong, but: the LED will always drop its designed voltage (3V for instance), as long as the supply current is high enough for it to light up. So the resistor is used mainly to lower the current coming in to the LED.
Geek and TheMG, who obviously knows all of this already, gives you what you need, the current draw from the LED
The needed supply current from the superbright LED: 200-500uA. If I got it right, (U/R=I) 50V/147k≈340uA, enough for the LED to light up.
With a 147k resistance in the circuit, calculating the power drawn from the supply (U*I=P), the LED would draw 50V*340uA=0,017W (17 mW).
You could use a 1/4 W resistor without a problem.

As I said, if I am wrong just correct me. I am just playing around with ohms law here.

[jacco vermeulen]

Option 3: add a winding to the main transformer.

[scottw]

I'm building an amp, and have decided to add a small transformer to power an LED

I did the same thing once, even added a pot so I could adjust to the desired brightness. The idea was to keep any of the LED's 'noise' out of the amps PS. But in reality it's not likely I could have heard the difference.


Scott

[kvk]

That ever happened to those little neon indicator bulbs used in old stereo amps? About the side of a miniature Xmas light with two little bars inside, glowed pale yellow when on. I think they could run off straight 120VAC (or close to it).

[Geek]

Hi,

Originally posted by lordvader
True, but I didn't like the idea of a massive power resistor to drop 50V down to 3V ...

Doesn't need to be

For example, in my tube work, I use a 470K, 1/2 watt resistor from the B+ of 250V for 500uA to a LED (if I don't use the heater winding). Works like a charm and only dissipates roughly 1/8 watt.

Originally posted by kvk
That ever happened to those little neon indicator bulbs used in old stereo amps? About the side of a miniature Xmas light with two little bars inside, glowed pale yellow when on. I think they could run off straight 120VAC (or close to it).

Neons are still plentiful. Go wonky after a while though, same as a Christmas light.

Cheers!

[TheMG]

Originally posted by Geek
Neons are still plentiful. Go wonky after a while though, same as a Christmas light.

Well the neons can actually last an insanely long time. Just like an LED you need to limit the amount of current going into it. The more current you put through it the shorter it's usable life will be.

Unfortunately I don't really have any numbers as to exactly how much current you should put through one.

The quality of the neon lamp can come into play as well. I've come across a few that had slowly leaked in air over time (for those that may not know, the neon gas in a neon lamp/tube is at a very low pressure, lower than atmospheric).

So yeah, with a quality lamp and well calculated current limiting resistor, you're in business and it will last years on end.