Rectifying Heater Voltage and Voltage Doubler?

[g1]

Dropping 50 or 60 Volts from a bias winding is not practical.

yes, if you have a bias winding, you can rectify it for positive alongside the negative bias supply. You will then have something like 50v. So you would need a much larger resistor. But it will work.

I don't think he's talking about a bias winding, he's tapping off the secondary HV.

I've decided to abandon using the heaters and wired up a half wave rectifier using the secondary AC, like some bias supplies, only positive.

[Enzo]

Bias windings are secondaries too, but I get your point. Tapping off the B+ winding makes it even more of a drag.

[J M Fahey]

Just reread this section, where you already answered some of our doubts:

I have 4 yellow LED's in parallel, each with an 820 ohm series resistor. I'm getting about 8.7 VDC with a full wave rectifier (two diodes) unloaded. They aren't bright enough. Powering them with 18 volts on a battery does the trick. I thought raising the voltage was the way to go. Guess I'll try lowering the resistors.

1) if 18V does the trick, then current through each is: (18-2)/820=20mA and that's the important parameter
2) get higher efficiency Leds, you'll have more light and be able to lower current to, say, 10mA ......... win win
3) even with what you have, you can lower resistors to (6.5-2)V/0.02A=220 ohms each and total current consumption of 80 mA
4) even better , use 2 LEDs in series plus (6.5-4)V/0.02A=120 ohms per string, for a total current draw of 40mA

Always with proper full bridge rectification, across the full 6.3V winding.

[Daver]

OK. Thanks for all the input. I'll give the floating bridge a go and report back.

[Daver]

So I wired up the floating full wave bridge. I'm only getting 3VDC even with no load. It's got to be a problem with the artificial center tap, right. This shouldn't be this hard. I've included a drawing so you know what I'm doing right or wrong.

[Enzo]

Two drawings, one with filter cap and one without. Does your made circuit include the filter cap or not. It needs it to get the voltage up.

[Tom Phillips]

Daver,

The problem seems to be that you have some of the diodes in your bridge installed backwards. Look up bridge rectifier circuit diagrams and compare them to your drawing. All should be well after you fix that and, as Enzo said, make sure the capacitor is connected properly.

You seem to be hung up about the artificial center tap. It is not a factor if you use a floating bridge circuit as described earlier in this thread. Don't worry about it any longer. Also know that artificial center tap circuits are used only for ground reference applications where there is little or no current flowing. Artificial center tap circuits are not useful as part of power producing circuits.

Tom

[Daver]

It has the cap. I couldn't remove my first drawing for some reason. Tom, I don't see that my diodes are in backwards????? Looks like a regular full wave bridge to me. Could you point out the problem more specifically? Thanks.

[Tom Phillips]

It has the cap. I couldn't remove my first drawing for some reason. Tom, I don't see that my diodes are in backwards????? Looks like a regular full wave bridge to me. Could you point out the problem more specifically? Thanks.

For some reason I cannot paste in the figure. Check out the drawings of bridge rectifiers in the tutorial at http://www.electronics-tutorials.ws/...e23.gif?81223b


Edit: My bad. I was looking at a different view of a schematic when I said that you had diodes in backwards. I agree that they are drawn correctly. The circuit should work as other members have described. So now we need to find a wiring mistake or a bad component or ??

[Daver]

Tom, I still don't see how mine is different. What am I missing?

[Tom Phillips]

Tom, I still don't see how mine is different. What am I missing?

See my edit in post #24. I agree that your drawing is correct. It's time to troubleshoot the actual circuit that you built because, for some reason, it is not working the way it should. The problem could be wiring, parts and maybe even test instruments. If you can't find the problem then consider building a standalone breadboard version of the circuit outside the amp and get that working. Then compare to your existing build.

Fun with circuits,
Tom

[g1]

Also, double check that minus end of bridge is not connected to ground anywhere, and that you are measuring the output with a floating multimeter.

[Daver]

The "-" isn't grounded. I have -3VDC there. So far the wiring checks out. 6.4 VAC across the two bridge AC connections, but 3VDC out. What is a "floating" multimeter? I'm using a Fluke 189 True RMS meter. Thanks!

[g1]

A floating multimeter does not connect to ground in any way. If you were measuring with a scope, the ground lead would force the - of the bridge to ground.
So you need to measure from the - to + of the bridge, not from either to ground.

[Daver]

D'oh! I've. Been measuring to ground. I know better. This has been a horrible weekend for many reasons. My head is not on straight. I will re-check tomorrow. Thanks.