Can someone explain these AC rectification types?

[MikeH]

I want to rectify the low voltage secondary on a PT into DC to Light LEDs and run a fan and maybe some other DC stuff.  I've seen both of these styles of rectification before, can someone explain the differences and pros/cons of the two?



I've seen variations on the second type with multiple diodes on each side as well.  Do different types yield different DC voltages and or wattage etc?

[gmoon]

Hi, Mike.

The top drawing isn't quite right. You can't ground the PT center tap and use a bridge in that fashion... Disconnect the center tap and it's OK.

The main difference between the two is the transformer itself. The full-wave setup on the bottom requires a transformer with +- windings...

[rustypinto]

Yea, the top drawing (4-diodes, or bridge rectifier) def. isn't right. Remove the ground symbol from the center tap for a single, positive, unregulated DC voltage. The only reason to ground the center tap in that case is if you use the negative terminal (shown wired to ground) of the bridge to make a negative polarity voltage.

With the correction of that drawing, both would be full-wave rectifiers, the only difference being that one can accommodate a center tapped power transformer.

[Nasse]

http://koti.mbnet.fi/huhtama/ele/index.php?si=ml11.sis&pa=

that page has some nice info...if you can read it... try translator like google or babelfish... but nice pics anyway

[MikeH]

Ah gotcha.  Ok so I should provide a little more info.  This is in a Matchless Hotbox.  I'm also using the secondary to run my heaters and pilot lamp, so imagine some heaters and a lamp across the outer windings.  I believe the center tap has to be grounded.  And then I want to rectify that to light LEDs.

I realize I could make the whole secondary rectified to DC and run my heaters that way, but that would require a lot of filtering and regulation and I'm working in an already existing design here, and space is at a premium.

So- I have a center tap, and it is grounded.  That part of my crude drawing is accurate.  Will the bottom scenario work for me?

[rustypinto]

Yes, bottom drawing.

Running the heaters and LEDs from the same secondary is noise prone, but if its all you have it will work (i've done it plenty of times, you just need lots of filtering).

[MikeH]

Yes, bottom drawing.

Running the heaters and LEDs from the same secondary is noise prone, but if its all you have it will work (i've done it plenty of times, you just need lots of filtering).

Like DC to ground filtering?  Will a 1000uf electrolyte do the trick?

[gmoon]

Depends on the transformer. AC filaments secondaries often have the center tap grounded for hum reduction, but aren't setup for center-tapped full wave rectification.

I.E., for example, instead of +-6.3V they are 6.3V total, side-to-side. The center tap is in the middle. Using a center-tapped fullwave setup (two diodes) will only get you ~half the voltage (the usual RMS calculations apply.)

[rustypinto]

I'm generally talking about having only one secondary (not multiple secondaries) to run the tube plates, lamp, any LEDs, and filaments.

But if this is a hotbox you should have two different secondaries. 1000uF is usually enough DC filtering (it helps if you can see how much ripple you are minimizing with a scope). In my humble experience, running fils on a well regulated DC supply minimizes noise.

[MikeH]

Thanks guys-  After a little more research I think these 2 scenarios will work:

(with the necessary limiting resistor in series)



Any comments/thoughts on which one will be less noisy/most effective?

[rustypinto]

I would go with the second scenario still. You can run into potential issues with the top drawing still if you have anything else referenced to that ground. I'm not sure what your trying to do still (building a hotbox, modding a hotbox, using the hotbox for something else?), but the bottom circuit is fairly safe concerning common ground references.

[gmoon]

If you're using the same transformer for each drawing, the results aren't equal. The first drawing utilizes the full side-to-side voltage of the secondary, and creates a ground reference in the middle.

The second is 1/2 the side-to-side voltage, with a ground ref at the lowest V. With a 6.3V transformer (not a +-6.3V), you'd get ~3+V.

2nd pic--but wired as AC, (without the diodes) then the ground ref is in the middle... this is the "normal" way to wire filaments.

Knowing the secondary voltages would help out...

[MikeH]

I would go with the second scenario still. You can run into potential issues with the top drawing still if you have anything else referenced to that ground. I'm not sure what your trying to do still (building a hotbox, modding a hotbox, using the hotbox for something else?), but the bottom circuit is fairly safe concerning common ground references.

Basically I'm building a hotbox (about 90% done) and after I've got it working, I want to see about adding some status LEDs, and a fan if needed.

It's weird; I've built tube amps and effects pedals (and I consider this one way more like building a tube amp), but I've never really combined elements of both.  Something so simple and trivial as lighting an LED in a pedal, becomes a big 'how the hell do I do that' in a tube design.

[MikeH]

The transformer secondary is 6 volts.  I don't know much about it; didn't come with a data sheet, but supposedly it is leftover stock from when matchless still made hotboxes.

Basically, I don't care how much voltage I get out of the rectification, as long as it's enough to power a couple LEDs.

2nd pic--but wired as AC, (without the diodes) then the ground ref is in the middle... this is the "normal" way to wire filaments.

Knowing the secondary voltages would help out...

The filaments aren't in my drawing, but they will be wired as typical in a tube amp, with pins 4 and 5 on side, pin 9 on the other; you know, the common setup.

[gmoon]

I gotcha. The filaments are still AC.

Then the second drawing should work. Probably better, in fact, for LEDs alone.

For that matter, with the right current-limiting resistor, you could power the LEDs with the AC--kinda like the xmas lights you plug into the wall. They only work for half the cycle, at best...but you won't find anything simpler...

(On second though, LEDs directly off the AC might be noisy...  )