Charge Pump question:

[JJGross]

Hi all,
I've built a power supply using the Weber pedal power transformer with dedicated outs for everything, but still have to use a wall wart for my wireless. I'd like to use one of the spare taps from the tranny to run my 12v AKG receiver. Would it be better to add a charge pump and just run that wireless on 17.whatever VDC or CP + some value of resistor to drop the voltage down to something closer to 12v? If so, what kind of value would get me in that ballpark of just under 18v source to around 12v delivered?

I know, awfully basic stuff, but I'm completely self taught so there are plenty of holes in what I know. Everyone starts somewhere ...

Cheers!

[Don Moose]

How much current does your receiver want, and does it want AC or DC?
The 11V windings are good for 300mA. How many do you have left over?

Bear in mind that a voltage doubler is a constant-power thing - you want (2*V)*A out, you gotta put in V*(2*A) (plus a bit for losses).

[JJGross]

The receiver draws 300mA @ 12vdc. I have an array of 8 regulated lines from each of the 11v taps from the transformer I intend to make available for my stompboxes. Measured voltage at the end of each line is about 10.2vdc. Each of these lines is rated at 300mA. Maybe overkill for what they all need, but I like the idea of lots of clean discreet power. That's why I bought the Weber transformer instead of a OneSpot or other wart-type solution. I'm also kinda cheap so a Power Brick is out of the question, especially since this will walk all over one of those anyway.


* note: look closely to spot the rectifiers on each line along the edge of the board *

Maybe I'm paranoid, but I don't like running things on 'almost' enough power. If I use one of the 11v taps I already have (which actually put out around 10v after the regulators) I'd think the receiver would at least distort, especially since the draw is right up there at 300mA. The receiver requires 12vdc, so running it with less volts isn't going to make it sound better and I'm not really in love with the sound of the AKG wireless as it is. It's what I have so I use it. Again I'm cheap so I won't buy a bunch of other wireless rigs to try out. I'll wait until this one dies, then invest in something good - or just go with a cord.

The other 9v tap that I haven't decided what to do with puts out a full 2A so that's the one I was thinking of for a charge pump since I'd have a lot of overhead on that line. It's the green pair coming off the tranny and not hooked up to anything yet. The wireless would be the only thing running on that tap and I'd make a separate board for it with it's own regulator & the proposed charge pump. Even with some expected current loss, there's no way I can imagine a charge pump + a regulator would eat up the current difference between the delivered 2000mA from the transformer and the 300mA draw of the wireless. That's an unused surplus of around 1700mA or almost 6x the draw on that line ... or would it?

Cheers,
- JJG

[R.G.]

The best thing to do within the limits you've set is to use the 9V winding to run a voltage doubler rectifier to get about +25Vdc, then regulate that to +12V for the wireless.

A single-chip charge pump doubler is only good for about 10ma. You'd have to use 30 of them in parallel. A discrete charge pump doubler can be made, but it would cause a lot of potential for noise if you mess up the physical locations of components and wires. Presumably getting rid of noise is why you're doing this, so this isn't a good option.

The individual windings can't produce enough current if they're only rated for 300ma; full wave rectification makes the winding "see" about 1.6 to 1.8 times the DC load.

So you make enough voltage with the 9Vac by doubling it, and feed that into a 7812 voltage regulator to make a clean +12V at up to 1A. Your wireless will only eat as much current as it wants from the correct voltage. Having more current capability is good.

Transformers have winding and can have taps. The Weber transformer has separate windings or secondaries. They aren't taps, as the word "tap" refers to a point somewhere in the middle of another full winding.

Just helping you fill in holes. I can help you find a "full wave voltage doubler" circuit if you can't find it on line to double the 9Vac.

[Don Moose]

What R.G. said.

Along with a bunch of other cool stuff, Duncan has this PSU Designer program that has a Full Wave Voltage Doubler among its topologies.

Definitely worth a download. (Windows only)

[JJGross]

Thanks for the help guys, looks like I have my homework cut out for me.

So far, the only schematics I'm finding are high voltage circuits for things like Jacobs Ladders, CRTs & Neon Ballasts. Way beyond what I'm looking for!

I'll let you know if I need you to throw something to me RG, but I'll try to find something on my own first. It ain't called DIY for nothing.

I'm finding plenty of schematics with generic layouts that don't have any values listed. Are there standard formulas to figure out what component values I should apply to these? I know things like C1 would be the first capacitor in the schematic, but that's not how I'll be looking it up on Mouser (or whatever).

Cheers!

[JJGross]

OK, what about one of these?
http://focus.ti.com/lit/ds/symlink/pt5041.pdf

Not cheap @ around $14, but it looks like this is made for what I need.
This and a couple caps & I'd be in business ... maybe?

Cheers!

[Enzo]

JJ, I could be wrong here, but I think you have a few misconceptions.

Does your receiver actually DRAW 300ma, or is that the size of supply it asks for? Get out the amp meter and insert it inline with the power connections and actually measure the draw. If it asks for a 300ma supply, it certainly is not going to draw 300ma. Is 300ma what it says on the wart that came with it? In the interest of science, take draw readings with the most LEDs lit up on the thing - each LED adds a few mas.

A "12v" power adaptor puts out 12v at full power draw. Sitting there not doing anything you might find as much as 17v coming out of it. Check ut - plug the wart into the wall, but leave the small end loose. Now measure the voltage there. Now connect it to the receiver adn measure the voltage. It will drop to something near 12v or a litle more.

My point here is that the voltage supplied by the wart is only approximate. The receiver internal circuits are run on regulated supplies. The thing takes the "12v" and knocks it down to 9 or 5 or whatever it needs inside. If you grab a 10v supply, it will still work, since the regulators inside are still there. Sure, you can go too low, but ball park is what you need. You could also run it on a 15v adaptor, but the innards would run pretty hot as the regulators try to dissipate the extra power. So for a unit marked 12v, something in the 10-14v range will suit.

Keep in mind as you add up the power needs of your stuff, that if the power adaptor for something lists some ma of current, that is what the adaptor puts out af full load, NOT what the device it powers uses. Chances are the unit under power draws half to a third of that amount. Measure each one. With pedals that run either adaptor OR battery, it is often easier to check current draw using the battery terminals - pop one of the two battery snaps off and connect an ammeter between it and the battery.

If I read you right, you are combining AC and DC imporperly. The winding on the transformer is 11VAC if I understand. COrrect me if I am wrong please. if you rectify 11 volts AC, you will get about 15 volts of DC at the filter cap. THEN it goes through a regulator which knocks it down a few more volts. The 11 volts coming in does not just sink to 10.5 or whatever. That 10v is the output of a regulator, and that regulator has several volts of headroom.

AC to DC and back can be approximated by a couple numbers based upon RMS and peak voltages. AC voltage is measure by RMS, which is a fancy way of saying average - more or less. The RMS voltage of an AC voltage is about 7/10 of the peak voltage. .7070707....... SO if you have the peak voltage, multiply by .707 and you have RMS. GOing the other way, if you have the RMS voltage, multiply by 1.414 to get peak Yes, it is really 1.4141414..... Really, in the shop, 1.4 and .7 are close enough.

If AC comes in RMS - as in 11v RMS - then why do I need peak? Because that is what the filter cap will charge up to when you rectify the AC into DC. 11 x 1.414 = 15.6. To be brutal about it, since the 11 is only two significant digits, then all our other numbers based on it can be no more accurate than that. SO we would call it 16vDC. But I am a rwevel, I usuallt compromise and use three digits. WHo cares...

SO that 11VAC winding is more than enough to make 12VDC. You even have enough headroom you can use a three leg voltage regulator liike a 7812. Looks like a couple rows of three-leggers on the board already.

From my previous math, if you just HAD to make 12VDC directly, just multiply if by .707. Gives 8.5vAC. SO a 9VAC transformer would do it. But you already have a perfectly good 11v winding.

Or have I completely misunderstood things?

[JJGross]

I just figured with a full 2A available on the only 9v winding from the transformer, it would be simple to just jack that up from 9v to 18v, rectify it & regulate it down to around 12 and be done with it.

I've already soldered up all my 300mA 11v windings to my rectifiers and regulated them down to 9v, each one for individual effects. I was hoping to not have to tear it apart to get power for my power hungry 12v wireless, but if that's what I have to do, that's what I have to do.

[Steve Conner]

Can't you just take two of the windings on your transformer and hook them in series?