Forumvibe Powersupply help

[pk1802]

Hey guys,

I am a long time reader, first time poster. I am getting ready to build a Forumvibe, and I am having a little bit of trouble with the logistics of a power supply. I picked up a 24vac wall wart at the local Salvation Army. It was super cheap and I figured I could somehow use it to power my Forumvibe build. I don't know much about converting or rectifying AC power to DC power, but I would like to learn. I would also like to try and keep the DC output above 18v or so. I have looked around on the forums and online, but most of the information I found is for 110+vac.

So, my question is, how do I get 18-24v dc from a 24vac wallwart?

I figured someone here would have the answer to my question.

Thanks in advance,
pk1802

[Jack5]

I built a forumvibe power supply using one of these kits:

http://www.jaycar.com.au/productView.asp?ID=KC5446&CATID=25&form=CAT&SUBCATID=588

It uses a lm317 series adjustable voltage regulator. I think the circuit is straight from the datasheet.   My DC output is set to around 15v IIRC.  I used a lower AC transformer too, 16v.

[R.G.]

I am a long time reader, first time poster. I am getting ready to build a Forumvibe, and I am having a little bit of trouble with the logistics of a power supply. I picked up a 24vac wall wart at the local Salvation Army. It was super cheap and I figured I could somehow use it to power my Forumvibe build. I don't know much about converting or rectifying AC power to DC power, but I would like to learn. I would also like to try and keep the DC output above 18v or so. I have looked around on the forums and online, but most of the information I found is for 110+vac.

So, my question is, how do I get 18-24v dc from a 24vac wallwart?

I figured someone here would have the answer to my question.

It's in general a good idea to go read Geofex - all of it. In this case, the info you want is in the article "Power Supplies Basics".

Your 24Vac wall wart will probably make over 35Vdc at low loads. You'll have to regulate that down. The Neovibe clone of the univibe as sold by GGG contains the rectifiers, filters, and a regulator on the board to make suitable power out of an AC voltage feed.

[pk1802]

It's in general a good idea to go read Geofex - all of it. In this case, the info you want is in the article "Power Supplies Basics".

Your 24Vac wall wart will probably make over 35Vdc at low loads. You'll have to regulate that down. The Neovibe clone of the univibe as sold by GGG contains the rectifiers, filters, and a regulator on the board to make suitable power out of an AC voltage feed.

Thanks for the quick response R.G.

I've read Geofex through a few times, but I guess I wasn't interested in Power supplies at that point. I will get right on that and report back if I need any more help.

Would it be easy to take the rectifiers, filters, and regulator from the neovibe and apply that to the Forumvibe, maybe on a piece of perfboard? Since I have already etched my Forumvibe PCB.

[pk1802]

I don't mean to double post, but I believe I have the Power Supply figured out.

Correct me if I am wrong. I can use the 24vac wall wart that I have and use a Diode Bridge, a 7815, as well as 2 1000uF 35v caps and 1 .1uF cap(for filtering I would assume).

R.G. mentioned something about the 24vac wall wart putting out more than 24vac at a low load. Would the 7815 take care of this problem?

[John Lyons]

Yes, the 7815 is a voltage regulator. The voltage after the regulator will be.....(wait for for it).....regulated to 15v and held stable
at that voltage no matter the load put on it (within reason).

[R.G.]

I don't mean to double post, but I believe I have the Power Supply figured out.

Correct me if I am wrong. I can use the 24vac wall wart that I have and use a Diode Bridge, a 7815, as well as 2 1000uF 35v caps and 1 .1uF cap(for filtering I would assume).

R.G. mentioned something about the 24vac wall wart putting out more than 24vac at a low load. Would the 7815 take care of this problem?

Small transformers have proportionately higher resistive losses in their copper wire, because the small core demands more turns for the same AC primary voltage. Their output sags proportionately more than bigger transformers.

A 24V/4A (call it 100 Volt-amps) may have an output of 24V at 4A, but with no load, when the resistance losses don't count, its output may be 25.2V. The manufacturer winds it to be 25.2V open circuit so that when it's fully loaded, the voltage sags down to the specified 24V at 4A.

However if a transformer was rated at 24V/100ma, it may be (and probably is) designed to have a no-load voltage of maybe 27V so that when it's fully loaded, it will sag down to 24V.

So a small 24V transformer may be really 27-28V, which means it provides 27*1.414 = 30.8V. And if the AC line runs 10% high, that could get to 33.8v. So yes, a 7815 is probably OK. The 7815 dies if the incoming voltage is more than 40V. This is the kind of thing you always have to check. A 7815 will also only dissipate about 2W if it's not on a heat sink of some kind. With 32.4Vdc in (that's 33.8V minus 1.4V for rectifier diodes), the voltage across the 7815 will be 32.4-15 =17.4V. So the max current it can let through without overheating is 2W/17.4V = 115ma. Yeah, a 7815 will do it, but it will get hot and probably ought to have a heat sink.

The lamp driver also runs from the unregulated voltage in the Neovibe. I haven't looked at the "Forum Vibe" recently, so it may. If so, it will be dissipating a lot because the lamp and driver will be running from quite a high voltage too.

It would be a lot better to find a transformer that would do 18-20Vac at 100ma if you can. Much less near the edge on several things.

[pk1802]

R.G. thanks for all your help. I think I'm starting to get a handle on how power supplies work. I'm am going to follow your advice, and make a different battle plan. I do have a 9vdc power supply on my pedal board, would a step up circuit be the better option, 9vdc to around 18vdc maybe?

I will take a look around and see if I can find any step up circuits.

[R.G.]

R.G. thanks for all your help. I think I'm starting to get a handle on how power supplies work. I'm am going to follow your advice, and make a different battle plan. I do have a 9vdc power supply on my pedal board, would a step up circuit be the better option, 9vdc to around 18vdc maybe?

It's more complicated, but it does work, according to reports here.

I will take a look around and see if I can find any step up circuits.

Oddly enough, there is an article at Geofex on how to step up 9V with a charge pump converter IC. 

[pk1802]

Oddly enough, there is an article at Geofex on how to step up 9V with a charge pump converter IC. 

I've already found it.

Thanks so much R.G.

[PRR]

> a small 24V transformer may be really 27-28V, which means it provides 27*1.414 = 30.8V.

Pentium bug? Finger-slip?

Slide-rule says about 38.2V, significantly off from "30.8".



Forteen-buck calculator has more digits: 27*1.414... = 38.18...V.

I have been spending too much time with scanner/camera to be sure of my result. (At one point I annotated the rule "3.82V", and didn't notice!)

And remember that I once wired a "400V" supply and it was headed for _800V_ and threatening to emit some very costly smoke.



> trouble with the logistics of a power supply.

1) Think (and read) BEFORE you throw money. True, it wasn't much money this time. And maybe you CAN do something with 38V. But it was a side-track from where you wanted to be.

2) The difference between "30.8V" and "38.2V" is the 35V MAXimum rating on most general-use regulators. Do not trust anybody else to do your sums for you. Is it like 31V, or like 38V, or are both of us wrong? Likewise as you shift to a very different plan, never 100% believe what you read or are told until you check it yourself.

[pk1802]

> trouble with the logistics of a power supply.

1) Think (and read) BEFORE you throw money. True, it wasn't much money this time. And maybe you CAN do something with 38V. But it was a side-track from where you wanted to be.

A lesson that I have no doubt learned. I won't be doing that again.

2) The difference between "30.8V" and "38.2V" is the 35V MAXimum rating on most general-use regulators. Do not trust anybody else to do your sums for you. Is it like 31V, or like 38V, or are both of us wrong? Likewise as you shift to a very different plan, never 100% believe what you read or are told until you check it yourself.

Very true, thank you PRR. If I can believe what you say.

[R.G.]

Pentium bug? Finger-slip? Slide-rule says about 38.2V, significantly off from "30.8".

Something about you have to both press the correct buttons on the calculator AND not be in a hurry enough not to notice that the numbers looked funny.

Yep, 38.2V. Subtract out 1.4 For rectifier drop and you get 36.8. Yep, the 7815 can only take 35V max, so whether it survives in this application depends on whether or not the high-line condition happens, and how much.

The 7815 is even more marginal, bordering on dead on contact. Find a plug-in transformer with 15Vac to 20Vac to run it. When I need something like this, I usually go check the bins at a Goodwill store. They often have a tub of unloved wall warts of all kinds and ratings.