power supply questions

[Marcos - Munky]

I need to build a power supply to use in my pedalboard. I bought a 12V 500ma transformer with 127V and 220V primary. Can I use this transformer? I need a power supply that can power as many effects as possible with this transformer. What I have in my pedalboard that need the power supply: Boss BF-2 Flanger, BSIAB2, wah/ge fuzz, Digitech Metal Master, Tim Escobedo's Pushme Pullyou, MXR Zakk Wylde, Jack Orman's Q&D Compressor II and Robovox (the one in my site). Can a power supply with this transformer power all of these effects, or if not, which ones can I power? And I never built a power supply before, could somebody draw me a power supply schematic using this transformer?

[gez]

Can a power supply with this transformer power all of these effects, or if not, which ones can I power?

If you have a DMM you can check the current consumption for each effect.  It's then a simple matter of adding it all up with a calculator to see what the total is.  Then you'll have your answer!  

I think there might be something in the FAQ about how to do this.  Or the archives.  If you can't find it just say and any number of people here will be able to walk you through it.

[cab42]

I just did the search.

Puretube posted a link to a list of power consumption for a range of commercial boxes

http://diystompboxes.com/sboxforum/viewtopic.php?t=30317&start=0&postdays=0&postorder=asc&highlight=&sid=68bc0887b23bff04987f3b1e63edcabd

Regards

Carsten

[Paul Perry (Frostwave)]

The transformer looks big enough to me, but 12v X1.4 (for peak volts) =16.8, so when you run it thru the 9v regulator, there is (16.8 - 9) drop across the reg = 7.8v, times 500ma gives almost 4 watts.
So your main problem, is going to be heatsinking the regulator.

[Marcos - Munky]

Thanks for the link. About the regulator, what about to use a diode bridge or maybe a few resistors to drop the voltage before use the regulator?

[Paul Perry (Frostwave)]

The trouble with using a resistor to drop voltage, is that it gives you  a 'less regulted' supply, meaning that if you change the load (by using more or fewer pedals) the voltage might go too low or too high respectively. Resistor dropping is only good where the load is constant, or the voltage doesn't matter much.

[bassmeister]

A short story about heat sinking; I built a robot controller board a while back. The motors ran on 24 V and the controller board ran on 5 V. The board (no motors running) took ~ 220 mA alone, which meant that the voltage regulator dissipated (24-5)*0,22 ≈ 4,2 W. At first I didn't think about it and didn't use a heat sink, but I noticed that the regulator was a bit hot when accidently touching it... . That was the start of learning more about the relationship between voltage regulation, current consumption and power dissipation. Well... P = U x I. Most regulators today has thermal overload protection, but they will not last as long if they work close to the limit. The same goes for probably all electronic components.

Your transformer should work just fine. Check out this Geofex article for more information http://www.geofex.com/Article_Folders/Power-supplies/powersup.htm. As an information rectifying diodes usually have a higher Vf (forward conducting voltage), but you will have a high enough secondary voltage anyway. Don't use your ordinary 1N4148 diodes for rectifying. They can't handle the current in this application.

I'm not familiar with all of the above effects, so can you specify which voltages you need, i.e. only + 9 VDC or ± 9 VDC or anything else? How many secondary windings does your transformer have? Parallelling two secondary windings doubles available current, and putting two in series doubles the voltage.

Also, don't forget safety earth!

[Marcos - Munky]

How I can measure how much mA does each effect need? So I can measure them and post here. I will need only +9VDC, but I think it will be good if I have a ±9VDC, it's not necessary, just to have a power supply if I build something that needs bipolar power supply. About the windings of my transformer, here is the only info that I found:

500mA Transformer

12VAC 330mA 110/220VAC (this is strange, the title says 500mA and the info says 330mA)

Measurements: A=54 mm, B=43 mm, C=36 mm, D=38 mm, E=64 mm ( what's this???)

http://www.planetsite.com.br/shop/produto_consulta.asp?PlanetSite=145&Produto=5147

[gez]

How I can measure how much mA does each effect need?

From here (I did say  :wink: ):

http://www.diystompboxes.com/cnews/FAQ.html


"How can I measure how much current my pedal is drawing?

From F Peña:
You have to interrupt the circuit measure current.

Do this:
Set your DMM on DC/mA.
Plug only the - side of the battery to the battery snap.
On the + contact of the battery, touch the red probe and on the free
contact of the battery snap touch the black one.
You should get the reading on the meter this way."

I usually use crock clips to connect everything.  Set your meter for the highest current setting and scale down accordingly.

[cab42]

One thing I did not found in my search. How do I measure how much current my power supply is able to supply?

I build a power supply kit many years ago, and I no idea how much current it supplys.

I read a beginner electronics text book recently and I recall something about measuring current in series with the circuit. Is this correct? Does that mean I have to break the circuit in order to measure? And how do I do that in practice?

Regards and thanks in advance

Carsten

[Nasse]

I think your transformer voltage and power rating is very ok for powering quite a many regular stompboxes, of course if you have plenty of them connected same time  or some consumes very much current it might too much.


Check your multimeter (if you have one) and it´s mA or A measurement range. And measuring current is just plain Ohm´s law, and the meter is connected in series with the power supply. If you forget the probes in current consumption sockets, and the meter setting to mA range, and try to check some voltage, don´t blame me, when the blue smoke comes out of your meter... If you are lucky, your multimeter has a fuse. I almost blew two meter learning this. Well the other was broken but I could fix it...

A transformer is quite safe way to drop high voltage but there are some safety rules. Like strong enough construction so your transformer and  hv wires don´t touch anything or get loose if or when the box is dropped or impacted, and enough space between them and other components and metal case, and some protective insulation, remembering it might rain some day or some damp nights and so on...

Both the primary and the secondary is preferably protected with a fuse. I think we should check some most useful and popular links to diy power supply construction. I think if you are in school or university learning electronics, they sometimes forget the practical side and are in a hurry to make you a scientist, or the safety is discussed later...

It is very easy to make bipolar regulated supply with a transformer with a single output winding, feeds few opamps or something easily, I think puretube has posted the link few times, and I once did a simple vero layout for 78xx and 79xx supply with cheap wall-wart, not the best but who cares if it works...

[Nasse]

One thing I did not found in my search. How do I measure how much current my power supply is able to supply?

I build a power supply kit many years ago, and I no idea how much current it supplys.

Just take more and more current out of your supply, and when the blue smoke comes out, back off the trim pot a little, and that is how much it is capable of.

Sorry, I could not resist...

Maybe you can quess the max current in the datasheet if you used a voltage stabiliser chip or a power transistor, and if you know your transformer...

[Marcos - Munky]

How can I know if my transformer have a single output winding? And it's output is AC, so I need to convert it to DC, right? How I do this?

[bassmeister]

How can I know if my transformer have a single output winding?

How many pins or cables comes out of it? You say in the first post that it's 127 or 220 V primary. To me, that sounds strange. Usually the transformer has either one single winding (two wires or pins) for one single voltage (115 or 230), or two windings (four wires or pins) so that you can choose if you want to connect the two windings in serial (for 230) or in parallell (for 115). Which country that uses 127 V, I don't know... Is it a typo? I'll try to be more clear tomorrow, right now I'm going out to drink beer  :wink:

And it's output is AC, so I need to convert it to DC, right? How I do this?

I don't think you read R.G. Keen's article "Power Supply Basics for Effects" in the link I posted if you ask this.

[Marcos - Munky]

Sorry, I didn't had time to read the whole article. But I printed it to read it later. Here in Brazil we have some places that use 110V (this is what the people say, it's really something near 120V) and some places that use 220V (something near 230V). My transformer have 3 wires in the primary side, a 0V, a 127V and a 220V. The secondary side have two red wires, don't have the central wire.

[bassmeister]

OK. I'd say just connect the 0 to one side of the power inlet. If you want to be able to select between 115 or 230 primary, you could have an SPDT switch on the primary side with the second power pin connected to the middle and each of the transformer primary windings to the outer ends of the switch. On the secondary side, I'd go for the solution on the bottom of the page in the link I posted. Choose a large capacitor (say 2200 uF) before the regulator for your + 9V rail and a smaller one for the - 9V rail since you only wanted - 9V for possible future use. A smaller capacitor means you can draw less current before ripple gets through.

Hope it turns out well.