How Much Current Does My Effect Draw?

[BrownPanick]

I've just recently gotten interested in building my own effects (and other general small projects).  I've built some smaller projects to start with that I've found on the Internet (stuff like a multi-voltage power supply, a small preamp, that sort of thing) but my technical knowledge on this topic is almost nil.  For my next project, though, I'd like to build a pedal board with a regulated power supply.  I was going to use a LM317T to create a 9 volt output, and from what I understand this will also output 1A guaranteed (more?), which should be more than what I need (but that is just a guess).  My question is, how do I determine how much current the entire effect is pulling?  One of my DigiTech pedals calls for a wall wart with an output of 9v/500ma, but do effects like this really use up to 500ma?  The second part of this post is regarding a transformer for my pedalboard's power supply.  I have a 12.6 VCT transformer (6.3 - 0 - 6.3), but it only outputs 300ma.  Do I need a different transformer to feed the whole thing, something that can handle more current, or does the 300ma feed the LM317T enough to convert the extra voltage to the 1A current it supposedly provides?

Sorry if this is such a remedial question.  I still have a long way to go to learn.

[JKowalski]

I've just recently gotten interested in building my own effects (and other general small projects).  I've built some smaller projects to start with that I've found on the Internet (stuff like a multi-voltage power supply, a small preamp, that sort of thing) but my technical knowledge on this topic is almost nil.  For my next project, though, I'd like to build a pedal board with a regulated power supply.  I was going to use a LM317T to create a 9 volt output, and from what I understand this will also output 1A guaranteed (more?), which should be more than what I need (but that is just a guess).  My question is, how do I determine how much current the entire effect is pulling?  One of my DigiTech pedals calls for a wall wart with an output of 9v/500ma, but do effects like this really use up to 500ma?  The second part of this post is regarding a transformer for my pedalboard's power supply.  I have a 12.6 VCT transformer (6.3 - 0 - 6.3), but it only outputs 300ma.  Do I need a different transformer to feed the whole thing, something that can handle more current, or does the 300ma feed the LM317T enough to convert the extra voltage to the 1A current it supposedly provides?

Sorry if this is such a remedial question.  I still have a long way to go to learn.

The adaptor that pedals call for will be above the actual current draw that they need. However, this is a safety margin which is a good idea to keep - in some cases. Some simple effects can draw 1-3 mA only but call for say a 50mA adaptor... You aren't gonna find a 1-3mA 9v adaptor so overkill is normal.

The best way would be to actually measure it yourself with a multimeter.

Actually, if I was in your place, I would just make a very quick educated guess looking at the pedals and make a pretty hefty overkill supply limited by size constraints from the layout of my pedalboard. If you only have maybe 1 digital effect and a medium amount of pedals, maybe 1-1.5 A. If you have alot of pedals but not many digital effects, maybe 2A. If you have alot of pedals and alot of digital effects, 3A?



The LM317 and LM780x series are LINEAR regulators. This means that the drop in voltage that they cause is lost power. It is not converted like a transformer would convert total power, it is just lost. Therefore the current rating of the transformer will be the current output of your regulator. If you want 9V 2A out, you need a transformer that supplies around 2A max no matter the voltage. When choosing a transformer, you want one that has the right current rating, and a voltage output which (when rectified, 9VAC does not result in 9VDC it results in 12.72VDC peak... factoring voltage drops from the rectifier bridge, you get a final 11.3 VDC (filtered)) results in a peak DC voltage around 2 volts higher than the output of your regulator (the regulator needs a ~2 volt margin to work correctly)

The 1A rating is the maximum current draw you should take from the LM317, since above that the regulator starts getting pretty hot from all the dissipated power.

If you wanted to save your power and convert say 20vdc 500mA to 5vdc 2A (unattainable 100% efficiency conversion) you would use a SMPS buck converter (very complicated in comparison) which would attain 75-95% efficiency. Maybe 20vdc 500mA to 5vdc 1.75A.

[BrownPanick]

I guess what I had in mind was doing something like:



   120 VAC =|)==== 12.6 VCT Transformer----Rectifier----LM317T----o  9 VDC output @ 1 - 1.5A

You don't think that something like this is a good idea?  I think I might have a 120 - 14 volt transformer that is rated for 3 amps, would the extra current capacity of the transformer be necessary?  I know the LM317 is going to generate heat, which is why I have a heat sink for it.  The whole idea is so I don't have to use batteries any more.  I don't want to use a wall wart because a) I don't like them and b) I want to do this as a project because it's there.    

[JKowalski]

I guess what I had in mind was doing something like:



  120 VAC =|)==== 12.6 VCT Transformer----Rectifier----LM317T----o  9 VDC output @ 1 - 1.5A

You don't think that something like this is a good idea?  I think I might have a 120 - 14 volt transformer that is rated for 3 amps, would the extra current capacity of the transformer be necessary?  I know the LM317 is going to generate heat, which is why I have a heat sink for it.  The whole idea is so I don't have to use batteries any more.  I don't want to use a wall wart because a) I don't like them and b) I want to do this as a project because it's there.    

Actually, you need:

120VAC -> Fuse -> Switch -> Low voltage transformer -> Bridge Rectifier -> Filter Capacitors -> LM317(with associated components) or LM7809 -> decoupling capacitor

If you are unclear with any part of if read up on power supplies some more. I believe geofex.com has some good info. Be careful! The 120VAC can kill you! Either while working with it or through some fault in construction later on. A VERY real danger. Have care in making sure all 120VAC wiring is sturdy, well sealed and identifiable.

As stated before, the linear regulator only drops voltage and therefore the max current in is the max current out. Choose a transformer with a current rating equal to the current rating you want out at 9V.

[BrownPanick]

Great, thanks.  That answers my question about the transformer rating and it's exactly what I had in mind.  I'm still a little unclear how to use my meter to measure the current draw from beginning to end of the entire effects pedal.  Do I measure the draw from ground to ... where?  The output jack?  The last transistor or ic in the circuit?

[rousejeremy]

Try this http://stinkfoot.se/archives/821

[PRR]

> I understand this will also output 1A guaranteed (more?)

No. "1A" is one of several limits which must be checked.

As a bad example: start with raw 35V DC. Drop to 9V. The (35V-9V)= 26V is wasted in the regulator. At 1A, this is 26V*1A= 26 Watts of waste heat. The '317 in tab-pack on huge heatsink at 26W will quickly get HOT and shut-down to save itself. With no heatsink, 1W is HOT and 2W will surely shut it down (though not usually fatally, like the old days).

As another (relevant) example: wire two cheap 9V batteries in series to make 18V. Regulate to 9V. Pull 1 Ampere. The poor teeny batteries will SAG BAD, probably below the 12V the '317 needs to work from, probably in seconds.

Power demand: run each box on common 9V battery. If it runs dozens of hours, it's maybe 10mA. If you have to replace every night, say 100mA. If they don't give you a batt-snap, or tell you to change batt after every set (transmitters are like this), then 200mA-400mA. Any more is too much for a 9V batt. How hot does the box get? Compare to a 3 Watt night-light in the same size box. (3W at 9V is 333mA.)

Most pedals are trivial when you use wall-power. Some pedals are pigs.

> the LM317T .... convert the extra voltage to the 1A current it supposedly provides?

No. It only wastes excess. It has no leverage.