combining 9V guitar pedals with a 12V lead acid battery

Discussion in 'The Projects Forum' started by Roob, Aug 30, 2015.

  1. Roob

    Thread Starter Member

    Mar 21, 2015
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    I want to power 9V DC guitar pedals using a 12V lead acid battery and would like some advice on this. I guess that I need to know the impedance of the pedals at the power input in order to make an accurate potential divider. I am concerned that if the power input impedance of the pedals is low then I will need a low value resistor shorting the positive battery terminal to the negative and this will then drain the battery quickly and heat the resistor. Any suggestions welcomed. Thanks
     
  2. MikeML

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    Oct 2, 2009
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    Most efficient way; use a step-down switch mode converter, avail on ebay for a few bucks., however the switch noise may get into your audio.

    Second best way; use a linear regulator like a LM317. Not likely to create any audible noise. Likely good enough with respect to efficiency.

    Third best way, use a 3.6V Zener to drop some voltage.

    Absolute worst way; using resistors...
     
    Last edited: Aug 30, 2015
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  3. blocco a spirale

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    Last edited: Aug 30, 2015
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  4. Roob

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    Mar 21, 2015
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    Excellent, thanks
     
  5. ian field

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    Oct 27, 2012
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    AFAIK: you can get a 7809 variant of the popular 3 terminal regulator, its probably worth having one per pedal even though most pedals draw less than the 1A rating - look at the manufacturers appnotes for tips on 3-terminal regulator decoupling capacitors.

    One thing you may have to watch out for - if you have any germanium transistor Fuzz-face type pedals, they probably ground the positive to the signal jack sleeve. Anything with silicon transistors or ICs probably grounds the negative to the signal jack sleeve. If you mix the 2 types on a single source of power, you're going to get a short circuit through the cable screens.
     
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  6. Roob

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    Mar 21, 2015
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    Is the fuse to protect from battery surges? I didn't think they did that. Am I protecting the pedals or the regulators? Should I choose something that can handle the current from all the pedals max draw or put one before each pedal? Thanks in advance
     
  7. blocco a spirale

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    The fuse is to protect the battery, and everything connected to the battery, from the damage that a short-circuit would cause. Lead-acid batteries can deliver very large currents that will melt wires.

    A single regulator will be sufficient for all the pedals, there isn't much to be gained by having one per pedal.

    The regulator should be bolted to a heatsink or metal chassis.
     
  8. Roob

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    Mar 21, 2015
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    Great. Got it. Thanks
     
  9. ian field

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    Just out of curiosity - how many pedals do you intend to power?

    anything that runs off a 9V PP3 battery, doesn't draw much current - but a lot of them in parallel might!

    Individual regulators needn't be such an extravagance - you can get 78Lxx devices in a TO92 package that are small enough to wire directly to the power jack inside the pedal.

    Even if all the pedals together aren't enough to overwhelm a TO220 cased regulator, each pedal will have a supply decoupling capacitor - the sum of all these will look like a short circuit for a tiny fraction of a second at switch on. The 3 terminal regulators have comprehensive safety features, but its generally better not to tempt fate.
     
  10. Roob

    Thread Starter Member

    Mar 21, 2015
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    I currently have 5 pedals, some quite hungry, 200mv is the largest I think. Potentially I could draw an 1A as I will also power a 9v battery amplifier.

    What can I do to avoid the problem that you mention of large currents whilst charging the decoupling capacitors? Does having more regulators help? Would I likely blow a 1A fuse at switch on do you think?
     
  11. ian field

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    As I said; the 3 terminal regulators have comprehensive safety features, these include current limiting and thermal shutdown, its just sensible practice not to run them outside their limits just because you can.

    The regulator requires local decoupling capacitors, and it copes with charging those at switch on - but add up all the capacitors in all the pedals and it can be a lot.

    With 5 pedals now, you may be close to the limit for 1 TO220 style regulator, if you add more later that could be a problem.
     
  12. Roob

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    Mar 21, 2015
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    Thanks for your help.

    I built a box that takes the 13.6V lead acid into a 1A fuse and into 2 voltage regulators (TI LM2940T 10V, 1A 11.5-26V input low dropout) each one has two outputs. It works. However, a strange thing: one of the pedals, a 200mA digital looper pedal, makes a rythmic background noise when recording (probably draws more current when recording?). It has been suggested that this sound can occur when under powered. Maybe a bit of an obscure question but if anyone has any thoughts...
     
  13. blocco a spirale

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    If you think the looper may be under-powered; check the input voltage is correct and/or try running it from a dedicated regulator.
     
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  14. MikeML

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    I warned you about switcher noise back in post #2. Likely, harmonics from the switcher are beating with some sample rate or bias oscillator...
     
  15. blocco a spirale

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    But, the LM2940T is a linear regulator.
     
  16. Roob

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    Mar 21, 2015
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    I was just double checking that myself....
     
  17. MikeML

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    Whoops.
     
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