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Old 9th April 2013, 06:46 PM   #1
Nicho is offline Nicho  United Kingdom
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Default Regulating battery to 9V

Hi,

I'm looking to make a rechargable battery pack that can power a few 9V effects pedals. I was thinking of using four of these LiFePo4 batteries to get a nominal 13.2V and regulating it down to 9V. Can anyone advise me of the best way to do this in order to achieve a nice smooth output voltage, while minimising additional current draw?
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Old 9th April 2013, 07:11 PM   #2
JMFahey is offline JMFahey  Argentina
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If voltage comes from batteries, it is already nice and smooth.
Use 3 packs instead of 4 for nice nominal 9.9V.
Your pedals will be happy.
If for any reason you *insist* on 9V (your pedals will not complain, by the way), you can add a series diode to lower it to nominal 9.2V , or even 9V at higher current consumption.
Zero extra current consumption , which will not happen with any other regulating system.
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Old 9th April 2013, 08:33 PM   #3
Nicho is offline Nicho  United Kingdom
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Quote:
Originally Posted by JMFahey View Post
If voltage comes from batteries, it is already nice and smooth.
I wanted to keep it that way. I suppose any regulator will do this will it?

Quote:
Originally Posted by JMFahey View Post
Use 3 packs instead of 4 for nice nominal 9.9V. Your pedals will be happy.
This was my first thought. Presumably the sound will be altered slightly? But I thought if they sounded better with 9.9V, great, if not I could adjust appropriate parameters to repair the damage. But while the batteries spend most of the time around 3.3V, I would probably operate them from about 3.6V fully charged to 3V cutoff, giving a 1.8V charge dependent voltage variation, which I wasn't sure would be ideal - or wouldn't it be worth bothering about?

Quote:
Originally Posted by JMFahey View Post
If for any reason you *insist* on 9V (your pedals will not complain, by the way), you can add a series diode to lower it to nominal 9.2V , or even 9V at higher current consumption.
Zero extra current consumption , which will not happen with any other regulating system.
Since my main (unspecified) goal was to keep the voltage constant, I don't think just a diode would work for me. Another idea I had was to use an (as low a current as possible) LED in series with a zener as part of the regulation, which would be on until the battery voltage dropped below a certain level, informing me when the battery needed charging.
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Old 9th April 2013, 10:42 PM   #4
Nicho is offline Nicho  United Kingdom
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Would this work?
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Old 9th April 2013, 11:32 PM   #5
JMFahey is offline JMFahey  Argentina
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Conceptually, yes.
The idea of using a MosFet is very good, because gate current is 0.
Practical problem is that most MosFets readily available need *around* 3.5VGS just to be turned on, "wasting" about what 1 cell supplies.

How much does a TL78xx need for itself?

Remember than the Zener itself needs a minimum current through it to "zene".

But keep throwing ideas, one of them might be the ticket.
Good luck.
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Old 9th April 2013, 11:40 PM   #6
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If you're going to go in that direction, why not budget that current for a regular voltage regulator? Using an LM317 with ~1k for the reference voltage, the quiescent current through the load then need be only a few milliamps to meet rated specs.
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Old 10th April 2013, 12:30 AM   #7
Nicho is offline Nicho  United Kingdom
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Thanks for the responses JMFahey and sofaspud. I could drop 3.5V accross the MosFet, and use a 50uA Zener, and LED to drop about 12.5 volts accross, or parallel 50uA Zeners if I need more current for the LED. I don't really mind wasting one cell of voltage - it means I need the fourth battery, but that would seem much better than a large current drain.

An adjustable regulator chip would also be a possibility - I will have to research them a bit. Is there any advantage to using these other than the ability to adjust the voltage (which might be nice)? Presumably the greater ripple rejection is not particularly important in the battery context.

My main concern with either form of regulation may or may not be related in some way to the term "transient response". Will there be a negative effect in this or in some related area.

Last edited by Nicho; 10th April 2013 at 12:46 AM. Reason: Ask an extra question!
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Old 10th April 2013, 01:08 AM   #8
Nicho is offline Nicho  United Kingdom
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I don't know if that last paragraph makes sense. I assume that the regulator must respond ex post to changes in current demand, and imagine (without much knowledge) the possibility of the lagging behind slightly, cutting corners, overshooting, etc., while the unregulated battery simply has lots of little electrons jumping up and down just waiting to go. Is there any truth to this or not?
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Old 10th April 2013, 01:16 AM   #9
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You might be better to use a battery like these:- Buy UltraFire Protected 18650 3.7V 2400mAh Lithium Batteries (2-Pack Grey)

These are 'protected' cells. They have a little built-in circuit that means you can't overcharge or over discharge them. Quite a lot cheaper, although the capacity tends to be a bit exaggerated with these Chinese cells. You'll get 6 for the price of 2 from the German site though. Delivery takes ~14 days however.

They do run ~3.6 to 4.2V fully charged, so an LDO regulator (MIC2941 is a good one) might be advisable, because 3 fully charged will be 12.6, which might be a bit too much for your pedals. It'd probably be OK, but nobody wants to say that for sure and have you blow up your pedals. Dropout voltage on that reg. is only 0.6V @ 1.2A out.

You won't find any practical difference using a regulated supply, the reg responds so fast.
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Last edited by counter culture; 10th April 2013 at 01:20 AM.
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Old 10th April 2013, 02:05 AM   #10
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Not to be rude, but you seem intent on making something quite easy, difficult.

JMFahey gave the best advice.

Most batteries do NOT have exhibit the same voltage through-out their discharge cycle. ONLY as an example, a '12'V lead-acid battery will vary from 14+V to 10+V.

Your stuff will be fine with 9V9. No worries at all.

The only wrinkle is adopting a battery charger to charge them.
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