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Battery Power to Wall-Wort

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Battery Power to Wall-Wort

Postby pitchwest » Thu Apr 09, 2020 12:45 am

Hi guys, I've recently found that I can't run my Zoom H5 off my computer's power without a loud enough hum. It only has two ways of being powered, one USB and the other battery. Batter is the only way I don't get the hum but I don't want to be constantly buying batteries. I have a Wall-wort 9V and 650mA output. I dropped the voltage to 2.5V with two resistors. The battery power is 2.5V coming from 2 AA batteries so I figured it would be simple. My Zoom H5 won't turn on with the Wall-wort power though. Any ideas why?

I'm at a loss but that's why I'm here! Any idea's guys?
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Re: Battery Power to Wall-Wort

Postby Wonks » Thu Apr 09, 2020 7:49 am

Is your external PSU AC or DC? It obviously needs to be DC.

What value resistors have you used? Is it a potential divider set-up or is it just in-line resistance? The resistor values will determine how much current can be drawn, and you may now not have enough available. An AA battery can provide about 50mA. So you'll need a maximum total resistance180 ohm to obtain that with a 9v supply.

And 2.5v is a bit under the nominal 3v from two batteries. I'd definitely go for 3v.

I assume this is being applied via the battery connections, rather than the USB port?
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Re: Battery Power to Wall-Wort

Postby James Perrett » Thu Apr 09, 2020 12:39 pm

One suggestion would be to use rechargeable batteries instead of normal AA's. You can often pick up a set of 4 rechargeables together with a charger for just over a tenner. Having 4 will allow you to have one pair charging while the other pair is in use. Just make sure you go for the low self discharge types like the Sanyo Eneloop or GP Recyko ranges.
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Re: Battery Power to Wall-Wort

Postby Martin Walker » Thu Apr 09, 2020 1:03 pm

Wonks wrote:What value resistors have you used? Is it a potential divider set-up or is it just in-line resistance? The resistor values will determine how much current can be drawn, and you may now not have enough available. An AA battery can provide about 50mA. So you'll need a maximum total resistance180 ohm to obtain that with a 9v supply.

And 2.5v is a bit under the nominal 3v from two batteries. I'd definitely go for 3v.

I assume this is being applied via the battery connections, rather than the USB port?

I think Wonks is on the money here - if you've chosen your resistors to provide 2.5 volts BEFORE you switch on your Zoom H5 then as soon as you do, that voltage will plummet as the Zoom draws current, so it won't power up.

You instead need to start with a largish series resistor (or preferably a smallish value potentiometer such as 1K or 5K) and then slowly reduce that pot's resistance ((or keep changing to a smaller fixed resistor) until the voltage on the Zoom side is 3 volts.

Hope this helps!


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Re: Battery Power to Wall-Wort

Postby James Perrett » Thu Apr 09, 2020 1:13 pm

If you use resistors you are likely to find that the voltage changes with load (unless you use really really low values and generate lots of heat). It is much better to use a voltage regulator of some kind.

However, a power supply with a 3 volt output is only about six quid from CPC if you don't want to follow my previous suggestion.
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Re: Battery Power to Wall-Wort

Postby Trevor Johnson » Thu Apr 09, 2020 3:00 pm

Have you tried a USB Power Bank? I use an Anker PowerCore+ 20100 USB-C for my portable device, (not Zoom though), and it is USB-C not mini-USB.

If you have one hanging about it may be worth trying, but I would be cautious in buying one to try, because I believe that with more recent power banks, their sensing electronics can sense load variation and may shut down.
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Re: Battery Power to Wall-Wort

Postby MarkPAman » Thu Apr 09, 2020 3:27 pm

We usually power our H4s at work from USB phone charger adaptors with no problems.
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