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Author Topic: 5V DC in to a 4.5V (3 x AA batteries) device  (Read 13570 times)
solarmon
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« on: August 30, 2012, 11:47:09 AM »

Hi,

One for the electronics gurus.

I want to power a sound recorder device using a 12V battery. This is the sound recorder module I have got:

http://www.ebay.co.uk/itm/170747526875? ... 329wt_1344

This makes use of 3 x AA batteries - i.e 4.5V. I originally got this 9V one:

http://www.ebay.co.uk/itm/170717668346? ... 040wt_1344

However, the playback button was a membrane one on the board and does not allow me to use a different switch (unless I somehow solder on the membrane contacts - which I don't want to do).

However, the replacement one uses 3 x AA batteries - i.e. 4.5V. So I cannot use the 12V to 9V converter I originally got for the 9V sound recorder:

http://www.ebay.co.uk/itm/290617583927? ... 903wt_1111

However, I also have this 12V to 5V converter:

http://www.ebay.co.uk/itm/310413238475? ... 467wt_1344

(are you keeping up at the back?!)

My question is: Is it safe to use the 5V supply to power a 4.5V device? Will the 0.5V difference be an issue (short or long term)?

Cheers,

SolarMon
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Gambot
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« Reply #1 on: August 30, 2012, 01:19:15 PM »

If it were my kit I would give it a try on 5V since if the recorder is designed correctly it should tolerate the extra half volt. The sound in the speaker might be a bit sharp though, but since it is not my kit I can't recommend it just in case it damages it.
The thing I could recommend, is to wire in a couple of say 100ma signal diodes in series and insert in series with the power lead (or if it is more convenient, one diode in the positive power lead connection to the board and one diode in the negative power lead connection to the board).
Each diode will drop 0.2V so with 2 in series you will drop 0.4V.
« Last Edit: August 30, 2012, 01:44:41 PM by Gambot » Logged

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Antman
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« Reply #2 on: August 30, 2012, 01:54:06 PM »

Given that new alkaline 1.5V Procells are a tad over 1.6V the unit should be capable of handling 4.8V at least.
So as per Gambot, put a 1N4001 or similar in series to ensure a 5V supply is safe.

But....

Be careful with switch-mode inverter modules and supplies as they may well only regulate to stated voltage with a minimum load current - often 10%. If the specification does not specifically state 0-3A (for example), then assume that the output may rise a bit above 5V on very low or no-load. So with no-load on a supply designed for 10% min load, you may get up to 5.5V output....
Now this may kill (say) TTL spec devices long-term as it is outside spec, although it is unlikely that your module uses such old technology  Cheesy

Antman
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jonesy
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« Reply #3 on: August 30, 2012, 02:07:31 PM »

A brand spanking new duracell AA will often have more than 1.5V.  I'm with gambot.  I don't think 0.5V is an issue, but it's not my gear to blow up!
I googled  '20s voice recorder module ' and a rook of pdf datasheets come up.  All of them had built in microphone and speaker, but looked remarkably similar with an anonymous black blob in the middle.  4.5V seems to be the minimum, 6V max.
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solarmon
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« Reply #4 on: August 30, 2012, 02:15:00 PM »

Hi All,

Thanks for all your feedback - I'll probably end up just try connecting it to to the 5V power source and see what happens.

There is actually a 12V sound recorder module - BRC-10M - as listed at:

http://www3.ocn.ne.jp/~melcom/products/module/voice_module/thumbnail.html

But I cannot find a source for it. Ideally, I would want to power directly off the 12V battery.

Cheers,

SolarMon
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Antman
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« Reply #5 on: August 30, 2012, 08:16:52 PM »

.... All of them had built in microphone and speaker, but looked remarkably similar with an anonymous black blob in the middle.  4.5V seems to be the minimum, 6V max.

That's so you don't see the charred crater when you try shoving 5V up where only 4.5V was designed to go  hysteria

Seriously, get a 78L05 (100mA max)  or 7805 (1A) regulator ic and you can power the original board from 12V as well.

Antman
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Pat_
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« Reply #6 on: August 30, 2012, 08:17:42 PM »

Just for the record, silicon diodes drop between 0.6V and 0.8V each. Schottky diodes around 0.3V to 0.8V. Depending on the current.
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Gambot
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« Reply #7 on: August 31, 2012, 01:16:15 PM »

Just for the record, silicon diodes drop between 0.6V and 0.8V each. Schottky diodes around 0.3V to 0.8V. Depending on the current.
Sorry, should have made a point of specifying Germanium diodes such as OA90 which drop 0.2 V whereas a Silicon diode at 0.7V drop would drop too much voltage for this application.
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wookey
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« Reply #8 on: August 31, 2012, 05:54:07 PM »

It'll probably be fine. That PSU is rather over-specced for this application, but again will 'probably' be fine. As Antman says a 7805 regulator would be more suitable with no risk of overvoltage. If you did add a diode to drop some voltage it really doesn't matter much which one. Anything powered by 1.5V batteries will have a wide input voltage range on the low side (i.e down to about 3V).
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Wookey
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« Reply #9 on: November 01, 2012, 05:31:49 PM »

Or if you are worried use a LM317T adjustable regulator and a couple of resistors or a trimpot to give you exactly what you want

Nick
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