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Author Topic: running 8.4V device on 12V? Bad Idea?  (Read 4374 times)
WALTERB
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« on: August 02, 2012, 08:08:32 AM »

I believe I know the answer before I ask, but I have a device that runs on 8.4V.  I want to run it on a 12V battery. Is there an easy way to step down the voltage?  I assume if I don't step it down it will ruin the device?

thanks.
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N3OX
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« Reply #1 on: August 02, 2012, 08:18:10 AM »

It will ruin many devices.  Some devices actually have a range but unless that is explicitly stated or you understand the innards of the device, you should assume 12V will kill an 8.4V device.

You can step down to 8.4V with a simple adjustable voltage regulator like a LM317 if you don't need a whole lot of current (1.5A or less)

There are many voltage regulators that will step 12V to 8.4V with varying levels of complexity and efficiency.

What's the device and how much current does it need?

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73,
Dan
http://www.n3ox.net

Monkey/silicon cyborg, beeping at rocks since 1995.
W5FYI
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« Reply #2 on: August 02, 2012, 08:22:47 AM »

Depends somewhat on the device, but it's easy enough to step down the voltage to a safe level. Voltage regulators, such as a 7808, will take 12V at the input and deliver 8V at the output. Circuits should be online and in the ARRL Handbook. Another approach is to use a string of diodes to lower the voltage. Each silicon diode in the series will drop .6V, so a string of six will drop 3.6V, enough to lower 12V to 8.4. Use something like 1N4001s or so, with a current rating high enough to handle your "device" current. A third approach is to use a voltage divider or potentiometer, but don't forget to figure in your device's resistance and current requirements when calculating resistor values. GL
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KA4POL
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« Reply #3 on: August 02, 2012, 08:23:01 AM »

A Zener diode will do. Question is what current does your device require and how accurate does the voltage have to be.
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AC5UP
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« Reply #4 on: August 02, 2012, 08:34:55 AM »

If the device draws less than 1.5 amps an LM-317 adjustable voltage regulator is an easy build. You'll find one at your local RagShag store along with their profoundly overpriced TO-3 / 3 amp regulators. If you're a junker, a 5 volt fixed regulator in a TO-3 package should be easy to come by and can regulate 8.4 volts by elevating the ground connection 3.4 volts above true ground with a potentiometer. The only difference between a "fixed" regulator and the "adjustable" version is the minimum voltage. For an LM-317 it's 1.2 volts, for an LM-7805 it's 5.0 volts.

Here's a typical data sheet:  http://www.ti.com/lit/ds/snvs365/snvs365.pdf   (see sample circuits on the last few pages)

...if the device could run on 12 volts there's a pretty good chance it wouldn't be labeled as 8.4 volts. 50% worth of overvolt is not a good idea.    Wink
« Last Edit: August 02, 2012, 08:37:06 AM by AC5UP » Logged

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WALTERB
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« Reply #5 on: August 02, 2012, 08:35:47 AM »

It will ruin many devices.  Some devices actually have a range but unless that is explicitly stated or you understand the innards of the device, you should assume 12V will kill an 8.4V device.

You can step down to 8.4V with a simple adjustable voltage regulator like a LM317 if you don't need a whole lot of current (1.5A or less)

There are many voltage regulators that will step 12V to 8.4V with varying levels of complexity and efficiency.

What's the device and how much current does it need?



thanks.  the device has nothing to do with ham radio, but I knew you guys would know the answer.  It an LED bike light system. It runs off its own battery pack that doesn't hold a charge very long. I wanted to hook it to a small 5AH AGM battery that is 12V.  Since its going on a bike voltage regulator also has to be small.  At some point I want to hook up a ham radio to the bike as well and use the same 12V power system if possible.

thanks
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N3OX
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« Reply #6 on: August 02, 2012, 09:27:49 AM »

thanks.  the device has nothing to do with ham radio, but I knew you guys would know the answer.  It an LED bike light system. It runs off its own battery pack that doesn't hold a charge very long.

That probably has some kind of regulator, probably a switching regulator, and it MIGHT actually survive at 12V but if it's expensive you should be careful Smiley

My bike light is made with a cheap LED track light bulb that is nominally a "12V" bulb.

http://n3ox.net/files/bikelight.jpg

It turns on around 9V at full brightness and I've tested it up to 16V with no ill effects.  It actually draws LESS current as the voltage is increased because the circuitry inside is designed to supply constant power to the LEDs inside. And weirdly, despite the fact that it's a LED light, the polarity of the input voltage doesn't matter...

If your LED light has the right kind of driver circuitry, it MIGHT work above its nominal voltage but then again it might just blow up Smiley   Safer to step it down.  Though you might want to do some web research into your exact make and model to see if a brave soul has ever tried it as high as 13V or so. 

I know some of those bike light systems can get really expensive, though, so I doubt too many people have intentionally set the voltage too high to see what happens.  But someone might have taken one apart to get a better idea of whether or not it could take more than 8.4V.
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Dan
http://www.n3ox.net

Monkey/silicon cyborg, beeping at rocks since 1995.
N3OX
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« Reply #7 on: August 02, 2012, 09:42:33 AM »

If you want to drop the voltage and get the most joules out of a single charge, a switching regulator is a good idea:

http://goo.gl/QVIlI

The full modules like the one above are more expensive than a linear regulator, and if you don't buy a module you need to source a bunch of different parts including an inductor to get the bare regulator controller chip to work.  But they are more efficient so they save some power and also you don't have to get rid of as much heat.
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73,
Dan
http://www.n3ox.net

Monkey/silicon cyborg, beeping at rocks since 1995.
KA4POL
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Posts: 1998




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« Reply #8 on: August 02, 2012, 10:07:52 AM »

A LED requires a constant current. So you need a constant current source like: http://users.telenet.be/davshomepage/current-source.htm
You do not have to have a certain voltage like 8.4. Your battery also does not have the 12 V for any charge.
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WALTERB
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Posts: 528




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« Reply #9 on: August 02, 2012, 10:11:50 AM »

thanks.  the device has nothing to do with ham radio, but I knew you guys would know the answer.  It an LED bike light system. It runs off its own battery pack that doesn't hold a charge very long.

That probably has some kind of regulator, probably a switching regulator, and it MIGHT actually survive at 12V but if it's expensive you should be careful Smiley

My bike light is made with a cheap LED track light bulb that is nominally a "12V" bulb.

http://n3ox.net/files/bikelight.jpg

It turns on around 9V at full brightness and I've tested it up to 16V with no ill effects.  It actually draws LESS current as the voltage is increased because the circuitry inside is designed to supply constant power to the LEDs inside. And weirdly, despite the fact that it's a LED light, the polarity of the input voltage doesn't matter...

If your LED light has the right kind of driver circuitry, it MIGHT work above its nominal voltage but then again it might just blow up Smiley   Safer to step it down.  Though you might want to do some web research into your exact make and model to see if a brave soul has ever tried it as high as 13V or so. 

I know some of those bike light systems can get really expensive, though, so I doubt too many people have intentionally set the voltage too high to see what happens.  But someone might have taken one apart to get a better idea of whether or not it could take more than 8.4V.

thanks. (cool bike and light by the way).

here it is, and it isn't cheap.

http://www.amazon.com/MagicShine-MJ-816E-version-improved-MJ-6006/dp/B005TY5PXY/ref=sr_1_2?s=sporting-goods&ie=UTF8&qid=1343927460&sr=1-2&keywords=bicycle+light
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AD4U
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« Reply #10 on: August 02, 2012, 10:23:45 AM »

As posted the simplest way is to series 5-6 silicon diodes with the proper current rating.  This will cost you less than a buck.

Dick  AD4U
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WALTERB
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Posts: 528




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« Reply #11 on: August 02, 2012, 10:29:22 AM »

As posted the simplest way is to series 5-6 silicon diodes with the proper current rating.  This will cost you less than a buck.

Dick  AD4U

cool thanks!
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KB1GTX
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« Reply #12 on: August 02, 2012, 08:28:09 PM »

MJ-6006 battery pack Specifications: Battery?4×18650 Li-ion battery (8.4V4.4AH)
Battery life:500 times cycle
Charger?DC 8.4V/1.8A
Charging time?3-3.5 hours
Discharging current??4A


How large is the 12v battery?   you alreay have 4,4 ah on the built in battery.
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