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Old Jun 11, 2005, 12:29 PM
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Reducing the output voltage on a 1 cell lipo

Can I put a resistor inline to reduce the voltage output on a 1 cell 1400mAh lipo to reduce the output from 4.15 volts to aprox 1.5 volts? and if so do you guys know what resistor I can get, say like at radio shack?
Thanks in Advance, Chris
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Old Jun 11, 2005, 12:33 PM
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Perth, Australia
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Hi Chris,
If you let us know what you are trying to power with the cell and how much current you require it will be easier to give you an answer.

Ron...
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Old Jun 11, 2005, 12:45 PM
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Cas, I am using the lipo for an onboard glowplug set-up for a three engine rc plane.
The lipo would be Ideal to save weight.
The 4.15 volts is to much, it just fry's the glow plugs. I think if we can get the voltage just close to 1.5 it would work great.
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Old Jun 11, 2005, 01:20 PM
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Perth, Australia
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Have you considered wiring the three glo plugs in series. 4.15 v divided by 3 = 1.38v.
With the volt drop due to the current flow of around 1A, the voltage per plug should be around 1.2v or slightly less.

Ron...
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Old Jun 11, 2005, 02:00 PM
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Hmm, No I havnt thought of that Ron. We allready got it wired up & ready to go where it runs through this onboard switch that runs through the reciever, so that we can manually turn the power on to the glow plugs thru the transmitter.
Would rather just try to convert the lipo down by using a resistor.
I know very little about resistors tho, and or the formula to what resistor to try. I called my local Radio Shack and they dont have a clue of what I should try.
Any guess on what might work?
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Old Jun 11, 2005, 02:02 PM
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Quote:
Originally Posted by Cas123
Have you considered wiring the three glo plugs in series. 4.15 v divided by 3 = 1.38v.
With the volt drop due to the current flow of around 1A, the voltage per plug should be around 1.2v or slightly less.

Ron...
Ron,
That's an excellent idea. Only problem would be if one glow plug burns out, it would break the circuit.

I would think that one smallish nicad would power all three and not weigh more than a lipo.
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Old Jun 11, 2005, 02:36 PM
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Calgary, Alberta Canada
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SoulTag and Gang,
Simple voltage drop is to use some diodes in series, you
will get approx a .6v drop over each diode. ie: 1N5400, I
use these to bring 20v to 16v for charging my 11.1v LIPo's
Correct voltage drop will show only under load.
Goog luck!
ricky
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Old Jun 11, 2005, 09:03 PM
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United States, WA, Puyallup
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I like the series set-up. Clean and simple. Wenwright's concern about one goes they all go is valid, but I'd chance it. Of course, it is not my model.

Second choice is the string of diodes - simple. You can compute a resistor value, but probably have to a little trial and error dickering.

Bill
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Old Jun 11, 2005, 09:18 PM
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Quote:
Originally Posted by SoulTag
Can I put a resistor inline to reduce the voltage output on a 1 cell 1400mAh lipo to reduce the output from 4.15 volts to aprox 1.5 volts? and if so do you guys know what resistor I can get, say like at radio shack?
Thanks in Advance, Chris
A resistor in line will reduce the amps available not the voltage. You need two resistors to work together but the wattage rating of the resistors make the weight of them an issue . See this link to figure it out link


Bill
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Old Jun 11, 2005, 11:06 PM
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I still say a single nicad would be your best bet.
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Old Jun 12, 2005, 04:31 AM
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Quote:
Originally Posted by poulsbobill
A resistor in line will reduce the amps available not the voltage.
Sorry Bill but that's not true. A resistor drops voltage. If you look at the link you posted you'll see "In electronics we use a resistor when we need to reduce the voltage applied to a circuit."

So it's possible to use a resistor but not a sensible idea in this case. As you'd want to drop about 2.5V and your 3 glow plugs in parallel could easily be drawing around 8 or 9A the resistor would need at least a 30 Watt rating with a good size heatsink. That's big, heavy and hot.

BTW at that current your 1400mAh li-poly cell would only last for about 10 minutes per charge, then you'll need 1.5 hours to charge it again. Also I hope your wiring/switch is correctly sized for the current.

A single NiCd/NiMH cell would do it easily without any additional resistors etc and will end up considerably smaller and lighter overall.

Steve
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Old Jun 12, 2005, 04:38 AM
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The string of diodes will work too, however you will need 4 to 5 diodes in series per glowplug. They will have to be 3 amp diodes as the current flow will be over 1A.

Ron...
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Old Jun 12, 2005, 04:56 AM
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Slipstick is right about the duration of the battery. The way around this is to have a microswitch that is actuated wnen the throttle servo gets below say 20%. Therfore the battery will be used only a small portion of the time while the motor is idling.
Having the battery connected when the motor is above this usually results in frequently burned out gloplugs in my experience.

Ron...
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Old Jun 12, 2005, 10:14 AM
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Hello,

it makes no sense to use a 3.7 V battery when only 1.2 V is required and then dissipate 2.5 V which is more than 2/3 of the energy stored in the battery.

Using a "switching regulator" will solve the problem. A good example is

http://focus.ti.com/lit/ds/symlink/pth04070w.pdf

This decvice has a 3 A rating and an undervoltage protection. You never know what it may be good for, when using lipos.

Gernot
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Old Jun 12, 2005, 12:31 PM
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Wow, I have learned alot. I think that I may go with the 1 cell ni-cad to simplify things. It sounds like by the time I put all those diodes in series I wouldnt be saving any weight at all.
Dr. Gernot, That little switching regulator looks like another great way to go, it only weighs 1.5 grams, plus the voltage protection is big, as I would have to have someway to keep from running the lipo down to far.
You guys are great, thank you so much for sharing your knowlege.
I have got to show you guys this 3 engine plane. I will try and get some pics to post.
Thanks again, everyone
Chris
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