# Buck converter help

Discussion in 'General Electronics Discussion' started by Gillesfizzog, Oct 11, 2016.

1. ### Gillesfizzog

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Jan 14, 2014
Hey,
I built a bicycle generator and it puts out up to100 volts DC. I want to charge my 12 volt battery so i need about 13.6 to 14 volts. However many of the buck converters online, at least that i can find, only have a max input voltage of around 60, So either i need help finding the right buck converter, or I try to reduce the voltage. Im producing up to 100 volts, but I need about 50 volts, so what if i put a load like a bulb or resistor in series with the converter. I assume i would need a 50 volt drop there. How much ohms of resistance would i need at that resistor to get a 50 volt drop?

Last edited by a moderator: Oct 11, 2016
2. ### Bluejets

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Oct 5, 2014
If you built the generator, then show the generator circuit.
Being a "generator" can take many forms and a solution may be in that section alone.

Charging a 12v battery does not require 50 volts.( or 100 for that matter)

3. ### AnalogKid

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Jun 10, 2015
Ohm's Law defines that the voltage drop across a resistance depends on the current through it, so selecting the resistor value and power depend on the current being drawn by the converter, and that depends on the output current. As the battery charges up, the converter output current will decrease, so the input current will decrease, so the voltage drop across the resistor will decrease and the voltage across the input to the converter will increase.

If it seems weird that when the current through a resistor goes down, the voltage drop goes down, while when the current through a power supply input stage goes down, the voltage across it goes up, that is a consequence of the difference between a constant resistance device (the resistor) and a constant power device (the power supply). When the two are in series, this happens.

Also, depending on the output impedance of the generator, its output voltage might sag when it has to deliver current to something other than a voltmeter.

ak

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4. ### davennModerator

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Sep 5, 2009
agree with bluejets

what sort of bike generator ? one of those small things that spins against the tyre ?
It may produce 50V under no load conditions a lot less with a load
and if it really is one of those generators, it will NEVER produce the 6A you were hoping for
maybe 100 - 200 mA

Dave

5. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

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Jan 21, 2010
I recommend you place a zener diode across the generator with a voltage a little under the max voltage of the buck converter.

If you ever expect to run the generator without a load you may be better off with a series regulator (but this will waste power when the generator is under load).

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6. ### Gillesfizzog

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Jan 14, 2014

Sorry about the lack of info, generator is a DC permanent magnet motor with a high amperage output potential, I have a pedal bike hooked up to it by means of a pulley

Last edited by a moderator: Oct 11, 2016
7. ### BobK

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Jan 5, 2010
Did you test the voltage under load? It may turn out to be way lower than you think.

Bob

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8. ### AnalogKid

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Jun 10, 2015
With one resistor, one zener diode, and one power transistor (with a 150 V or higher rating) you can make a "power zener" circuit, basically a low-precision pre-regulator, that limits the voltage available to the buck converter.

ak

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9. ### Gillesfizzog

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Jan 14, 2014
Great info,
Hears a picture of my newer idea, I wonder if i put two bulbs of the same wattage rating, in series like so to theoretically waste around 45V so that i dont go over the 50V max input of the charge controller. Not the most efficient im sure but hopefully simple and easy for now.

10. ### Gillesfizzog

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Jan 14, 2014
I just realized I can get a mppt controller with a max voltage input of 100v

11. ### Chemelec

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Jul 12, 2016
Remove some windings to get a Lower Output Voltage.
Probably Better than using a Regulator.

12. ### davennModerator

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Sep 5, 2009
L1 will severely limit the available current that can used by the controller and any load
and L2 wont be doing anything useful as it is in parallel with the load