Author Topic: regulating 12 volt battery output to 12 volt  (Read 5856 times)

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(unknown)

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regulating 12 volt battery output to 12 volt
« on: February 10, 2010, 09:09:45 AM »
The problem is the thermostat ceases to work when the batteries get up to peak charge voltage(15volt). Also the thermostat temp set point varies with the voltage which is important since I want to control an incubator for eggs with this thin. I have tried using and lm338 voltage control IC but the thing sucks about 1.5 volts even when the voltage is around 12 which will not work.


So I need a regulator that will accept 12 to 15 vols and put out 12 volts at about 5 amps(to run a resistance heat source)


thanks

Joseph

« Last Edit: February 10, 2010, 09:09:45 AM by (unknown) »

electronbaby

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Re: regulating 12 volt battery output to 12 volt
« Reply #1 on: February 10, 2010, 09:36:26 AM »
use a 7812 regulator (good for 1 amp) to control your thermostat, and use it to drive a low current relay to supply your resistance heater current directly from the battery. There is no reason to use the regulated voltage for the heating element is there?
« Last Edit: February 10, 2010, 09:36:26 AM by (unknown) »
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(unknown)

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Re: regulating 12 volt battery output to 12 volt
« Reply #2 on: February 10, 2010, 09:51:50 AM »
The 7812 also steals about 1.5 volts so it would not work when the batteries are around 12v. Also the resistance heater I currently use is a 120 volt seed heat mat which draws juice proportionate to voltage(does not get hot at 12v) so I need to run through an inverter or some kind of dc voltage booster(which I don't need).

Joseph
« Last Edit: February 10, 2010, 09:51:50 AM by (unknown) »

electronbaby

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Re: regulating 12 volt battery output to 12 volt
« Reply #3 on: February 10, 2010, 10:00:00 AM »
if the heat mat you are using will not work at 12v, then you need to go to a higher voltage. If you will be using an inverter, then simply build your control circuit to work at 120vac. There will be somewhat better regulation through the inverter, and this will give you more voltage headroom to work with. It sounds like you want to heat with 12vdc...good luck...may be easier to go another route.
« Last Edit: February 10, 2010, 10:00:00 AM by (unknown) »
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ghurd

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Re: regulating 12 volt battery output to 12 volt
« Reply #4 on: February 10, 2010, 01:39:48 PM »
DC-DC Converter?

eBay has a lot of them.


Not sure if this is "perfect" or how it works, but item #300393836451, 3-30V in, adjustable output 1.3-18V.


Use something like that to a relay or power fet, which controls a DC heater.

A 120V heater won't make heat at 12V.  Need to go with something 12V, or make something 12V.

G-

« Last Edit: February 10, 2010, 01:39:48 PM by (unknown) »
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joestue

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Re: regulating 12 volt battery output to 12 volt
« Reply #5 on: February 10, 2010, 09:04:28 PM »
Wire up the thermostat with a 12 volt zenar, and wire up the 5 amp resistance load directly to the battery.

« Last Edit: February 10, 2010, 09:04:28 PM by (unknown) »
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(unknown)

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Re: regulating 12 volt battery output to 12 volt
« Reply #6 on: February 11, 2010, 09:18:07 AM »
I would rather not run the inverter 24/7 since it is winter and my system is fairly small, so running everything at 120vac is out. I don't need to heat with 12v I just want to use the 12v thermostat I have to turn off the inverter to which the heat mat is plugged.


thanks

« Last Edit: February 11, 2010, 09:18:07 AM by (unknown) »

(unknown)

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Re: regulating 12 volt battery output to 12 volt
« Reply #7 on: February 11, 2010, 11:10:25 AM »
Looking at the specs for the voltage regulating IC I  think this may have the same problem as the others were at 12v in it would not put 12v out.

thanks
« Last Edit: February 11, 2010, 11:10:25 AM by (unknown) »

(unknown)

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Re: regulating 12 volt battery output to 12 volt
« Reply #8 on: February 11, 2010, 11:11:37 AM »
To use a zener the input would have to be 2 volts higher than the output.


thanks

« Last Edit: February 11, 2010, 11:11:37 AM by (unknown) »

ghurd

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Re: regulating 12 volt battery output to 12 volt
« Reply #9 on: February 11, 2010, 11:42:02 AM »
Some DC converters are Boost.  Or Buck-Boost.

Meaning some can be set to put out 12.0V with 8 to 30V input.

That's the type of DC-DC converter you are looking for.
« Last Edit: February 11, 2010, 11:42:02 AM by (unknown) »
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(unknown)

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Re: regulating 12 volt battery output to 12 volt
« Reply #10 on: February 11, 2010, 01:31:19 PM »
I think I will try the LM2940 and run the inverter straight from the battery. I think this may be the cheapest solution. This think only steal 1/2 a volt. So as  long as the battyery stays above 12.5 volts it should be fairly accurate.


Joseph

« Last Edit: February 11, 2010, 01:31:19 PM by (unknown) »

joestue

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Re: regulating 12 volt battery output to 12 volt
« Reply #11 on: February 11, 2010, 09:21:36 PM »
Not true. this is a textbook shunt regulator.


You need to size the zenar for worst case case however. At most your resistor can only drop say .3 volts under the full load of the thermostat... which would be what, milliamps?

and therefore at the full 15.5 volts, the resistor will be dropping 3.5 volts, so you might need a 1/2 watt or even a 2 watt zenar, depending on what resistor you choose, and what voltage drop you consider acceptable.


If .5 volts drop is unacceptable then get a dc-dc converter. I don't know of any FET based linear regulators that you can get off the shelf, might try making your own.. but they tend to oscillate. It might be less effort to build a more intelligent thermostat anyway?

« Last Edit: February 11, 2010, 09:21:36 PM by (unknown) »
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