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# small 12v heater element control Answered

Hi, looking for a bit of advice from electronically literate folks...

I'm planning a low-temp water heating device that will keep a water/yeast/sugar solution at around 38C to produce optimum levels of CO2.

so far what i have in mind is using a cheap 12v car immersion heater (the type used for making hot drinks on the go), powered by 4 x 3.7v lithium batteries in series (giving me roughly the right voltage range).

The part i'm now trying to figure out is regulating the heating of the solution. I'm considering using an LM35 component to monitor the temperature via contact on the outside of the vessel. What i'm stuck on is what other components would i need in the circuit to take the  voltage output of the LM35 and use it to control the 12v immersion heater to maintain a temperature of around 35 degrees?

Any advice would be much appreciated, thanks!

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## Discussions

how big a vat of liquid?

Why batteries not household power?

Sterno?

Other options?

Two posts due to first post (this one) not showing up immediately.

Batteries are 3000mah. immersion heater is unlikely to be more than 120W

water needs to be maintained at 38C whereas the immersion is designed to boil water.

I'm trying to figure out what other components are needed to achieve temp-controlled switching of the immersion.

There are no such examples on the datasheet.

If you need 10 amps for the heater than logic says 3 amps won't do you any good.

would you care to explain your logic?

120 w / 12 = 10 A

3000 mah is 3 amp hours

So, downunder isn't _quite_ right in his math.

Amp hours aren't the same as amps.

_IF_ the batteries can run the amps, they will last 3/10ths of an hour. (time spent on).

Short answer: use different batteries or different heater.

Not much to explain really:
The heater needs 10 amps, the batteries deliver a max of 3 amps - without the missing 7 the batteries will be in a short circuit and fail within a few seconds.
Even if you have batteries that can deliver 10 amps they would not run for long.
To run your heater at full capacity you need 10 amps, to run it for 2 hours the battery needs to provide 20AH at least.

Of course you could limit the power requirements of the heater for example by using two or more in series but it would be far easier to build one to match.
Still leaves the question of usability and run time on batteries.

ok so i finally found it after a fair bit of digging:

http://electronics-diy.com/electronic_schematic.ph...

pretty much exactly what i need.

I only posted here as a possible shortcut to all the searching in the hope that those with more experience might point me in the right direction.

Oh well.

step 1.