[Lab] Help? I can't wrap my head around transistors / mosfets.

[Michael Grant]

What's the current required for heater?
If you don't know, measure the resistance of the heater and apply i = V/R.

This will allow you to choose a capable switching device.
Likely a logic level MOSFET will be the ideal choice, switched via series
resistor from the AVR.

And which Arduino are you using?
Most are 5v but some are 3.3 volts.

Michael Grant
-- 
http://krazatchu.ca/


On Mon, Nov 25, 2013 at 6:20 PM, Ken McKinnon <klmckinnon at rogers.com> wrote:

> I started writing a response, but figured someone out there in google land
> would have a better explanation.
>
> Try http://sunburst.usd.edu/~schieber/psyc770/transistors101.html
>
> If I recall, the arduino is a 3.3 volt device, so you may need something
> like a optocoupler in between your arduino and the transistor that would
> need to be at a bit higher voltage to turn on completely.
> As an aside, a PWM control would probably control the heat to a much finer
> degree, with a lot fewer parts....
> Good luck on your project, if you need more or that link isn't precise
> enough to get you on path, ping again.
>
> Ken
>
>
> On 2013-11-25 4:37 PM, Justin Slootsky wrote:
>
>> Hey everyone, I'm having trouble understanding transistors / mosfets, and
>> what I need.
>>
>> I'm working with Chris on a heating project, and we're looking at using
>> an Arduino to control heating pads in a mitten to keep his hand warm.
>>
>> The heater will run on 7.2V, which will generate enough heat to keep warm
>> but not enough to burn.
>>
>> What we would like to do is to regulate the power going to the heating
>> pad based on the value returned from a temperature sensor.
>>
>> I forsee us turning the heater on and off based on threshold temperatures
>> inside the mitten.  In the future, it might be advantageous for us to be
>> able to turn the power half on if it is near the desired temperature but
>> for now a binary on/off based on an output from the Arduino will do.
>>  (software will determine the on/off cycle based on the value returned from
>> the temperature sensor).
>>
>> My understanding is that we need a mosfet or a transistor or something
>> for two reasons
>> 1) The Arduino won't send 7.2V power. (even if we use one that has a
>> direct battery line, it is too much power to send to the Arduino)
>> 2) The power draw would be too high anyways.
>>
>> Can anyone help me with what I need in order to connect this up?
>>
>> Thanks
>> Justin
>>
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