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

[Justin Slootsky]

The current requirement at 3.6V is .576A 
The current requirement at 7.2V is 1.152A 

We'll likely be using a lilypad Arduino (sewable), but we do have a 3.3V and a 5V Trinket available to us if required. 

----- Original Message -----

| From: "Michael Grant" <michael at krazatchu.ca>
| Cc: "Justin Slootsky" <justin at slootsky.org>, "lab" <lab at artengine.ca>
| Sent: Monday, November 25, 2013 7:24:05 PM
| Subject: Re: [Lab] Help? I can't wrap my head around transistors / mosfets.

| 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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