Simple Circuit to Amplify Voltage to Power Heating Pad

Projects General Guidance
1

Hi,

I am trying to create a circuit to power a heating pad that will be used in a small robot as a "tail" that leaves a heat trail behind it as it moves. I am using Arduino Uno to run the robot.

I have been testing a couple different heaters, but the best one for my purpose so far seems to be a 9.4 ohm Kapton resistor from Minco. The issue that I am facing is that I am not able to get the heater to a high enough temperature. I am currently powering it using a TIP120 Darlington transistor and a 5V output from the arduino in the manner shown in the (crudely drawn) circuit diagram I have attached. The resistor connected to the base of the transistor is connected to pin 11 of the arduino. I am using the following code to operate the heater:

int heatPin = 11;

void setup() {

  pinMode(heatPin, OUTPUT);

}

void loop() {

  analogWrite(heatPin, 255);

}

I am using analogWrite so that later I could use this to control the temperature.

As the circuit is currently set up, I am only achieving about 2.7 V across the heater and it is only heating to 40C. I want to get the heater hotter, and the way to do this seems to me to be to amplify the voltage across it. I have seen many suggested ways of doing this but I want to know what the simplest way of achieving this would be, as the resulting circuit will need to fit onto my small robot. My best guess of how this could work would be using a small op-amp (like this one: SparkFun OpAmp Breakout - LMV358 - BOB-09816 - SparkFun Electronics), but is that overkill? Ideally I would want the voltage to be at least 5V, but maybe as large as 10V.

It is also a possibility that amplifying the voltage may not be the only way to improve the heater's function. Any input on how to improve my circuit, and specifically if an op-amp would be a good solution, would be greatly appreciated!

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2

How are you powering the system? Heaters tend to need a lot of power.

If you are leaving a heat trail you should probably insulate the top surface of your resistor to minimize heat loss to the air.

If only 2.7V of your 5V supply is dropping across your heater then the rest is dropping across your transistor. Switch to a logic-level MOSFET which generally has a much lower on-resistance and will drop more of the voltage across your heater.

3

A 9.4ohm load should NOT be powered from Arduino's 5volt rail.

What is the power rating of this resistor.
How do you power your robot.
Leo..

4

johnwasser:
How are you powering the system? Heaters tend to need a lot of power.

I am currently testing the circuit by powering it from a desktop computer, but I want to use batteries to power the heater on the actual robot. Thanks for the tips about insulation and using a MOSFET!

Wawa:
A 9.4ohm load should NOT be powered from Arduino's 5volt rail.

Why is this? (I am very new to all this!) How should it be powered?

I do not know the power rating of the heater because the specs are not available online (it's a heater from an old project my professor did), but comparing to similar heaters available from Minco now I think it is probably about 50 Watts.

5

Hi..

To provide 50 watts into a 9.4 ohm resistor needs 21.6 volts at 2.3 amps ( power = V^2 / R or = I^ 2 x R)
A decent power FET will switch this easily - but you'll need a voltage translation circuit to supply 8 - 10 volts to the gate of the FET for a clean switch from the arduino's 5v output, and , of course, if you're going to run this for any length of time, a pretty juicy battery...

regards

Allan.

6

allanhurst:
A decent power FET will switch this easily - but you'll need a voltage translation circuit to supply 8 - 10 volts to the gate of the FET for a clean switch from the arduino's 5v output

Not needed if a logic level fet is used.

21.6volt 50watt is a lot.
A 24volt truck battery should do it.
Leo..

7

It is down to the current your heater takes. A USB connector can only supply half an amp. At 5V a 9.4R resistor will take just over half an amp. In the long term this will not do your USB connector any good. The heater is taking 2.5W from the USB which is why it is not getting very hot.

To get it hotter you need more voltage to drive more current through it. Look up ohms law.

8

Assuming Lithium Polymer (LiPo) batteries to minimize weight and size, you'll need about six cells in series to get the voltage (nominal 3.7V per cell). You can then use a 5V buck converter to power the Arduino.

Figure $100 for a 5 Ah (5000 mAh) battery. Since you are drawing 2.3A, each battery will last about 2 hours.