Help with understanding Heat Pad schematic

Other Hardware General Electronics
1

Hello, I'm trying to build a little insulated enclosure with a heating pad to ferment some yogurt at warm temperatures. I found this example circuit diagram to control the heating pad on this website. I've also attached the diagram.

I don't understand the purpose of adding the diode in between the two ends of the heat pad, is it to act like a voltage divider so the pad doesn't draw too much current? Is it necessary for the operation of the circuit? I've seen other examples where they don't have the attached diode.

I also don't really understand the addition of the MOSFET, why can't I just wire the heat pad directly to the Arduino?

Thank you!

1 Like
2

This schematic looks very bad to me.

The pointed heat pad can drawn until 600 mA and I don't think it's good idea to plug it in Arduino 5 V since the regulator can get very hot. I think the pad should be connected directly to a 5 V power supply.

About your questions:

1 - The diode seems to be a flyback but I can't understand the reason for it since the pad is a resistive not a inductive load.

2 - The MOSFET is needed because digital pin can delivery about 40 mA only,

2 Likes
3

If I connect the heating pad to a separate 5V supply and a MOSFET, which parts of the circuit would you recommend attaching to a breadboard?

4

Hi,
this diode is recommended when the MOSFET or transistor load is an inductive load.
It is used to dampen the reverse voltage spike caused by the FCEM (Counter Electromotive Force) it produces when switching off inductive loads.

Inductive loads are motors, relays, coils in general, etc.
I don't know if this PAD is an indicative load or not.

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5

This module can solve the problem.

The best option is a module where the MOSFET has logic level (like IRLZ44N) but it can be hard to find.

6

This is the PAD model.

7

Tks

8

If do you want know how to build the circuit it should be like this:

led circuit
led circuit1001×578 26.2 KB

mosfet
mosfet443×600 163 KB

The positive from PAD is connected directly to 5 V from power supply.

1 Like
9

Thank you! How did you determine the resistance values?

10

Check it:

The 100R will limit the current on gate about 50 mA. Could be a bit higher since Arduino can delivery only 40 mA.

P.S. I think you have marked the wrong post as solution since the answers to your initial questions are on post #2.

1 Like
11

The pull down resistor should be connected directly at the PWM pin, not where it is drawn in post #8.

The gate resistor should be between the PWM pin and the gate of the MOSFET, as it is drawn in post #8.

a7

12

@alto777 this question is explained on link available on post #10.

...Then remember that if the gate resistor is placed to the left of the pulldown resistor, you get a voltage divider circuit that will reduce the voltage to the gate.

If you have chosen a gate resistor that is at least 100 times smaller than the pulldown resistor, then the reduction in voltage is so small that it doesn’t matter. But if they are a bit closer in value, the voltage on your gate will be lower than the pin voltage.

With 10k pull down the voltage over the gate will be 4.95 V.

13

So what? That is an interesting fact and does not change where the pull down resistor shoukd be placed.

Why would you ever throw away anything like that?

The linked article ends up recommending the proper placement. Pull down or, in the case of high side switching, pull up the PWM pin directly. Attach the gate resistor there and run it to the gate.

a7

14

In this case there's no problem.

The recommendation is to a second case.

But if they are a bit closer in value, the voltage on your gate will be lower than the pin voltage.

The solution? Switch places between the two so that the pulldown resistor is connected directly to the output pin.

There's some difference between my view point and your so let's stop here.

15

ahh I see, thanks for the help

16

Should be post #2 :smiley:

17

Why not just do it in a way that is correctly for any case? At least then there is less to remember, OK here, not so OK there…

Ppl come here to get their projects working, and one hopes to learn things.

a7

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