Hi All,
I have recently brought a heating pad from amazon (link) and now I'm looking at getting a MOSFET to control the voltage from 5 to 12V.
I had a look for MOSFET that I could possible use. I see this on amazon and was wondering would this be okay to control the Heating Pad, as I don't have much knowledge of MOSFET, so I'm not fully sure about this.
Many thanks
The key for using a mosfet with an Arduino is the use the type known as Logic Level Mosfets, typically denoted by the L in there product code such as the one you show irL88.
That device will handle up to 30 V at 60Amps so should be ok for your heating pad.
You need to complete the circuit like the pic shows, where typically Rin= 220R and Rgs = 10K
Seems rather expensive at £6 but buying one off, think most of that is carriage.
Just to confirm about the connections.
+Vin = ardunio pin
+Vdd = power supply (5V - 12V)
Lamp = Heating Pad
Also, is Diode required as I see some examples online with Diode. Just asking if it's necessary to have one.
thank you
The diode would be used when there is an inductive load (in other words... a coil of wire where some electric field energy can be stored up). It is called a kickback or back emf diode since the inductive load (like a relay) will kick back the voltage held in to coil upon the removal of voltage. The coil will discharge the voltage as fast as it can in an inverse polarity to the way it was energized, normally creating a very high voltage spike.
This behavior can exceed the max parameter rating of some transistors making them decide to stop working.
Most heating pads act like resistive loads... so a diode here would be very optional.
Would A diode like this be used for the heating pad.
Try looking at the datasheet
I have recently brought a heating pad from amazon (link) and now I'm looking at getting a MOSFET to control the voltage from 5 to 12V.
That's not going to happen, at least not the way you worded it.
The EFFECTIVE voltage to the load (pad) can be controlled , as already described with a low side switch.
You are not changing the voltage used. It will always be 12V.
The pad replaces the "Lamp" in the schematic posted.
The mosfet does need a protective diode as already mentioned. (cathode to +Vcc(pad), anode to mosfet DRAIN.)
What you are changing is the EFFECTIVE voltage by controlling the duty cycle of the mosfet .
Have you measured the load current with a meter yet (at the voltage you plan to use for the pad) ?
Do you have a meter ?
Do you know how to do that ?
We have the datasheet but it is useless with the heating pad current or some spec that allows us to calculate that. The resistance would help but it will not give us an accurate current value but is better than nothing. Once we know the load current that we can use the datasheet to calculate power dissipation etc
The mosfet does need a protective diode as already mentioned. (cathode to +Vcc(pad), anode to mosfet DRAIN.)
Actually, I fail to see how the heater mentioned would be inductive, but like I said... adding the diode won't cause trouble... it just won't do anything (much).
You might be right.
Ok. So the MOSFET will change the duty cycle to allow different voltage to reach the heating pad. Got that
raschemmel:
Have you measured the load current with a meter yet (at the voltage you plan to use for the pad) ?Do you have a meter ?
Do you know how to do that ?We have the datasheet but it is useless with the heating pad current or some spec that allows us to calculate that. The resistance would help but it will not give us an accurate current value but is better than nothing. Once we know the load current that we can use the datasheet to calculate power dissipation etc
The heating pad will be powered by a 9V battery (image) so the range will be between 5V to 9V. I haven't measured the load current since I don't have a meter. I'm currently asking on amazon for details relating to the resistance so current can be calculate using V/R = I.
For my own interest to measure the current it would basically involve connecting a ammeter in series with a 9V battery and heating pad. Right?
Many thanks for the help in advance
It's not that simple. You are not going to measure that voltage but the load will behave as if it were that voltage. It has to do with duty cycle and power supply voltage.
That pad is about 9.9 watts, a 9V transistor battery will last about 1 minute :o
This would work and cheaper:
That happens to be the same mosfet for which I linked the datasheet already.
Satvir019:
MOSFET will change the duty cycle to allow different voltage to reach the heating pad. Got that
For my own interest to measure the current it would basically involve connecting a ammeter in series with a 9V battery and heating pad. Right?
Not sure if this will help but a pwm control signal on the gate of a MOSFET Encodes an analog value in the digital pwm signal. The analog value is the average of the switching power across the load. The duty cycle controls this average from 0 to 255 ( no power or 255 "steps"where a "step" = an incremental average voltage. If you put a large enough electrolytic capacitor across the load you can read the filtered voltage as an analog voltage.
@ Outsider,
The jury is still out 
Just for the record I read the number off the photo posted by the OP at the beginning of the thread, but you made no mention of that and somehow arrived arrived at same part number . Coincidence ?
No, the PWM at the Gate turns it on or off. the MOSFET is not made to operate in a linear manner.
It and the load are thereby, on or off.
That's all there is to it.
@RunawayPancake,
Duh.
Ok, so I forgot to mention you need a resistor in series between the load Vcc and the cap to measure the analog voltage equivalent across the load. Without the RC LPF you would only see on or off , but you already knew that's what I meant anyway because you already know how to convert pwn to analog , don't you ? What you should have said was:
"Ahem, aren't you forgetting the resistor ?"
The OP obviously doesn't know this and your are just going to confuse him by ignoring what I think you know I meant.
@OP ,
Connect a 4.7k ohm resistor from 9V to the + terminal of a 4.7uF cap.
Connect the - terminal of the cap to the drain of the MOSFET.
If you then put a meter across the cap you can plot the analog voltage with respect to the duty cycle! where duty cycle is the X axis and voltage the Y axis.
If you put a Diff Probe across the pad and display it on ch-1 of a scope while viewing the + terminal of the cap on ch-2, you will see the PWM switching on ch-1 and the filtered analog voltage on ch-2.
If you add a voltage divider you can read that voltage on an analog input and print it to the serial port with the corresponding PWM value (0 to 255) and the plot will show a linear correlation between the pwm value and the filtered analog value.
It will look like a ramp.
outsider:
That pad is about 9.9 watts, a 9V transistor battery will last about 1 minute :o
This is similar to heating pad that was available on amazon. Still waiting for an answer for the resistance of the pad. So far nothing.
Also, the link you provided for the heating pad has a link to the MOSFET that is required. If I don't hear anything from amazon I buy this one and see how it goes.
raschemmel:
Duh.
Connect a 4.7k ohm resistor from 12V to the + terminal of a 4.7uF cap.
Connect the - terminal of the cap to the drain of the MOSFET.
If you then put a meter across the cap you can plot the analog voltage with respect to the duty cycle! where duty cycle is the X axis and voltage the Y axis.
If you put a Diff Probe across the pad and display it on ch-1 of a scope while viewing the + terminal of the cap on ch-2, you will see the PWM switching on ch-1 and the filtered analog voltage on ch-2.If you add a voltage divider you can read that voltage on an analog input and print it to the serial port with the corresponding PWM value (0 to 255) and the plot will show a linear correlation between the pwm value and the filtered analog value.
It will look like a ramp.
I just want to clarify this as I'm a bit confused. I will be using a 9V supply as mentioned in my previous post. Also I don't have 4.7K ohm resistor, I have 1K, 10K and 220 ohm resistors, so can I use one of this (10k?) instead. Furthermore, I have taken all the information you gave and turn it into a diagram. Please can you check the diagram to make sure I did all the connections correctly.
I like to ask about the duty cycle. The voltmeter will measure the voltage, but for the x-axis what will this value be? Where will this come from?
Furthermore, you said to add a voltage divider I take you are referring to the one for the gate terminal of the MOSFET. For this when the value 0-255 is applied, am I reading the gate voltage that's being applied to the MOSFET?
Many thanks
NO.
Your circuit schematic has numerous errors. see attached.
(the 10 k resistor and cap were in the wrong place)
For clarification, previous reply was addressed to Runaway Pancake.
Given that someone else posted the power rating of your pad (which you failed to include) as 9.9W, I have taken this rating to be correct and calculated accordingly . If you have information to contrary please advise.
Given P = 9.9W,
I must be P/V = 9.9W/9V = 1.1 A
Given I = 1.1 A,
R must be V/I = 8.18 ohms
Your 9V power supply must be rated for at least 1.5A
You have provided NO information.
You have NOT provided ANY ratings or specs for heating pad.
You have NOT provided ANY part number for MOSFET.
Without the part number of the one you said is recommended, I have temporarily supplied a part number
You have NOT provided ANY details about your 9V power supply (current rating etc)
The ameter goes in series between the 9V power source and the load (the heating pad)
The heating pad goes between the ammeter and the mosfet drain.
I have shown where to connect ammeter "+" and "-" leads.
Current and resistance are calculated from the 9.9W rating posted by someone else since you provided no information.
Voltage divider values should yield 4.7V at 100% duty cycle (pwm value = 255)
I like to ask about the duty cycle. The voltmeter will measure the voltage, but for the x-axis what will this value be?
Not sure if you understood my previous post about plotting voltage with respect to duty cycle.
Obviously this can only be done if you have the RC LPT to convert the pwm (on/off waveform) to analog voltage.
Y Axis (Voltage)
^
|
|
|
-----------------> X Axis (duty cycle)
Duty cycle = 0, Voltage = 0 etc
9V/2 = 4.5V
100% duty cycle /2 = 50% duty cycle
THUS ==> duty cycle = 50% = Voltage = 4.5V
DO THE MATH
9V/255 "steps" = 0.03529411764705882352941176470588 Volts/per step.
Thus Voltage = [duty cycle} * [255 counts] * [0.03529411764705882352941176470588 Volts per count]
SO , 50% Duty cycle * 255 counts = 127.5 counts (must be integer) => 127 counts
127 counts * 0.03529411764705882352941176470588 Volts per count = 4.4823529411764705882352941176471 Volts.
Thus if you read the analog input and multiply the analog counts by 4.7V/1023 (0.0045943304007820136852394916911 Volts) you will get the voltage.
so 1023 /2 = 511.5 (must be integer) => 511
511 * 0.0045943304007820136852394916911 = 2.3477028347996089931573802541544 (half of 4.7V)
Keep in mind, the above calculations are based on a full scale voltage of 4.7V at 100% duty cycle due to the voltage divider values.
Normally the analog range is 0 to 5V. That doesn't apply for you because your range is 0 to 4.7V.
If you use Clear Terminal instead of the arduino IDE serial monitor, you can capture the serial prints of the duty cycle and analog voltage measured and then after cycling through all 255 counts you can click
"File\Save buffer to file " in the Clear Terminal window and save the serial output , which you can use to plot the values in Excel (analog voltage with respect to duty cycle)
Thank you very for the clarification.
raschemmel:
You have provided NO information.
You have NOT provided ANY ratings or specs for heating pad.
You have NOT provided ANY part number for MOSFET.
Without the part number of the one you said is recommended, I have temporarily supplied a part number
You have NOT provided ANY details about your 9V power supply (current rating etc)
Sorry about not providing any information for the heating pad. There was nothing on the amazon site and when I search online they are of different length. The 9.9W someone stated would be similar to the one I brought as it's similar in size. So I will take this one for the time being. I have just placed an order for a multimeter so I would be able to give you more accurate value, once I get the device.
MOSFET that I stated at the start and that was shown for heating pad was MOSFET IRL8721. So this is the one I will be using. The datasheet for this is here.
Gate Threshold Voltage - Max: 1.35V Min: 2.35V
RDS(on)max :8.7mOhm @VGS = 10V
The values shown in the diagram for R4 = 4.3K and R5 = 4.7k, could you please explain how to calculated them.
Many thanks
