Hello and thanks for taking time to possibly help me.
I need to build a device to switch about 75W of 12 VDC power from an Arduino. I also want the switching to be completely isolated from the Arduino. After some research I came across this posting.
I am planning on turning an electric heater on/off. I would like to keep the option open to PWM it but that may not be necessary (more nice to have). The power source is a 12V battery (so max of ~14.8VDC). The heater is rated ~70W. When in use, the heater could remain on for hours.
Cheaper parts may be available but I am more interested in availability and having something that works than saving money.
If possible, I may want to use PWM on pin 3, with the default frequency of 500Hz.
I adapted a schematic from the above internet link. I have attached a JPG file to this post.
The diodes are probably not needed for a heater but I have decided to include them just so the circuit is also compatible for other things. If I ever get this right, I don’t want to have to start all over again.
The IRL40B209 has Rds On of 1.25mΩ. If I assume power switching of less than 100 Watts and voltages between 10 and 15 Volts, are my calculations correct that I have less than a quarter Watt of heat to dissipate? If that is true, the heat sink I will use anyway isn’t even necessary. (10A * 10A * 1.25mΩ = 0.125W ?)
So, my questions are: Does this circuit look viable for the described switching? Should it be possible to use PWM at 500Hz?
Looks reasonable, and yes you don't need D1 for a resistive load, only for an inductive load like a motor or relay.
I make the current 6A, and power dissipation < 0.1W
With PWM you'll have to checkout the switching speed, as switching losses may easily come to dominate.
If so that can be cured by adding a MOSFET driver which will drive the MOSFET gate several orders of
magnitude faster.
You could drive the MOSFET gate with more like 10V if you want.
MarkT - Do you mean potentially use a MOSFET Driver instead of the optocoupler? When you say drive with 10V, you mean using different resistors on R2/R3 so as not to divide the incoming voltage in half, but rather leaving it closer to the source voltage?
TomGeorge - I am not sure what you are saying. That it Looks good is always well heard, but what were the schematics supposed to be? All I see is a small black & white X.
I adapted a schematic from the above internet link. I have attached a JPG file to this post.
To quote the OPs post #3
That it Looks good is always well heard, but what were the schematics supposed to be? All I see is a small black & white X.
How can you adapt a schematic and not understand it.
Is see X1 and X2, but there is a lot more in the diagram.
Looks good == I can't see any problems with it.
What is your electronics, programming, arduino, hardware experience?
Tom....
Edit, Sorry I now understand (I think), what are you using to view this forum?
Do you literally see x's not an image.
Download the attached file, all I have done is put your schematic up on the forum so those with platforms that have problems with attachements can see what you are talking about.
Yes TomGeorge, we seem to have a communication Problem. Of course I could understand the schematic which I adapted. I literally see a small black and White, single "X" Image. I am using Internet Explorer 11 on this Laptop. That "X" is what I have often encountered while browsing the Internet and the Image is no longer available. I had one year of rather intensive electronic Training followed by three years experience with the Army but it has been over 30 years now. I am nothing more than an occassional electronic Hobbyist. I am a professional programmer in many different languages but never done professional work in C.
I wondered what you meant by "OPs schematic". I wondered if "OP" were something like "Eagle", but I guess "OP" is an Abbreviation for "original poster"? I still Need to work on understanding the Forum Jargon.
Thanks for the comments and especially for coming back and Clearing up the communication Problem.
MarkT - Do you mean potentially use a MOSFET Driver instead of the optocoupler? When you say drive with 10V, you mean using different resistors on R2/R3 so as not to divide the incoming voltage in half, but rather leaving it closer to the source voltage?
As well as the opto coupler, the coupler drives the gate driver, the gate driver drives the MOSFET gate hard.
TomGeorge - I am not sure what you are saying. That it Looks good is always well heard, but what were the schematics supposed to be? All I see is a small black & white X.
Lars81:
What is the purpose of the zener diode in this circuit? If omitted, should R3 remain unchanged?
If I understand correctly, this zener restricts the gate voltage to 18V (even though maximum voltage from the battery is 14.4V), is that so?
I would say originally the circuit was setup for generic use.
That is the supply could be any voltage you had.
Yes the zener keeps the max gate voltage to 18V, which should have the gate switching the MOSFET fully ON.
R3 is to pull the gate to source when there is no gate drive, usually it is 10K.
It may have been dropped to 2K2 so only 6V was applied to the logic level MOFET.
A zener is a pretty standard way to protect a MOSFET gate from overvoltage of whatever origin, be it
supply noise, static, inductive rebound of the source electrode on fast switching. The gate oxide on even
a big beefy power MOSFET is extremely delicate and will be punctured by the briefest of over-voltage events,
given its a layer of silica a few nanometres thick.
Its very easy, for instance, to damage a MOSFET in handling if anti-static precautions aren't employed. Some
devices have built in protection on the die and its a great idea that's not used enough.
