Connecting a MOSFET to a DC Power Supply

Thread Starter

brown1

Joined Nov 13, 2012
21
Hi Bordodynov,

I appreciate your effort. I have been trying to buy those particular Zener diodes, but am having no luck. There is a 13 week wait and I have to buy thousands... :O :)

Would this be an ok alternative? http://www.conrad.com/ce/en/product...er-Diode-DO-35/?ref=detview1&rt=detview1&rb=1
The breakdown voltage is the same, ie 15V.
The impedence is 11 ohms at 5 mA, rather than 10.
And reverse leakage current is a little less at 36 mA , instead of 50mA.
Is it close enough?

Cheers
Alex
 

Bordodynov

Joined May 20, 2015
2,906
You can use any zener with an operating voltage of 10-15V. Chosen your transistor is very cool. I chose transistor so that the power transistor is less than 1 Watt. I also chose a very low cost.
 

Thread Starter

brown1

Joined Nov 13, 2012
21
Low cost is always good... Since I'm having to pay for most things myself...

Thank you both for your help and advice... I am half way through my labview programme and it should be ready for when the other stuff arrives...

I will post updates soon.
Cheers
Alex
 

Bordodynov

Joined May 20, 2015
2,906
These are my circuits are slow. I advise to use a modulation frequency of 12 Hz. Then there is no sound effects. For fast modulation (tens of kilohertz) is necessary complexity of the circuit. But I think you do not need.
 

Thread Starter

brown1

Joined Nov 13, 2012
21
Hello All,

Sorry for the long delay... The parts only arrived very recently... I was hoping to finish today, but not quite... This is as far as I got....
P1040059.JPG

I had to build the box for it too... Funny how long drilling a few holes can take... I pinched the metal box from the scrap, so had abit of work to make it right...
I will finish it over the weekend...

Cheers
Alex
 

Thread Starter

brown1

Joined Nov 13, 2012
21
Well, I finally wired it up and I cannot get it to work properly.

The pin from the arduino produces a nice clean top-hat function, but by the time it gets to the isolator is a very strange shape. Sort of triangular.

I will post some pictures of the oscilloscope trace tmrw...

Cheers
Alex
 

Bordodynov

Joined May 20, 2015
2,906
The maximum control frequency my circuit 1 kHz. I will not knowingly use the recommended frequency of 12 Hz. To use the higher frequencies is necessary to use additional elements. At least this diode and PNP transistor.
If you want to use a higher frequency, then say so. Then I will give a more complex scheme.
 

Thread Starter

brown1

Joined Nov 13, 2012
21
Morning Borodynov,

Thanks for your reply. The Arduino PWM pins that I am using have either a frequency of 490hz or 980hz. https://www.arduino.cc/en/Reference/AnalogWrite . Am I pushing the limits of your circuit? I don'T really have an alternative, but to use the Arudiuno to do the switching.

I guess I could write some labview software to switch the pins fully on and off at a much slower rate. However, I am concerned about running in to timing issues.

I am going to look at the circut this morning and see if I have not made a mistake.

Thank you once again.
Alex
 

Bordodynov

Joined May 20, 2015
2,906
You have used a transistor that you've mentioned? Or the one that I recommend. The maximum frequency is determined by the capacitance of the transistor and the charge-discharge currents. I do not use in their projects microcontrollers and therefore does not give Council for their use.
 

Thread Starter

brown1

Joined Nov 13, 2012
21
Hi Bordodynov,

Thank you for your new circuit design. I will order the components tmrw.

A quick update...
My soldering was not perfect and resulted in some short circuits.
Also, I had to change the pulldown resistors, because I am using 12 V power source rather than 24 V, which I will do in the future. That was my own stupid fault for not thinking.

So now it works for a single power supply. The top-hat function going to the opto-coupler is the same as the pulse coming from the PWM pin, which it wasn't before. Changing the PWM value to the Arduino pin changes the Average current going to the load. So i am pleased.

I will now test it for 3 independent power supplies. Then PID control them with labview.
Thank you once again.
Alex
 

Thread Starter

brown1

Joined Nov 13, 2012
21
I double checked the circuit and the single power supply that was working, was wired incorrectly. I had wired the PWM signal from Arduino to the source of the transistor. I have now switched it to the gate and now nothing works.

I have slowed the pulse down to a frequency of 1 Hz. That was not as easy as it should be.

So I upload the the input to the transistor gate, which is 0-5V 1 Hz
VoltOutputFromArdiuno1Hz.jpg

Then the voltage:
VoltAtDrainWrtGround.jpg

Point on the circuit where voltage is measure Vsd
Circuit1.png .
 

Thread Starter

brown1

Joined Nov 13, 2012
21
So just to be clear...

We have the source of the FET connected to the PWM pin of arduino and the gate permanently grounded. I must say, that I do not understand this circuit anymore... :( :)

Please excuse the editing, it was quickly done in Paint

start.png

Thanks
Alex
 

Thread Starter

brown1

Joined Nov 13, 2012
21
I think, I have not explained myself very well. The oscilloscope traces on my post #35 are from the circuit with the PWM signal connected to the gate and the source connected to ground, as in the original design.

Now, I understand, you suggest that I have made a mistake with my soldering. This is possible, but I simply cannot find it. I have looked at it for too long... :)

I will give it to someone else to check for me. I hope they can see it with a fresh pair of eyes. :)

Cheers
Alex
 

Bordodynov

Joined May 20, 2015
2,906
The use of bipolar transistors to connect optocoupler (alternatives). Transistor 2N7000 easy to disable. I'm in their designs using FDV301N transistor with integrated protective zener diode.
Transistors any low power.

12Vmod.png
 
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