I wonder if anyone could answer a couple of my questions?



1. Can I PWM a constant current power source with a Mosfet? Or will the circuitry in the power supply trying to keep a constant current cause a problem? If so, should I use a constant voltage source? For your information the load draws about 7 amps when 12 V DC is put across it.

2 Is the floating voltage pulse signal from the arduino to the Mosfet driver ok, or should I connect it to common ground via a pulldown resistor, as I did with the driver output?

3. I am using the USB connector to power the arduino, is this ok? Or do I need to us another 9v ACDC power supply and connect the 0V neutral wire to the common ground?

4. Thanks.

Alex

 

looks nice however; no need for a mosfet driver use a 10Ohm resistor. make sure that you use a logic mosfet on @ TTL high on the gate.
pull dwn depending on driver capabilities from mpu and start up but 47K is always ok.
take rep rate PWM in account (the mosfet has a build in cap at gate. You are ok when freq < 50KHz )
feeding MPU look at spec's from than board.

 

Thanks Pickbuster,

For your prompt and reassuring response. I am a beginner, so I will have to read and digest what you said, so I don't ask something too stupid in reply. Its late here, so I am just off to bed and will have a really good think about it tmrw.
Cheers
Alex

 

You can PWM the current through the load. The CC power supply will merely go to its high voltage limit and await the next opening of the mosfet current path. The load usually limits it's own current because it has resistance. If that is true, making a CC power supply is redundant. If you really need the power supply to limit the current, you already have the right configuration.

Your arduino needs to share a ground connection with your CC power supply.

 

yep.. just like this where input is directly to the arduino PWM pin.. no need for the driver or 9v supply..
replace solenoid in diagram with your load..

maybe you want to explain what the CC supply is for.. non-dimmable LED driver maybe? and you want dimming via the PWM/Mosfet?

 

See

 

Thank you very much for the helpful responses... Especially Bordodynov for taking the time to make a design and put it into a simulator (I think that is what Splice is...)

Bordodynov, what the advantages of your design? I have not seen an optocoupler before or it used to isolate a micro-controller. I think this will be useful for me to know. Right now, I will go with the simplest circuit possible or should I change to yours?... I really am a beginner and fear complexity...

Mcvyer: I am using a constant power source, because it is the only thing I have been given. Infact, it only has two knobs for setting DC voltage and current, although the resistor draws what it will. What you say about powering an LED is essentially the issue, but instead of an LED it is a home made heater. For fun I have attached a picture of my first attempt. I have two more to build.. It can only get better ... I hope

The wire is Kanthal, 18 guage. I found NiCr was not maleable enough to form a stable wire structure. It is electrically isolated from the metal frame (3 rods) by the 3 ceramic washers...



Thank you all once again.
Alex

 

Optocoupler I used for its intended purpose. For galvanic isolation. This improves security. The PC uses a switching power supply. On the USB can be potential. Optocoupler isolates your power unit from the computer. For the heater does not need PWM 50kHz. Enough units hertz and kilohertz.

 

Just a quick update: Once I grounded the arduino to the ov voltage of the power supply it worked. I think I had what you call a floating voltage from the PWM output pin before the grounding. I am very happy to have learnt something and thank everyone for their help.

Bordodynov: Thank you for the explanation. For the time being ie until I start getting weird problems I will not isolate the arduino with the photocoupler. However, I think I may have to for the next thing I wish to do.

ah, but it is not over.... I wish to run 3 of these heaters, each with their seperate power supplies, with a single arduino. My question is: how do I avoid the grounding issue mentioned above? Can I connect the Arduino to 3 seperate grounds? Will I need 3 photocouplers to isolate the arduino from each power source?

Here is a picture of what I would like to do... It is very ugly at the moment and not very clever, but it must just work reliably. After that, I can think about being elegant.

Cheers
Alex

 

I see that you continue to include a block for "MOSFET driver." What is in that block? As already posted, an Arduino will drive a logic level MOSFET directly. And yes, you can drive as many MOSFET gates as you need with only one Arduino output; the MOSFET gate is driven by voltage as opposed to current (although you might want to include a current limiting resistor between the output of the Arduino and the gate of the MOSFET to preclude the possibility of damaging the Arduino if the MOSFET should fail.)

 

The opto-coupler divorces your arduino from the higher power circuits. Translation: You don't have to ground the arduino to the load supply if you use an opto-coupler.

 

oh sorry, I never explained about the mosfet driver... I am using it, because the mosfet requires 10 V at the gate to switch it on and my Arduino can only supply 5V...

Thanks for the extra info about the optocoupler... It looks like I should use that... It will take time for me to buy some... Can I fit it directly into my circuit or must I use Bordodynov's circuit and repeat twice for the other heaters? I am a beginner so the simplest solutiont is best.. for now

Cheers
Alex

 

Use the Russian circuit, one for each load. The 2N7000 will pass 2/3 of an amp with a 4.5 volt gate signal, way more than the 0.01 to 0.02 amps required by the opto.

 

pick up some logic level mosfets while you are at it and drop the driver

 

pick up some logic level mosfets while you are at it and drop the driver

That's what I meant when I said to use the 2N7000. It's good enough for an opto driver at 4 to 5 volts, even if it keeps getting better all the way up to 10 volts Vgs.

 

ok guys...

Although the original circuit worked for a single heater, I am going to take your advice and use Bordodynov's circuit. It will take a few days for me to order and make, so I will get back to you all next week.

In the future, I will want to build my own power supplies to standardise them. At the moment they supply different voltages, but all less than 15V.... I was thinking of making a 20 V DC power supply, will the circuit above still be useable or will I have to change components?

Thanks
Alex

 

Bordodynov's circuit is a textbook example of just what you need. It's just about universal if you buy parts that can survive the voltage and current of the load.

 

You need to remember the breakdown voltage gate-source. Normally, it is equal to 20V. But it may be less. To protect the transistor need to use a Zener diode. I recommend to the zener 15V. I will give a modification of the scheme on Monday.

 

Thanks Everyone....

I have already ordered some of the components and they should arrive by Wednesday. I am really excited...

I am wriing some Labview VI's to incorporate everything together... Hopefully I have that ready for Wednesday and I can let you know how it goes...

Cheers
Alex

 

Transistor http://www.irf.com/product-info/datasheets/data/irfp064npbf.pdf

24V.



48V