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Topic: Voltage control of external circuit? (Read 1 time) previous topic - next topic

zyzly

Hello all! :D

I am trying to build a circuit whereby the arduino board controls the voltage that is coming from a battery to a load, cant use PWM to imitate a change of voltage and the 5v output from the arduino is too low, i am operating in a range of about 5-20 volts.

How would I go about doing this?

raschemmel

Quote
I am trying to build a circuit whereby the arduino board controls the voltage that is coming from a battery to a load, cant use PWM to imitate a change of voltage and the 5v output from the arduino is too low, i am operating in a range of about 5-20 volts. 
FAULT !
INSUFFICIENT DATA !

STATE OBJECTIVE (Project Objective, ie: what are you trying to do, NOT what you are telling us but what
EXACTLY is your reason for your question ?

Quote
cant use PWM to imitate a change of voltage
What does THIS mean ? What are you trying to imitate ? What change in voltage ?
Why do you think you can't use PWM ? Do you know what an RC LP FILTER is ?

Quote
5v output from the arduino is too low, i am operating in a range of about 5-20 volts.
Operating WHAT ?
What does this mean ? Voltage of/for WHAT ?

Start over.

Project Objective:
Hi, my name Joe Blow. I am trying to control a [blank] with an arduino (fill in blank).
I want to [blank, blank] (explain what you are trying to do.

zyzly

Apologies for the lack of data! I am building a miniture heat gun with adjustable temperature output (think of a soldering gun with the ability to change the temperature), so far I've planned to do this by using the arduino to control the voltage coming from an old laptop adaptor which I am using to power the circuit. I was thinking of using a potential divider circuit with a digital pot but this will waste a lot of energy?

Thank you! and sorry again for the bad first post

raschemmel

It sounds doable. Can you draw a schematic ? (I could but I want you to do it because you're the one asking the question). I assume you mean 19Vdc ?

Do you know what a MOSFET is ?

avr_fred

Still lacking parameters.

You've stated the voltage you're looking to control is 19 volts. Okay, fine. But what is the current? Milliamperes? Amps? Kilo amps?  :smiley-lol:

The typical laptop supplies are anywhere from 65 to 100 watts. At 19 volts, that's 3.4 to 5.2 amps. Is that enough for your heater? Do you know it's wattage? If so, at what rated voltage?

Since most digital pots are good for a few milliamps, I think you need to take that of the list of possible solutions.

Now a question for you. If you said your heater is 19 watts at 19 volts, it will consume 1 amp. If I apply 50% voltage, 9.5 volts, I'll have a heater that dissipates 9.5 watts, right? Yes, of course it will.

Now, what would happen if I applied 19 volts but I turned the voltage on for 1 second and off for 1 second in a continuous fashion? What would the *effective*  wattage be with that 50% duty cycle pulse applied to my heater?

If you're not the OP, please allow him to respond, please ?

zyzly

#5
Dec 12, 2014, 05:44 am Last Edit: Dec 12, 2014, 05:46 am by zyzly
9.5 Watts average?

The reason I said i couldent use PWM was that the voltage the arduino supplies is lower than the power source and I did not want to fry it.

I was experimenting with transistors acting as a switch to rapidly switch the circuit on and off, but that wasent with PWM. If i hook up a transistor to the PWM output to control the current through the circuit with the load, would the shape of the voltage be similar to that of the pwm signal from the arduino?

I have a diagram but its hand drawn and not with me, ill post a picture of it in the morning :D

Thanks for your help guys!

avr_fred

Yep, you nailed it. The power (watts) directly relate to the duty cycle of the pwm waveform. 25% on time = 25% power from your heater. 50% on time = 50% power and so on, it is linear.

You connect the output of the Arduino pwm output to an n-channel logic level MOSFET, like this one. check the product listing for a tutorial on how to connect your Arduino and load to the MOSFET.

So, once you know your heater load, calculate the amps and then the dissipation in the MOSFET which would be 0.035 * i^2. The .035 is the on resistance of the MOSFET, i is your heater current squared. More than a watt or so will most likely require a heat sink. The usage time will is a factor as well, the shorter the on time, the lower the temperature rise.

raschemmel

Or you could just use Google to research your question before posting.

PWM transistor tutorial

skypickle

...n-channel logic level MOSFET, like this one. c...
I am amazed that I can pump 60v and 30 amps through that little MOSFET. That's more power than a hair dryer. How does that little thing not burst into flames or melt?

raschemmel

Quote
I am amazed that I can pump 60v and 30 amps through that little MOSFET. That's more power than a hair dryer. How does that little thing not burst into flames or melt?
Did you actually try it ?

allanhurst

You have to take mosfet current ratings with a pinch of salt. That's a pulse rating. Look carefully at the datasheet

But they're still pretty amazing.

Allan

Southpark

Yep..... and also check out the power dissipation rating 'Pd'.

raschemmel

Quote
I am amazed that I can pump 60v and 30 amps through that little MOSFET. That's more power than a hair dryer. How does that little thing not burst into flames or melt? 
Oh it will if you don't use a heatsink.

tinman13kup

Oh it will if you don't use a heatsink.
With liquid nitrogen coursing through it.
Tom
It's not a hobby if you're not having fun doing it. Step back and breathe

raschemmel

Quote
With liquid nitrogen coursing through it.
Yeah, I forgot to mention that. Good point.

@OP,
Did you or did you not try running 30A at 60V through that mosfet ? (I suspect not).

FYI, I was working in a lab one day and a 2N3906 transistor (TO-92 CASE) blew up and the shrapnel banked off the two furthest walls and then skidded to a stop on the floor near where I was working.
The piece sounded like a BB when it hit the wall and there was a cone shaped crater in the face of the transistor.

This is why safety glasses are recommended when working with exposed electronics.
If someone had been sitting right in front of that PCB when it happened and the piece hit their eye, I suspect it would have been embedded in their eyeball.

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