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31  Using Arduino / Motors, Mechanics, and Power / Re: Mosfet capacitance? on: February 15, 2014, 04:40:48 am
The geared motor is the one I've been testing the most with, and it makes an annoying whining sound at the PWM frequency at anything but ultrasonic frequencies.
32  Using Arduino / Motors, Mechanics, and Power / Re: Mosfet capacitance? on: February 15, 2014, 03:13:55 am
I found this interesting presentation which explains mosfets in great detail while being fairly easy to follow:
http://www.fairchildsemi.com/Assets/zSystem/documents/collateral/onlineSeminars/Understanding-Modern-Power-MOSFETs-PPT.pdf

On page 28, slide 56... I'm not sure how to use the equation there.

Pd = ((Vin * Iout) / 2) * Fsw * ???

That first bit seems easy enough.

(12v * 1A) / 2) * 46,875hz = 281250

But then the rest of the stuff on the right... 

I think Idriver should be 9mA, so I input .009 for that.
And Qgs2 isn't on my datasheet, but Qgs is.  I guess I can substitute that though it will be a bit higher.  So that's 0.3nC. 
And Qgd is 2.4nC...

(I'm getting all these values from here: http://www.irf.com/product-info/datasheets/data/irlml6344pbf.pdf)

But after I add Qgs and Qgd... Should I divide them by 1,000,000,000 to convert to C?  Or leave them as nC?

If I don't divide them I get (2.7 / .009) = 300.  Double it, and the final result for Pd is 168750000 so that must be nW?  If I do divide them I get 0.0000003.  Then if I double that, and plug it in, I get Pd = 0.16875.  So that must be watts?

It seems low-ish... but maybe it's right?  12v * 1A is only 12W to begin with.
33  Using Arduino / Motors, Mechanics, and Power / Re: Mosfet capacitance? on: February 14, 2014, 07:49:03 pm
Quote
Thanks for all the links, but what is your motor ?

I would like to use this board for a number of different projects, so I don't have a specific motor, but one I was testing the h-bridge driver I mentioned with was this one:
http://www.aliexpress.com/store/product/Zgb37-dc-gear-motor-zytd520-motor-eccentric-shaft-37mm-24v200r-min/202210_852537734.html

That one requires around 1A.

I would also like to drive this vibration motor:
https://catalog.precisionmicrodrives.com/order-parts/product/324-401-24mm-vibration-motor-13mm-type

That requires around 150mA.


Quote
Why would you want 22kHz or 44kHz for the motor ? The default Arduino PWM frequency is around 500Hz.
I doubt if the motor coils act different above 5kHz.

If you use a low PWM frequency then the motor will make noise.  You will hear the coils resonating at the chosen frequency.  22khz and 44khz are ultrasonic... beyond the range of frequencies you can hear, so the motor will run quietly.


Quote
You also need a 100 ohm resistor to the gate to protect the ATmega chip for current peaks. That will slow down the switching of the mosfet and it gets hotter.

Yes, that's why I said the gate would be driven with 9mA.  I'd have a resistor on there to limit the current to that.


Quote
Can you use transistors?

A mosfet is a transistor.  Are you saying you think a BJT would work better?

I don't know if they have any special gate driver circuitry in them, but I'm using logic level mosfets.   I haven't used BJTs, and I don't know all their idiosyncrasies.  Using logic level mosfets makes things simple.  Well, simpler.
34  Using Arduino / Motors, Mechanics, and Power / Re: Mosfet capacitance? on: February 14, 2014, 07:06:30 pm
Quote
If you want to PWM fast, you'll need gate currents of the order 100mA+

I'm using a Teensy 3.1 for this project, so I'm limited to 9mA @ 3.3v.  If that's insufficient to switch my mosfet at the PWM speeds I desire I may as well go for a full h-bridge driver, it would be easier to implement and take up just as much space on the PCB.  I already have one in mind which handles 44khz PWM just fine.

That said, here is one of the FETs I was looking at:
http://www.irf.com/product-info/datasheets/data/irfml8244pbf.pdf

This one supposedly can handle a lot of current.  And I notice that it requires a significantly higher gate charge than a lot of other FETs here:
http://www.digikey.com/short/z8qnr

So, it should switch more slowly than those, right?

But I've done the calculations, and while I don't know how the lower voltage affects the charge rating my calculations seem to indicate this one would switch so fast that 46khz should not be a problem. 

The rating for this FET is 6.8nC @ 4.5v.  That's 0.0000000068C.  My current is 9mA which is 0.009A.

0.0000000068C / 0.009A = 0.00000075 s

And 1s / 46,875Hz = 0.000021 s

0.000021 s / 0.00000075 s = 28.4

So this FET ought to switch 24x faster than my PWM speed.  If I were supplying 9mA @ 4.5v

Of course we're not switching a 4.5v we're switching at 3.3v.  3.3v / 4.5v =  0.73, which multiplied by 28.4 gives 20.8. So if my hunch is correct, even at 3.3v it will still switch 20x faster than my PWM speed. 

And my intuition tells me that if it's switching 20x faster than it needs to then it will only be in the bad state 1/20th of the time., and that this should not be so long to cause it to pop.  At least if my current is below its max current rating by at least 80%.

Am I in the ballpark here or am I way off?

I can reduce the PWM speed to 22khz if I have to.  I haven't noticed the motor making noise at either 22khz or 44khz. 
35  Using Arduino / Motors, Mechanics, and Power / Mosfet capacitance? on: February 14, 2014, 12:10:39 pm
I'm trying to select a suitable mosfet for a variety of tasks.  I want one which can handle as much current as possible, but I want to keep my board small too.  Typically I'd just choose one with a high continuous current rating and calculate the power dissipation, but one of the things I want to use this for is to PWM a small 12v motor at up to 46,875Hz.  The motor would draw around 150mA at most I think.

I've been trying to figure out how to calculate how much heat is going to be generated switching a logic level mosfet with 3.3v and around 9mA of current. 

I've been looking at several on Digikey:
http://www.digikey.com/short/z8qnr

And when sorting by price, I found this one stood out with an awfully high continuous current rating:
http://www.irf.com/product-info/datasheets/data/irfml8244pbf.pdf

Seems to good to be true, so there must be some catch but I have yet to spot it.

Anyway, I've hit a bit of a road block.  I believe the gate charge time will allow me to calculate how  much time the mosfet is going to spend not fully on or off, but the rating is in "nC".  Nano charge?  What the heck is that?  Is that just another word for farads?  I've looked everywhere and it does seem to be nano-charge but I can't find any explanation of what this charge unit is.
36  Using Arduino / Motors, Mechanics, and Power / How will a motor's speed differ if using braking or not with PWM? on: February 12, 2014, 01:00:30 am
I've seen mention of braking making PWM on motors more linear, (I believe it was mentioned on Pololu.com in this motor controller section) but I have been unable to find any further information or graphs of how braking is supposed to affect motor speed.

I did notice that with a motor controller, disabling braking did seem to affect the speed at which the motor turned.  I did the test a couple weeks ago but I think the result was the motor spun a lot faster than normal at that PWM speed.

Anyway, is there any sort of graph out there showing how the speed differs?  I would understand if when ramping the speed down it wasn't linear, since the motor would have inertia, but when speeding it up or running at a steady speed, I'd think it ought to run the same speed regardless of whether there's braking or not. 

Though I suppose if one did brake when not accelerating that would make the motor run at a slower speed than otherwise....  But I'm having trouble deciding with which method a 50% duty cycle would result in a speed half of the maximum.   Would it be with braking?  Or without?  Seems like it ought to be without.  With braking maybe it would be 25% of max speed instead of the expected 50%?

Anyway, I just need to run a motor at 8 different speeds in one direction, which is why I ask, it seems like  using a motor controller for that is overkill, but if I remember with my test with braking off I was not seeming to get different speeds.  I guess I'm gonna have to run some more tests soon.  But if there's some info out there, maybe an equation to use to linearize the speed if it's not linear, that would be useful.
37  General Category / General Discussion / Re: For sale: Two Gravitech MP3 modules w/ Arduino Nanos - $75 on: January 09, 2014, 05:02:48 am
Thanks.   And in that case I'll just edit the post . smiley
38  General Category / General Discussion / Re: For sale: Two Gravitech MP3 modules w/ Arduino Nanos - $75 on: January 09, 2014, 04:47:42 am
By clicking the red X that says Remove?
39  General Category / General Discussion / For sale: Two Gravitech MP3 modules w/ Arduino Nanos - $75 on: January 09, 2014, 04:23:58 am
Hey guys,

A project I've been working on took a new turn and I've got some spare parts to sell.

The two mp3 modules, and one of the nanos are brand new and have never been used. (Though they were removed from their static pouches.)  All three are from Gravitech, and the Nano is a 3.1.  The other Nano is a Funduino brand 3.0.

The mp3 modules were originally $40 ea.  The Nano 3.1 was $35, and the 3.0 was something like $12.

I'll sell the whole lot for $75, with free priority mail shipping in the US.  That's less than $30 per MP3 module and $8 for each of the nanos.  You save 30%!

Email me at scswift@gmail.com if interested.  I'll remove this post once they're sold.
40  Using Arduino / Audio / Re: Voltage divider on AREF to read mic? on: December 27, 2013, 05:04:17 am
I found the answer to my question:
http://forum.arduino.cc/index.php/topic,20090.0.html

The minimum aref is around 1v.  So that probably wouldn't be low enough to read a source of 10mv or less with any accuracy. smiley-sad
41  Using Arduino / Audio / Voltage divider on AREF to read mic? on: December 27, 2013, 04:25:54 am
Let's say I had a board running on 3.3v and I wanted to sample analog voltages from an electret or dynamic mic.

From what I've read they output at most a few tens of mv.  Obviously if I just attached one of these to an analog pin I would not have much success reading them.  And typically one might use a mic preamp to allow them to be read.   

But let's say that I don't have or can't use one of those.

Is it possible to stick a voltage divider on AREF to bring Vcc down from 3.3v to say, 50mv, and then get  8 or 12 bits of analog precision reading that mic directly?

(I guess the mic would output +- mv, so that would have to be offset to stay positive too.)

42  Using Arduino / General Electronics / Re: How to calculate voltage drop across a resistor? on: December 19, 2013, 04:55:57 pm
But what would a good value for this resistor be?    My intuition tells me that in doing this I will be creating a voltage divider that reduces the input voltage.  As the input voltage determines the volume of the output, I probably don't want to reduce it too much.  (though if the amplifier has some gain it can overcome this)  I have no idea what the resistance on the positive side is though.  So I'm not sure how to calculate this voltage drop.   
43  Using Arduino / General Electronics / Re: How to calculate voltage drop across a resistor? on: December 19, 2013, 02:10:52 pm
Btw in case you didn't read my other post where I was discussing this same project, I ended up frying the mp3 module, so I cannot perform any more tests at the moment with that, though I do have some other sound boards with leds that I can try this technique with.

I don't know how I fired the mp3 module.  It seems like I put 12v into it somehow but I can't for the life of me figure out how.  Fired the Arduino Micro I was using too.  Using a spare Nano I had now.
44  Using Arduino / General Electronics / Re: How to calculate voltage drop across a resistor? on: December 19, 2013, 02:07:08 pm
To reduce the noise input you need to reduce the input impedance of the amplifier. A simple way is to put a resistor to ground on the input at the amplifier right at the amplifier, that is as cloce to the input pin as you can.
It worked for me with the very bad multiplexing noise from this project.
http://youtu.be/pL0pMAPkkVw

When you say the input to the amplifier, do you mean on the line in?  And which side should this resistor be on?  One wire on the line in seems to go to ground already, the other seems to go to a capacitor based on the readings I'm getting on my multimeter.

Also how does adding a resistor to ground reduce the input impedance?

And what size resistor would you suggest?  I assume not one too small.  1K?  10K?
45  Using Arduino / General Electronics / Re: Calculating inductance on: December 19, 2013, 01:07:07 pm
Oh great I fried my Arduino Micro too at the same time as I fried the MP3 module.

This doesn't even make sense.! 

Let's say I had accidentally disconnected the ground to the amp.  I don't think I did, at least not at the time I fried it.  But let's say I had. 

In that case the path to ground would have been through the audio line through the mp3 player ground that is attached to the Arduino ground pin.  But those are just traces.  No current should have flowed through either chip as a result of that.

At the same time, I have a 0.1uf capacitor in series on the + audio line.  So if I managed to put 12v DC there, it should have been stopped by the cap.

I wish I knew what the hell I did to cause this.  But I guess the only thing I can do right now is grab one of my other microcontrollers with built in sound and test that with the amp and see if the same thing happens.  I hate to kill a $50 board though. :/
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