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Topic: generating a -10v...+10v control signal from an arduino (Read 5169 times) previous topic - next topic

P18F4550


Grumpy_Mike

We have already established that a negitave voltage is not needed for this.

oric_dan

Quote
We have already established that a negitave voltage is not needed for this.


Ditto-ed.

However, for reference, the 7662 chip is a nice 8-pin negative-voltage generator, and has
a wider voltage range than the older 7660 chips,

http://www.google.com/webhp?source=search_app#hl=en&tbo=d&sclient=psy-ab&q=7662+converter

Gadget999

i was trying to do something similar a while back

http://arduino.cc/forum/index.php/topic,76821.0.html

Jonnym

#34
Nov 25, 2012, 12:51 am Last Edit: Nov 25, 2012, 12:55 am by Jonnym Reason: 1
does this one look better??


Grumpy_Mike

Quote
does this one look better?

Yes.

Why don't you use proper symbols for transistors?

Jonnym



Jonnym

alright...

next set of questions... resistor sizes.

I am using a  2N3904 transistor, which may not bethe correct thing to use, but maybe it is. i don't know for sure...

the way i understand this, R1 would be the current limiting resistor for Arduino. but i can seem to find what the current needs to be to operate the get the transistor to 'turn on' (yes, i sure there is a very technical term for this :)  )

i bet i would plug the need current and something like 5v's into Ohm's law and i would get something helpful???

here's the data sheet
http://www.fairchildsemi.com/ds/2N/2N3904.pdf

i'm going to guess 10k and 5v for ohm's law values... maybe? 

thanks ya'll



be80be

#39
Nov 25, 2012, 02:28 am Last Edit: Nov 25, 2012, 02:34 am by be80be Reason: 1
If you want to guess a 1k to 2.2k would be better if you want the right one. Read these
http://www.physics.unlv.edu/~bill/PHYS483/transbas.pdf
http://www.rason.org/Projects/transwit/transwit.htm

Jonnym

#40
Nov 25, 2012, 03:13 am Last Edit: Nov 25, 2012, 08:35 pm by Jonnym Reason: 1
great links...

working through the numbers, i am going to assume a 200ma collector current, which would give me something different than an HFE of 30. but i'll use 30, since this looks to be rough numbers anyways.

200/30 = 6.67ma

so there's a bit of "double it for safety margin" to make sure the base is saturated.

6.67 *2 = ~14ma

reading the unlv.edu pdf

R1 =V/Ib = (5V - 0.75V)/0.014A = ~305?

and choose the closest standard... nice.. thanks

_J


dhenry

Quote
assume a 200ma collector current


That's a lot of current for what you are trying to do.

The general design process will be around how fast you need the valve to read / its input impedance. From there, you decide your pwm's period/frequency and you get the desired rc values.

I would use 10k for R2/R3, 4.7uf for the cap and 1k for the base.

Jonnym

fantastic...

so R3 = 10k, this value comes from R/C calculator. same with C1

how about R2, where does this value come from?

thanks
_J

dhenry

I was guessing that your pwm is likely 500hz, and your valve has minimum speed requirement (near dc control signal). So I wanted to achieve a time constant of 50ms. That provides me with 10k/4.7u on the r/c filter.

I used 10k for the pull-up for convenience: the charge up time constant is now 100ms and the discharge time constant is 50ms. Not a big deal.

Jonnym

thanks for the explanation, i am really trying to work through all the steps in this project to remove some of the mystery surrounding this stuff. but it seems, the more i learn, the more mystery presents itself. so yea - i guess it's all just magic :)

_J


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