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Topic: +/-15v input (Read 1 time) previous topic - next topic

gijs

i am looking into interfacing with modular synth voltages (CV). but am not sure if the way i want to do it is safe. let me know what you think..

the voltages would be between +15V and -15V.




Grumpy_Mike

That circuit will not allow you to see the full input range, it will only chop off any input to the 0 - 5 V range.
Is it important you see the full rang? If so you need some op amps to scale the voltage and add an offset.

gijs

#2
Jan 10, 2011, 04:35 pm Last Edit: Jan 10, 2011, 05:01 pm by gijs Reason: 1
the 20k potentiometer should do the offset, but i could put a potentiometer on the CV input to act as a attenuator.

what i was wondering is if the voltage for instance would be +15V then the voltage on the pin is 5.25V, and if it is -15V it would be -0.25V. is this correct.. and if so is this bad?
the atmega328 datasheet says absolute maximum is +0.5 and -0.5, so its under that.

Grumpy_Mike

#3
Jan 10, 2011, 06:32 pm Last Edit: Jan 10, 2011, 06:34 pm by Grumpy_Mike Reason: 1
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the 20k potentiometer should do the offset

No.
With -15V at one end of a potential divider and +5v at the other with equal resistors you will get -10V on the arduino's input.

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+15V then the voltage on the pin is 5.25V, and if it is -15V it would be -0.25V. is this correct.

I  think not please show your working. With +15V on the input and the pot at ground you have two 10K resistors and so +10V on the input.

In practice the two diodes should protect the input from over voltage but as I said before the resistor network is not going to do enough.

gijs

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With -15V at one end of a potential divider and +5v at the other with equal resistors you will get -10V on the arduino's input.


something like this?:
CV            POT            PIN
7.5            2.5            5
5            2.5            3.75
2.5            2.5            2.5
0            2.5            1.25
-2.5            2.5            0

what i am unsure of, is the BAT46 diode. it says it has a forward voltage drop of 0.25v
(http://nl.mouser.com/ProductDetail/STMicroelectronics/BAT46/?qs=sGAEpiMZZMuIUjt4yeP9c4vDnG6dExKEZObNAFndhYk%3d). does this mean that the voltage on the microcontroler pin will always stay under 5.25V?
if i understand correctly this is what this "lsimaster" is saying over here:
http://www.ccsinfo.com/forum/viewtopic.php?t=41358&postdays=0&postorder=asc&start=0 (second post from the top).

i know you explain things here: http://www.thebox.myzen.co.uk/Tutorial/Protection.html, thanks for that!
but i want to use the protection as a functional thing. so it will be protecting almost all the time if i send in a waveform that is clipping.. but in the datasheet of the atmega it says it can handle 5.5v and -0.5v but don't go to these voltages to often, because then it "may effect device reliability" (see text on image above) . so if i go to 5.25V and -0.25v allot will this be bad? it is over the standard voltage 5v and under the absolute maximum of 5.5V..

Grumpy_Mike

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something like this?:
CV            POT            PIN
7.5            2.5            5

Yes that is what I said it doesn't compensate for +/- 15V it just chops it off

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does this mean that the voltage on the microcontroler pin will always stay under 5.25V?

Yes

The over voltage rating on the data sheet come from the fact that there are internal diodes in the same configuration as you have. They can only stand about 1mA of current so you are supplementing them by using external clamping diodes.

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so if i go to 5.25V and -0.25v allot will this be bad?

No that will be fine.

gijs

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Quote
does this mean that the voltage on the microcontroler pin will always stay under 5.25V?


Yes


..so it matters what diodes i use.. as long as the forward voltage drop of the external clamping diodes is under the forward voltage drop of the internal clamping diodes i am safe..


Grumpy_Mike


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