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tronicgr
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 « on: February 26, 2008, 07:08:25 07:08 »

Hi to all,

I'm trying to design a simple circuit that converts 0 - 5volts analog to a range of -11volt to +11volt.

I need this voltage level conversion to interface my AVR that outputs 0-5v PWM (with RC filter) to a AC motor controller that need the -11volt to +11 volts signals to position large motors... (-11volts is one direction, 0volts is center, +11volts is the other direction and in my AVR the signals are 0volts one direction, 2.5volts center, 5volts the other direction...)

I know that it can be done with a opamp but can't find such ready circuit around.

Any suggestions?

Regards, Thanos
Greece, Athens
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titi
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 « Reply #1 on: February 26, 2008, 12:47:06 12:47 »

Hello,

For your voltage converter circuit, you need to make with LM358 or other Operational Amplifier, a substractor of 2.5v with a gain of 4.4.
If you have 5v, it is easy to have 2.5v, between GND and 5v, you put 2 resistors in serial by example 4.7k at the middle you have 2.5v
you make Vout=(Vin-2.5)*4.4
Example:
Vin=0    => (0-2.5)*4.4=-11v
Vin=2.5 => (2.5-2.5)*4.4=0v
Vin=5v  => (5-2.5)*4.4=+11v

To have +11v -11v, you need LM358 supply with +15v -15v.
This small drawing explain that better.

Best regards.
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tronicgr
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 « Reply #2 on: February 27, 2008, 03:31:23 03:31 »

Hi Titi,

I don't understand how to apply the 4.4 gain...

Best Regards, Thanos
Greece, Athens
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xfx7950
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 « Reply #3 on: February 27, 2008, 07:52:11 07:52 »

me too
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kang2008
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 « Reply #4 on: February 27, 2008, 09:45:28 09:45 »

Posted on: February 27, 2008, 04:29:11 16:29 - Automerged

8.2k/1.8k = 4.55 gain
Vout=(Vin-2.5)*4.55
Example:
Vin=0    => (0-2.5)*4.55-11.375v
Vin=2.5 => (2.5-2.5)*4.55=0v
Vin=5v  => (5-2.5)*4.55=+11.375v

or

8k/1.8k = 4.44 gain
Vout=(Vin-2.5)*4.44
Example:
Vin=0    => (0-2.5)*4.44=-11.1v
Vin=2.5 => (2.5-2.5)*4.44=0v
Vin=5v  => (5-2.5)*4.44=+11.1v

Posted on: February 27, 2008, 04:42:50 16:42 - Automerged

i am a korea deaf you have know mathematics.
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titi
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 « Reply #5 on: February 27, 2008, 11:35:14 11:35 »

Hello Tronic and xfx7950,

if you cannot see the picture, it is because, you are new on sonsivri, and to have access to the link, you need have 7 posts.
(See the rules)

h**p://rapidshare.com/files/95400020/VoltageLevelConverterCircuit.gif

(Replace ** with tt) if i put the full link you could not see it (need 7 posts).

Best regards.
 « Last Edit: February 27, 2008, 11:38:21 11:38 by titi » Logged
tronicgr
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 « Reply #6 on: February 29, 2008, 09:45:47 09:45 »

Hi,

I can understand the second part of the schematic is the LM358, butI can't figure what is the first part on the schematic... Is it a comparator?

Regards, Thanos
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titi
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 « Reply #7 on: February 29, 2008, 10:24:37 10:24 »

Hello,

The first part of the schematic is a follower, this is for generate a 2.5v with low impedance, and after you subtract the 2.5v with the second part of schematic (the substracter).
If you connect directly the middle of the two 4.7k to the substractor you don't have 2.5v because the current go into 1.8k.
An other solution is to use a ref diode like LM385 2.5v or TL431 to make 2.5v with low impedance and connect the ref diode directly on the 1.8k (more expensive).

The substract make ((voltage on 1.8k pin +) - (voltage on 1.8k pin -)) * (8.2k/1.8k)
If you make (voltage on 1.8k pin +) varies between 0 and 5v you have Vout varies between  -11.3v to +11.3v.

Best regards.
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kang2008
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 « Reply #8 on: February 29, 2008, 06:35:44 18:35 »

titi~  this error 's schematic of 2.5 opamp. i have a schematic test ok
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titi
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 « Reply #9 on: March 01, 2008, 07:29:05 07:29 »

Hello,

the schematic modified by kang2008 is not good.

The first opamp need have the feedback on pin "-" and the input on pin "+".

The first stage is a voltage follower circuit with gain of 1.

Regards.
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