How do I convert 0-10V to -5V +5V differential?

Need to arithmetically add -5V. Try a couple of Rs and a pot to -10V supply?

I'm a complete amatuer

- so I think what you are saying is that I need to provide a -10V supply and combine the signals somehow?

If someone could do a quick ascii sketch of the layout, that would help.

Thanks, Steve

Hi, Steve. First off, many signal generators have a dial that can control the DC level of an AC signal. It's usually labelled "DC" or "DC Offset". See if you have that adjustment on your signal generator. If you do, your problem is solved. Just tweak the dial to get the appropriate DC offset. If you don't have that adjustment, you might want to scrounge around and find a signal generator that does -- most do, and this is the easiest and best solution.

If your signal generator is homebrew, or you don't have a DC Offset adjustment available, and the input to your galvo amp is high impedance, you might be able to get away with just a DC blocking capacitor like this (view in fixed font or M$ Notepad):

` C1 .-----------. ` +|| | | ` 0 - 10V .-----------||-------o-------o---oIn | ` 30 Hz | || | | | | ` / \\ 100uF | .-. | Galvo | ` (A C) /-/ 10K| | | Amp | ` \\_/ 1N4733 ^ | | | | ` | Vz=3D5.1V | '-' | | ` | | | | | ` | V | | | ` | /-/ | | | ` | | | | | ` | | | | | ` | | | | | ` '--------------------o-------o---oGND | ` | | ` '-----------' created by Andy=B4s ASCII-Circuit v1.24.140803 Beta

This may not work for you if the galvo input isn't high impedance, or if you need the laser to remain in one position. There will also be some minor distortion of the input voltage, even once the DC voltage stabilizes. But it's easy, the components are available at Radio Shack or any other hobbyist source, and it's all passive components -- no extra power supply. Just one cap, two zener diodes and a resistor. Note that some galvo amp inputs require a DC path to GND, so don't forget the 10K resistor, or the galvo might drift away.

If you want instantaneous, precise translation of your 0 to 10VDC staircase into a -5V to 5V staircase, which will work from DC to audio frequency, you'll need an op amp and split supplies (+/-15V, for instance). Since the galvo uses these, they're frequently available on the terminal block or external connector. Just RTFM (Read The Fine Manual). You would use the op amp (it really doeesn't matter which one

-- an LM741 available at Radio Shack would work just fine here) and a handful of resistors to make what's called a difference amplifier. This just subtracts 5V from your input voltage. It would look something like this (view in fixed font or M$ Notepad): ` ___ ` .---|___|---. ` | 22K | ` | | ` 22K | +15V | ` ___ | |\\| | ` 5VDC o---|___|-o---|-\\ | Output ` ___ | >----o---o ` 0 to 10Vo---|___|-o---|+/ -5V to 5V ` 22K | |/| ` | -15V ` .-. ` 22K| | ` | | ` '-' ` | ` =3D=3D=3D ` GND created by Andy=B4s ASCII-Circuit v1.24.140803 Beta

The circuit works by subtracting the 5V (you can make the 5V by putting a pot between the + supply and GND) from your input signal. That's why it's called a difference amplifier.

But judging from your own evaluation of your skill level (complete newbie) this might be a bit of a stretch.

Let us know if any of these solutions look good, or if you need more advice.

Good luck Chris

Thanks for you all your help - I have all the parts to build the op-amp solution at home - the only thing that puzzles me a bit is the

+15V/-15V power to the 741 - how do I create +15 and -15V from a standard transformer.

Is there a book on this somewhere...?

:)

Steve

--- View with a non-proportional font like Courier:

+V | | Vin>--[R1]--+----------|+\\ | | >--+-->Vout [R2] +-----|-/ | | | | | GND +--[R3]-----+ | | | -V [R4] | 5V

R1 = R2 R3 = R4

-- John Fields Professional Circuit Designer

--- Since the 741 can take an input supply of plus and minus 22 volts, max, you can easily get less than that that from a simple, unregulated, half wave supply using a 12V transformer, like this:

+--[IN4001>]----------+---->+17V | | | | MAINS>--+ || +--[-17V | || | | | P || S | |+ R || E [C1] [C2] I || C |+ | | || | | | MAINS>--+ || +-------------+-------+---->GND

Since your output is only going to swing between +5 and -5V, and the

741 needs about 3V above and below that to let that happen, you've got about 9V of headroom to either rail so C1 and C2 can be fairly modest, depending on what else you're going to use the supply for. What can you tell us about the load?

-- John Fields Professional Circuit Designer

Mr. Fields has given the best solution. The OP might want to add a couple of protection diodes to keep the inputs in the range of the supplies and the caps from being reverse-biased at turn-on, like this (view in fixed font or M$ Notepad):

` D ` o------o-------->|----o-----o------o+14VDC ` | +| | ` | C --- - D ` o------|---. --- ^ ` | | | | ` 10.2VAC | '----------o-----o------o ` from | +| | ` Wall Wart | C --- - D ` | --- ^ ` | D | | ` '--------|

mp

Hi, Mr. Fields. It's possible the two extra diodes are "gilding the lilly", but possibly not.

First, the whole circuit without the extra diodes: ` ___ ` .--|___|--. ` ph A D | | ` o-----o----->|---o-----------------------. | ` | +| ___ | |\\| | ` | C1--- in(-)o-|___|-o--|-\\ | ` o-----|---. --- ___ | >---o---o ` ph B | | | in(+)o-|___|-o--|+/ ` | '------o---. | |/| ` | +| | .-. | ` | C2--- =3D=3D=3D | | | ` | --- GND | | | ` | D | '-' | ` '-----|

Back in the day, it used to be somewhat common practice to use split supplies with a 4016 or 4066 for low level +/- analog input voltages. Combined with a CA3130 op amp and some logic ICs, you could do some Q&D signal conditioning.

Thanks for the spot, Mr. Fields. It's good to know we have a few pros like yourself, Mr. Holme and Mr. Popelish here among the hoi-polloi, keeping us all honest. ;-)

Chris

--
That\'s interesting. I didn\'t even see that path!

Looking at it a little more closely, though, the voltage which will
be developed across the cap, with an average current of 3mA for half
a cycle of 60 Hz will be:

           I dt     3E-3A * 8.3E-3s
      E = ------ = ----------------- ~ 0.053V  
            C          4.7E-4F

So that\'s only 53mV away from ground.

I don't know how precise it must be? I have drawn an standard opamp schematic here, important: use courier as font to read it (or copy it to notepad and use Courier there): ----- ---| 10k |--

+5V | ----- | | ----- | |\\ |

--| 10k |---| \\ | ----- |- \\ |

0-10V | \\---------- -5 to +5V out | ----- | /

--| 10k |-- |+ / ----- | | / | |/ --- | | 10k | | | | --- | --- gnd, connect gnds of 0-10V, +5V, opamp power supply gnd - You need a fixed +5V, generate it with a 78L05 or something else. Depends on how precise you want to to be. Supply the opamp with +/-

15V. Any opamp will do, use one that does not need a cap in the feedback, or use a small cap across the feedback resistor.

Feel free to ask any questions, by email: snipped-for-privacy@hoeben.com as I do not check this newsgroup often and may miss answers.

Regards, Pieter Hoeben