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Electronics => Projects, Designs, and Technical Stuff => Topic started by: TERRA Operative on November 28, 2021, 02:24:33 pm

Title: How to get a variable +5v to -5VDC from 24VDC supply?
Post by: TERRA Operative on November 28, 2021, 02:24:33 pm
Hi all.

I am currently building a jig to test HP 10811 OCXO's.
As part of the Electronic Frequency Control, I can supply it within a range from -5VDC up to +5VDC for the full EFC adjustment range.
The circuit will be supplied +24VDC (+20-30VDC is acceptable for the OCXO according to HP).

Is there a relatively simple way to have a single potentiometer or similar to vary a voltage output from -5VDC to +5VDC? I don't need any appreciable current, but the output voltage needs to be stable within 100uV.

Thanks!
Title: Re: How to get a variable +5v to -5VDC from 24VDC supply?
Post by: capt bullshot on November 28, 2021, 02:34:12 pm
Look at page 7 of this datasheet for a reasonably simple and stable +/- 5V supply:

https://www.ti.com/lit/ds/symlink/ref02.pdf (https://www.ti.com/lit/ds/symlink/ref02.pdf)

Connect the potentiometer ends to these voltages and tap the -5V ... +5V from the wiper. If required, add a simple Op-Amp buffer circuit. I'd recommend using a 10-turn pot.

Additionally,  you'd need a +/- 12V supply from your 24V, I'd use one of these simple and cheap SIL packaged DC/DC converters - add some filtering as they are quite noisy.
Title: Re: How to get a variable +5v to -5VDC from 24VDC supply?
Post by: TERRA Operative on November 28, 2021, 02:37:38 pm
Hmm, I'm not sure how that will work?

I need a single output that can vary anywhere across the -5VDC to +5VDC range, not two separate outputs for +ve and -ve..
Title: Re: How to get a variable +5v to -5VDC from 24VDC supply?
Post by: capt bullshot on November 28, 2021, 02:38:36 pm
See my edit
Title: Re: How to get a variable +5v to -5VDC from 24VDC supply?
Post by: TERRA Operative on November 28, 2021, 02:40:34 pm
Aaahh yep, like a voltage divider type arrangement.
Title: Re: How to get a variable +5v to -5VDC from 24VDC supply?
Post by: mawyatt on November 28, 2021, 03:01:44 pm
Use an op-amp (OP07), good 5V reference and a couple 10K 1% resistors and a 10K or 1K 10 turn pot. Connect two 10K resistors to op-amp configured as inverter with one 10K resistor to Vref to Vin - on op-amp, other 10K from V- to Vout on op-amp. Now connect pot top to Vref, bottom to ground and wiper to op-amp V+.

With pot set to bottom op-amp output will be -Vref, with pot set to top output will be +Vref, with pot set to mid-range output will be ~0 volts.

Basically what this circuit creates is a Bi-Phase Modulator, where the op-amp gain changes from +1 to -1 with the pot setting.

Edit: The concept was used long ago as the first term in the Pade Approximation of a pure time delay when the pot is replaced by a RC network, also used a variable phase shifter with same arrangement. Here's brief simple analysis of the circuit in DC for OP and more general solution.

Best,
Title: Re: How to get a variable +5v to -5VDC from 24VDC supply?
Post by: ledtester on November 28, 2021, 03:10:36 pm
If you can produce +/- 5V rails I would just use an op-amp in a voltage follower configuration. The V+ input is connected to the wiper of a pot between the two rails to set the output voltage.

Depending on the op-amp used, you might need +/- 8V rails so the op-amp can output voltages in the +/- 5V range.
Title: Re: How to get a variable +5v to -5VDC from 24VDC supply?
Post by: TERRA Operative on November 28, 2021, 03:19:32 pm
Quote from: mawyatt on November 28, 2021, 03:01:44 pm
Use an op-amp (OP07), good 5V reference and a couple 10K 1% resistors and a 10K or 1K 10 turn pot. Connect two 10K resistors to op-amp configured as inverter with one 10K resistor to Vref to Vin - on op-amp, other 10K from V- to Vout on op-amp. Now connect pot top to Vref, bottom to ground and wiper to op-amp V+.

With pot set to bottom op-amp output will be -Vref, with pot set to top output will be +Vref, with pot set to mid-range output will be ~0 volts.

Basically what this circuit creates is a Bi-Phase Modulator, where the op-amp gain changes from +1 to -1 with the pot setting.

Edit: The concept was used long ago as the first term in the Pade Approximation of a pure time delay when the pot is replaced by a RC network, also used a variable phase shifter with same arrangement.

Best,

Something a bit like this?
Title: Re: How to get a variable +5v to -5VDC from 24VDC supply?
Post by: mawyatt on November 28, 2021, 03:50:39 pm
Yes exactly. Just posting something above while you posted :-+

Note the more general solution where one can create a variable phase shifter using a RC network.

Best,
Title: Re: How to get a variable +5v to -5VDC from 24VDC supply?
Post by: TERRA Operative on November 28, 2021, 03:53:33 pm
Makes sense, thanks! :)
Title: Re: How to get a variable +5v to -5VDC from 24VDC supply?
Post by: David Hess on November 28, 2021, 09:11:07 pm
It looks like you have it handled.  I will just add:

Quote from: TERRA Operative on November 28, 2021, 02:24:33 pm
I don't need any appreciable current, but the output voltage needs to be stable within 100uV.

That is 1 part in 100,000 so plan on using a 10 turn poteniometer and a precision operational amplifier and reference to get reasonable stability.  The OP-07 operational amplifier is the most economical choice.

Note that even with a 10 turn precision potentiometer, you will not achieve 1 in 100,000 settability.  That would imply a resolution of 10,000 counts per turn which is not going to happen, but 1 in 100,000 stability is possible.  If you want better settability, then coarse and fine poteniometers will be required.
Title: Re: How to get a variable +5v to -5VDC from 24VDC supply?
Post by: asdf336 on November 30, 2021, 01:10:22 am
Could be as simple as 2 buffers.  One fixed (say 12V), one trimmable from 7-17V.