Getting relay control voltage and dual opamp supply from same two windings

[mansaxel]

I'm about to build a little project that uses both opamps and relays, and since I've both got a finished design for the opamp part, and also have bought a bucket of relays (literally) the voltage requirements for these two components are fixed;

• The opamps want +/- 15VDC
• The relays want 24VDC

I've doodled a circuit together that I hope will work for supplying this from a single transformer, with center-tapped secondary (attached). Will this work? Is there a clearly better way with minimum of fuss?

[bdunham7]

Are the grounds isolated from each other and will the +24V circuit not interact with the other in any way?  There's obviously a half-cycle potential between the op-amp ground and GNDA.

[Ian.M]

You'd probably be better off with an 18V-0-18V secondary, and a single bridge rectifier.  That will give you about +/-24V unregulated with respect to the center tap, the positive side of which which will suit the relays well enough as they aren't too fussy and will easily tolerate +/-20% variation from their nominal coil voltage, then use the +/-15V regulator section you've already designed to feed the OPAMPs.  Hammond Mfg transformer division's Design Guide For Rectifier Use will help you work out how big a transformer you need - add up the coil currents for all the relays that can be on at the same time, in whatever combination draws the most current and apply the formula from the section 'FULL WAVE Capacitor Input Load' to get the secondary current due to the relays, then average the OPAMP section's positive and negative suply currents and apply the'FULL WAVE BRIDGE Capacitor Input Load' formula for the secondary current due to the OPAMPs, and add the two calculated secondary currents together to get the total RMS secondary current so you can choose a transformer.

N.B. If the relay duty cycle is low, and the max on time is under 5 minutes, it may be acceptable to moderately overload the transformer when the maximum number of relays are on, as long as the long-term average relay current + the OPAMP current does not overload the transformer.

[mansaxel]

Are the grounds isolated from each other and will the +24V circuit not interact with the other in any way?  There's obviously a half-cycle potential between the op-amp ground and GNDA.

Thanks. I've already identified that -- the GND symbols are different for a reason! 

[mansaxel]

You'd probably be better off with an 18V-0-18V secondary, and a single bridge rectifier. 

Obviously. Yes, these relays are NOS STC telephony ones, nominal 24VDC and will pull nicely from perhaps 18V and up. So, 20V will do.

If I have to buy new transformers I can as well get a 18-0-18 one.

[Terry Bites]

Here's a trick from the old days.  If your relay still holds in at 15 V you're fine. Test the relay pull in and release voltages.
https://www.homemade-circuits.com/how-to-make-relay-driver-stage-in/