Relay control

On a sunny day (Thu, 8 Aug 2013 06:54:13 -0700 (PDT)) it happened snipped-for-privacy@gmail.com wrote in :

How much 'over voltage'? Usually DC relais have a bit of margin. Are you driving with transistors + diodes (flyback), or some driver chip?

On Thursday, August 8, 2013 7:24:13 PM UTC+5:30, snipped-for-privacy@gmail.com wro te:

o. Due to possibility over voltage on the 12V line I am supplying the relay s with 12V 300ma LDO regulator (250ma drop out) which has 3.3V compatible e nable pin. Since I need to ON or OFF all four relays in parallal, can I use the regulator's enable pin to switch the 12V power ON or OFF instead of dr iving the relays through a transistor or FET or a relay driver? Are there a ny potential issues with this as most of the relay driving schematics I hav e seen use drivers?

nd step up from 3.3V to 12V? I read somewhere that switching converters can not be used for inductive loads.

Can go as high as 16V. I am looking at the options whether to use driver tr ansistor / FET is required or control relays using regulator's enable pin.

There's no reason why you shouldn't use the regulator's enable pin to switch power to the relays. Be sure to protect the regulator from back EMF from the relays as they switch off.

Cheers

On a sunny day (Thu, 8 Aug 2013 08:15:22 -0700 (PDT)) it happened snipped-for-privacy@gmail.com wrote in :

Well any emittor follower zener combination would work too. As far as switching is concerned why not use the software? I mean off is off, for a relay: no drive no current.

Be very sure to protect the regulator from back EMF when it switches the relays off. If you don't know what that means, ask now!!!

Look at the relay data sheets. There's a good chance that 16V will be fine, even though it means nearly twice the dissipation in the relay coil.

If you use the regulator make sure that you have enough heat sinking for it to work when your supply is at 16V.

Thanks. Each relay has diode for protection from back emf. On the other side, any issue if I use 3.3V to 12V boost converter to drive inductive loads like relays?

A diode across the relay actuator coil. Or a transzorb, actually might be better.

You may well have stability issues if you use a converter that's designed assuming a resistive load. That means that you'll probably have to tune the compensation on the converter to match the load. But I don't know of any fundamental reason why it can't be done.

Meaning: don't buy from National or anyone else that won't share a model, understand how a boost converter works, and make sure to at least simulate the circuit on SPICE or similar, with a good model of your relay coils.

I don't see why not in principle, using the switcher's enable pin to switch the relays. Four 360ohm relays at 12V is 1.6W and with SMPS (in-)efficiency probably around 600mA from 3.3V, so the TPS61040/1 may not be man enough.

As Jan said, you could maybe use the micro as a switcher if you have a spare ADC or comparator input, but I wouldn't bother unless you're making enough of these (thousands?) such that the component saving covers the development costs. Unless you just want to do it for fun, of course.

Cheers

On a sunny day (Thu, 08 Aug 2013 18:39:11 +0100) it happened Syd Rumpo wrote in :

Actually what I ment is that if the micro controls the relays, set all control pins to zero, why change the relay supply? To use the micro's PWM and control from there I agree with you, an emittor follower would be simpler I think.