+-18v +48 PSU need LOW VOLTAGE RAIL for relays and LEDs

Hey guys,

I'm planing a PSU for my API 312 clone. So the usal +-opamp-supply and 48v rail aren't a problem, as I made similar PSUs for other projects (green...).

But for my API project I want to use relays (like in the Fabio-Design) and found some available which are rated for 6v or 12v.

So how do I get something in between 6v and 12v out of a 18VAC trafo? I'd prefer the 6v soltion the reduce power consumption.

Using half-wave rectification is what comes to my mind first, but I think to recall that it is not recommended because it can drive the transformer into saturation or something.

And just to use a regulator to drop the 21 something volts after the rectifier to 12 or even less volts would really stress the regulator, hmm?

So any ideas for a more elegant solution?

Thanks for your help. Sebastian

JanusRec said:

Hey guys,

I'm planing a PSU for my API 312 clone. So the usal +-opamp-supply and 48v rail aren't a problem, as I made similar PSUs for other projects (green...).

But for my API project I want to use relays (like in the Fabio-Design) and found some available which are rated for 6v or 12v.

So how do I get something in between 6v and 12v out of a 18VAC trafo? I'd prefer the 6v soltion the reduce power consumption.

Using half-wave rectification is what comes to my mind first, but I think to recall that it is not recommended because it can drive the transformer into saturation or something.

And just to use a regulator to drop the 21 something volts after the rectifier to 12 or even less volts would really stress the regulator, hmm?

So any ideas for a more elegant solution?

Thanks for your help. Sebastian

Click to expand...

The high tech solution is dc-dc switcher that will efficiently drop unregulated DC down to lower voltage with more current.

A lower tech solution that I used to generate a high current 5V supply from a similar too high voltage transformer, was to add a transistor switch in series with the low voltage supply rectifier, so it would stop charging the low voltage supply at a few volts above the 3 terminal regulator. This gave me a similar power efficiency benefit as a full HF switcher, without the complexity or HF noise.

Note: I have encountered issues with too much inductance between the transformer and the switch causing a voltage spike, but I have used this in production with good reliability before.
=====
Another technique I have used if there is only one relay, is to use a separate rectifier and undersized reservoir capacitor so the cap is only holding enough voltage to keep the relay from dropping out. When power is removed the relay drops out almost instantly, which is useful for bypass relays. 

LEDs can be wired in series to recover some efficiency loss.

JR

JohnRoberts said:

LEDs can be wired in series to recover some efficiency loss.

JR

Click to expand...

Yep,

a good solution here is used in many console channel strips- hang a number of LEDs in series from a constant current source (1 transistor, 3 diodes, handful of resistors) to ground (or to +Ve if current sink) and then use switches to short out LEDs that are needed to be off. Needs a switch to give inverse of logical switching (i.e. ON is OFF) but is a good use of power. Red/Grn/Yell V drop is ~2V. White/Blue- depends on device.

Start at 10mA and check brightnesses from there.

Mark

JanusRec said:

So how do I get something in between 6v and 12v out of a 18VAC trafo? I'd prefer the 6v soltion the reduce power consumption

Click to expand...

Relays don't care much about their exact supply voltage. Your 18VAC trafo will give 25-26V after rectification, which is Just Fine for 24V relays. Hard to get simpler than that, and for most relay families the 24V version eats no more power than the 12V or 6V ones. (48V and up coils do have higher power requirements in most relays; some ultra-miniature 24V ones eat a bit more than their 12V counterparts).

If you get clicks in your audio from switching the relays off your main smoothing cap, add a simple RC filter. The higher the coil voltage the lower the relay current, so all else being equal 24V relays should disturb your supply less than 12V or 6V models.

If you have a tube of 12V relays that you absolutely must use, them in series and drive them with a current source as described in this thread. Same works for LEDs, as Mark said.

JDB.

Ok, thanks so far. Of course you're right about the relay power consumption being equal for 6,12 and 24v relays. And the LED series is a very good idea -thanks. First I thought about bypassing the unused LEDs with a 200R restistor in my voltage source circuit, but the the CCS idea sounds even better. Never built one though. Is there a better design than the Simple transistor current source with diode compensation on wikipedia? It uses only two diods (@Mark) and I didn't find anything usefull on the forum.

So could I use my - let's say 18v regulated - positive rail on the channel board to feed my audio circuit and a current source + relays without getting crosstalk, clicks and similar problems? Given that I use a proper bypass caps of course. Because I thought about having an independend power rail for the whole LED and switching stuff to prevent those proplems from the beginning.

I did a 4 rail PSU layout (still untested, but you could be the first .. actually need to get this etched super quick myself, but anyway... this PSU will give you a bipolar, +48V and an extra positive rail for LEDs, relays, etc, all from one dual secondary power trafo.  I just tried to download the etch file myself and rapidshare gave me a "no more free download spots available message" so if you are interested and can't download it I can put it in the gmail account.

Edit: forgot link to thread:
http://www.groupdiy.com/index.php?topic=36462.0

JanusRec said:

Is there a better design than the Simple transistor current source with diode compensation on wikipedia?

Click to expand...

It does the job well enough for your purposes. For more on current sources have a look at

http://www.groupdiy.com/index.php?topic=33088.0

Walt Jung's articles on current sources (linked from Reply #14) are particularly interesting.

JanusRec said:

So could I use my - let's say 18v regulated - positive rail on the channel board to feed my audio circuit and a current source + relays without getting crosstalk, clicks and similar problems? Given that I use a proper bypass caps of course. Because I thought about having an independend power rail for the whole LED and switching stuff to prevent those proplems from the beginning.

Click to expand...

It Depends, as always.

Is it possible to have clickless operation with a current source from the unregulated supply, at the buffer caps just after the bridge rectifier? Yes.

Is it hard to get it working and clickless? Not very.

Will every single implementation of this scheme be clickless, no matter the layout or the presence/absence of any additional decoupling? No, of course not.

Hints:
- tap power and ground for the relays directly at the buffer caps. In other words: try to make the power and ground path that is shared between the 312 and the relays as short as possible. Look up "star grounding" if you need more information.

- keep the wires to relays and switches as far away from the audio wiring as possible. Consider twisted pairs for the wires going to and from the relays. You don't care much about capacitance between wires in the relay circuit, but you do care about loop area which could form an antenna to couple to your audio circuits.

JDB.