+24v Relay Power and +/-18v Audio from One Wallwart

[thermionic]

Hi,

I'm involved in developing a limited-run product. Obviously, an external CE / UL-approved wallwart is an attractive option for power. Bearing in mind that the relays alone will take anything up to 250mA, and wallwarts with AC outputs becoming an endangered species, it appears that the only option will be to opt for an SMPS wallwart. I will need a maximum of 100mA per rail for the audio.

My current thinking is as follows:

1.25A SMPS Wallwart - directly to relays

1.25A SMPS Wallwart also feeds 2x+/-24V Dual DC-DC Converter - to - LM317 / LM337 (traditional design, set to output +/-18v).

The analogue circuitry will be divided into 2 halves, with each being fed by a different LM317 / 337 pair

The drop from +/- 24v to +/-18v is likely to create a fair amount of heat...

The easiest way to do this would be with an external AC adaptor, skipping the DC-DC convertors, but I just can't find one powerful enough... The option outlined above isn't cheap, or efficient. Can anyone think of a better way?

Thanks in advance.
Justin 

 

[gemini86]

Why don't you opt for a switching regulator since you're going smps anyway. Add lots of filtering.

 

[thermionic]

Thanks.

I'm looking into using a circuit that lowers the relay voltage to its nominal 'hold contact' level after initially being thrown (any ideas gratefully received). This should save a lot of power (not to mention reliability), enabling use of a 18 VAC / 500mA wall-wart. The analogue circuitry can come off a voltage doubler / 2-diode rectifier set up (with plenty of filtering around the unregulated side) to give +/- 18v, with relays being fed off a separate supply.

Justin