Hi,
I need to power a logic circuit and a bunch of relays for a preamp I'm working on. I've come up with this design for a +12V / +5V DC power supply from 6+6VAC transformer. Does it look like it will work? I've been trying to simulate it in TinaTI but I'm struggling with the modelling the mains into a trafo.
Any pointers would be welcome.
Simon.
I need to power a logic circuit and a bunch of relays for a preamp I'm working on. I've come up with this design for a +12V / +5V DC power supply from 6+6VAC transformer. Does it look like it will work? I've been trying to simulate it in TinaTI but I'm struggling with the modelling the mains into a trafo.
Any pointers would be welcome.
Simon.
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As a general rule, you can't have one secondary feeding two different rectifier arrangements - there are exceptions but they need care. Your circuit might work, but probably not as you expect - for example the lower supply may actually be a half-wave rectified 12V even though it superficially looks like a full-wave rectified 6V.
Have another think about it. Note the voltages and currents for both phases of the mains.
Have another think about it. Note the voltages and currents for both phases of the mains.
There is a simpler, and completely orthodox solution requiring only one rectifier bridge:
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You can buy relays in 5V or 12V. You can buy logic in 5V or 12V (CMOS). Pick one and go with it.
Yes, the plan posted by Elvee is THE Obvious Way (after you have seen it) to do what you ask.
All power supply questions should start with power required!! If you need 1mA of this and 1A of that, or vice versa, or 2mA of each, it often leads to a practical (cheap) answer of low brain-strain. In days of incandescent pilot lights, we ran gobs of low-speed CMOS logic as negligible extra load on the lamp supply. While getting 5V logic from 50V amp-power looks wasteful, some of the PIC processors draw so little power that a resistor/Zener works fine; OTOH running a 80286 from dropped 50V would warm the room.
When I was designing the preamp I could only find 12V versions of the relay (Omron G6SK) in stock with the suppliers I use. You prompted me to check again and Mouser stock the 5V now... so thanks. That only leaves me with two 12V 1.2W incandescent bulbs for the VU meter. I'll see if I can get 5V versions of those, leds are a possibility but wouldn't be my first choice for colour.
If I stick with 12V relays then I need 12V 500mA and 5V 250mA.
That's based on 20x12V relays @ approx 20mA switching current (assuming I'll never switch more than half at one time) plus two 1.2W bulbs - I need minimum 500mA (including losses and a bit of headroom if I change things around).
I need 6V to drive a motorised potentiometer (I'm going to try it at 5V, if not use the unregulated supply) with a max current of 150mA and a couple of MCUs probably worst case 40mA each. So >250mA to be on the safe side.
With 5V relays I'll need about 900mA at 5V and I'll need to do something about the 2x1.2W 12V bulbs.
6V incandescents should (still??) be readily available, and light-up fine if dimmer with 5V and greatly increased life.
80mA of 5V logic can be dropped from a 12V supply with "only" 0.56 Watt waste heat (perhaps less than the heat from your lamps). A naked TO220 regulator will manage without a heatsink, though I'd feel better with a few square inches of fins.
80mA of 5V logic can be dropped from a 12V supply with "only" 0.56 Watt waste heat (perhaps less than the heat from your lamps). A naked TO220 regulator will manage without a heatsink, though I'd feel better with a few square inches of fins.
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