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Old 29th December 2006, 10:42 AM   #1
borges is offline borges  Norway
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Default Power supply - how to avoid getting hurt

Hi,

for my CD player project I'm about to build my own power supply. But before I send of the PCB files to production I'd like to know if this is safe.

Here's what I'm doing:
- Two primaries for 115/230V operation
- Five secondaries, custom made for my V/A needs
- All wound around in a single toroid transformer
- One secondary is always in use
- An MCU controls relays on the other 4 secondaries
- Primaries are always connected to the mains connector
- The relays brake and close the circuit after the regulator
- Simple three-legged regulators are used to save space and $

My question is: Does this sound sane? Is it safe to have the mains connector permanently connected to the primaries of the transformer? What happens if I pull the mains cable and stick my finger in the connector?


Any help appreciated!


Cheers,
BÝrge
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Old 29th December 2006, 03:04 PM   #2
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I would always use a separate traffo for trickle/standby power. Firstly, and most importantly, it can be fused correctly for the load, rather than having to be rated for the full consumption of the amp, and secondly, it saves, (a bit of), power. you can then switch the mains directly to the traffo, better as relays don't like switching DC much. You also need a way of isolating the mains, so a switch, (it can be on the back panel) is obligatory.
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Old 29th December 2006, 03:19 PM   #3
borges is offline borges  Norway
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Good idea about the switch, Al. I'll put it in.

The reason I'd like to switch DC is that the PSU is for a CD player. And switching AC may lead to any kind of DC on the power rails of the DAC. That could send out signals that might ruin equipment like the following amp and speakers.


Thanks for the advice!


BÝrge
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Old 29th December 2006, 04:18 PM   #4
moamps is offline moamps  Croatia
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Hi borges:

Your plan for the power supply design sounds sane enough. The same principle is in fact used by many audio/video equipment manufacturers for the equipment that needs little power from the mains. The only difference is that these manufacturers do not normally use relays to switch on power but transistors or microprocessor-controlled regulators. You can also avoid using relays in your design by directly controlling the regulators via MCU (e.g. when using LM317/337 types, by switching of/on voltage on the adjust pins of each regulator).

Regards,
Milan
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Old 29th December 2006, 04:24 PM   #5
borges is offline borges  Norway
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Nice one Milan!

I am indeed using LM3x7's. Do you mean you just leave the Vref pin floating? That should be easy to manage with some MOS devices.

Or maybe I should switch to LDOs with explicit enable pins.

Do you see any ill effects in having the primaries constantly (through a switch on the rear panel) to the mains connector?


BÝrge
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Old 29th December 2006, 04:40 PM   #6
AndrewT is offline AndrewT  Scotland
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Close rate the mains fuse.

Check running temperature of the transformer. Maybe add or utilise the thermal breaker in the transformer.

LDO is specific to low drop out types.
You can use any reg with a shut down pin.

You could use solid state relays and time them to come on and/or off in a particular order. Or use a series of triacs from timers.
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Old 29th December 2006, 05:34 PM   #7
moamps is offline moamps  Croatia
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Quote:
Originally posted by borges
....I am indeed using LM3x7's. Do you mean you just leave the Vref pin floating?...
No, I didn't mean the Vref pin should be left floating. What I meant is that the Vref pin should be connected to the ground. As a consequence, the output voltage from the regulator is not zero but 1.25V (when off), which is not an issue in most cases. The picture below (taken from the National's 317 datasheet) shows a controlled regulator.

Quote:
Do you see any ill effects in having the primaries constantly (through a switch on the rear panel) to the mains connector?
No, I don't see any issue with that (my CD player is switched on constantly and is still very much alive and kickin').

Regards,
Milan
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Old 29th December 2006, 06:37 PM   #8
anatech is offline anatech  Canada
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Hi borges,
I agree with Al. A separate "housekeeping " supply can leave your digital stuff running if you want. You can electrically turn it off. Milan's diagram is exactly how it is done.

Studer/Revox is one example of a manufacturer that does this. Then you only need an AC rated relay to turn on your analog stuff. Just like in most receivers today.

Also, all my equipment is turned on as needed and has been for years on end. No problem. I'm immune to lightning storms.

-Chris
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Old 29th December 2006, 08:34 PM   #9
borges is offline borges  Norway
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Thanks guys!

Looks like I'm going to redesign the supply with regulators with enable/disable (LM2941 and LM2991). And the digital part I can keep running off a simple 7805. I don't want to run my DAC on +-1.25V.

BÝrge
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Old 3rd January 2007, 08:17 PM   #10
KISS is offline KISS  United States
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Something you might want to consider is the reduction of phantom power. For instance have a small regulator for a few watts. Use something like a PIC micoprocessor to deal with the turn on signals.

Add a turn on signal to the mains. When you detect a turn on signal for an aux supply. turn on the mains. Allow a delay, then turn on the appropriate regulator. If the system is on already then no delay is necessary.

There are some new switchmode regularors on the market that are rated for a few watts power for phantom power.

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