Power supply - how to avoid getting hurt

[borges]

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,
Børge

 

[pinkmouse]

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.

 

[borges]

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!


Børge

 

[moamps]

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

 

[borges]

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?


Børge

 

[AndrewT]

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.

 

[moamps]

....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.

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

 

[anatech]

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

 

[borges]

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.

Børge

 

[KISS]

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.

Ideas are free.

 

[borges]

Yes, I could use such a double switching scheme. What I have done for now, though, is to use one transformer for both the main power and the MCU power. I'm using an AVR MCU. Then I'm using LM2941/2991 regulators with an on/off pin.

The primary winding is always connected to the mains plug, and the secondaries are always connected to the rectifiers and caps. Then the switching is in the regulators.

Perhaps not the ideal solution but I'm having space constraints that forbid a separate transformer for the MCU. So that rules out a mains relay too.

But my main consern is to make it safe. So if the mains cable is pulled out and a finger goes in, I don't want anybody to get a shock from it. Do you know of recomended safety circuits at the mains side of things?


Børge

 

[nemestra]

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?

Also add Voltage Dependent Resistors (VDRs) to suppress mains spikes on the primary side.

As to what happens when you remove the mains lead and stick your fingers in - nothing. The magnetic field in the transformer collapses within a few milliseconds. By the time the plug has cleared the socket (no matter how fast you pull it) any danger has disappeared.

James

 

[borges]

Good to know. I suspected as much, the inductance in the primaries are DC shorts. But I have limited experience in this, so I want to be safe before I send off the PCB files.

James, you don't happen to have a schematic of those resistors, do you? Also, should I apply X1/X2/Y (or whatever they are called) capacitors?

Thanks,
Børge

 

[borges]

Re: Re: Power supply - how to avoid getting hurt

Okay, I know it is a stupid question, but does the VDR go in series or in parallel with the mains?

I'm looking for white papers and application notes on how to build the mains side of my power supply so that it may be possible to obtain the necessary approvals. It is still a DIY project but I want to do things properly, especially when I'm messing around with 230V.

Børge

Also add Voltage Dependent Resistors (VDRs) to suppress mains spikes on the primary side.

 

[AndrewT]

Hi,
VDRs and similar goes between live and neutral.
they work by changing to a low resistance when the transient voltage goes above set voltage. They do have a finite life if asked to work doing real spike arresting.

X caps also go live to neutral, but they work by becoming low reactance to high frequency. The lower voltage rated version is proof to over 2000V and is self healing so should last forever.

Y caps go from live to earth and from neutral to earth (imagine the tops of the Y being the line connections and the bottom of the Y being the earth connection) one on each line. It is usual for the Y cap to be about 10 to 100 times lower value than the X cap. This is to minimise the leakage to earth.

 

[borges]

Thanks Andrew!

And I guess the fuse goes on Live between the mains+VDR+XY arrangement to the primary of my transformer?

So the VDR acts a bit like a zener diode for AC, is that it? For 230VAC, the VDR should be spec'ed for 275-ish?

Børge

 

[AndrewT]

Hi,

guess the fuse goes on Live between the mains+VDR+XY arrangement to the primary of my transformer

no guessing, absolutely.

For 230VAC, the VDR should be spec'ed

for UK 240Vac which can go to 254Vac then 275Vac is a must.
For 230Vac you need to contact your supplier and ask what the voltage supply tolerance limits are.
You may find that 250Vac is OK for the lower supply voltage.
I have a small stock of 250Vac that I cannot use.

 

[nemestra]

for UK 240Vac which can go to 254Vac then 275Vac is a must.

Yes, VDRs go in parallel to the mains and after the fuse. I've used 275Vac VDRs on 240Vac UK mains with no issues.

Regards,

James

 

[borges]

Thanks guys, the picture is getting clearer now!

Børge