Four PSU questions

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Hey all, first off: thanks for a great forum! I've learned a lot just by cruising around for the past year or so. However, I have three lingering questions that I haven't yet found answers to, either by cruising or searching. If these are dumb questions, be gentle to the noob...

1. I've read that you can place a varistor across switch contacts to prevent popping and increase the life of the switch. However, for a DPST switch, I'm still not sure across which contacts to place it! The only info I've found stated the varistor should be connected "in parallel," which leads me to think I'll need 2 varistors, one across each of the throws.

2. For the AnTek AN-0212 transfo, can I wire the secondaries in series to get both 24V and 12V like the attached pic (from ESP)? I'm hoping to build a MooseFET preamp and use the 12V for remote volume control (a sine qua non for my wife).

3. I've read that a 300VA transfo is the cutoff for inrush worries, because the windings of a 300VA or lower transfo will prevent it. What I'm wondering is, if I connect two 200VA transfos to the same inlet, do I need to worry about inrush? I'd rather not have to have a soft start module, for a variety of reasons, if I can avoid it.

4. Finally: I'm planning to also build ESP's Project 101 for my amp. I'm thinking I'll just use a steel flat to separate the PSU from the amp section, to prevent hum. My question is: should the bridge rectifier be on the transfo side of the flat, or the amp side? I'm assuming the caps will be on the amp side...

Any answers, suggestions, instructions, etc. are most welcome! Cheers~
 

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1/ Good question and I'll be the first to admit I'd have to research that. Instincts suggest that the varistor should be across L-N and placed after the switch (and mains fuse for safety). Switch contacts are sometimes suppressed with a 'snubber' which could be in the form of a 0.01uf cap in series with say 100ohm and placed across the contacts. The cap should be class 'X' rated. DPDT... hmm... two snubbers.

(with a good switch that's suitably rated you shouldn't get any popping or noise even without a suppressor)

2/ The AnTek is described as a 0-12, 0-12 transformer, so that means two separate identical 12 volt windings. In series and you get 0, 12 and 24 volts AC or more conventionally written 12-0-12. That means you can generate a DC supply of around -/+17 volts DC or a single DC supply of 34 volts.

So your diagram becomes like this. You can have either what is to the left of the line, or what is to the right, but not both.

Capture.JPG

3/ No inrush worries for normal mains outlet socket feeding 400va. A UK 230 v 13 amp outlet doesn't even blink with 650va and higher.

4/ You can't beat fresh air separation for reducing hum. Bolting the toroid to a steel baseplate shouldn't be a problem as long as the sensitive parts of the preamp are some distance away such that they are not affected by the radiated field from the transformer.
 
Thanks Mooly. Quick responses:

1. I've picked up that most people don't use a varistor, so obviously it's unnecessary more often than not. Sounds like its necessity is brought on by other errors that I don't plan to commit! :D

2. I can't have my cake and eat it too?! Bummer. I guess I've got to get two transformers then.

3. Sorry, I should have been more clear. I'm not worried about inrush for my outlet, but for the slo-blow fuse. Is there still nothing to worry about?

4. What exactly are the "sensitive parts" of the amp?! I suppose this is really what I was trying to ask in the first place.

Again, thanks!
 
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Whether you need two transformers depends on what you are trying to do.

OK, I've just looked up what a MooseFET preamp is,
DIY Audio Projects Forum • MooseFET Preamplifier

This is a single rail design and so you can use your transformer to generate all the supplies you mention. The 24 vac from the transformer will go to a bridge rectifier and reservoir cap to provide an unregulated 34 volts dc that you feed to the LM317. That is using the transformer as in the right of my diagram.

You have a couple of choices for deriving a second lower voltage supply for the remote. You could add a 78xx (5v, 9, 12, etc) type regulator feeding it from the stable 24 volts feed from the LM317, or you could add a second LM317 to be fed from the unregulated supply (just as the main one) and use suitable voltage setting resistors to determine its output voltage.

Assuming I'm looking at the correct project then you definitely are nowhere near even needing to think about inrush problems with such a low powered design as this. A 200va transformer is massively oversized for a design that draws around 250ma in total. A small 30va rated tranny would be more than adequate (assuming the remote part draws little average current).

Sensitive parts of an amp are the parts around the input. Again, looking at the project and I can't see any real issues.
 
I fit soft start to all transformers bigger than 80VA.
That allows me to use close rated fusing.

eg.
160VA 230:25Vac transformer.

It needs a fuse rated @ T2A to allow reliable starting on a UK supply. The USA/Canada 110/120Vac supplies require double the fuse rating.
But it will run on a T800mA fuse for ever, if you adopt a soft start.

If you run two 160VA transformers without a soft start, you will need either a T3.1A, or T4A, mains fuse.
If one transformer gets abused, then that fuse will pass 8A for a long time before it ruptures. How much damage could there be if that 8A passes for 30minutes?

Had a close rated fuse of T800mA been fitted to each transformer, then an 8A fault current will blow that fuse in such a short period (milli-seconds) that there is no possibility of consequent damage. There won't be enough heat to set your cable/curtains/carpet on fire.

That's the advantage of close rated fusing. You fuse for the normal maximum current when the equipment is operating normally. The close rated fuse ruptures quickly when abnormally high current passes due to some accident/incident.
 
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A varistor is sometimes used to protect circuits (during operation) from incoming mains transients.

To protect a switch from arcing at the point of switching off (or switch bounce when switching on) you need to add a snubber across the output or across the switch. For a DPDT switch you only need one snubber if it is across the output, or two if they are across the switch contacts. The reason you only need one across the output is that in this case you don't know and don't care which switch contact first breaks the circuit; by the time the second one opens the circuit has already been broken. In many cases a simple X rated capacitor is sufficient, as the transformer itself will provide enough loss to damp the circuit. 5nF or 10nF should be fine.

There is another advantage in putting the cap/snubber across the output and not the switch: the cap lasts longer as it only sees mains when the equipment is switched on.
 
Thanks to all three of you.

Mooly: yeah, 200VA would be way overkill for a preamp! That question was regarding an amp build I also hope to attempt (Rod Elliott's P101). And thanks for the option. I was under the impression that getting 12VAC from 24VAC wasn't quite that simple, but I'll look into it a bit more.

AndrewT: That makes a lot of sense. There are lots of Chinese soft start boards on eBay. Are these of tolerable quality, or should I stick to building my own?

DF96: Thank you!! That's exactly what I was wondering. Cheers~
 
I looked at a few very cheap to not quite so cheap soft start and speaker delay/protection kits from the far east.

I bought a dozen speaker delay/protection kits from the cheapest because they gave sufficient info to assess their quality and I determined they could be made to be reliable, if the relay actually breaks a fault current. The relay is the final protection and I won't know until after my speaker is damaged!

I ended up making my own soft starts because I could not find any kits, nor PCBs that I thought were reliable enough. I will not use a direct to Mains power feed into a mains powered soft start.
 
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A varistor is sometimes used to protect circuits (during operation) from incoming mains transients.

To protect a switch from arcing at the point of switching off (or switch bounce when switching on) you need to add a snubber across the output or across the switch. For a DPDT switch you only need one snubber if it is across the output, or two if they are across the switch contacts. The reason you only need one across the output is that in this case you don't know and don't care which switch contact first breaks the circuit; by the time the second one opens the circuit has already been broken. In many cases a simple X rated capacitor is sufficient, as the transformer itself will provide enough loss to damp the circuit. 5nF or 10nF should be fine.

There is another advantage in putting the cap/snubber across the output and not the switch: the cap lasts longer as it only sees mains when the equipment is switched on.

What do you mean by output?
 
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