PSU fuse placement

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Hi guy's

I'm going to build a dual rail power supply for some op-amp projects. I've got all the necessary 'bits and pieces' and have done the basic layout and am happy with my work so far. But, one question remains...

Where to place the (live) 'line' fuse? What is 'best practice', to place the fuse between the mains supply and the 'On-Off' switch, or between the switch and the primary transformer windings?

An engineer friend of mine say's between the switch and the transformer. The reason I'm seeking further input is that the switch is a SPST 240v(6A) (led indicator) with three contacts and 'logic' tell's me to put the line fuse before the switch.

What do you think? Does it actually make any difference?

--greg
Still learning, don't shout at me, educate me.
Starting the fire is easy; the hardest part is learning how to keep the flame!

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Of course it makes a difference - a chance less for a disaster. Your 'logic' is just fine.

Dor

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Thanks!

I suppose I was looking for a little reassurance that I was 'right' and my engineer friend was a little 'complacent'.

I've always used 'off the shelf' supplies or batteries before, this is my first attempt at building my own supply.

--greg
Still learning, don't shout at me, educate me.
Starting the fire is easy; the hardest part is learning how to keep the flame!

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I suggest a power entry module that has the familiar IEC receptacle (like the one on your computer PSU), fuse and even a switch maybe. But at least the fuse is built-in in that case. Some of those modules may even have mains voltage filtering.

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I think that UL and/or the National Electric Code would say, if you are in the US, which you are not.

Here, I think that the "safe practice" is to make the fuse the very first thing.

Jim

Jim Wagner Oregon Research Electronics, Consulting Div. Tangent, OR, USA http://www.orelectronics.net

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Quote:
I suggest a power entry module that has the familiar IEC receptacle

Agreed, and already purchased!
Quote:
Here, I think that the "safe practice" is to make the fuse the very first thing.
Exactly what I was thinking. And, btw, have you implemented my 'steps' for the Mac OS X 10.5.8 Eclipse/AVRplugin/GCC toolchain yet? I spent all weekend working on that for you guy's! :twisted: And where is Kurt? Did he completely give up before I solved the problem for him?

--greg
Still learning, don't shout at me, educate me.
Starting the fire is easy; the hardest part is learning how to keep the flame!

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Greg. You are in the UK. Don't all UK power plugs already have a fuse built into them ... maybe my memory is hazy.

Cheers,

Ross

Ross McKenzie ValuSoft Melbourne Australia

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I would actually (and do) for having the fuse between the switch & the fuse.
It would be relatively safer to change the fuse if the power to the fuse is disconnected (but only if the switch/fuse is in the active of the line circuit).
Common sense would suggest that you would of course disconnect the equipment from the wall supply, however, that is the problem with common sense, it is not so common!

Charles Darwin, Lord Kelvin & Murphy are always lurking about!
Lee -.-
Riddle me this...How did the serpent move around before the fall?

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Depends on the fuse as well probably. If it blows quickly then I guess there would be a (possibly) negligible timeframe for the switch to melt or whatever. However if it takes longer then the opposite may happen and the result I'm not at all sure of.

Having it after the switch could prevent the fuse from blowing while it's off, but that's about the only reason I'd have it there.

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Quote:
Having it after the switch could prevent the fuse from blowing while it's off

Run that past me again!

Charles Darwin, Lord Kelvin & Murphy are always lurking about!
Lee -.-
Riddle me this...How did the serpent move around before the fall?

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LDEVRIES wrote:
Quote:
Having it after the switch could prevent the fuse from blowing while it's off

Run that past me again!

haha yeah ok I didn't think that one through. No current flow in either case. I'm just gunna go hide now.

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Shhh...don't tell anyone and we will keep it between you and I. :)

Charles Darwin, Lord Kelvin & Murphy are always lurking about!
Lee -.-
Riddle me this...How did the serpent move around before the fall?

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The IEC plugs/leads I can 'acquire' from work are moulded with a built in fuse, yes. However, these plugs are fused at 5 amps. My transformer is rated at 1 amp, hence my need to insert a 1 amp quick blow fuse in the supplies chassis/case. Further, I plan to install a resettable poly fuse in the downstream circuit after the secondary windings but before the filter capacitors. As this supply is designed to operate 'experimental' op-amp circuits, I want it to be as bullet proof as possible.

--greg
Still learning, don't shout at me, educate me.
Starting the fire is easy; the hardest part is learning how to keep the flame!

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The transformer rating of 1 Amp, is that the primary or scondary rating? If it is the primary, I am assuming that the output current is about 10 Amps, which is pretty grunty for a "op-amp" supply, with a fairly elaborate regulator as I assume it will be a dual tracking supply. If it is only a 1 Amp output, thenthe input fusing only needs to be about 150 mA.

Quote:
The IEC plugs/leads I can 'acquire' from work are moulded with a built in fuse, yes. However, these plugs are fused at 5 amps.

Does that mean, if the fuse blows , the whole "IEC plugs/leads" (?) get thrown out. No replaceable fuse. :shock:

Charles Darwin, Lord Kelvin & Murphy are always lurking about!
Lee -.-
Riddle me this...How did the serpent move around before the fall?

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Good question! It's a 240v / 12-0-12 centre tapped 24VA supply. So it's the output thats 1 amp. I'm just using 78xx/79xx regulators and 25v caps (on the pre-regulated side), and yes, it's dual rail.
The IEC cables are fully moulded so, yes, if they blow, they get chucked out.

Quote:
If it is only a 1 Amp output, thenthe input fusing only needs to be about 150 mA.
Where do I go to locate this kind of information? Nothing I've read so far explains that relationship.

--greg
Still learning, don't shout at me, educate me.
Starting the fire is easy; the hardest part is learning how to keep the flame!

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Quote:
Where do I go to locate this kind of information? Nothing I've read so far explains that relationship.

Don't they teach maths any more?

24VA @ 24V = 1A
24VA @ 230V = 0.104A hence a 150mA fuse is suggested.

Ok. No transformer is 100% efficient. But then no fuses are 100% accurate either.

Your mains fuse is really for when you stick an iron bar straight into your PSU case.

If the secondary has a major current surge, you could use a 1A fuse there.

But if your intention is to produce a dual rail supply, you want both +ve and -ve rail to shutdown simultaneously. The LM7805 and LM7905 will act independently.

I suggest that you sense the currents well below 1000mA. e.g. a variable overload setting. This should shutdown both rails when tripped.

David.

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Quote:

Don't they teach maths any more?
Couldn't say for sure. I left school over thirty five years ago.
But now I see it written down, it makes perfect sense.
I need to buy a book on supply design, and stop relying on the internet for schematics :)
Thanks for the other info... very helpful.

--greg
Still learning, don't shout at me, educate me.
Starting the fire is easy; the hardest part is learning how to keep the flame!

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I would make the fuse bigger. On initial power-on the inrush current can exceed the nominal current by quite a bit. My rough guesstimate would be at least 500mA for your 25 watt power supply, especially as it is a quick-blow fuse.

I just grabbed a switching power supply I had lying around (12V, 2.1Amp) and its primary fuse is 2 Amps.

Putting the fuse in front of the prower switch, as suggested has also the simple advantage that the power-indicator in the switch shows the 'real' power situation. Fuse blown - LED dark. Otherwise your power switch will show 'power' when there is none.

Markus

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Thanks, I was thinking about the initial power surge/inrush so have ordered some 200ma and 500ma fuses today :) I'll start with the 200ma and if that does blow immediately on power up, I'll upgrade it to the 500ma. I've still got the 1A incase the demand is even higher. Just I'll just have to see.

The consensus of opinion is the line fuse should precede the on/off indicator, so I'll do just that.

--greg
Still learning, don't shout at me, educate me.
Starting the fire is easy; the hardest part is learning how to keep the flame!

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I don't know if anyone is remotely interested, but I finally got around to finishing the supply project. There was lots of cussing and burnt fingers along the way plus about as many major beginners cock up's as I could throw at it :)
The line fuse does go before the switch :) The second switch, that is, the first one got quite damaged as I was trying to unsolder the primary feed from the fuse as I soldered it up wrong, whilst the switch worked fine, the indicator light would not go out! I've also totally destroyed one -ve regulator by swapping the input and output leads (D'oh) and burned myself because it got so hot.
I've finally got the output ripple to below 10mV on both rails and when loaded the supply regulates at +/- 100mV so I'm pretty chuffed. Even the supply case looks ok, following my 'How to cut holes in plastic' thread.
Thanks again for all your help and advice. I'd probably have killed myself if not for your help.

--greg
Still learning, don't shout at me, educate me.
Starting the fire is easy; the hardest part is learning how to keep the flame!

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Pictures or it didn't happen! :)

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You disbeliever you...
(Um, I've never attached a picture...)

Yup, that worked. Red LED for +ve Green for -ve. :) You don't want the ripple trace on the scope do you? I've no idea how to capture that on my Mac's ;)

Attachment(s): 

--greg
Still learning, don't shout at me, educate me.
Starting the fire is easy; the hardest part is learning how to keep the flame!

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Hi Greg,

I think I see a reamer from DealExtreme ... nice tool.

Cheers,

Ross

Attachment(s): 

Ross McKenzie ValuSoft Melbourne Australia

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Quote:
I think I see a reamer from DealExtreme ... nice tool.
Indeed, I'd almost given up on it ever arriving. It took TWO MONTHS to be delivered! Still, as you say, Nice Tool :)
You may also notice in /near the top left of the image, my first ever AVR development board, with sticky breadboard and RTC with battery backup :)

--greg
Still learning, don't shout at me, educate me.
Starting the fire is easy; the hardest part is learning how to keep the flame!

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You are going to heatsink those regulators later?

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Quote:
You are going to heatsink those regulators later?
Mwhahahahahah *cough*

--greg
Still learning, don't shout at me, educate me.
Starting the fire is easy; the hardest part is learning how to keep the flame!

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gregsmithcts wrote:
Indeed, I'd almost given up on it ever arriving. It took TWO MONTHS to be delivered! Still, as you say, Nice Tool :)

It seems that shipment speed is hit or miss with Dealextreme. I've have had between 3 and 9 weeks from order to delivery. I suspect they negotiate low shipment tariffs and get low priority shipment in return.

Markus