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Electrical device safety, fusing, heat

E

Eel

Jan 1, 1970
0
Hi, I got snuckered into putting together a diverse set of pc boards +
devices as part of what is supposed to be a software project. I'm
going to deliver with a big disclaimer that I ultimately don't know
what I'm doing, but I was wondering if anybody had some quick answers
for some hopefully simple questions.

1. Are there any safety guidelines for hooking a device up to 110 VAC
current? Is this a matter of just making sure the connections are
insulated and properly grounding anything that might come into contact
with a stray voltage? I need to hook wall current up to a 24 VAC
transformer that will drive a stepping motor via a 3rd party pc board.
There are a couple of other boards that get their power from a
computer.

2. Any links or info on specifying a fuse? I have 24 VAC at 2 amps,
so that would be something like .6 amps for 110 volts? When should I
use slow blow fuse?

3. Is it all right to mount everything on metal box with a few vents
and close it up? 24 volts at 2 amps is about 50 watts. It seems like
I could put a 60 watt light bulb into a box and close it up no
problem. Is that the case?

4. Any good books on this topic? When I was a student (a while back)
"The Art of Electronics" by Horowitz and Hill was popular. It seemed
a bit "off balance" in that in one sentence they were telling you not
to use electrical tape to insulate connections and in the next
sentence they were talking about quantum electrical theory. Is the
new edition any good? Any better books?


Thanks.
 
J

John Woodgate

Jan 1, 1970
0
1. Are there any safety guidelines for hooking a device up to 110 VAC
current?

Just a few thousand. Try UL standard UL60950.
Is this a matter of just making sure the connections are
insulated and properly grounding anything that might come into contact
with a stray voltage?

And a few thousand other things, like creepage and clearance, fire
enclosures, mechanical strength....
I need to hook wall current up to a 24 VAC
transformer that will drive a stepping motor via a 3rd party pc board.
There are a couple of other boards that get their power from a
computer.

2. Any links or info on specifying a fuse? I have 24 VAC at 2 amps, so
that would be something like .6 amps for 110 volts? When should I use
slow blow fuse?

The voltage is not even 110 V, it's 120 V. You are starting a long way
back. You do need a slow-blow fuse to feed a transformer, but you can't
calculate its value just by using the transformer current ratio. You
have to take into account inrush current. Under certain circumstances
the current at switch-on is limited only by the DC resistance of the
primary winding of your transformer. Calculate that current and then
look at fuse I^2t curves to choose one that will withstand that inrush
current for half a period of 60 Hz, i.e. 8.3 ms. Check that it will also
NOT withstand twice your 0.5 A normal operating current for more than a
few minutes at most. This latter is a rough guide: there are tests that
have to be done to determine whether the fuse rating is correct.
3. Is it all right to mount everything on metal box with a few vents
and close it up? 24 volts at 2 amps is about 50 watts. It seems like I
could put a 60 watt light bulb into a box and close it up no problem.
Is that the case?

UL 60950 will tell you all about that! There are strict limits on the
permitted temperatures of various parts.
4. Any good books on this topic? When I was a student (a while back)
"The Art of Electronics" by Horowitz and Hill was popular. It seemed a
bit "off balance" in that in one sentence they were telling you not to
use electrical tape to insulate connections and in the next sentence
they were talking about quantum electrical theory.

How dreadful!
Is the new edition
any good?

I expect you would find it much the same.
Any better books?

Not that will tell you the answers to your questions, AFAIK.

There are few books that tell you about how to design products to meet
safety requirements. One problem is that publishers would be reluctant
to publish such books because of possible liability if blindly and
inappropriately following any advice or recommendation in the book led
to an accident. You can learn design requirements from the relevant
standards, such as UL 60950, but it's a very steep learning curve.
 
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