Fuses

Discussion in 'Electronic Basics' started by howa, Mar 20, 2006.

1. howaGuest

Howdy:

I have 3 power supply circuits that I am going to put in one box.
Each draws less than 1 amp off the primaries, and sure enough I have no
problems when I use a 1A fuse.
However,
When I split the power 3 ways off the power jack, and insert a 1A fuse
on each branch, the fuses blow.
Same with 1.5A.
3A fuses, all is good again.
I thought that on parallel circuits the voltage for each node is the
same and the current splits....
But I am missing something.
Can someone tell me why I can't use the 3 x 1A fuses?

TIA,
-howa

2. Tom BiasiGuest

Maybe its me tonight but I can't visualize what you are doing. I sounds like
you are just putting your 3 power supplies in a box and using common AC and
fusing the supplies individually. If that's the case, the operation would be
no different than before you put them in a box. If you are fusing the common
feed to all three then a1 Amp fuse wont do it.
Tom

3. howaGuest

Here is a simplified block,
with ~ representing the wall AC,
f1-3 representing the fuses,
and r1-3 representing the loads of each power supply.

<pre>
[-------|--------|--------|
| | | |
| f1 f2 f3
~ | | |
| r1 r2 r3
| | | |
[-------|--------|--------|
</pre>

I am assuming this statement is true:
The configuration shown is the same as if I had 3 x individual devices
on the same power strip,
or 3 devices in one box with the input AC split 3 ways.

If this is true, then it would mean that if I had the 3 individual
devices with the
correct fuse and put them all on the same power strip and turned them
on at
the same time, then each of the fuses would blow, even though if turned
on
one by one, each would work correctly. ??

Is this because:
a) f1-3 are just wire, and in effect the same point?
b) the transformers have a spike upon power-on that somehow their
neighbors "see"?

TIA for any clarification.

4. Greg NeillGuest

What transformers? You didn't indicate any transformers.
For that matter, you didn't indicate what the loads were
*really*. I get the impression that r1, r2, and r3 aren't

5. howaGuest

The circuit is a 220VAC/2x35VAC transformer, followed by a full wave
rectifier, and a 10KuF cap.
After that, there is a dummy load (100K resistor).

6. Greg NeillGuest

Well, maybe as you've suggested there's some interaction
between the transformers during the initial turn-on
inrush. You should also verify that there's no shorts
to ground for any of the transformer windings.

Have you considered using slo-blo fuses?

7. ehsjrGuest

At turn on, the capacitors are virtually a dead short
circuit, and draw a (relatively) huge amount of
current. With 10K uf caps and ~ 100V (70 * 1.4)
the draw is too high for a 1 amp fuse, even if the
supply was a single transformer, regardless of the
fact that you don't blow the fuse that way. This
inrush current creates high frequency transients,
which I suspect are the cause of the interaction
when you have all three supplies connected.

You need to handle the inrush current problem on
each supply. Simply putting in a 3 amp fuse may
be incorrect. The fuse needs to be sized to
protect the transformer. Perhaps a 1 amp fuse
is necessary - don't know, because we don't
have the specs on the transformer. If it is,
you'll need a soft start. That is easily
accomplished by installing an NTC thermistor
in the primary side.

Ed

Assuming that putting in a 3 amp fuse

8. howaGuest

Thanks for all the great info.
Ed, what you are saying is what I have stumbled onto.

I measured the current with a DVM I have and found a wide variation on
the power-up spike.
The current I measured was certainly below the 1A fuse's capability,
but I do not think the
DVM has the ability to see the startup spike - however, it did show a
variety.

This predicts (correctly) that I should be able to blow out the 1A fuse
given some number

I found a document (worth googling for) on the internet by a company
called LittleFuse
called "Fusology". It is quite complete, and one of the points I took
out of it was that
at room temp, a fuse's rating had to be de-valued like this:

1A / .75 = 1.33A needed fuse

Other factors like the spike, etc., can influence fuse selection.

So, in summary the 1A fuse was working by "mistake" and based soley on
numbers a
1.25A or 1.5A fuse is more appropriate.

I have found that in practice, when considering the inrush current, a
2A slo blo fuse seems
to work properly. The transformer manu. suggests a time delayed
circuit, so I suspect I
will research the NTC thermistor and then swap out the 2A fuses with
1.25A or 1.5A units.

--howa

9. Phil AllisonGuest

"howa"

** WAKE UP !!!!

You ALWAYS have to use "slow blow" fuses in AC supply circuits employing
iron transformers and derived DC supplies - dumbo.

......... Phil

10. howaGuest

Hi Phil,

I was using slo blo's all along, I mentioned it for clarification on
the semi-final solution.
I also mentioned the LittleFuse Fusology reference, two things that I
thought might
Remind me what you have added to the discussion?

And thanks for the kind words, sweetie, you made my day ;>

11. Phil AllisonGuest

"howa"

Phil Allison

** WAKE UP !!!!

You ALWAYS have to use "slow blow" fuses in AC supply circuits employing
iron transformers and derived DC supplies - dumbo.

* Shame you did not mention it right off.

Shame you did not mention the VA rating or type of transformers being used -
no way to suggests suitable fuse sizes without that info.

Shame you do not know how to post to usenet with googlegroupos*

** Some basic facts.

I saw none from you.

* Click on "options" first, then "reply" - so usenet folk can see the
previous text and know who you are relying to.

........... Phil