Fuses

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

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.

[-------|--------|--------| | | | | | f1 f2 f3 ~ | | | | r1 r2 r3 | | | | [-------|--------|--------|

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.

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 simply resistive loads.

Yes, your absolutely right. 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).

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?

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

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 of tests. In about 7 tests I had a blown fuse.

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

"howa"

** WAKE UP !!!!

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

........ Phil

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 add value to someone reading over the thread in the future. Remind me what you have added to the discussion?

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

"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