Fuses - Slow Blow, Anti-Surge, Time Delay?

[icon]

Hi

I'm blowing a lot of fuses on a power supply I've built which should only draw max 630mA on the 240V side, but as a conseqence of the the 10,000uF or so of capacitance draws a momentary 3-4A which is vaporising the 1A fuses I have. Doing a search with the usual suppliers reveals Slow Blow, Anti-Surge and Time Delay options. Are they all names for the same thing? Or which do I want?

Thanks
John

[Bloch]

http://www.schurterinc.com/var/schurter/storage/ilcatalogue/files/document/datasheet/en/pdf/typ_SPT_5x20.pdf


x = 630mA

1.5 x In min.       60 min
2.1 x In max.      30 min
2.75 x In min.   250 ms2.75 x In max.    ]80 s
4.0 x In min.       50 ms
4.0 x In max.        5 s
10.0 x In min.      5 ms
10.0 x In max    150 ms

Normally they are stamped with T

[Jeff1946]

Jumper the fuse and see what smokes. 

[IanB]

Maybe just try a 5 A fuse?

[Kremmen]

3 solutions:

1: increase the mains fuse or make it slower
 + fuses don't blow so easily when not desirable
  - fuses don't blow so easily even when desirable

2: increase the main fuse and install a second one to the output
  + main fuse withstands the starting as above, yet provides protection to the innards of the supply
  + output fuse can be dimensioned to blow quickly when max current is exceeded
  - more expensive, more stuff to install

3: incorporate an inrush limiter or PFC to the circuit
  + can keep original fuse
  + lower inrush current = lower stress to components and mains, no line surges
  - more complex and costly solution

[icon]

Hi

I have a 3A fuse in the mains plug (which doesn't blow) and a fuse on the secondary side. The fuse that's blowing is at the mains input of the device. I'm planning on Kremmen's option 1b - make it (the input fuse) slower. My question was which of [Slow Blow|Anti-Surge|Time Delay] do I need, or do they all do the same thing?

I did jumper the fuse - with my multimeter. Which is how I established that it was drawing 3-4A when first switched on. No smoke - without a load on the output, input supply current rapidly settles down to 10ma or so.

Cheers
John

[amyk]

I don't think you're actually drawing 630A of inrush current, that's 150+ kW

You should add a NTC. They can be found in scrap computer PSUs for free.

[Bloch]

The sole purpose for a main/ primary transformer fuse is to protect the transformer.


So we need to know that size transformer you have.


I don't like to just use a BIG fuse and then be happy. It is better to make some kind of slow chargeing the big capasitor.

[Bloch]

My question was which of [Slow Blow|Anti-Surge|Time Delay] do I need, or do they all do the same thing?

To get an answer you need to look at the datasheet :-)


My ques is that you have a 5*20mm fuse .....
If you look here http://uk.rs-online.com/web/c/fuses-sockets-circuit-breakers/fuses/cartridge-fuses/ There are a menu - Fuse Speed


You will want a T or a TT fuse.

[icon]

I don't think you're actually drawing 630A of inrush current, that's 150+ kW

Where did that come from? [Oh, from me, first post; apologies.] I said 3-4A (ie between 3A and 4A, as near as I can measure).

I don't like to just use a BIG fuse and then be happy.

Quite. I have a 3A fuse in the plug, which would only serve to prevent the wiring in the walls from catching fire. Hence me looking for a different option to protect the primary.

In fact the one I'm trying is a (6.3x32mm) 800ma fuse with a nominal I²t value of 8. If I understand correctly this should tolerate 4A for 0.5s, which I think would cover the initial inrush.

Regards
John

[cybergibbons]

You said 630A in the original post - presume you meant mA?

[Time]

Why can't you use a time delay fuse?  I think those are specifically meant for inrush conditions but I could be wrong.

[T4P]

Now i understand the differences, Slow Blow has a just a blob on, Time delay has a spring mechanism that is slower.
Anti surge is also slow blow

[icon]

You said 630A in the original post - presume you meant mA?

Ah rubbish, so I did. 

Edited.

John

[Time]

time delay wont break unless the over current lasts over a certain time length.  this is to accommodate the inrush current.

[westom]

If I understand correctly this should tolerate 4A for 0.5s, which I think would cover the initial inrush.

  Even in the 1950s, this problem was solved by an inrush current limiter.  Search the electronic catalogs for them.  An inrush current limiter will also increase reliability.

  Others now do same with active circuits that also perform power factor correction.

[amyk]

Something like these: http://www.ametherm.com/inrush-current/

[ejeffrey]

I say use a slow-blow fuse *and* an NTC inrush limiter.  Too much inrush will cause unnecessary stress on your transformer and input caps, so the NTC is good for your equipment reliability even aside from nuisance fuse and circuit breaker trips.   Even so, there will be some inrush.  A slow blow fuse won't hurt anything, and if it lets you get away with a lower fuse rating, it is better.

[jahonen]

Apropos, I just installed a softstart addition to two isolation transformers (545 VA toroid). I finally got bored to change the fuses (sometimes several times) on each restart My approach was to use just 5x10 ohms resistors in series of the primary, and relay which shorts out those resistors after 500 ms or so. Although NTC is simple, I don't personally like idea of a hot component and it doesn't work on brief disconnect. It took a little iteration to get the switching action quick on disconnect, so that softstart works properly even if power is only briefly disconnected. Seems to work very well. Theoretical maximum inrush current is therefore 230 V/50 ohms = 4.6A, which seems to be low enough for the fuse to handle. Relay and time delay circuit (just some discrete components) is powered through a "cap dropper" supply, using a 330 nF X2-grade capacitor.

Now I can use 2.5A slow-blow fuses (nominal for the transformer), before even 3.15A slow-blow fuses usually blew on power-up.



Note that fuse shown on the softstart PCB is not the main fuse, that one is just protecting the softstart resistors in case the output is shorted.

Regards,
Janne

[Kremmen]

Heh, it's not that long ago i did a similar inrush limiter for the 4 kVA power transformer on on the z-axis drive of my cnc mill. My approach was to use an RC delay to trigger a small thyristor that switches on a bypass relay that short circuits the limiter resistor. Also this solution is immediately ready for restart after power out, no need to wait for hot components to cool down. Additionally the circuit has its own power supply and main contact relay that opens in case of power failure to the unit, so there is no risk of burning the limiter resistors in case of any problems.

[SeanB]

I did the same with my Variac, as the switch on surge was enough to trip the supply breaker. I used 3 parallel 33R 10W resistors, shorted by a relay contact from a 24V relay driven via a capacitive divider and a bridge rectifier. My safety was a 110C thermal fuse on one of the resistors. Been working now for over 20 years, and is protected by a 10A fast blow fuse.