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Fuse blowing - can't figure out why

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Hey all,

I've built a small dark-activated switch using a photoresistor a comparator and a relay for a 60W light bulb (at 230V, 50Hz).

In series with the bulb and relay, I've connected a 250V 2A fuse to protect the relay. Thing is - this fuse keep blowing every now and then, and I can't figure out why... Without the fuse, the bulb was never blown...

Am I missing some sort of a ripple-filter I'm not aware of maybe?

Thanks!
 

Hi,

I think your circuit has some problem. Please send the circuit for debugging...

Sudan
 

Hi, the fuse does'nt supose to blown off but cheak may be the is a short circute some where.
 

Also, be aware that the resistance of incandescent lamps are very low when cold. It can be as much as 1/10th of the hot resistance. Therefore can draw up to 10 times the current at start up, as it will draw when hot. If you are using "fast-blo" fuse and the lamp is switched on when the line voltage is at peak (230x1.4=322V) it will instantaneously draw much more than 2A. "Slo-blo" fuses are designed to allow for this brief current surge.

Attached are a couple of graphs, that someone made, to show the steady state relationship of voltage, current, resistance, and heat of a 120VAC 100W lamp.

Ken
 

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    z9u2k

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KMoffett said:
Also, be aware that the resistance of incandescent lamps are very low when cold. It can be as much as 1/10th of the hot resistance. Therefore can draw up to 10 times the current at start up, as it will draw when hot. If you are using "fast-blo" fuse and the lamp is switched on when the line voltage is at peak (230x1.4=322V) it will instantaneously draw much more than 2A. "Slo-blo" fuses are designed to allow for this brief current surge.

Attached are a couple of graphs, that someone made, to show the steady state relationship of voltage, current, resistance, and heat of a 120VAC 100W lamp.

Ken
Click to expand...

I think this just might be it. I don't know if the fuses I'm using are fast-blow or not (all I know is that they're ceramic), but during device selection I didn't take that initial inrush current into consideration.

Anyway, I now have to make sure my relay is able to withstand that current peak, or I may have to replace it...

I suspected it might have something to do with current ripples on the power line, but couldn't figure why only that fuse is blown (I have a secondary 200mA fuse on the same line for the device's internal power supply, which never blew). This should explain it.
 

Hi z9u2k,
You should pls a NTC forseen (your TV, and PC power supply, has it too :)): MegaSurge Inrush Current Limiters w*w.ametherm.com
K.
 

Unless the lamp is something special, inrush current limiting is overkill...as you are still going to want a fuse there. I've used NTC limiters on expensive projection lamps because of their replacement cost. Just make sure that your 2A fuse is a "SloBlo" or "Time delay" fuse.

Also, since you source in 230VAC (remember peak=230Vx1.4=322V) you might want to use a fuse with a higher voltage rating.

Ken
 

you might want to use a fuse with a higher voltage rating
Click to expand...
250V rating with fuses means AC (Vrms), see any fuse datasheet.

The fuse characteristic is clearly coded on most standard fuses, by the way. 2A slow blow would be O.K. for an incandescent lamp.
 

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