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Triac Short Circuit Protection

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Suraj143

Active Member
Hi all!

I'm using BT136 triac.

For the output of the triac I use two wires to the AC bulb (One is from T1 & other from L wire).

My problem is

By mistake if I short the two wires will it blow up the triac? If so any precautions to make it safe against short circuitry.

Thanks
 

mvs sarma

Well-Known Member
Hi all!

I'm using BT136 triac.

For the output of the triac I use two wires to the AC bulb (One is from T1 & other from L wire).

My problem is

By mistake if I short the two wires will it blow up the triac? If so any precautions to make it safe against short circuitry.

Thanks
Generally the TriAC is wired after the load . the sequence is Live, on/off switch, Load (bulb/fan), TriAC with its controlling circuit , then return to Neutral.With this, the fear of TriAC blowing will never arise, perhaps.
I have been seeing circuits like this only.
 

mvs sarma

Well-Known Member
As Jim B pointed out correctly, any device would blow if applied directly across L and N of the public mains, for what ever reason . Nothing else on earth might protect in such case, except perhaps POLYFUSE

Why searching for anyone out there-- you messages will be viewed befor the day. Are you in a hurry or what Suraj?
 

MrAl

Well-Known Member
Most Helpful Member
Hi,

Yes i agree that the triac will blow out. Use a fuse in series (fast blow)
and hope that helps. If it doesnt, at least it will blow probably before
the mains, and then you can replace the triac.
You should be careful anyway when dealing with this kind of circuit.
If you are shorting the two wires out then you are not being very
careful about this.

Unfortunately there isnt even any way to turn the triac off with
an electronic current detect circuit, so a fast fuse is your
only hope here. You could over spec the triac (use a much bigger
size triac) as these big ones are pretty hardy and might withstand
the current for the time it takes to blow the fuse. The fuse
should be sized carefully too, using the lowest current rating possible.

I dont know what kind of current you are trying to switch here either.
 

Nigel Goodwin

Super Moderator
Most Helpful Member
OK, you're not going to stop the TRIAC failing 100%, but you can increase it's odds - first use a MUCH higher rated TRIAC (say 25A or 50A instead of 10A), secondly you can buy special fuses for TRIAC's, which 'some of the time' will fail before the TRIAC goes S/C - but these cost more than the TRIAC's, so it's not a cheap option.

I've repaired many disco light units over the years, and basically when a bulb goes it takes a TRIAC out, most of the time! - it's not cost effective for a manufacturer to over rate the TRIAC's enough, or to use the special fuses.
 

Hero999

Banned
If you'rere desining it yourself, you could make sure the TRIACs are easy to replace by using sockets but that adds another concern for reliability.
 

MrAl

Well-Known Member
Most Helpful Member
Hi again,

Hero:
That's an interesting idea...i wonder if someone makes a socket for a
TO220 for example? I know the bolt would still have to be removed
though, but at least no soldering.
 

Hero999

Banned
You could use an eight pin dill socket and only use three of the pins or 6 if you've got two TRIACs in your circuit.
 

Suraj143

Active Member
Thank you guys for comments then I'll use a high rate triac if it possible I'll use a fuse too.

Code:
when a bulb goes it takes a TRIAC out, most of the time!
Nigel can you tell me how is this happens?
 

Nigel Goodwin

Super Moderator
Most Helpful Member
Thank you guys for comments then I'll use a high rate triac if it possible I'll use a fuse too.

Code:
when a bulb goes it takes a TRIAC out, most of the time!
Nigel can you tell me how is this happens?
As bulbs fail they generally go S/C, either with the filament shorting the contacts out, or by an arc across the contacts - it's a VERY common occurance.

Like many here, I've made quite a lot of spare cash over the years replacing blown TRIAC's in light units :D
 
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