# Triac Fusing Current Value check

#### belkzak

##### New Member
Hi Guys,

I havebought a triac which have a value of fusing current = 166 A2S for Tp=8.3ms mentioned in the data sheet

I am wondering how can i check this value? Also , for the current pulse width triggering the TRIAC (0.5ms).

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#### kubeek

##### Well-Known Member
What do you want to check? I understand this as the maximum I2t for a fuse that protects the triac, if you use higher rated fuse than the triac can be destroyed.

#### belkzak

##### New Member
Yes Kubeek, i understand also that it is the value for the fuse to protect the triac from damage, which means also the higher current that it can support i guess. but how to check it?

#### Nigel Goodwin

##### Super Moderator
Yes Kubeek, i understand also that it is the value for the fuse to protect the triac from damage, which means also the higher current that it can support i guess. but how to check it?
You check it by destroying it - however, it's certainly NOT the value for a fuse, which won't protect a triac in any case, and certainly not one that's anywhere near the maximum rating.

I've repaired countless light units over the years, always with failed triacs, and always because a bulb has blown. Fuses aren't to protect the electronics, they are to reduce the chances of fire or serious damage - and the triac almost always blows when the bulb goes. The fuse 'might' blow as well, but not until the triac has already failed.

Supposedly you can buy fuses intended for triac protection, but they cost a LOT more than the triacs do, and the triacs still fail anyway - just not every time a bulb blows, possibly once every three or four times?.

#### belkzak

##### New Member
You check it by destroying it - however, it's certainly NOT the value for a fuse, which won't protect a triac in any case, and certainly not one that's anywhere near the maximum rating.

I've repaired countless light units over the years, always with failed triacs, and always because a bulb has blown. Fuses aren't to protect the electronics, they are to reduce the chances of fire or serious damage - and the triac almost always blows when the bulb goes. The fuse 'might' blow as well, but not until the triac has already failed.

Supposedly you can buy fuses intended for triac protection, but they cost a LOT more than the triacs do, and the triacs still fail anyway - just not every time a bulb blows, possibly once every three or four times?.
Would you please, then, temll me what is this value I^2T used for ? and anyways, how can i just confirm that itis 166 A2S (this test is just a part of a validation plan for this triac)

#### Nigel Goodwin

##### Super Moderator
Would you please, then, temll me what is this value I^2T used for ? and anyways, how can i just confirm that itis 166 A2S (this test is just a part of a validation plan for this triac)

No idea, it's not of much use for actually using the device.

#### JimB

##### Super Moderator
Many years ago (pre-internet) when the discussion turned to the subject of fuses to protect transistor circuits, the usual answer was:

"A transistor is the fastest fuse on three legs"

ie Trying to protect a solid state device with a fuse is a waste of effort.

JimB

#### KeepItSimpleStupid

##### Well-Known Member
Here https://forum.digikey.com/t/fuse-i2t-rating-melting-point-and-breaking-capacity/1271 is a blurb on I^2T

You can buy fuses with I^2t ratings. They are designed to protect semiconductors like triacs and SCR''s. I often combined a semiconductor fuse (It was in the SCR power unit) with a load fuse. There was a 25A SCR unit with an expensive like $25.00 each fuse and I combined it with a appropriate load fuse costing <$1.00. The application made it easy to blow fuses.

#### rjenkinsgb

##### Well-Known Member
I2T is basically a rating of how fast a device will heat to the point of damage or failure at some current level.
It's I x I x T; Current squared multiplied by time.

Hypothetically, if a fuse has a lower I2T rating than the device it's protecting, the fuse will fail first. However the device will also have experienced the high fault current and been rapidly heated itself, which can cause degradation and shorten its life or make it more likely to fail in a future overload, even it it appears OK after an initial fault.

If it's something likely to experience faults, a rather higher rated device (so much higher I2T rating than the minimally sized one) but still with the lowest current ultra-rapid fuse with the lowest I2C rating that can survive normal operation is your best option.

See the data here for some typical small ultra-rapid (eg. FF, uR or gR rated) fuses:

Note there are two I2T / A2S ratings - As current is still passing until the arc quenches, I'd consider the pre-arc one unimportant; the second "Clearing" I2S is the total energy let-through, which is the critical value.

#### belkzak

##### New Member
Is there a way to check if this value 166 A2S declared in datasheet? it's actually the point i am stuck in !

#### rjenkinsgb

##### Well-Known Member
Is there a way to check if this value 166 A2S declared in datasheet?
You state it's in the datasheet for the triac (it is in the section you show), and the value is given for each fuse in the fuse data I linked to ?????

Remember it is a maximum figure, the fuse should have an I2T rating (A2S is the unit) the same or preferably lower than the triac requirement.

The triac has an absolute maximum rating of 15A RMS; depending on the real-world load requirements you could use a fuse from that list rated anything up to 16A, which has a total (cleared) I2T rating of 140 A2S.

If the load is rather lower than 15A (which is good design practice), you can use a suitably lower rated fuse to suit the load.

If you mean, how to verify the datasheet figure:
Test some thousands of devices with timed, controlled, high controlled overloads, then test the survivors for total functional life vs. devices that have never been overloaded.

#### belkzak

##### New Member
You state it's in the datasheet for the triac (it is in the section you show), and the value is given for each fuse in the fuse data I linked to ?????

Remember it is a maximum figure, the fuse should have an I2T rating (A2S is the unit) the same or preferably lower than the triac requirement.

The triac has an absolute maximum rating of 15A RMS; depending on the real-world load requirements you could use a fuse from that list rated anything up to 16A, which has a total (cleared) I2T rating of 140 A2S.

If the load is rather lower than 15A (which is good design practice), you can use a suitably lower rated fuse to suit the load.

If you mean, how to verify the datasheet figure:
Test some thousands of devices with timed, controlled, high controlled overloads, then test the survivors for total functional life vs. devices that have never been overloaded.
Yes, I wanna verify how far is this value 166 A2S correct!

#### KeepItSimpleStupid

##### Well-Known Member
This,
I think will help understand the concept.

I have used a 3AG fuse load rated and an (I^2)t fuse (in the instrument) saved us a lot of money. The wiring was inside a vacuum system.

#### kubeek

##### Well-Known Member
I have the feeling that if you know what the fusing integral actually means then you should have no issue devising a method to test it.
166 A2s over a period of 8.2ms. That means the current is sqrt(166/0.0082)=142A.
Apply constant current of a bit less than 142A for exactly 8.2ms and then keep increasing it to see at which current the triac dies. Absolutely pointless test in my opinion, but you do you.

#### belkzak

##### New Member
I have the feeling that if you know what the fusing integral actually means then you should have no issue devising a method to test it.
166 A2s over a period of 8.2ms. That means the current is sqrt(166/0.0082)=142A.
Apply constant current of a bit less than 142A for exactly 8.2ms and then keep increasing it to see at which current the triac dies. Absolutely pointless test in my opinion, but you do you.
One more question please, i got a request from my client to calculate the i2t for a pulse width triggering the triac of 0.5ms , i am wondering how to do that! Is the i2t really related to the current on the gate to trigger the triac ?!

#### alec_t

##### Well-Known Member
I wanna verify how far is this value 166 A2S correct!
Why? Even if you verify it for one device, the next device you pick up will have a different value because of manufacturing tolerances.

#### rjenkinsgb

##### Well-Known Member
One more question please, i got a request from my client to calculate the i2t for a pulse width triggering the triac of 0.5ms
For the gate energy? Square the current and multiply by time.

Though that has zero relationship to the I2T rating of the device power electrodes...