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one opto-coupler and 3 BT139 Triac

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arvinfx

Member
Hi

Can i drive 3 triac in parallel style connect ( T1s to T1s and T2s to T2s and Gates Pin to Another Triac Gates Pin) and drive three them whit only one Opto-coupler ?:confused:
 

Ubergeek63

Well-Known Member
you can not, in general, drive triacs in parrallel-they do not current share well resulting in catastrophic failure

if you force current sharing you can connect the main terminals in parrallel, but this is very clumbsy.

you can not tie the gates together as one tends to fire first and will rob the others of gate drive, again resulting in catastrophic failure
 

MrAl

Well-Known Member
Most Helpful Member
Hi,


Are you trying to create an 'OR' circuit where any gate drive can turn the circuit on,
or are you trying to increase the current rating of a triac by using three of them
in parallel?
From what you say it sounds like you are trying to put three of them in parallel in order
to increase the total current rating and therefore be able to drive something that
uses a higher current than any one triac is rated for. Unfortunately, this is a problem.

The problem starts where any one of the three may turn on first, taking a high current
before the others can turn on, but that's not the worst of it. The bigger problem is
that any one of the triacs might force the other two to turn off if the gate drive
is a pulse rather than a continuous drive signal. This could happen because the
other two may be robbed of all of their original current and so their conduction current
falls below the holding current and they turn off. This means one triac carries the full
load for the entire half cycle, and that's not good. This of course means that a gate
that pulses can not be used so a continuous gate signal is mandatory.

Ok, so we know now that a continuous gate signal is needed, but also as someone
said the gate voltages may be different so that means each gate has to be driven
individually. Also, driving the gate negative for any conduction angle is a good idea
to keep gate current and power low.

Next, one triac may turn on before the others and carry the full load for some time
period. Now they are quite hardy devices, but this still isnt a good idea so perhaps
some resistance in series with each main terminal. A small amount resistance may
help match the triacs up a bit better. Im not sure what your total load current is,
but since each device is rated at 16 amps you probably dont want to use too much
resistance or it will eat up too much voltage and also some energy. If you were to
use 0.1 ohms for example, that would drop 1.5v at 15 amps.
Also, some fuses in line with the main terminals is a good idea too. Perhaps rated for
15 amps.

Ok so here is the list:
1. Resistors in series with the gate to allow individual gates to get their required voltage
2. Continuous gate drive mandatory using a negative gate signal only, and pulse drive not allowed
3. Small resistors in series with main terminal to help match devices
4. Fuses in the main terminal lines

Note that this discussion is more or less theoretical, as i have never had to do this
in real life.

Also, the best idea is to purchase a triac that has the rated current so that you can
use a signal device rather than three in parallel, which basically is not a good idea.
Since each device you mention has a rating of 16 amps each, you may be able to
find a 45 or 50 amp device that can handle the full load. If not, you would still be
better off to make your own triac out of two 50 amp SCR's, which has been done
quite a bit in the past and always works fine. You could drive the gates perhaps
with two opto couplers.

If you do attempt the three triacs in parallel, at least test them to make sure they
all turn on and stay on once installed. If the construction works at all, you can test
each triac for temperature rise to see if any one triac is taking much more current
than the other two. If one gets very hot while the other two are cool that means
you may have to replace that one with another one that has similar turn-on
characteristics as the other two.

Lastly, GE makes a manual for SCR and TRIAC operation in parallel and other configs,
and i think it is the sixth edition of their scr and triac manual. You might find one
on Amazon or somewhere like that or maybe even online. This should have more
information about this topic.
 
Last edited:

arvinfx

Member
Thanks for you attention

My Current is 10 amp and i need 3 triac to reduce the temprature on my triac hit sink ( this is necessary for my project) . I use BTA40 and it is huge for my use. so can u send me link or complete name of GE manual?
 

MrAl

Well-Known Member
Most Helpful Member
Hi again,


Sorry i dont have a link to that manual. You would have to look for the sixth
edition of their manual on triac's and scr's, but i dont have the exact name
handy either. Perhaps on Amazon?
 
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