capacitive load on triacs

[raseel]

hai,
iam designed a triac circuit to controll a capacitive load. The triac
used is BT139. I am not included a snubber circuit.Is that necessory
for capactive load??
Then how is the design the snubber components?? iam expecting a
positive reply..

RASEEL

 

[Robert Baer]

hai,
iam designed a triac circuit to controll a capacitive load. The triac
used is BT139. I am not included a snubber circuit.Is that necessory
for capactive load??
Then how is the design the snubber components?? iam expecting a
positive reply..

RASEEL

"Snubber" circuite are used when inductors are being switched, due to
the ringing / high voltages created by inductors (dI/dT).
So.. look at a circuit like the following: power supply feeds a
resistor to a switch to ground (the triac) and the capacitor is in
parallel with the switch.
Start with switch open - capacitor charges to supply voltage = is
triac rated for that voltage (or more)?
Then close the switch (turn on triac) - large current flows from
capacitor thru triac, limited only by inherent resistances of the
circuit = is the I*I*T rating of the triac exceeded (pronounced "eye
squared tee")?
Sadly that data appears to be very absent these days; 30-40 years ago
that crucial design info was *published* as a part of the data sheet
specification of (power) thyratrons, ignitrons, SCRs and triacs.
That info is similar to SOAR or Safe Operating Area in power
transistors and is absolutely necessary so that a designer can prevent
device destruction.

 

[Larry Brasfield]

"Snubber" circuite are used when inductors are being switched, due to the ringing / high voltages created by inductors (dI/dT).

The dI/dt rating is more likely to be important if a capacitive
load is being switched. An inductive load tends to be a
gentle one with respect to dI/dt since inductor current is
hard to change fast.

(To the OP
You need to be sure that something limits the rate at which
current rises when the triac is first turned on. Until it has
been fully turned on, it can be harmed by currents that are
well within the device current rating. Inductance is often
added in the load circuit to limit dI/dt to the device rating.
For your BT139, that is 10 to 50 A/uS depending on the
load current direction and the gate drive polarity.

(Cut the rest, with which I agree.)

 

[Alan Turner]

Hi

"Snubber" circuite are used when inductors are being switched, due to
the ringing / high voltages created by inductors (dI/dT).
So.. look at a circuit like the following: power supply feeds a
resistor to a switch to ground (the triac) and the capacitor is in
parallel with the switch.
Start with switch open - capacitor charges to supply voltage = is
triac rated for that voltage (or more)?
Then close the switch (turn on triac) - large current flows from
capacitor thru triac, limited only by inherent resistances of the
circuit = is the I*I*T rating of the triac exceeded (pronounced "eye
squared tee")?

Perhaps it may be possible to fire the triac near the zero crossing of the
mains voltage. In this case the current will be limited to C*dv/dt. This
assumes that the initial charge on the cap is zero of course

I suppose it depends if the OP wants an "on-off" control, or one with a
variable firing angle. If it is an on-off situation, the OP might consider
a solid state relay.

Regards,
Alan

 

[Fred Bloggs]

(To the OP
You need to be sure that something limits the rate at which
current rises when the triac is first turned on. Until it has
been fully turned on, it can be harmed by currents that are
well within the device current rating. Inductance is often
added in the load circuit to limit dI/dt to the device rating.
For your BT139, that is 10 to 50 A/uS depending on the
load current direction and the gate drive polarity.

Another example of Brasfield moronic advice. Hey , Larry, you try hard
to be the big anal-yst but it's not working too well for you- you just
don't have the evolutionary endowment. To listen to a moron like you one
would suspect that a 50A/us current rise limiting at 1mA would blow the
thyristor...what a confounded idiot and fake you are. It looks like you
are a total failure at actually understanding anything. All you know how
to do is regurgitate specs you can't fathom- pathetic.

 

[Larry Brasfield]

Derf transform applied.

(To the OP
You need to be sure that something limits the rate at which
current rises when the triac is first turned on. Until it has
been fully turned on, it can be harmed by currents that are
well within the device current rating. Inductance is often
added in the load circuit to limit dI/dt to the device rating.
For your BT139, that is 10 to 50 A/uS depending on the
load current direction and the gate drive polarity.

[derf] To listen to [derf] you one would suspect that a 50A/us current rise limiting at 1mA would blow the thyristor... [derf]

Your "1mA" figure has no factual basis in this thread.
My advice to the OP could easily be pertinent.

 

[Fred Bloggs]

Larry Brasfield wrote:
[...snip moron Brasfield editing...]

Your "1mA" figure has no factual basis in this thread.
My advice to the OP could easily be pertinent.

You are running 10/10 on worthless and inapplicable "advice" so far.
Keep using your little specious pseudo-intellectual vocabulary- just
love it- a sure sign of a nobody trying to be a somebody the way you
misuse those words. You are a good-for-nothing loser. You should find
companionship with a certain sub-population of similarly worthless
trolls on SED- lots of non-degreed technician types here- long on OT
opinions and sickeningly short on anything worthwhile or constructive.

 

[Winfield Hill]

Larry Brasfield wrote...

Robert Baer wrote ...

The dI/dt rating is more likely to be important if a capacitive
load is being switched. An inductive load tends to be a
gentle one with respect to dI/dt since inductor current is
hard to change fast.

(To the OP
You need to be sure that something limits the rate at which
current rises when the triac is first turned on. Until it has
been fully turned on, it can be harmed by currents that are
well within the device current rating. Inductance is often
added in the load circuit to limit dI/dt to the device rating.
For your BT139, that is 10 to 50 A/uS depending on the
load current direction and the gate drive polarity.

A bit misleading in the OP's case. The di/dt issue addresses the
case of repetitive low-gate di/dt drive while expecting fast triac
current rise. This involves conduction-spreading, an issue that's
reduced with more aggressive gate drive. Usually a more relevant
issue arising from high peak currents when switching capacitive
loads is the need to model the transient power dissipation, and
analyze this against the "Transient thermal impedance" curves.
This reveals the triac's transient thermal mass, which must absorb
the transient energy without exceeding Philips' modest 125C maximum
junction-temp spec for the BT139. As with the dI/dt issue, adding
series inductance is sometimes helpful because the part's available
thermal mass increases by the square root of the increased duration
of the heat pulse.

 

[Larry Brasfield]

Larry Brasfield wrote...

A bit misleading in the OP's case. The di/dt issue addresses the
case of repetitive low-gate di/dt drive while expecting fast triac
current rise. This involves conduction-spreading, an issue that's
reduced with more aggressive gate drive. Usually a more relevant
issue arising from high peak currents when switching capacitive
loads is the need to model the transient power dissipation, and
analyze this against the "Transient thermal impedance" curves.
This reveals the triac's transient thermal mass, which must absorb
the transient energy without exceeding Philips' modest 125C maximum
junction-temp spec for the BT139. As with the dI/dt issue, adding
series inductance is sometimes helpful because the part's available
thermal mass increases by the square root of the increased duration
of the heat pulse.

Mr. Baer's post to which I was responding adequately mentioned
the pulse energy issue. I don't think it is misleading for me to add
another consideration, especially when I indicated agreement with
the rest of Mr. Baer's advice, including that issue.

Have you seen any data on how safe dI/dt varies with gate drive?
My understanding is that the gate does not reach all portions of the
junction pair that is being turned on and that conduction spreading
is what gets most of the device area conducting. Given the relative
magnitude of the load and gate currents, at least when dI/dt is an
issue, I would expect the spreading rate to be independent of the
initial gate drive.

 

[Robert Baer]

The dI/dt rating is more likely to be important if a capacitive
load is being switched. An inductive load tends to be a
gentle one with respect to dI/dt since inductor current is
hard to change fast.

(To the OP
You need to be sure that something limits the rate at which
current rises when the triac is first turned on. Until it has
been fully turned on, it can be harmed by currents that are
well within the device current rating. Inductance is often
added in the load circuit to limit dI/dt to the device rating.
For your BT139, that is 10 to 50 A/uS depending on the
load current direction and the gate drive polarity.

(Cut the rest, with which I agree.)

I can see that one might think that dI/dT is important, but I*I*T is
far more important!
If you check out the units, (I*I*T)*R ==> energy (aka power).
And it is a high amount of power that can literally vaporize devices.

 

[Winfield Hill]

Larry Brasfield wrote...

I would expect the spreading rate to be independent of the
initial gate drive.

If you study various manufacturer's writings on the issue, you'll
learn that high-current fast-risetime gate drive is important in
high dI/dt applications. I haven't made comparative measurements
myself, but I've read enough to give me respect for the argument.

 

[Fred Bloggs]

Mr. Baer's post to which I was responding adequately mentioned
the pulse energy issue. I don't think it is misleading for me to add
another consideration, especially when I indicated agreement with
the rest of Mr. Baer's advice, including that issue.

Hey, idiot- this is your standard cop-out: first you pretend to post an
all encompassing description and then you later claim some mousy excuse
whenever anyone points out your omissions. Looks like you have a pat set
of excuses on the ready- along with the "I have nothing to prove"
weaseling- you are a worthless usenet troll and snake who gets whacked
early every time.

Have you seen any data on how safe dI/dt varies with gate drive?
My understanding is that the gate does not reach all portions of the
junction pair that is being turned on and that conduction spreading
is what gets most of the device area conducting. Given the relative
magnitude of the load and gate currents, at least when dI/dt is an
issue, I would expect the spreading rate to be independent of the
initial gate drive.

Think it might be a diffusion capacitance charging issue, retard? Your
confusion here does not gel with your claimed prowess in that eb-zener
"puzzle" you were pseudo-intellectualizing over earlier. It is a very
sorry situation when a person your age insists on living in a fantasy
land when you should be reconciling your ignorance, incompetence, and
overall worthlessness as a human being. Go away and stay away.

 

[Larry Brasfield]

[Derf transform applied.]

Mr. Baer's post to which I was responding adequately mentioned
the pulse energy issue. I don't think it is misleading for me to add
another consideration, especially when I indicated agreement with
the rest of Mr. Baer's advice, including that issue.

[derf] first you pretend to post an all encompassing description and then you later claim some mousy excuse whenever anyone points
out your omissions.

False. My first post on this thread raised a single issue without
making any claims that other issues did not exist or matter. To
the contrary, I acknowledged the existence and relevance of other
issues with my quoting note: "(Cut the rest, with which I agree.)"

Many people consider the excision of material irrelevant to the
points being made to be good Usenet practise, as do I. Your
transformation of it into an excuse for spew is pathetic.

[derf]

Have you seen any data on how safe dI/dt varies with gate drive?
My understanding is that the gate does not reach all portions of the
junction pair that is being turned on and that conduction spreading
is what gets most of the device area conducting. Given the relative
magnitude of the load and gate currents, at least when dI/dt is an
issue, I would expect the spreading rate to be independent of the
initial gate drive.

Think it might be a diffusion capacitance charging issue [derf]?

I doubt it for reasons I doubt you could discuss intelligently.
However, the notion of charging the diffusion capacitance
is rather amusing. Do you think it exists before "charging"?

[derf]

Go away and stay away.

Your obsessive repetition is fascinating. Again, No.

 

[Fred Bloggs]

False....

Ehhh- shut the F__K up, you whining little fake- looks like you're
drowning, poser, get a clue.

I doubt it for reasons I doubt you could discuss intelligently....

You have this persistent hang-up on intelligence because you do not
possess any- tough sh_t pseudo-intellectual, you will just have to work
with the hand God dealt you, and that "ain't" much...

Your obsessive repetition is fascinating. Again, No.

Carry on, little trooper, just like your mommy and daddy taught
you...you're essentially a little weakling loudmouth and a wet rag as an
engineer poser...and guess what?- mommy and daddy were wrong- you are
truly a p.o. crap...

 

[Ken Smith]

A bit misleading in the OP's case. The di/dt issue addresses the
case of repetitive low-gate di/dt drive while expecting fast triac
current rise. This involves conduction-spreading, an issue that's
reduced with more aggressive gate drive.

In crowbar applications, the worst capacitive load case, the gate drive
really should have a rise time under 100nS. You want to hit the part with
nearly the makers limit for gate current.

[...]

junction-temp spec for the BT139. As with the dI/dt issue, adding
series inductance is sometimes helpful because the part's available
thermal mass increases by the square root of the increased duration
of the heat pulse.

Also, the inductance decreases the energy that has to be eaten in the SCR.
A small impedance prevents the current from rising to a huge number until
after the voltage on the SCR is decreased. The inductor doesn't even have
to remain an inductor at the full current. An inductor that is lossy to
high frequencies and saturates as the full current seems to work fairly
well to keep an SCR alive. I've used a 0.25 inch toroid of 3F3(I think
from memeory) material with a few turns of #14 wire. The current waveform
looked something like this:

^ Way up off screen
! on the scope
............*......
............*......
............*......
............*......
............*......
............*......
..........**.......
.......***.........
...****............
**................

I think what happens is that the lossiness of the inductor lets an initial
current flow because the loss makes it look like a parallel RL circuit.
This initial current gives the SCR a chance to have its morning coffee
before it needs to do any real work.