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'[PIC] triac driving a monitor'
2004\03\28@122611 by Anand Dhuru

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Hi folks,

I'm facing a strange problem. I have designed a PIC based IR receiver that
responds to a key on a commercial IR remote. Basically, it toggles a pin for
every key press on the remote.

This pin drives the gate of a triac thru' a 220 ohm resistor.

Another pin has a switch that does the same thing for local (manual)
control.

And, I've used a transformerless design for the power supply.

Now, if I connet an incandescent bulb to the output, the circuit works
flawlessly. But, when I connect an old monitor I'm using as a TV to the
output, the unit switches on reliably, both thru' IR or the switch; however,
once on, it does not switch off every time (does so perhaps once every 8 to
10 keypresses).

It is as if the monitor, when on, might be locking up the PIC, as it doesnt
seem to respond either to the IR or the local switch. Again, if I keep the
monitor off (or switch it off), the circuit works very well on-to-off and
off-to-on.

I realize driving this kind of a load with a Triac is a bad idea. Is there
any way around this without using a relay? I would like to retain the
transformerless design, if I can.

Thanks for any advice, in advance.

Regards,

Anand Dhuru




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2004\03\28@123648 by Wouter van Ooijen

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> Now, if I connet an incandescent bulb to the output, the circuit works
> flawlessly. But, when I connect an old monitor I'm using as a
> TV to the
> output, the unit switches on reliably, both thru' IR or the
> switch; however,
> once on, it does not switch off every time (does so perhaps
> once every 8 to
> 10 keypresses).

Try powering the IR and PIC from a battery + regulator, sharing only
ground with the rest of the circuit. If this helps, add more filtering /
decoupling to your normal power circuit. If it does not help you might
suspect some inductive coupling trouble. Check: could it be that the
gate of the triac becomes a volatge source it the load you use?

Wouter van Ooijen

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2004\03\28@125105 by Anand Dhuru

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Hi Wouter,

> decoupling to your normal power circuit. If it does not help you might
> suspect some inductive coupling trouble. Check: could it be that the
> gate of the triac becomes a volatge source it the load you use?

This is what I was suspecting, since I have no problems with a resistive ( a
100 watt bulb) load of similiar power requirement.

Would the absence of a snubber network be the cause?

Regards,

Anand



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2004\03\28@125728 by Spehro Pefhany

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At 07:36 PM 3/28/2004 +0200, you wrote:
> > Now, if I connet an incandescent bulb to the output, the circuit works
> > flawlessly. But, when I connect an old monitor I'm using as a
> > TV to the
> > output, the unit switches on reliably, both thru' IR or the
> > switch; however,
> > once on, it does not switch off every time (does so perhaps
> > once every 8 to
> > 10 keypresses).
>
>Try powering the IR and PIC from a battery + regulator, sharing only
>ground with the rest of the circuit. If this helps, add more filtering /
>decoupling to your normal power circuit. If it does not help you might
>suspect some inductive coupling trouble. Check: could it be that the
>gate of the triac becomes a volatge source it the load you use?

Yes, the gate of a triac does become a voltage source when it is carrying
a load.

>Best regards,

Spehro Pefhany --"it's the network..."            "The Journey is the reward"
spam_OUTspeffTakeThisOuTspaminterlog.com             Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog  Info for designers:  http://www.speff.com

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2004\03\28@140753 by Hopkins

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You may be getting interference from the line drive section of the monitor -
try shielding the monitor in a metal grounded shield along with good
decoupling of the pic supply line.

or try moving the pic outside the monitor to a distance of a meter or so on
extension leads to prove if the monitor is interfering with the pic.

I have seen the IR section of TV system shielded in some cases.
*************************************************
Roy Hopkins   :-)

Tauranga
New Zealand
*************************************************


{Original Message removed}

2004\03\28@175522 by M. Adam Davis

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The triac only turns off when there is no /current/ flowing through the
junction in either direction.  The TV may well be providing enough
inductive kickback each AC cycle to prevent the proper turn off of the
triac.

The solution to this is a snubber.  A diagram using a snubber can be
found here:
www.ubasics.com/adam/electronics/doc/triaccir.gif
Make sure the capacitor is rated for direct line voltage connection, and
use a 2 watt resistor even if you aren't dissipating that much energy.

A snubberless triac circuit only works reliably with an entirely
resistive load, such as an incandescent lamp.  There aren't many loads
today that can be characterized as entirely resistive...

-Adam

Anand Dhuru wrote:

{Quote hidden}

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2004\03\29@154332 by Peter L. Peres

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Anand, your monitor probably has strong parasitic radiation due to decayed
filter capacitors and jams the remote receiver. Try to scope the IR rx
signal. An easy way is to put a 2mA LED in series with its pullup resistor
on the output. But this will not show glitches.

Peter

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2004\03\29@154336 by Peter L. Peres

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> The triac only turns off when there is no /current/ flowing through the
> junction in either direction.  The TV may well be providing enough
> inductive kickback each AC cycle to prevent the proper turn off of the
> triac.

A TV, monitor, or computer SMPSU will have capacitive lead in the current
so there is no inductive kickback from it. However the triac/thyristor
must be steered with DC (on all the time it is to be on) because the real
conduction angle of the rectifier bridge in the smpsu can be as low as 0.5
degrees at low load. This means using a lot of power for the gate (a lot
when compared with what the PIC needs). Also as Spehro said the triac gate
becomes a voltage source when the triac is on. But I think that trouble is
jamming from the monitor to the IR receiver.

Peter

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2004\03\29@224234 by Anand Dhuru

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Peter, I suspected that as well (from my earlier experience with Girder and
UIR), but in this case when the remote misbehaves, so does the local/manual
momentary switch. I therefore think the issue is to do with the fact that I
do not have a snubber circuit across the triac.

Even so, I still cant figuire out why the PIC should not respond to the
inputs. That too, its not as if the PIC is hopelessly locked and *has* to be
reset to recover; it would respond to an input (IR or the switch) if you try
a dozen times!

I did try the snubber from the URL given by Adam. The values which seemed to
work were 100 ohms and 0.47 uF. But now, the monitor LED flickers at times
even when the triac is off, the current being passed on by the snubber
itself.

Perhaps another combination of R-C might wok better?

Regards,

Anand


{Original Message removed}

2004\03\29@230728 by Rich

picon face
Please excuse my butting in.  But I wonder if you know whether your triac is
firing in all four quadrants.

{Original Message removed}

2004\03\30@153133 by Peter L. Peres

picon face
>Even so, I still cant figuire out why the PIC should not respond to the
>inputs. That too, its not as if the PIC is hopelessly locked and *has* to
>be reset to recover; it would respond to an input (IR or the switch) if
>you try a dozen times!

The pic's code might play an important role (how robust it is wrt
debouncing heavyly jammed inputs). F.ex. I'd start by adding 5nF caps
across each pic input and ground directly at the chip. Troubleshooting
problems like your can be hard. I would try to substitute the old load
(lamp) while leaving the circuit where it is, and the monitor plugged in
next to it, in the *present* wiring configuration. See if it still makes
trouble and if switching the monitor off fixes it.

>I did try the snubber from the URL given by Adam. The values which seemed
>to work were 100 ohms and 0.47 uF. But now, the monitor LED flickers at
>times even when the triac is off, the current being passed on by the
>snubber itself.

Snubbers are not snake oil. It is not very likely that the snubber turns
on anything in the monitor. I use a significantly smaller snubber, maximum
0.047uf + 220ohms. It is much more likely that something else turns on the
triac. Try to remove the pic from its socket and put a jumper in to ground
the triac gate in the socket, see if that fixes the current flickering.

Peter

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