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PICList Thread
'Help with triac circuit'
1999\04\06@001945 by Brian Gracia

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Hello All,

I am building my first pic circuit.  I need to control a triac that
controls a water sprinkler valve.

The ratings on the valve solinoid are V= 24V ac
                               Inrush= .4 A
                               Holding= .2 A

I am going to buffer the pic to the triac using a optocoupler.  Can someone
(anyone) offer help with or without a circuit?

Brian

1999\04\06@014120 by Ron Fial

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I you want to build a 'consumer' quality circuit, i.e. pretty good reliability, you could get away with a triac output optoisolator.  Try 4N40IS-ND, which is $1.23 in singles, 14ma into it (from your output pin, to its LED, then via a 250 Ohm resistor to +5).  This optoisolator will switch 300ma continuous, and it is a 400V part to boot.        Better yet, try the NEC part (digikey PS3901LNEC-D -- $2.26 each) which needs only 5 ma and switches 1.2Amps.  You should have a snubber across the solenoid coil.  A snubber is (in this case) a 120 Ohm resistor in series with a 0.1uf to .5uF 400V capacitor, to absorb any inductive spike from the solenoid coil when the triac turns off, or somebody pulls the plug in the middle of the AC cycle.    Sure, the triac turns off at the zero crossing, but that is when voltage is minimum, not current.  This circuit should work for 2-3 years, till the solenoid gets old, stuck in the 'off' position, and the higher current resulting burns out the triac in the optoisolator.

If you want to build a industrial quality circuit, you would use the optocoupler to 'drive' a larger triac, such as a 400V 10Amp  sensitive-gate device (e.g. digikey S4010LS2-ND ).  The optoisolator's triac is connected  to the MT2 of the big triac, and the other side of the optoisolator's triac is connected to the gate of the big triac, but through a 180 Ohm resistor.  The snubber goes across the solenoid, not across the triac.  In 10 years, this circuit will still be working OK, even after the solenoid is smoking cause its stuck in the un-actuated position.

These are the cheap, simple and robust solutions.

You can also get a commercial AC solid state switch, with built in logic level drive.  These run about $18 each and up , but use two SCRs instead of a triac, and are much more reliable driving an inductive load.  For example, look at CRYDOM CC1000-ND (digikey #),  over 3 volts will operate it, up to 2.5 Amps switching, 24-140VAC rating, about $18.00   Regardless of scr (more reliable) or triac models, you should always put in the snubber circuit.

Incidentally, just to save you some grief.  Some people get the idea of running the 24V ac through a bridge rectifier, which turns it into pulsating DC, then using a single SCR to trigger the solenoid.  But this means that a solenoid meant for AC gets DC through it, and the current flow will be so high that the solenoid will burn out, or self-magentize and not release.  So if you have an AC solenoid, make sure you run it on AC only.

Regards,
 Ron Fial
---------------------------------

At 10:56 PM 4/5/99 -0500, you wrote:
{Quote hidden}

1999\04\06@072607 by Byron A Jeff

face picon face
>
> Hello All,
>
> I am building my first pic circuit.  I need to control a triac that
> controls a water sprinkler valve.
>
> The ratings on the valve solinoid are V= 24V ac
>                                 Inrush= .4 A
>                                 Holding= .2 A
>
> I am going to buffer the pic to the triac using a optocoupler.  Can someone
> (anyone) offer help with or without a circuit?

This is pretty easy I think. Run down to the nearest RatShack and get yourself
a MOC3010 optocoupler triac. It has the standard LED on one side and a low
current triac on the other. Lighting the LED fires off the triac.

The triac in the 3010 is then used to turn on a high current traic, like the
6A jobs that the RatShack have in the same rack as the 3010. The total
package is about $4.

I've used this combo sucessfully in my Christmas Light Controller. Be sure
to take a look at the example circuits on the backs of the packages...

The two things you need to be aware of: one is that the solenoid coil acts
as an inductor, so you'll need to use the inductive circuit on the back of
the 3010 package. I've never done this because all I've played with are lights
which are all resistive. The other is that the top part of both triacs must
be connected to the hot lead of the AC. It doesn't work if they are connected
to the neutral lead. This always drives me absolutely nuts when I'm trying
to get the circuit working for the first time.

Good Luck to you. Hope this helps.

BAJ

1999\04\06@111506 by Reginald Neale

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Ron said:

>Incidentally, just to save you some grief.  Some people get the idea of
>running the 24V ac through a bridge rectifier, which turns it into
>pulsating DC, then using a single SCR to trigger the solenoid.  But this
>means that a solenoid meant for AC gets DC through it, and the current
>flow will be so high that the solenoid will burn out, or self-magentize
>and not release.  So if you have an AC solenoid, make sure you run it on
>AC only.
>

This is quite true, but you can still use an SCR and a bridge. Just put the
load in series with the AC side of the bridge. You do lose about (3 x
0.7)V, which could be significant at 24V.  We have a product that works
this way. Never been a problem, but our product runs at 115V where the
voltage loss can be tolerated better.

Reg Neale

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