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switching 120vac on/off

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IamSmooth

Tue Jan 29 2013, 10:52PM

Registered Member #190 Joined: Fri Feb 17 2006, 12:00AM
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Posts: 1567

I have a four-way valve I'm thinking of getting that uses 120vac to move the solenoid. The only states are SideA closed/SideB open if there is no voltage; SideA open/SideB closed if there is 120vac. I was thinking of building a 555 timer to control a mechanical relay every 5 minutes (or whatever the interval) and switch back and forth. This requires that I build a voltage supply for the circuit using the wall voltage. Is there another, simpler solution? Are there any solid state chips that can open and close using AC voltage? If I have to build a voltage supply for the 555 timer I guess I could get a solenoid that uses 24vdc and then use a mosfet as my switch.

Electra

Wed Jan 30 2013, 12:10AM

Registered Member #816 Joined: Sun Jun 03 2007, 07:29PM
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You can get industrial timers to do that sort of thing; some are electronic and have their own inbuilt power supply from 110 or 240v. Older ones are motor driven and operate a micro switch from a cam, after a delay set by a knob on the front.

Kind of expensive if you have to buy them new, but surplus/used ones are common and can be found cheap.

If youâ€™re building your own then you could also use a solid state relay, which wouldnâ€™t use much power. Then if you want to avoid using a mains transformer to power the timer, search for Transformerless power supply circuits. These usually use a capacitor to drop the voltage. Though the main hazard is there is no isolation and you must treat you circuit with the same care as you would with anything else at mains voltage. As can easily happen if the mains connections are reversed.

klugesmith

Wed Jan 30 2013, 02:07AM

Registered Member #2099 Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1716

As an alternative to a mechanical or solid-state relay, you could use a triac with an optoisolated triac gate driver (MOC something, 6 pin DIP, $1 or so).

Your 120 volt AC solenoid will probably work fine on 120 volts DC, which can be switched with any old high voltage MOSFET or BJT. Be sure to have a clamp diode in parallel with coil!

The main thing that makes a solenoid or relay coil "AC" is the presence of a shading loop on part of the magnetic pole, so the armature doesn't start to spring back at every zero crossing.
A comparison of AC operating current with the coil's DC resistance will tell you whether its inductive reactance is important. Lacking an AC ammeter, you could temporarily run coil with a series resistor, and see if the AC V_coil + V_resistor = V_across_both.

Much easier than getting 120 volts DC is to use pulsating 120 volts from a bridge rectifier. No "smoothing filter" capacitor, which would charge to V_peak and overvolt your coil.

Sulaiman

Wed Jan 30 2013, 06:56PM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140

Be very cautious using DC instead of AC for solenoids or relays,
the ac types commonly have quite low resistance windings, relying on inductance to operate at low current.

Relays are ok but for repeated switching of an inductive load I would use a Triac.

There are opto-triacs for firing larger triacs,
that work either immediately (irrespective of ac phase)
or those that have internal circuitry for zero-crossing switching ... less emi/rfi.

There must be an unused wall-plug ac/dc psu somewhere around,
much easier than building one.
If not, most 'Sunday Markets ' have old ones for a quid usually.

IamSmooth

Sat Feb 16 2013, 08:05PM

Registered Member #190 Joined: Fri Feb 17 2006, 12:00AM
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Posts: 1567

After doing some more research I find I need two timers with an overlap

Timer one is two minutes long. One minute in timer two switches from off to on and cycles every two minutes. After timer one has been on for two minutes it switches off. Both timers repeat cycle every two minutes. As you can see there is a one minute phase difference between the two of them.

Is there a timer that can do this or do I have to build one? The actual time will be different, but the operation will be the same.

Patrick

Sat Feb 16 2013, 08:28PM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

well i dislike moving parts, and avoid them when ever possible, so im partial to the TRIAC idea, even radio shack as a lame triac meant for 200 volts i think. (you can reverse parallel two SCRs, though a triac is more preferable.)

remember, though i think many of us have used a AC switch for DC, that the DC switches are designed to break both steady current and voltage with beefy metal contacts, but AC ones are largly meant to break peak current and voltage occasionally, with lighter wimpy contacts.

its mostly a life span question for the switch, so long as you arnt selling this to a lay person and it isnt a fire hazard...

(most of our crap made here, is for ourselves anyway)

IamSmooth

Sat Feb 16 2013, 10:28PM

Registered Member #190 Joined: Fri Feb 17 2006, 12:00AM
Location:
Posts: 1567

Patrick
You are referring to the switch, right? I'm asking about the timing device that will control the switch as i described. Any suggestions?

Patrick

Sat Feb 16 2013, 11:23PM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

Oops! my mistake... my brain is tired.

but, i always have a prefereance for the 555 or 556 timers, or if you want it to be unnesscarilly complicated and expensive you can use a pic or stamp.

i have found 55x timers to be more than reliable at many timing tasks.

radioshack sells a 12.6V, 300mA trnsformer, so you can low voltgae power the timer from a 120 socket, then power the solinoid from 120 as well, usuing a triac/relay...

mY brain is exhauSted, time for the zleep......

IamSmooth

Tue Feb 19 2013, 09:31PM

Registered Member #190 Joined: Fri Feb 17 2006, 12:00AM
Location:
Posts: 1567

Patrick wrote ...

well i dislike moving parts, and avoid them when ever possible, so im partial to the TRIAC idea, even radio shack as a lame triac meant for 200 volts i think. (you can reverse parallel two SCRs, though a triac is more preferable.)

I will probably use a triac for switching the solenoid valve on and off. Do I need a snubber or should I get a "snubberless"?

Here is a link to some triacs at digikey

I have never used one. I just need some help with the terms:

Is the voltage off-state the maximum voltage? If I'm using 120vac, would a 400v work? What is the difference between voltage and current gate trigger? I plan on having a voltage source connected to the gate through a resistor. Do I just have to make sure I don't exceed the max on either?

BigBad

Wed Feb 20 2013, 02:14AM

Registered Member #2529 Joined: Thu Dec 10 2009, 02:43AM
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Posts: 600

Coincidentally, I've just ordered some triacs and optos.

I've also just got a relay thing working for a sous vide controller using an arduino; works great, although I had to run a fairly low cycling frequency (30 seconds) to avoid killing the relay too young.

I think for switching AC inductive loads, provided you don't switch them very often relays are usually better; although, depending on the relay, they may only give 100,000 cycles or so, that may be enough. Relays are very simple to work with, and normal failure modes will not take out your drive circuitry; they have great isolation.

Relays don't like DC much, they can have much shorter lives with DC, the electrodes tend to sputter and stuff.

TRIACs they work really well with resistive loads, but are a right pain with inductive loads, they tend not to switch off properly. You can use snubber circuits, but the snubber circuits are really geeky to design, and the models for how big the capacitors and resistors need to be are not totally reliable, there tends to be throwaway lines like: 'however the design must be tested'.

The snubber circuits form a RLC oscillator, so you can actually get higher voltages when the snubber is present than without, so you need a higher rated TRIAC than your line voltage.

FETs and thyristors are both much easier to work with than TRIACs.

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