Question re: Electrical Theory and Terminology

[rp2813]

I think the transformer that powers the irrigation timer control here may be failing, but I'm having trouble finding a replacement with the same specs.

 

Here's what I have: 

Input is 120VAC, 60Hz, 38W

Output is 24VAC @ 1250mA

 

The closest thing I've found has an output of 24VAC @ 750mA

 

I've also seen one that's 24VAC 20VA, and another that's 24VAC 40VA.  I think the 40VA one also indicated 1660mA, but can't remember for sure.  Almost all of the transformers I pulled up state they can be used for alarm systems and closed circuit cameras, but irrigation timers are not mentioned.

 

Is there a difference between mA and VA, or are they simply different terms for the same values, and is there a formula to translate one to the other?  I tried finding information on line but was unsuccessful.

 

The transformer I mentioned above with 750mA is marketed by an irrigation supply and is supposedly the suggested replacement for my system.

 

I've seen a couple that are 24VAC with mA listed at 100 or even as low as 50.  Do I need to concern myself with that mA value at all?  What would be the issue if it's significantly smaller or larger than the value listed on the current transformer?

 

To provide background on why I think the transformer is failing, I ran the system the other night and all solenoids on every station made a loud chattering sound.  Loud enough to be heard across the street, and in the case of the stations in the back yard, loud enough to wake the neighbors.   The sprinklers worked, but on a couple of stations they would briefly cut out and then re-start.  Some of the solenoids were replaced within the last year or two, and they're just as loud as the older ones.  I know they can't all have decided to fail at exactly the same time.

 

On line research about this problem suggested the "common" wire may be faulty, but that doesn't seem to be the case.  The system is an old analog "RainDial" brand, and although I've read that there's little to go wrong with a transformer, I think that in this case it may very well be the culprit.

 

Any advice and/or explanation of the electrical terms above would be appreciated.

 

Thanks,

 

Ralph

 

 

 

[perc-o-prince]

Hey Ralph,

I can't really answer your question, but I wonder if this may help. It's used, but....

Chuck

http://www.ebay.com/itm/7R11-POWER-...ultDomain_0&hash=item20d180451c#ht_400wt_1798

 

[perc-o-prince]

And, maybe this one is a direct replacement? Is 1250mA the same as 1.25A?

Sorry if you've already checked these...

Chuck

http://www.ebay.com/itm/120V-24V-Tr...03132420&meid=2699caeab3fd40ff90efe56d9fa7c1f

 

[rp2813]

Thanks Chuck.

 

I checked the ebay listings last night and that's where I found the items I mentioned above.

 

What I failed to state in my OP is that this transformer plugs into a standard household AC outlet, and is a large blocky thing similar to but larger than those for small cordless appliances and gadgets.

 

The correct type may have two terminal screws where a pair of thin wires can be attached (see picture), or a cord that's hard-wired as an integral component of the transformer.

 

Something like this:

 

 

[cornutt]

'VA' stands for "volt-amps", which is a way of measuring the power output of a transformer. If you divide the VA number by the transformer's voltage and then mutiply by 1000, you will get an estimate of the mA output (probably a little high, depending on what kind of application the transformer wil be used in). And yes, if the transformer's output isn't high enough, it probably won't work -- the output voltage will drop and it may not be enough to make the device it's powering work, or the transformer itself may overheat.

BTW, the ones that plug directly into a receptacle are commonly called "wall warts" by musicians.

 

[rp2813]

Thanks David.

 

So if I want to duplicate 24V 1250mA, I should be looking at a wall wart that's 24V and ideally 30VA, correct?

 

So far I've only seen 20VA and 40VA.  Would 40VA cause any harm?

 

[kb0nes]

Higher amperage capability will cause no harm, other then being possibly costlier and larger physically. The current is defined by the supplied voltage and the load. Having a larger transformer only means it will be less stressed so don't sweat trying to find an "exact" match.

As David mentioned the VA rating is basically the output voltage times the rated output current which gives an idea of the total output of the transformer. VA is basically an AC equivalent of watts which of course also is equal to amps times volts. VA doesn't consider power factor of the load which somewhat simplifies calculations when dealing with a non-resistive load.

 

[cornutt]

Yeah, a 40VA transformer will be fine.

 

[rp2813]

Thanks Phil.

 

Now I know I have some leeway in deciding on a replacement.

 

[rp2813]

Update

I took a $10 gamble and ordered a replacement transformer.

 

I hooked it up yesterday and it didn't resolve the problem with the chattering solenoids.

 

This may require a new thread on the subject of automated irrigation systems.

 

[rp2813]

Well, whaddaya know --

I ran the sprinkler system night before last with the original transformer and all was quiet.

 

Maybe it just needed to wake up from a deep sleep over the winter, or maybe it had something to do with my shutting off of the water supply to fix a spigot in the back yard.

 

The pipes are so old here, just shutting off the water and the resulting fits and bursts from faucets and toilets after the water has been restored can easily generate problem-causing crud -- or break it loose as the case may be.

 

I'll keep my fingers crossed that the solenoids continue to behave, even though irrigating this year will be very limited since we've just had our fourth consecutive dry winter and water reserves are well below normal.

 

[sudsmaster]

Is it possible that in the process of switching transformers back and forth a partial short in the leads was eliminated?

 

You could also put a VOM on the system to see if full voltage was making it all the way to the valves.

 

Or you could just sit back and rest easy now that the system is working again, at least until next spring.

 

 

 

 

[rp2813]

No relaxation after all.

 

The digital readout started a routine of flashing and quivering when I tried to run a semi-automatic cycle night before last.  The solenoids made an even louder chattering sound, and the valves wouldn't open

 

I googled "Rain Dial RD-900 Display Flashing" and got a few hits.  The control is toast, based on the flashing and the chattering, which are both common indicators of failure.

 

The good news is that this type of control seems to be popular and easily repaired.  I thought for sure it was a throw-away item and checked prices at OSH yesterday.  Most were well over $100 for a model that served at least eight stations, which is what I need.

 

For $55 and my old rebuildable control, I can get an identical refurbished control.   Turnaround time is just a few days.   I sent the control off this morning.

 

Meanwhile, I had a spare control that had its own issues, which is why I replaced it, but it's workable, though only on fully automatic mode, and there's no chattering.  The controls are easily changed out -- it only takes ten seconds or less -- so for the interim the slightly demented control will do the job.

 

Also, per the refurbisher's site, the 24 volt transformers rarely fail.  It cost me $10 to find that out.  If only the display had started flashing first . . .

 

[sudsmaster]

It's always  something.

 

I hope you get it all fixed in time to cut back the water usage 20% as compared to 2013...