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24VAC power supply question about VA spec

Discussion in 'Power Electronics' started by Kewpy, Oct 26, 2013.

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  1. Kewpy

    Kewpy

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    Oct 26, 2013
    I'm trying to use a Digital twin timer (Sestos Model: B2E-2R-24) and am not sure if the power supply I have will be ok for it. The specs for the timer say: AC/DC 12-24V ±10% <4VA (Actually, the < sign has an angled line under it as well, but couldn't type that)

    My question is: My power supply is a 24VAC, 30VA and I'm wondering about the "VA" spec. Does the <4VA on the Timer mean that it'll "use" less than 4VA, or will the power supply's 30VA output overload it and burn it out?

    Thanks,

    Kewpy
     
  2. davenn

    davenn Moderator

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    Sep 5, 2009
    hi Kewpy
    welcome to the forums :)
    that symbol means less than/or equal to 4VA ( VA = Watts)

    it should work fine on your power supply, as it will only draw the current needed to operate.

    cheers
    Dave
     
  3. Kewpy

    Kewpy

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    Oct 26, 2013
    Hi Dave
    Thanks for the answer. As someone with only very basic knowledge, I kind of thought that, but lo and behold, I ended up somehow smoking the timer (that I waited for several weeks to arrive thru EBay)

    Again, as a total novice, my ability with a multi-meter is limited to testing continuity and voltage. So, I applied power to the timer, it turned on, and I proceeded to test which of the 10 terminals were the relays and whether any of them passed the 24v thru (to power the device I want to turn on and off via the timed output)

    I do have the schematic for the timer, but didn’t understand the layout with confidence, so I figured I could explore it with the multimeter and find out what was what.

    However, the second time I powered it on; it started smoking and died, hence my question about whether my power supply killed it.

    Is it possible I hurt something by testing it with the multi-meter?
     
  4. BobK

    BobK

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    Jan 5, 2010
    The fact that they specified a power supply to less than a specified wattage is totally bizarre. Any normal circuit will draw as much power as it needs, and therefore you would NEVER specify that a power supply must supply less than specific wattage, only that it would supply greater or equal. If indeed, that is what killed it, their circuit design sucks better than a Dyson vacuum.

    Bob
     
  5. Kewpy

    Kewpy

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    Oct 26, 2013
    BobK: If I understood DaveN's response, the spec given (<4VA) is just a "heads up" as to what the timer's max. wattage "draw" would be. (if I understood correctly)

    Just to make sure I'm quoting correctly the specs and requirements of the timer, here is where the instructions for the Sestos B2E-2R-24 are loctated: http://www.sestos-hk.com/english/download.asp (the instruction file is the 2nd one on the list: B2E-EN.PDF)
     
  6. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

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    Nov 28, 2011
    It should be obvious that the "<4VA" marking means that the unit DRAWS 4 VA or less, not that the power supply must be RATED for 4 VA or less; as you say, Bob, that interpretation makes no sense, so you use the interpretation that does make sense.

    To try to answer the original question: the fact that your power supply is rated for 24VAC and 30VA implies to me that it's just a transformer powered from the AC mains supply. If so, that specification means that the output voltage is 24VAC when the transformer is loaded at 30VA. Transformers are not regulated, so their output voltage will be higher than specified if the AC mains voltage is higher than nominal, AND if the transformer is not loaded by the specified amount.

    So if it's a transformer, it's quite possible that its output voltage could have been as much as 30VAC, perhaps even more, which COULD have damaged the unit if it wasn't designed with reasonable safety margins.
     
  7. Kewpy

    Kewpy

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    Oct 26, 2013
    Yes, it's a common AC plug in transformer, so that explanation makes the most sense. I do have some 12V transformers that I could use instead, so that may be safer to use, given your explanation of the voltage.

    Also, as I mentioned, am I safe to check the various screw terminals on the back of the timer (with the multimeter) to see what opens and closes and which (if any) pass voltage thru? I understand on the timer's diagram the "NO" and "NC" indications, but I want to see for myself that they open and close with each timed event. However, there's also some nomenclature that I don't understand - perhaps some of it's functions would be more helpful for my project, than just a contact opening and closing.

    In a nutshell, I'm opening and closing a 24V soleniod valve on a timed schedule, (to purge a water well filter) but ONLY when the thermostat is calling for heat (on my GeoThermal heat pump system)

    Should I start a new thread if I need help understanding the timer? (I did provide the link to it's instruction sheet a few posts up) If so, is this the proper section?

    Thanks,
    Kewpy
     
  8. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

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    Nov 28, 2011
    Yes, definitely use a 12VAC transformer.

    Yes, you can test the contacts with a multimeter.

    I think you should spend a reasonable amount of time playing around with the timer to get a feeling for how it works. I think I get the general idea from the data sheet, although I think there's one mistake in the timing diagram. When you get to that point, let me know if you see a mistake there, and we can compare notes.

    I assume you can set the timings so that things happen quickly during testing? It seems to be fairly programmable. Once you get a feel for it, you should be able to tell whether it will do what you want or not.
     
  9. Kewpy

    Kewpy

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    Oct 26, 2013
    The plan is pretty simple: Activate (power on) the 24VAC soleniod purge valve for approx. 5 seconds, then remove power for "X" amount of time.

    Meaning, the "off" time will initially be say every 5 minutes, but as the well runs cleaner and cleaner, gradually increase the off time to maybe as much as a 12 hour interval (or maybe even longer). (hence the Sestos "Dual" Timer)

    The only other part is that I want the timer itself to either be unpowered and then only powered up when the T-stat calls for heat (via a 24VAC output from the T-stat/Zone controller) and run thru it's timing cycle(s), or have the Timer "on" (and Timing) 24/7 and have the timer close a contact, between the 24VAC from the T-stat/Zone controller and the soleniod purge valve. (as programmed)

    I actually have the purge valve working (temporarily) now without the timer: Every time the T-stat calls for heat, the zone controller activates a zone valve on the Heat Pump, to which I've also wired in parralel my 24VAC soleniod purge valve.

    The only downside to this arrangement is that the purge valve is activated all the time the Heat pump's zone valve is powered, thereby wasting some of water thru the purge valve. However, the longer the system is being used (now that heating season is here), the cleaner the well gets, thereby reducing the need to purge the filter.

    Hopefully I didn't confuse you with all that!

    Thanks,
    Kewpy
     
  10. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

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    Nov 28, 2011
    I kind of get it. As I said, play around with the timer (when the replacement arrives) and see what it can do, and try to fit it into your scheme. I don't think it can automatically adjust its timing duration though.

    If you want more help, you need to explain it from the ground up. I have no idea even what the system is used for! Draw up a diagram of the system, explain its purpose, and describe the functionality you want. Include a timing diagram like the one in the Sestos manual, showing the various signals (signal from thermostat, signal to purge valve, and any others).
     
  11. Kewpy

    Kewpy

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    Oct 26, 2013
    Will do, and thank you for your expert advice so far. Oh, BTW, I measured the voltage on the transformer I was using and it was 27.6VAC, so that would put it a bit over the "+10%" spec, and as you said, that could've been the culprit. As for the timing, I'm not looking for any "automatic" adjustment of the two time durations. It'll be strictly a manual change, as field conditions warrant.
    Thanks again,
    Kewpy
     
  12. Kewpy

    Kewpy

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    Oct 26, 2013
    Which Transformer: AC or DC?

    The specs for the timer indicate AC/DC 12-24V. Now that I know I should use a 12V transformer, should I use 12VAC or 12VDC? (I have both)

    Is one preferable? If not, is there any difference in power consumption, inasmuch as it'll be on 24/7?

    Thanks,
    Kewpy
     
  13. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

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    Nov 28, 2011
    Neither is preferable, as far as the timer and power consumption are concerned. I would use AC because it leaves the DC power supply available for other things, and other things most often want DC.
     
  14. Kewpy

    Kewpy

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    Oct 26, 2013
    Thank you!

    The replacement timer should be here today. And just to be sure and set my mind at ease: I can safely use the multimeter to check continuity across the various terminals even when the timer is powered up, correct? The small amount of power the multimeter uses to do that won't hurt any of the componets in the timer? (obviously you can tell I'm quite the novice when it comes to this sort of thing, and am a bit "gun shy" seeing how I smoked the first one!)
     
  15. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

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    Nov 28, 2011
    That's right. The outputs of the timer are (presumably) coming from the contacts of electromechanical relays - they're able to switch several amps of current. So you will not damage them by just testing them with a multimeter on resistance or continuity range.
     
  16. Kewpy

    Kewpy

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    Oct 26, 2013
    While I am waiting for the mail, I made up the attached schematic to help you understand what I'm trying to achieve. Hopefully I made it clear enough.
     

    Attached Files:

  17. Kewpy

    Kewpy

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    Oct 26, 2013
    Not going to work.

    Well, have the new timer in, it works ok, but unfortunately, it looks like this timer isn't going to do what I need.

    I originally read it's "features" and saw it was a "Twin" timer that could have it's set values be different for the A output period and the B output period.

    However, what wasn't said was that the time "units" (hours, mins. sec) are NOT independently selectable! Meaning if you set it for "Seconds", it's Seconds for BOTH events! (Same if you set it for Min. or Hours). You can't set it for Seconds for the "A" period, and Hours for the "B" period.

    I need it very short (5 seconds or less) for period "A" and the "B" period value needs to be (initially) minutes and eventually leading up to be several hours.

    Meaning, at first, I want it to be "on" for <5 seconds, (which will never change) and then "off" for say 10 minutes. Down the road, I want to be able to increase the "off" time to be say 30 minutes, then, later on, maybe 1 hour, then maybe 6 hours, all the way to eventually being off for 12 hours. (and repeat the cycle)

    And of equal importance, it needs to be only functional when the rest of the system is running. (When the Thermostat calls for heat-See drawing in earlier post)

    There were some "kits" that I had earlier made inquires about, but didn't follow up when I found this SESTOS timer; thinking it's an industrial item and probably more reliable for long term use.

    Any suggestions would be greatly appreciated.
    Thanks
    Kewpy
     
  18. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

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    Nov 28, 2011
    Well, how dumb is that. Pretty dumb!
    OK, I'm happy to try to help. But I need to understand the whole system. I simply will not design something for a system I don't fully understand; it's a characteristic of my personality I suppose. I always need to understand the big picture.

    Thanks for the schematic diagram. That's helpful. But I also need a diagram of the whole overall system, with all its input and output sources - I assume there's some kind of water feed, perhaps from a rainwater collection tank or something? And the water is being used somewhere, I guess? And there's a heater and a thermostat, and a purge valve that has something to do with cleaning the filter - perhaps allowing backflow through it?. So that's all I know so far; I need to understand how it all fits together. Can you draw a detailed diagram that explains the system as clearly as possible? And write up a description that goes with it.
     
  19. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

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    Nov 28, 2011
    I just re-read the thread. I think I get it. You have a controller that controls a heater, and when the heater is activated, you want to activate the purge valve for five seconds initially, then every X amount of time after that; when the heater is not powered, nothing happens on the purge valve.

    You can probably use a "time delay relay" to provide your adjustable pulse to the purge relay. Set the dual timer for minutes, and make it generate a one minute pulse; feed this through a relay that activates for a few seconds.

    These relays are available on eBay, no doubt, and from electrical supply companies. I don't know whether you can get AC ones, but have a look.

    In other words, you can use the dual timer for the OFF time, and the relay to provide the short adjustable pulse to the purge valve.
     
  20. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

    8,393
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    Nov 28, 2011
    This is a 12V DC adjustable time delay relay:

    http://www.ebay.com/itm/DC-12V-Dela...868?pt=LH_DefaultDomain_0&hash=item3cd485c754

    If you use 12V DC instead of AC with your dual timer, you can use this module.

    I think the way to connect it all up would be to use a relay with a 24V AC coil which is connected across the heater, with the contacts of that relay switching the 12V DC supply on to the timer, and the timer contact switching +12V onto that board. The contacts on that board would then connect between the purge valve and its power supply, which seems to be 24VAC.
     
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