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Thread: Less than 24v effect?

  1. #1
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    Less than 24v effect?

    Good evening guys, my question is, how sensitive are the Johnson controllers at a >24v output from a xformer? My last call today, was a server room no cool, it was a york unit, first thing I noticed was the xformer was wired for 230, but was really 209. Could the >24v have any effect on the control voltage side? I rewired it for 208, and my controls guy was like everything seems to be doing what it should

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  2. #2
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    If you meant less than 24 volts...the yes I have seen control systems go haywire with low voltage.

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  4. #3
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    Quote Originally Posted by BALloyd View Post
    If you meant less than 24 volts...the yes I have seen control systems go haywire with low voltage.

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    Yes that is what I meant, but the control voltage was at 22.6. But thank you for your knowledge.

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  5. #4
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    Quote Originally Posted by Matt Woodbridge View Post
    Yes that is what I meant, but the control voltage was at 22.6. But thank you for your knowledge.

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    18 volts or less and you are screwed. 22.5 shouldn’t be an issue, but 24-27 is better

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  7. #5
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    Quote Originally Posted by JCIman View Post
    18 volts or less and you are screwed. 22.5 shouldn’t be an issue, but 24-27 is better
    Thank you my man, im going back in the a.m. to take another look at it

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  8. #6
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    Just check out the cut sheet for the device in question. Most give a valid range like 18-30vAC.

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  10. #7
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    Good day Matt,

    Just doing an off the cuff calculation, the primary miswiring would have reduced the output voltage by about 10% ... which is very close to what you measured (22.6VAC). Also on a side note... a 24VAC output transformer is the output voltage at the maximum rate current of the transformer. So... if you use a larger VA rated transformer than what you actually need then output voltage will be larger than 24VAC. As a consequence if you are measuring 24VAC on a 24VAC rated transformer then you are at the transformer's maximum power output... and so the resulting output voltage can droop if your load increases for some reason. I usually spec at least 50% what I need to be on the safe side and if the transformer size is too big, etc then maybe I will allow only a 25% increase.

    Cheers,

    Sam

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  12. #8
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    Had a Honeywell spyder today that we couldn’t download. Long wire run and 5-6 vavs on the circuit. Two vavs within 10’ would accept the download this one wouldn’t. Swapped controllers etc. Ended up being voltage drop had 24.5 at xfmer and 22.4 at vav. Temporary xfmer and then it would download.


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  14. #9
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    Quote Originally Posted by bigguy158 View Post
    Had a Honeywell spyder today that we couldn’t download. Long wire run and 5-6 vavs on the circuit. Two vavs within 10’ would accept the download this one wouldn’t. Swapped controllers etc. Ended up being voltage drop had 24.5 at xfmer and 22.4 at vav. Temporary xfmer and then it would download.


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    I'm glad you made it work bro, thats awesome

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  15. #10
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    I see it all the time on RTUs, I swap the high voltage wire from the 230 to the 208v terminal wiring(some older transformers showed 200v terminals and it works the same) to get 24-26v ...I see plenty of problems of anything below 22v when all the contactors and relays activate . In some cases it gets below 20v and contactors / relays chatter then fail..

    Don’t feel bad, some units have been misdiagnosed For over 10 years because other parts swappers don’t notice the obvious of checking for the correct voltage output first..

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  17. #11
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    In good old Europe electronic devices usually work correctly, if the supply voltage is between 90% and 110% of the nominal value.

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  19. #12
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    I've seen low voltage problems too. I sometimes wish transformers put out 25 volts. Kind of like a car system is called 12 volts but the engine really puts out 14.

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  21. #13
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    Quote Originally Posted by incontrol View Post
    I've seen low voltage problems too. I sometimes wish transformers put out 25 volts. Kind of like a car system is called 12 volts but the engine really puts out 14.
    Yes I usually like to see about 26, but it is what it is

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  22. #14
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    It depends on your incoming high voltage...I have seen 490v put out 27v where 460 has 24.5v ....215v put out 26v where 208v has 24v . You get the idea...but the voltage also drops under use and how many components are energized at the time...
    Some buildings can have 245v incoming with nothing running but drop to 232v with everything running...

    You could have a weak transformer or bad coils on the relays/contactors/ corroded connections giving a higher resistance pulling the voltage lower..I’m fine seeing 23v , I recheck everything at 22v or lower with everything energized ...

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  24. #15
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    Quote Originally Posted by Unlimited1 View Post
    It depends on your incoming high voltage...I have seen 490v put out 27v where 460 has 24.5v ....215v put out 26v where 208v has 24v . You get the idea...but the voltage also drops under use and how many components are energized at the time...
    Some buildings can have 245v incoming with nothing running but drop to 232v with everything running...

    You could have a weak transformer or bad coils on the relays/contactors/ corroded connections giving a higher resistance pulling the voltage lower..I’m fine seeing 23v , I recheck everything at 22v or lower with everything energized ...
    Yes for sure, 22 is usually good? My old boss would always say look for 25+

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  25. #16
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    Quote Originally Posted by Matt Woodbridge View Post
    Yes for sure, 22 is usually good? My old boss would always say look for 25+

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    My meter is high end and very accurate . A lot of cheaper meters don’t give the same results..as mentioned I look for problems at 22v and below. Incoming high voltage can have corroded connections.. I always use a good known Multi tap test transformer That I got from an older unit that was replaced and recheck...However if you have 22v and everything is working good, it is what it is...

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