Question about measuring/verifying  120v rac output - Page 1
Forum section: electronics / beginners

author: DrkMith
author_rank: Newbie
timestamp: October 14, 2014, 06:19:02 am
content: Hello, I'm new here but thought I would try and seek some help with a problem I need to figure out.I have a hydronic boiler that has been giving me trouble. Last week I was called out to fix the unit. When I got there it would not light. I did the usual troubleshooting and found that the gas valve was not letting anything through. The gas valve is just 2 solenoids, and one was not opening. The following day the valve was replaced, however 2 days later it had failed again. I helped my boss over the phone and found that it seems that the coils had failed.I talked to the engineer who designed the boiler and found out that the gas valve is powered by 120v rectified Ac.He said that is the diodes failed that it could be receiving non rectified Ac and would cause the coils to burn out. I am going to replace the valve and the controller tomorrow, but wanted to test that theory before I replace the parts.So there is the back story...Here is the question...How do I find out if the controller is sending out 120v ac or 120v rac?I wish I had any test equipment available to me besides a ryobi multimeter and a no name brand basic one.With what I have available how can I test the output of the controller.I might be able to purchase a new better multimeter tomorrow if the price is not outrageous if there is one that would help...(fluke most likely)Thank you in advance for helping me out, sorry if this is a very noob question. Not to worry, I am on a new quest to learn what I can and hope to contribute here when I can.-Justin

author: RedOctobyr
author_rank: Contributor
timestamp: October 14, 2014, 05:02:31 pm
content: Quote from: DrkMith on October 14, 2014, 06:19:02 amI'm new too, and I'm not sure of the answer. But as you haven't gotten any other responses, I'll give it a shot.An oscilloscope would *show* you the wave, and it would then be clear what you're dealing with. But a scope is not available.If you mean rectified AC in the sense that both halves of the sine wave are >0V, vs half being >0V and the other being <0V (like normal AC), perhaps your multimeter could kind of give you a hint?Measure it with the meter set to AC, and with the meter set to DC. If you read, for instance, 120V AC (when set to AC), but 0V DC (when set to DC), I'd guess it's just standard AC. If you measure 0V or low-voltage AC (when set to AC), but maybe ~60-120V DC (when set to DC), I'd be more inclined to think it's rectified AC.The engineer you talked to might be able to shed some light on a way to test it, or what you're looking for.DISCLAIMER: This is based only on my simple understanding, and may well be wrong. There are people here that can doubtless give you a good answer. The first two lines of the picture at this site may help illustrate what you're (presumably) dealing with. Half-phase rectified waves would look different than full-phase rectified waves. Sorry I can't be of more help.https://www.nde-ed.org/EducationResources/CommunityCollege/MagParticle/Physics/MagnetizingCurrent.htm

author: DrkMith
author_rank: Newbie
timestamp: October 14, 2014, 05:18:10 pm
content: I'll give that a shot, thanks!I just want to find out what I can while the controller is still installed since it won't output anything when it is not installed. But I will open it up and look for any damage/failed parts before I send it back to the manufacturer.

author: DrkMith
author_rank: Newbie
timestamp: October 15, 2014, 05:18:43 am
content: Well I got the boiler up and running again, just replaced the valve and controller. Did measure the voltage of the original controller and got about 50v in ac mode and 104v in DC mode. The new controller had about the same voltages. I checked the bridge rectifier in the controller and from my limited knowledge seems OK. But better safe than sorry just replace both parts. I am thinking that the valve must have stuck closed and caused the coil to burn out trying to open it. I Re-adjusted the gas pressure to make sure it was well bellow the max pressure to be safe. Here are some pictures of the valve... It does say 120vrac o it... Interestinghttp://imgur.com/a/lOjCw