Indicator circuit for 240vAC heating element

Discussion in 'The Projects Forum' started by JohnnyD, May 5, 2009.

  1. JohnnyD

    Thread Starter Well-Known Member

    Aug 29, 2006
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    0
    Hi,

    I work at a plant nursery where we have some very young plants germinating on heated beds. They are really simple - just 240v in, thermostat, 240v out plus a temperature probe. The trouble is, they are so simple they don't even have a light to tell when they are on or off. We recently had one running off of a dodgy extension lead and we found out one morning that it had tripped the circuit breaker in the fusebox and they had all been off all night.

    What I need is to add a light to the control box which tells when it's on or off but I need to be able to tell if the heating element is making a circuit or if there are any breaks in the wire. Hence, I can't just put a 240v neon indicator in parallel with the thermostat controlled output because I still won't know if there's any current going through the element.

    I have looked around on the internet and found a very simple circuit using only 3 diodes, 1 resistor and an LED but I am not sure if this is too simple or if it really will work with only these components.

    Basically, there are 2 diodes in series with the heating element, then another diode, the LED and a resistor in parallel with these 2 diodes but pointing the other way. I hope that makes sense, I don't have time to draw and upload a diagram right now but I will try later.

    What I eventually want is to have a board with 4 or 5 of these current detecting circuits and then have these connected to 4 or 5 regular 240v mains sockets so whatever I plug into them (ie the heated beds) I can tell whether it's working or not.

    Thanks a lot, this will really save us some worrying each night as our whole business depends on these plants growing properly.

    Cheers,

    John.
     
  2. KMoffett

    AAC Fanatic!

    Dec 19, 2007
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    The attached is a version of what you were talking about. The big issue is that the diodes must be rated to handle the heater current.

    Ken
     
  3. mik3

    Senior Member

    Feb 4, 2008
    4,846
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    Use a panel mounted ammeter to measure (and thus indicate) the current the heater draws and a panel mounted voltmeter or neon lamp to check the voltage.
     
  4. CDRIVE

    Senior Member

    Jul 1, 2008
    2,223
    99
    Ha, you must be an old timer. You took the words right out of my mouth. An excellent axample of KISS! :D
     
  5. KMoffett

    AAC Fanatic!

    Dec 19, 2007
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    OK, this is not KISS. ;) I'm not sure how "mission critical" the heating is, but I couldn't help but play with an alarm idea. Basically, if the thermostat applies power to the heater, but there is no heater current, the alarm sounds.

    Darn, I forgot to stick a neon across the relay.

    Ken
     
  6. KMoffett

    AAC Fanatic!

    Dec 19, 2007
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    You are absolutely right...it can't work. I fouled up my truth table and was thinking the diode ANDing function would work. What I really wanted was an XOR. And it does work.
    Thermostat OFF/ no Heater current: no alarm
    Thermostat ON/ Heater current: no alarm
    Thermostat ON/ no Heater current: alarm
    Thermostat OFF/ Heater current: never happens

    I reconfigured my schematic per your instructions, but I don't see how that will work when the thermostat is on, but there is no heater current. It looks like the relay contacts short the b-e junction, silencing the alarm whenever the thermostat is ON, powering the relay. ??? Or, am I missing something.

    Ken
     
  7. CDRIVE

    Senior Member

    Jul 1, 2008
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    Well so much for having the ability to delete a bogus post. I deleted that post because I misinterpreted the logic of your circuit. The things that I found wrong still stand but my fixes were wrong. Anyway, I put this together and the logic is as follows:
    Thermo closed and no fault: LED OFF.
    Thermo open and no fault: LED OFF.
    Thermo closed with fault: LED ON.
    Thermo open with fault: LED OFF.

    I used a LED but you can replace it with a Radio Shack peizo.
     
  8. studiot

    AAC Fanatic!

    Nov 9, 2007
    5,005
    513
    Hey guys the poster has 240 volt mains and therefore different and more stringent regulations about connection compared to your 120v.

    Johnny,

    Don't mess about with mains if your business depends upon it. Breach of the electricity at work regulations could shut you down as well.

    Either get a competent electrician (this is not an electronics problem) to wire in mains failure alarms

    or buy plug in ones similar to these

    http://www.ultrasecuredirect.com/acatalog/mains_power_failure_alarm.html

    There are plenty to choose from cheaply.
     
  9. CDRIVE

    Senior Member

    Jul 1, 2008
    2,223
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    I will not argue this point but this is an educational forum. As such information posted here should be treated as same. When I reply to a post I have absolutely no idea what the geographic is. For all I know, he lives on a farm, an island, or some other remote place with next to no regulations. Maybe it's a town so small that everyone knows each other, including the fire and building inspectors. I know a place only 5 hours away from me that the Mayor, Fire Chief and Ferry Operator are all the same guy! You would not believe what the ferry actually is. OSHA would have a heart attack!:D
     
  10. CDRIVE

    Senior Member

    Jul 1, 2008
    2,223
    99
    Ken, your logic looks OK but I would be concerned that using the 4N33 as an Emitter Follower might be putting you close to the limit of being below the level considered logic H for the CD4070. The CD4070 data sheet lists logic H as 3.5V min @ 5V Vdd. The Darlington is going to output only 3.6V!
     
  11. studiot

    AAC Fanatic!

    Nov 9, 2007
    5,005
    513
    The cheapest legal option would be to use one of these mains power monitors, per heater. Just plug it in and plug your heater into it. I carry a slightly more sophisticated one round for servicing. They are great.

    It doesn't provide an alarm but does constantly indicate power drawn.

    http://www.maplin.co.uk/Module.aspx?ModuleNo=223573

    CDrive,
    I'm sure it's the policy of this forum to offer legal solutions. Illegal wiring has been discounted before in older discussions.
    Jonny posted at 2100 GMT, which suggests Europe, Australia or South Africa as places with 240 volts. However I can't see soil heater being much used in the last two locations so that that brings us to Europe. 2100 GMT is a bit late for Eastern Europe/Russia so I guess Western Europe. Much of Europe has 200 to 230 volts, so I'm going for Britain or Ireland.
    Concealing one's location makes it much harder to offer help for hardware supply and regulation conformity.
     
  12. KMoffett

    AAC Fanatic!

    Dec 19, 2007
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    CDRIVE,
    Looks good!

    studiot,
    You're right.

    JohnnyD,
    For a commercial operation, that pays for workman's comp and liability insurance, amateur solutions, even well conceived ones, are a risky choice.

    Ken
     
  13. JohnnyD

    Thread Starter Well-Known Member

    Aug 29, 2006
    79
    0
    Wow, lots of replies!

    Thanks for all the suggestions. I'll look into each one later when I have some more spare time.

    The whole liability insurance / doing things properly isn't an issue. I'm sure any health and safety officers would have a field day with some of the stuff that goes on round here! I just need a cheap (but safe obviously) way of having some kind of indication that the heated beds are definately working. They are normally set to around body temperature so you can't really tell if they're on or not by feeling them. The circuit that CDDrive attached looks good and would also fit my long-term plan of having sensors for all kinds of things all over the farm hooked up to a monitoring system on a PC :) I think I'll buy one of those mains ammeter things just to test them all once and see how much current they draw which will help me with this circuit and getting the right diodes.
     
  14. mik3

    Senior Member

    Feb 4, 2008
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    No, I am not very old. I am 22 and I have been studying electronics for 7 years now. :p
     
  15. CDRIVE

    Senior Member

    Jul 1, 2008
    2,223
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    Then you're simply wise before your time. ;)
     
  16. mik3

    Senior Member

    Feb 4, 2008
    4,846
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    I just like electronics and that's all. :cool:
     
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