Time delay relay to start a refrigerator

Discussion in 'The Projects Forum' started by circa27, Jul 12, 2011.

  1. circa27

    Thread Starter Member

    Feb 22, 2011
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    I need a time delay relay to control my refrigerator. It needs to delay engaging for 5-10 seconds after its turned on. Time off delay doesn't matter, all else is irrelevant.

    I was told putting a decent sized cap around 2200uF between the 85 and 86 pins would do this but I fail to see how that would work. Any suggestions?
     
  2. praondevou

    AAC Fanatic!

    Jul 9, 2011
    2,936
    488
    Do you want to build your own circuit or buy a plug and play relay? There are plenty of "delay on make" relays you can buy. Just google it.

    I'm not a refrigerator specialist. What are pins 85 and 86?
     
  3. circa27

    Thread Starter Member

    Feb 22, 2011
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  4. praondevou

    AAC Fanatic!

    Jul 9, 2011
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    There may be others that have a better service :D

    Putting a huge capacitor on your relay coil won't help, only cause a current spike your power supply has to cope with.

    How do you want to control your relay?
    What refrigerator is it? The usual we have at home 120VAC?
     
  5. circa27

    Thread Starter Member

    Feb 22, 2011
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    Yeah I didnt think so haha. I found this too, just no price listed yet and I like to learn to do things on my own! But this would work too http://www.airotronics.com/site/product-delaymake_TBC.php

    Its battery powered, thats why its 12v. Not plugged in. So its on a basic On/Off switch now, I just want a delay to turn it on. Unless someone can recommend a cheap Canadian based supplier! My usual place is closed at the moment :/
     
  6. Jaguarjoe

    Active Member

    Apr 7, 2010
    770
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    Here is the heart of an on delay timer:

    http://clarkson-uk.com/555-timer/operation/frames3.html

    I don't know if the 555 can supply enough current to directly drive a Bosch relay which is somewhere around 166ma. If not, you can always put something like a 2N2222 transistor between them, or, just pick a better relay.
    Make sure you put a flywheel diode across the relay coil.
    If this is going into an automobile, beware that the 12v electrical system in a car is horrendously noisy and full of transients, some approaching 60 volts. That will kill every sensitive component connected to it, like a 555 IC.
    Look at the e-books on this site, I think there's lots of 555 stuff there.
     
  7. circa27

    Thread Starter Member

    Feb 22, 2011
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    Thanks man thats perfect. So for the 0.01 uF cap on the right side of the diagram, is that necessary? I understand all of that, really helpful. However, the + and - on the left side of the diagram, is that for the trigger input? And then the + on the right side goes to the component, which is grounded somewhere else?

    And lastly, can I just use a vregulator to avoid voltage spikes?
     
  8. CDRIVE

    Senior Member

    Jul 1, 2008
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  9. circa27

    Thread Starter Member

    Feb 22, 2011
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    There is no 7 pin? haha

    Okay so I know how a 555 can be used as a pwm, could it be used to basically have the frequency set really low (a fraction) so essentially it would always be 'ON' for the high side of the wave, and the low side of the wave would be just 5 seconds or whatever I need? I know the f=1.4/(R1+2R2)C1 and that the high/ON side of the wave is 0.7(R1+R2)C1 and the low/OFF side of the wave is 0.7(R2)C1

    So with that, If R2 was relatively small, and R1 was relatively large, it would be off for a few seconds and on for hours? Since I'd only have the lights on for a couple hours at most, it would never reach another off on cycle

    I'm new to this 555 bidness, can someone guide me from this post?
     
    Last edited: Jul 14, 2011
  10. Moonstar78

    Member

    Jul 7, 2011
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    Does this thing works on a computer monitor?

    Can i have the values of the components you used.

    Thanks you and more power!
     
  11. Moonstar78

    Member

    Jul 7, 2011
    39
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    Does this thing works on a computer monitor? Can i have the schematic and components values you used.

    Thanks and more power!
     
  12. CDRIVE

    Senior Member

    Jul 1, 2008
    2,223
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    This will probably work for you. You would have to cut into the low side of the relay coil as shown.
     
  13. circa27

    Thread Starter Member

    Feb 22, 2011
    38
    0
    You are great. What program did you use for that? Is that the same idea that I was getting at with the delay on the low side wave and then a constant high side wave? Minus using the 555 of course..

    For the diode, will a 1n4001 or 1n4002 work? And where the 12v+ source is at the top, that would connect to what looks like the 85 pin on the relay coil and then the other end of the coil towards Q1 is the 86 pin of the relay coil? And finally, is Q1 just a basic transistor not a mosfet or anything?

    That diagram is perfect if it works like it shows! Doing this tomorrow, so please get back to me on all those questions I had!
    Thanks a ton man
     
  14. CDRIVE

    Senior Member

    Jul 1, 2008
    2,223
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    Designsoft's Tina Classic
    I don't know what "wave" you're referring to but if you're asking if this is 'Low Side Switching' then yes.
    I'm attaching an updated schematic with a Radio Shack parts list.
    Those node designations don't mean anything to me, as I don't have your frig or the schematic of your frig. If you have one please post it. My schematic is based on an unqualified assumption that one side of your thermostat is connected to the +12V source and the other terminal is connected to the high side of your relay. If one side of your relay coil is connected to board or chassis ground then it's high side switched and my circuit should work. You must confirm this though.
    No, a BJT would not be suitable here. Q1 is a MOSFET.

    Here's an explanation of high side and low side switching.

    High Side Switching: Your relay currently has one end of the coil always connected to negative ground. When the thermostat closes it completes a current path from the +12V rail to the high side of the relay coil.

    Low Side Switching: Your relay coil has one end always connected to the +12V rail. The other end of the coil (low side) is connected to one terminal of the thermostat, While the other terminal of the thermostat is connected to ground.

    It is more likely that you have High Side Switching but you must confirm this.

    FetDelay Theory Of Operation:
    This circuit is based on a (before modifications) High Side switched relay coil. K1 coil must be lifted off ground and connected to the Drain of Q1. Once this is done your circuit will be both High and Low Side switched. When the thermostat closes +12V is applied to the high side of K1's coil. Q1 is initially OFF so the relay doesn't energize at this time. C1 begins to charge through R1 creating an R*C time constant. As C1 charges the Gate voltage of Q1 increases to a point where Q1's Drain-Source junction begins to conduct current through K1's coil. When the Gate voltage reaches about +3.5V K1's contacts will close. This occurs at about 6 seconds after the thermostat first closed. When the thermostat opens C1 will be quickly discharged through D2. D1 prevents damaging back emf from K1's coil.
     
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