Antique payphone - coin relay - boost 12 VDC to 130 VDC?

Discussion in 'The Projects Forum' started by KeithB, Sep 19, 2010.

  1. KeithB

    Thread Starter New Member

    Sep 19, 2010
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    I'd like to generate 130 VDC to power a Western Electric 1A coin relay used in antique Western Electric three-slot payphones, preferably in a reliable, small-footprint circuit. The input power source would be a 12 VDC 1A wall-wart. Following ideas from ladyada's MintyBoost projects, I reviewed the Maxim MAX1605 boost converter which seems (at first glance) to be one possible candidate for this project. Not being either an EE or even a circuit designer, I'd like to learn what values of commonly available external components are necessary to fulfill this need.

    The 1A coin relay inside the payphone moves contact switches and acts as a servo to shift a small plastic paddle in one of two directions (to either retain or return the coins) depending on whether it receives +130 VDC or -130 VDC. Could I simply reverse the two connections to this relay using a DPDT relay, or would dual positive and negative supplies be necessary?

    Thank you in advance for any advice or suggestions you might provide. Your help will be gratefully and generously shared with other telephone collectors wishing to keep these beloved relics of the 20th century alive and working as originally intended, albeit it within their own homes. :)
     
  2. Kermit2

    AAC Fanatic!

    Feb 5, 2010
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    A 12 VDC relay can reverse the 130 VDC voltage.
     
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  3. KeithB

    Thread Starter New Member

    Sep 19, 2010
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    I was fairly certain I could do that, but just don't have enough real-world experience to feel safe about it, whether I can successfully test it on a breadboard or not. As the old saying goes, "I know enough to be dangerous". ;) I also know enough to be safe and ask questions of those more experienced, fortunately. :D
     
  4. Kermit2

    AAC Fanatic!

    Feb 5, 2010
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    A simple 130VDC supply from a doorbell transformer.

    R3 is referenced in the write up but it should be R2
     
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  5. marshallf3

    Well-Known Member

    Jul 26, 2010
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    Home Depot is selling mostly 16V doorbell transformers and ripping people off with the prices anyway.

    Go to any HVAC (heat and air) wholesale house and get a 24V one, they're used to power thermostat circuits.
     
  6. KeithB

    Thread Starter New Member

    Sep 19, 2010
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    For 11 bucks, this 24VAC @ 20VA doorbell transformer is available at HomeDepot. At about 2"x2"x2" in size, it should fit easily into a project box/container. The voltage doubler circuits are certainly simple enough, thank you. I was going to ask what other common projects might use such a supply, but apparently it's popular for manufacturing colloidal silver.

    Thank you all VERY much!!

    (I'm actually ashamed :rolleyes: I didn't find that power supply circuit earlier.)

    For $5 more, this White-Rodgers HVAC transformer outputs 40 VA.
     
    Last edited: Sep 19, 2010
  7. Kermit2

    AAC Fanatic!

    Feb 5, 2010
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    Yes, it was one I've seen at a site for manufacturing colloidal silver. But will work just as well for this application I think. I removed references to the silver manufacturing stuff to prevent confusion about the application here.

    Nice job with the googling!
     
  8. marshallf3

    Well-Known Member

    Jul 26, 2010
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    I'm paying $6.50 for the 2 or 2-1/2 amp ones from my local HVAC guy but it could be an exception to the rule. I've even got a multi-tapped primary one sitting around I paid $7.50 for, it's got 480/277/240/208 & 120V input taps and 3A of 24 VAC output. I haven't a clue what I'm going to do with it though, just wanted it on the shelf.
     
  9. KeithB

    Thread Starter New Member

    Sep 19, 2010
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    Fortunately, the payphone's coin relay shouldn't require ridiculously clean DC power, or terrifically high amperage. Now I'll have to pull out the breadboard and start building this supply for testing with the coin relay.

    I really appreciate the help. Thank you VERY much. :)
     
  10. eblc1388

    Senior Member

    Nov 28, 2008
    1,542
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    Instead of building boost converter to obtain DC 120V, here is a simple scheme to get that.

    You just need to use two AC/AC walmarts. Depends on where you are T1 primary should be rated to your local mains voltage.

    The second walmart is connected in reversed with low AC in and high AC out. A simple rectifier will then convert the AC voltage into DC voltage suitable to drive the relays.

    If you want DC12V for other part of your project then you can add a bridge rectifier and smoothing capacitor to obtain it from the first walmart.

    [​IMG]
     
  11. KeithB

    Thread Starter New Member

    Sep 19, 2010
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    I think one significant difference will be selecting the appropriate current-limiting resistors. I'd imagine the Western Electric coin relay requires more than the 300 μA current the silver guys use. If I arbitrarily selected 250 mA, I calculated it would require 260 Ω resistors. Unfortunately, reviewing the power dissipation formula, it appears sizable power resistors (> 15W?) would be required.

    Did I miss a decimal point somewhere? :confused:
     
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