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Thread: class II supply

  1. #11
    Join Date
    Jul 2013
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    Renton, WA
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    586
    Back in '71 the RR3 were about $2 ea IIRC. Over $25 now, but have a number of spares for home use snagged from surplus store onesies and twosies over the years.

    Think the GE switches were 50 cents each back then, probably still have the old Graybar receipt somewhere.
    Any switches I've added in the last few decades have been snap action microswitches.

    I ordered a couple of 1A max 24 Vdc supplies from ebay/China ($5.98, including shipping). Only 2 settings, 0.5 and 1A, so will go with the 0.5 A. The shipping rates for a 2# package from China are still a wonderment to me, less than a postage stamp here.

  2. #12
    Join Date
    Apr 2008
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    Ann Arbor, Michigan
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    170505-0737 EDT

    junkhound:

    My guess is that the supply you ordered will have a moderately low internal output impedance, and hold fairly close to 24 V until current limiting at 0.5 A. Thus, an external load of 50 ohms will probably have about 24 * 0.5 = 12 W dissipated in the load.

    I would guess that this much power dissipation on a continuous basis would burn out the relay coil, a stuck closed control switch.

    Suppose the power supply internal output impedance is raised to 500 ohms, a resistor, then maximum steady state coil voltage is about 2.5 V. Coil power dissipation is about 6.25/50 or 120 mW. Instead with a 200 ohm internal impedance we get about 0.5 W dissipation. Possibly tolerable. But still a very long recharge time from a fully discharged 250,000 ufd capacitor.

    Instead of increasing source impedance it might be desirable to add a time controlled limit on how long current could be supplied. For example, this might be a relay NC contact in series with the 24 V supply that opened after 100 mS of on current over 0.1 A, with an automatic reset time of possibly 1 second.

    .

  3. #13
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    Dec 2007
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    NE Nebraska
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    Quote Originally Posted by junkhound View Post
    Own home - Low voltage lighting,, using GE RR relays. After 45 years the class II 24 Vac transformer finally failed, internally shorted.

    45 years ago power electronics just about did not exist, now one can get a 85V-265Vac to 24 Vdc converter for $10 !
    Can set current limit to less than 1/2 amp on the unit to comply with class II, etc.

    Question is: Is the use of one of these NEC compliant? It is UL listed.
    I tend to interpret NEC to my own advantage (or ignore it altogether in some cases), but wondering how others would interpret this use. Standard cord plug-in, 24V output on external screw terminals.
    45 years ago we didn't have art 411 for low voltage lighting systems.

    Today 411.4 would require all parts of the system to be listed for use together as a system.

    I have no idea how to comply with your 45 year old system as it probably isn't listed, no longer meets current listing requirements, or no longer has listed components of that system available.

  4. #14
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    170508-2205 EDT

    kwired:

    How does 411.4 apply?

    A 24 V relay coil is not a light.

    .

  5. #15
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    NE Nebraska
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    Quote Originally Posted by gar View Post
    170508-2205 EDT

    kwired:

    How does 411.4 apply?

    A 24 V relay coil is not a light.

    .
    Wasn't thinking about controls, but OP is asking about replacing a transformer that failed, and power supplies for LV lighting does need to be listed as part of the system, so unless this is for control power only it probably still applies.

  6. #16
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    170508-2405 EDT

    kwired:

    If junkhound used the GE transformer sold for a power source for RR relay coils, then it was a Class 2 power source. This had an unloaded AC output voltage of about 24 to 28 V, and when rectified and filtered the DC voltage is in the mid to high 30 V range.

    At this moment my unloaded DC voltage is just under 39 V.

    In junkhound's application I believe he has in excess of 70 watt-seconds of energy storage at the DC level. It will be well above 70 if he is running at my voltage.

    My guess is that junkhound may control as much as 50,000 W with one control switch 50 diodes, and 50 RR relays. Needs a good size breaker box. Is this on a 15 kVA power company transformer? I doubt it. In years passed my transformer was a 25 kVA unit until it leaked. The replacement is 50 kVA and I don't exceed 2 kVA much of the time. Most of the time I am below 24 to 39 kWh per day. I have a gas range top, a gas clothes dryer, and gas heat.

    .
    Last edited by gar; 05-09-17 at 12:43 AM.

  7. #17
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    NE Nebraska
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    Sorry, should have looked to see exactly what a RR relay was before presuming this is a low voltage lighting system.

    I now see he only has a low voltage control system for his general lighting.

  8. #18
    Join Date
    Jul 2013
    Location
    Renton, WA
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    Update:
    So, after a week or so the light switches again fail to operate, and the LED indicator on the 0.5A 24 Vdc supply is dim.

    Ohm reading on feed to relay circuits was 4 ohms.

    Aha, dendrite in the alum electrolytic capacitor ?

    Hooked up a 20A 24 Vdc supply temporarily, all works, take the 20A supply off, now system works with the 1/2 A supply.

    Must have burned the dendrite open, time to replace the caps (> 30 years old) before another dendrite forms.


    Am assuming a continuous 4 ohm load on the original class II xfmr apparently overheated it to failure. Dendrite likely made a complete path sometime during the night, as the lights worked when we went to bed and were dead in the morning.

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