Derate AC to DC

Discussion in 'The Projects Forum' started by Lightfire, Apr 20, 2011.

  1. Lightfire

    Thread Starter Well-Known Member

    Oct 5, 2010
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    Hello,


    Somebody posted/replied at my thread saying-"You should de-rate AC."

    Which seems to be so familiar. Because I have a push button switch rated as 250 V for AC and 30 volts for DC.

    So, how to derate AC ratings to DC ratings????

    Thanks!!!
     
  2. t_n_k

    AAC Fanatic!

    Mar 6, 2009
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    Normally that's left to component (switch) manufacturers to specify. The user then wisely keeps to the spec.

    Do you understand why there might be a difference in the AC and DC ratings?
     
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  3. mcgyvr

    AAC Fanatic!

    Oct 15, 2009
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    Without the original context/thread its hard to answer this question. In general when talking about switches you derate for DC not AC. Because AC has a zero crossing point and it is easier to quench an AC arc than it is for DC. DC switches/breakers are larger than AC rated devices because of this.
    250V AC/30V DC is clearly DC derating not AC.

    But tnk put it right also.. You find a product/switch/etc.. that is rated for the voltage and current levels you are using. Manufacturers have gone through the testing already to determine if their device is capable of interrupting the voltage/current levels safely. You should only use products that are already rated for what you are trying to do.
     
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  4. CDRIVE

    Senior Member

    Jul 1, 2008
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    I quickly breezed through the data at this sight and it's loaded with absolutely exquisite graphics for the student. Cut-away views of many types of switches, relays and solenoids abound! Really cool!
    Whether or not these pages answer the specifics of your question, they are still worth your time to read. You can use the Forward - Back buttons or the index on the left of the page.

    http://www.tpub.com/neets/book3/8n.htm

    http://www.tpub.com/neets/book3/9.htm
     
    Last edited: Apr 20, 2011
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  5. CDRIVE

    Senior Member

    Jul 1, 2008
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    For every positive voltage vs. time point on a sine wave there will be a an equal negative voltage vs. time point. Since unlike numbers (when added) subtract and since these voltages and currents are equal but opposite, the average is zero.

    http://www.carlingtech.com/support/learn_more_switch.asp?page=switches_amp-rating

    I think my power company now owes me a complete refund, plus interest, for the past 34 years! :rolleyes:
     
    Last edited: Apr 20, 2011
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  6. Lightfire

    Thread Starter Well-Known Member

    Oct 5, 2010
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    But most of my switches that I have purchased do not specify if the ratings were either AC or DC. The ratings were 300V/16A but it is bit obviously AC. So, if we will convert it to DC, what would be the rating?

    I have used that 300V/16A in most of my 12VDC projects. ;)

    Thanks!
     
  7. CDRIVE

    Senior Member

    Jul 1, 2008
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    From what I've read the general rule of thumb seems to be close to: ACval/5.
     
    Last edited: Apr 20, 2011
  8. Lightfire

    Thread Starter Well-Known Member

    Oct 5, 2010
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    ACval=Ac value???????????? means ac value divide to 5 and that would be the dc ratings??/
     
  9. CDRIVE

    Senior Member

    Jul 1, 2008
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    Yes, that's what I read. Keep in mind that the rating of switches are made with thousands and hundreds of thousands open/close cycles in mind. I would imagine that a 16A-AC switch could handle 10A for limited intermittent use. Other opinions are welcomed though.

    Keep in mind that incandescent lamps draw more current when turned on cold.
     
  10. mcgyvr

    AAC Fanatic!

    Oct 15, 2009
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    My opinion is that if a switch has no DC rating it should not be used on a DC circuit.
     
  11. BillB3857

    Senior Member

    Feb 28, 2009
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    Not only that, but all of the electricity we get MUST be sent back to the utility! Otherwise, our Ground Fault Interrupter's trip. How do we apply for our refund?
     
  12. CDRIVE

    Senior Member

    Jul 1, 2008
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    Please try to keep in mind that Lightfire's world is probably nothing like yours. For him, power at the outlet is not a constant in life. Two years touring the finer locations of South East Asia gives one a different perspective. Admittedly, it was a lifetime ago but I seriously doubt that there's been an industrial revolution since I left. While I was there I saw some of the wildest examples of "Field Expedience" that I will ever see in my life time. ;)
     
  13. RFactor

    Active Member

    May 1, 2009
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    As far as DC goes, I think it may depend on how the switch works, if it is a slide switch and it is slid slowly you could draw an arc and degrade the contacts after a few operations. If it is spring loaded and disconnects quickly, it might last longer. It would also depend on what material the contacts are made of, as in: how much did it cost. As mentioned, with AC, the zero crossing would end any arcing quickly. And I also agree, if it is not rated for DC then don't use it.
     
  14. CDRIVE

    Senior Member

    Jul 1, 2008
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    That's a broad statement. Does that include lighting a LED or two? Honestly, if I had a bank of 100 (20mA) LEDs I wouldn't hesitate to use any AC wall switch (snap toggle) in my house. I figure the switch would wear out mechanically long before the contacts demise. Put a simple snubber across the contacts and I'll hang a bunch more LEDs on it.

    Sorry Ron, I couldn't help myself! :D
    Do ya miss me? :p
     
  15. RFactor

    Active Member

    May 1, 2009
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    The original poster mentioned a 300V, 16A switch, so I assumed we were not talking about powering some LEDs.
     
  16. CDRIVE

    Senior Member

    Jul 1, 2008
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    This thread is a separate but continuation of another of Lightfire's topics.
    http://forum.allaboutcircuits.com/showthread.php?t=52412

    He's a young fellow who lives in Manila, where components are not in abundance. Currently, his projects seem to revolve around 12V battery backup lighting. He will be lighting small automotive (bayonet base) lamps. The photos that he posted look like brake, backup or parking lamps. Admittedly, the current per lamp could add up to a significant number. Since small automotive lamps put out substantial light I wonder if his system would provide enough emergency lighting if he wired them in series - parallel. It would cut his current demands in half.
     
  17. mcgyvr

    AAC Fanatic!

    Oct 15, 2009
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    I don't care if he is from Manilla or Manhattan. Safety should still apply. He obviously has access to the internet on a regular basis so he can't be that bad off.
    And when you aren't capable of making the proper judgements on your own you must rely upon the manufacturers stated ratings.

    You shouldn't judge if your circuit worked or not by "lightfire" or "no lightfire" :)
     
  18. CDRIVE

    Senior Member

    Jul 1, 2008
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    I'm not going to get into a debate over the "Nanny State" mentality here. I'm not a big fan. As a child I played with Mercury and lived in a house with lead paint and asbestos siding. I made countless slingshots, bow and arrows and played cowboys and indians. By the age of 14 I graduated to home made gun powder. I'm still here at 65. :p

    You will note what Lightfire posted below. The switch is rated a 16A AC. Divide that by 5 = 3.2A. I would not have any reservations using that switch at 3.2ADC . Don't forget that, while it's a max rating, those ratings pertain to thousands of switching cycles.


    That's why I told him to divide the rating by 5. ;)

    I haven't a clue of what this means. Is it a riddle? :confused:

    FYI: My last post included a schematic. Its sole purpose was to cut Lightfire's current consumption by 50%. Emergency lighting doesn't have to be bright. It just needs to be bright enough so that you can move around without breaking your neck while conserving as much power as you can. I'm a Hurricane veteran 5 times over. I have experience with total darkness. Besides that, it included a snubber which can reduce the damage from contact sparking significantly. After all, that switch can handle 16A-RMS, that's equivalent to 16ADC. Once the contacts are closed that switch will handle 16ADC with no problem what so ever. It's only the opening and closing of the switch that's an issue when used with DC. Mind you, I'm not telling him that he can use that switch @ 16A but I am telling him that he can very safely use it @ 3A, especially for a purely resistive load. With a snubber I'd push the envelope even further but I'm not telling him to do so.
     
  19. Lightfire

    Thread Starter Well-Known Member

    Oct 5, 2010
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    Actually, I am not living at Manila, I am living at Laguna. Sorry if I lied, but manila and laguna were same characteristics. i guess laguna is belong to metro manila. :D
     
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