Switches and voltage

Discussion in 'General Electronics Chat' started by questilong, May 3, 2016.

  1. questilong

    Thread Starter New Member

    Apr 19, 2016
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    Hello, in my road I have another problem (
    I am looking for a switch to use to turn on/off a bleeder resistor,
    but I can't find standard sized switch for 470VDC...

    Here is my schematic : I want to find a switch I could put in safely at the location called "bleed"
    [​IMG]

    S3 (bleeder) switch will be this one (i have those left) 3A/250V :
    http://www.mouser.fr/ProductDetail/Carling-Technologies/110-B-63/?qs=6sqnkTSM%2bhyUK67ktDTkhw==

    Can I use one of these as the bleeder switch ? or will 470VDC be too much ?

    By the way I don't understand how voltage rating for switches work... for AC and DC... help is greatly appreciated.
    Thank you
     
    Last edited: May 3, 2016
  2. BR-549

    Well-Known Member

    Sep 22, 2013
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    I would use a relay.
     
  3. questilong

    Thread Starter New Member

    Apr 19, 2016
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    Thank you,
    but It is too expensive to find a relay rated for 500VDC, that is why I want a switch.
    Unless I am missing something, because I am not that experienced in electronic.

    Also the switch I have is 3A 250VAC
     
    Last edited: May 3, 2016
  4. BR-549

    Well-Known Member

    Sep 22, 2013
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    It's just not a good idea to put that kind of voltage on a switch or hook up wire.

    I'm sure you can find an in-expensive one. Relays are mostly very reliable. A small circuit board type will work.

    Just an opinion.
     
  5. BR-549

    Well-Known Member

    Sep 22, 2013
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  6. questilong

    Thread Starter New Member

    Apr 19, 2016
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    Thanks again,
    I wanted to go for a relay first (to make it automatic, so when the power is shut down, the bleeder is switched on)
    But I could not find a relay (cheap & small enough) with a coil rated for 470VDC+
    I wanted to do this :
    [​IMG]

    I can't understand why contact on the relay will withstand this voltage, but not a switch contacts ?

    EDIT : THANK YOU FOR THE RELAYS, I could not find this category !
    So now I understand, you idea was to power the relay coil from an other power source (I have acces to 5VDC in this amplifier ;)

    They are rated "Dielectric strength 1000VAC" so If I want a switch I need to find something with similar specs ?
     
    Last edited: May 3, 2016
  7. mcgyvr

    AAC Fanatic!

    Oct 15, 2009
    4,771
    971
    That relay will NOT work.. Its contacts are NOT rated to switch 470VDC.. Its not rated to switch DC at all
    Typically the DC ratings (if approved) are a fraction of the AC ratings as a DC arc is harder to extinguish thus causes more contact damage/heating,etc..

    Dielectric strength is the minimum guaranteed voltage potential it can take across any insulation before arching to adjacent pins. This is not the same as its DC contact voltage rating.
     
  8. questilong

    Thread Starter New Member

    Apr 19, 2016
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    First Thank you very much.
    Yes after reading other Fujitsu relays I found that relays rated for 440VAC are rated for 300VDC max ;)
    I was going to post what I found, but you where faster.

    Now I understand better dielectric strength is different than contact rating.
    BUT what I find Interesting is the fact that switch&relays rated for 120VAC 6A, are also rated for 240VAC 3A (ohm law).
    So in theory 500VAC is possible at 1.5A, and 1000VAC is possible at 0.75A !
    Since my 470VDC is 0.5A max (protected by a fuse). So maybe 500VDC 0.5A is possible ?

    I start to think it's impossible.... but what do you think ?

    EDIT : maybe OHMs law is not applicable, since the Fujitsu "contact rating" is 10A 250VAC (but the "MAX switching voltage" is rated at 440VAC)... I am lost...
     
  9. mcgyvr

    AAC Fanatic!

    Oct 15, 2009
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    Possible and rated/approved are 2 different things.. For a professional application you must choose components that are "rated or approved" to what you need or spend extra money during testing to ensure that the component will also work safely under the conditions you are putting it through..

    max switching voltage is the maximum voltage open circuit voltage (no load) voltage that its safely allowed to switch.

    ohms law doesn't apply here.. Its all about contact design and materials and how they are effected by the arcs generated during switching,etc...
    You can't apply a formula to adjust the rating.
     
  10. #12

    Expert

    Nov 30, 2010
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    I have several thoughts.
    First, 100 ohms is a strange size for a bleeder resistor. It should be more like 100k@ 5 watts, or 220k@ 2 watts, or 560k@ 1 watt, and always connected.
    Be sure to observe the voltage rating on the resistor. IIRC it takes a 2 watt resistor to avoid arcing at 500 volts.
    Second, it is common practice in my trade to throw a transformer in and power a relay coil from that source, but I don't think using a relay is the right way to do this job.
     
  11. BR-549

    Well-Known Member

    Sep 22, 2013
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    That was the first result of a "500 VAC circuit board relay", on google.

    Change it to VDC. Pardon me.

    Maybe a nice big knife switch on the front panel.
     
  12. BR-549

    Well-Known Member

    Sep 22, 2013
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    100 ohms. That's not a bleeder, that's a heater.
     
  13. questilong

    Thread Starter New Member

    Apr 19, 2016
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    BR-549 : I will make it part of the circuit I think it's the only solution ;)
    Yes you are right I should not assume, safety is what I want ;)
    I think I will just make it part of the circuit then...

    The second schematic is not by me, it's from a google search, it was just to illustrate what I wanted to do at first, not the values just the circuit :)
    By the way I will use a 100K resistor,
    but since I will make it part of the circuit (un-switchable) maybe I should make it bigger than 100K to put less load on the power supply ?
    (even if it bleed the capacitors slower)

    What do you think ?
     
    Last edited: May 3, 2016
  14. #12

    Expert

    Nov 30, 2010
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    That's why I spent the time to calculate some options for you.
    A 560k resistor with a 50 uf cap has a time constant of 28 seconds. Allowing more than a minute to bleed down a cap is the usual way to do it.
    Just look out for that arcing voltage limit. Read the specs on the resistor!
     
  15. BR-549

    Well-Known Member

    Sep 22, 2013
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    What is switching S2? Is that for tube warm up, before power?
     
  16. questilong

    Thread Starter New Member

    Apr 19, 2016
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    Thank you I will get back to you if I have some troubles with it !
    Yes it's for tube heathers warming, for S2 it's a Carling SPDT (rated for 6A 250VAC) this on was not a lot of thinking, because this switch is used by Marshall shince the 1960 (for 650VAC), and amplifiers build with it still work today 50years later !
    So I just used the same switch. I know pretty lame sorry
     
  17. #12

    Expert

    Nov 30, 2010
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  18. BR-549

    Well-Known Member

    Sep 22, 2013
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    questilong,

    I have no idea of what you are doing. If you are refurbishing an old rig, and you already have HV at the control panel (S2 for instance)........then by all means, get a HV switch.

    This used to be common.

    But if you are building something new, it's just good safety to keep HV away from panel.

    I'm not trying to rag on you.

    It's a good project.
     
  19. #12

    Expert

    Nov 30, 2010
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    That's a good point. Front panel switches should have no chance of killing the customer when they fail.
     
  20. BR-549

    Well-Known Member

    Sep 22, 2013
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    "for S2 it's a Carling SPDT (rated for 6A 250VAC) this on was not a lot of thinking, because this switch is used by Marshall shince the 1960 (for 650VAC)"

    That does not make sense to me. Are you using a switch or a relay at S2? If it's a relay........what are the specs?
     
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