RELAY CONTACTS VAC and VDC Explanation Please

Discussion in 'General Electronics Chat' started by renegadegas, Aug 6, 2016.

  1. renegadegas

    Thread Starter Member

    Dec 2, 2012
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    Hi Everyone,

    Thanks for looking at my post and I hope you will be able to help.

    My understanding of relays is limited. What I know is you can have coils that require a DC or AC supply
    which when energised will change the positions of the output contacts (eg, NO will go Closed and vice versa).

    Take for instance a 24VAC relay coil from the link below. Reading the product information, I realise it states the
    contact voltage VAC is 250 VAC and 3 lines below that it states Contact VDC as 30VDC. Why do we have VDC on the contacts when
    we are putting in VAC? Is there any rectification going on inside the relay to get VDC because to me we can only put in 24VAC and I'm not expecting VDC on the contacts. Or are they stating we can ahve both DC and AC on the contacts and if so which pins do we get them from?I'm only expecting VAC on the contacts information but not both. I am very confused. Your help is deeply appreciated.

    http://uk.farnell.com/omron-industrial-automation/my4in-24acs/relay-4pco-24vac/dp/186600
    Thank you in advance.
     
  2. MaxHeadRoom

    Expert

    Jul 18, 2013
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    That is showing the DC and AC current rating of the contacts, these are different values due to more extreme arcing etc that can occur when switching DC.
    Also contacts are usually rated for inductive or resistive loads, with some de-rating for inductive on both AC and DC.
    Max.
     
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  3. renegadegas

    Thread Starter Member

    Dec 2, 2012
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    Thanks for your quick reply but when they refer to the contacts, they are referring to where the coil contacts meets the output contacts and not the pins right?.
    So if I want to drive a load of 24VAC what does these VDC and VAC contacts information have on my load?

    When you state switching DC, what does it mean? We've put 24VAC into the coil so we expect 24VAC on the output pins right?

    Thanks very much again.
     
  4. MaxHeadRoom

    Expert

    Jul 18, 2013
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    Generally the coil supply and the contacts are treated totally separate and are isolated from each other, particularly in the case of this relay , the coil current will exhibit a certain value based on the voltage applied and the resistance/inductance (DC/AC) of the coil.
    In the case of the one linked to, it shows a 24vac coil.
    Max.
     
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  5. #12

    Expert

    Nov 30, 2010
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    Whole lot of, "No".
    The coil does not touch the output contacts in any way. The output contacts are rated for either AC or DC current and voltage which you supply. Relays do not rectify. If you want the contacts to carry AC volts, use the AC voltage and current ratings of your relay as the maximum you can do.
     
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  6. MaxHeadRoom

    Expert

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    One relay that used to exist in older automobiles used the same Common terminal for the coil and the one contact, thereby only having 3 terminals for terminal economy purposes. But this config is generally rare.
    Max.
     
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  7. renegadegas

    Thread Starter Member

    Dec 2, 2012
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    Thanks for your reply.
    I do understand it's the magnetic field that is created by the coil being energised that changes the state of the output pins.
    All I wanted to know was how important the information of VDC and VAC printed onto the relay is because if I was using a VAC rated relay, why should they even print onto the relay a VDC information. I dont think VDC info should even be on the case as to me, it creates an impression you can have VDC output somewhere.

    So if I had a 24VAC rated coil I should expect 24VAC on the output pins and not worry about the other information about VDC printed on he coil, right?

    Thanks
     
  8. MaxHeadRoom

    Expert

    Jul 18, 2013
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    As I posted earlier, the AC/DC rating applies to the contacts ONLY, you have a choice to switch AC or DC with this relay, Regardless of the coil, the manuf does not know the intended use.
    The coil and the contacts are ISOLATED from each other.
    Max.
     
    Last edited: Aug 6, 2016
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  9. #12

    Expert

    Nov 30, 2010
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    You can, right after you connect your DC supply to one of the output contacts.
    NO! You can expect the voltage you connect to one output pin to come out the other output pin.
     
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  10. renegadegas

    Thread Starter Member

    Dec 2, 2012
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    Please look at the picture attached. The relay on the left has some info printed on the bottom right
    about 5A 250AC and 5A 30VDC. Can someone shed some light on it please.

    My brain is getting fried :) 20160803_201236.jpg
     
  11. MaxHeadRoom

    Expert

    Jul 18, 2013
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    It means the contacts can switch 5AMPS up to 250vac or 5AMPS at 30vdc.
    These are maximum rating for DC and AC for the contacts.
    If you were to switch 5amps at 250vDC you most likely would get a sustained plasma arc and burn out the contacts.
    Max.
     
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  12. #12

    Expert

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    If you are using AC voltage to power the load, the limit is 250 volts AC RMS and the current limit is 5 amps.
    If you are using DC to power the load, the voltage limit is 30 volts and the current limit is 5 amps.
     
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  13. renegadegas

    Thread Starter Member

    Dec 2, 2012
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    Sorry Max for being a pain. My understanding of your answer is that I can have switched Ac or DC provided it's stated in the data sheets or printed on the case regardless of supplying AC to the input. Right?

    Cheers
     
  14. #12

    Expert

    Nov 30, 2010
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    right.
     
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  15. MaxHeadRoom

    Expert

    Jul 18, 2013
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    Yes, regardless of what you supply the coil with, the contacts switch anything up to the rated AC or DC value.
    Max.
     
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  16. MaxHeadRoom

    Expert

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    I think you have to get around to the fact that anything you supply to the coil is totally separate and isolated from the contacts.
    Max.
     
  17. BillB3857

    Senior Member

    Feb 28, 2009
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    The contacts (the switching part of the relay, not the coil) can safely handle 5 Amps at 250 volts AC or can handle 5 amps at only 30 volts DC. As explained in earlier posts, the possibility of arc over when the relay is de-energized is greater in a DC circuit, so the voltage rating is reduced. As to why they put both ratings on the lable... the relay can be used for either. Otherwise, there would need to be two different part numbers with two different labels for the same physical device. The manufacturer thinks that anybody using such a device would be able to decide upon their individual needs.
     
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  18. renegadegas

    Thread Starter Member

    Dec 2, 2012
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    My boss recently asked me to trace a wire to a control panel and let him know what power was needed to power the
    load which was not present ( Just a hanging wire close to a Tap). I suspected it was a solenoid and I traced the wire back to a relay but
    I got confused as to what sort of power the solenoid needed because the relay had AC and DC written on the case. I knew the supply to the relay was 24VAC but the output was the problem. I guessed and told him it was 24VDC. I don't know if I was right.

    Thanks Max and Expert
     
  19. MaxHeadRoom

    Expert

    Jul 18, 2013
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    If the load says 24vac then this has to be the nature of the supply, so it sounds like you could be wrong with 24VDC? How do you know the nature of the solenoid if not present?
    How come your boss gave you this task?
    Max.
     
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  20. Tonyr1084

    Active Member

    Sep 24, 2015
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    So far you've gotten good answers. The problem has been in your understanding of what things mean. Let me take a slightly different approach; maybe this will clear it up for you.

    A relay consists of two distinct parts. First, there's the switch, the C (common), the NC (normally closed) and the ON (normally open) portions of that switch. The second part of the relay is the coil. ALL the coil does is either throw the switch from the NC to the NO - or not. When you energize the coil it will switch from C - NC to C - NO (moving the contact between the common and the normally closed switch from that position to the other, the normally open)

    The coil is rated in its own voltage - be it AC or DC, be it 120 volts or 12 volts or 5 or 3 or some other voltage - depending on what the relay was designed to use as a control voltage. Other than that the coil does nothing more. No rectification, no addition or subtraction to the contacts.

    The switch (the contacts) is simply a switch. Like any switch, it's going to be rated to handle certain loads. So the switch (the coil you have) can handle switching up to 5 amps on 120 volts AC (NOT DC). OR it can switch up to 5 amps on 30 volts DC. That's the maximum rating of the switch.

    So if you're planning on controlling a circuit with 12 volts DC but not drawing more than 5 amps then the switch can handle the load. However, if you're going to be attempting to switch more than 5 amps - the switch will not handle the load. The contacts can pit and burn out or can weld shut, rendering the relay useless.

    Same is true if you're using a low voltage to control a much higher voltage. Suppose you're using an Arduino. It runs on about 5 volts. A coil rated for 5 volts DC will be needed. The switch (whatever its rated for) can handle (in the case of your relay switch rating) 120 volts AC and up to 5 amps.

    You can use different voltages to control different voltages. However, the relay does not condition or change in any way the sources you're using the switch to control.

    Your relay can't handle switching more than 5 amps. Period! If you're using 120 volts AC - you're fine to switch 5 amps. But if you want to switch - oh, lets say 48 volts DC - at 5 amps - your relay can't be expected to live very long because you're over the capacity of the switch rating.

    If I'm reading your relay correctly, the coil is controlled by 230 VAC (not DC).
     
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