Solid state relay switching

Discussion in 'General Electronics Chat' started by bobbie, Jan 26, 2010.

  1. bobbie

    Thread Starter New Member

    Jan 26, 2010
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    Hi guys,
    I am learning about electronics. I am making a 12 volt battery switching devise. I am using a 40 amp 24 - 240 dc volts output, 5-32 dc volts input solid state relay, mounted on a heatsink. Standard square type with two locking screws.

    My concern is my understanding of how these SSR work, as they are mosfet output. If I connect two 12 volt lead acid batteries in series, with a control volage on the input, the SSR operates as is expected. It switches and I get 24 volts across the batteries.

    However, if I decide to connect the batteries in parallel, I get a constant connection with either the positive or negative terminals if I use the SSR to switch the same terminals on or off. Therefore, by my understanding, the mosfets should not be open when there is no control voltage at the input, but the connection is closed. How can this be?

    Regards,
     
  2. Blakus

    New Member

    Apr 3, 2008
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    I would look at the specifications for the relay, since you quote 24-240vdc as the switching capability. I guess if it would switch lower they would havew said ??
     
  3. bobbie

    Thread Starter New Member

    Jan 26, 2010
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    Hi Blakus,
    The problem as I see it is that the SSR is always on when I attempt to connect or disconnect the negative or positive terminals. If these do not reach 24 volts difference there should be no voltage switched at all, hence no connection. This is where my understanding needs to be improved.
     
  4. Wendy

    Moderator

    Mar 24, 2008
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    SSRs come in both AC and DC flavors. An AC unit will stay conducting until the current reverses.

    The other thing to note is SSRs failure mode is shorting. It is possible you have a bad device.

    The AAC book has a section on SSRs...

    http://www.allaboutcircuits.com/vol_4/chpt_5/5.html
     
  5. bobbie

    Thread Starter New Member

    Jan 26, 2010
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    Hi there Bill,
    thanks for the reply. I have a couple of these SSR's and they both exhibit the same bahaviour. I got them from the internet:-
    http://www.sureelectronics.net/pdfs/DC-RR014.pdf

    I am not certain of the construction technic, but as you article suggests, they might be transistor output.

    As they are switching the batteries in series but not in parallel, I am inclined to think that they are working OK. So, the issue is either finding out how these SSR's are manufactured; maybe a clue as to their true operation - most unlikely as I have not received a reply to this question from the supplier. Or, whether the solution is an electronics theory one. Very likely too.

    I am thinking that as both outputs connections for the load are say zero volts negative rail, this SSR is unable to activate and thus operates as "ON".

    Quite perplexing for me at the present time.

    regards
     
  6. Wendy

    Moderator

    Mar 24, 2008
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    Your link is dead from here.

    Can you show a picture of the parts, and possibly the datasheet? Many cases you can up load it as an attachment.
     
  7. bobbie

    Thread Starter New Member

    Jan 26, 2010
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    Thanks Bill, link works for me.

    Here is the datasheet
     
  8. Wendy

    Moderator

    Mar 24, 2008
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    I don't see why it should matter, but the minimum spec for the power side of this SSR is 24V. For whatever reason it requires the 24 to operator properly (which makes no sense to me).

    It is also polarity sensitive. You must have the power leads connected to the correct polarity for it to work.

    This is the first SSR I've ever seen with a schematic. This is unusual.

    What kind of current do you plan on switching? Most MOSFETs will work equally well or better for this kind of application. If you plan on switching the + side you would need a P-Ch. MOSFET.
     
  9. bobbie

    Thread Starter New Member

    Jan 26, 2010
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    Hi Bill,
    I'm glad this has someone else stumped! I am just looking at switching batteries from 24 volts to 12 volts up to 40 amps to run a couple of 24v DC motors. Its more of an experiment for me.

    What I do not understand, is why, when I try and connect the two negative terminals (batteries in parallel), the SSR appears to short out. My understanding would be that at the very least it would not switch at zero volts at all. Maybe I should look for a SSR that allows a minimum zero load output?

    Regards
     
  10. Wendy

    Moderator

    Mar 24, 2008
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    Can you show the circuit you are trying to attempt?
     
  11. retched

    AAC Fanatic!

    Dec 5, 2009
    5,201
    312
    The switch has a 3-25mA rating. In parallel, what is the rating of your batteries? You may be saturating the switch, causing a closed circuit.

    from the datasheet:
    [ed]
    This could be a problem:
    [/ed]
     
    Last edited: Jan 26, 2010
  12. bobbie

    Thread Starter New Member

    Jan 26, 2010
    22
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    Hi Bill,
    here is a sketch - not so artistic! - of the battery switching I am playing with at the moment. I have had a few goes not realising that only certain file types are accepted, which makes my sketch a bit smaller. Enjoy!!
     
  13. bobbie

    Thread Starter New Member

    Jan 26, 2010
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    Hi retched,
    I thnk the 3-25ma is for the switch that is optically isolated from the mosfet on this SSR, so I don't see how that might be influencing the output load; which is where I am trying to switch the batteries over from 24 volts to 12 volts. I am just using old auto lead acid batteries. I am switching the SSR's from the PWM.

    The issue is probably due to the polarity as Bill mentioned. As the SSR needs min output 24v load, placing a negative line onto the positive terminal may be dampening the internal circuitry somehow. I probably will need to try a zero volts load SSR, as zero volts, to me, implies no polarity with a SSR. Although, zero volts in reality may have polarity? Not sure how to reconfigure the circuit to switch the motors and batteries, to get what I am playing with. Needs a bit more thought!

    Thanks for the comments.

    Regards
     
  14. Wendy

    Moderator

    Mar 24, 2008
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    If you open the input pins does it turn off?

    You have a delete function on this forum for duplicate posts. Unusual for forums like this, but this site is unusual in several ways.
     
  15. bobbie

    Thread Starter New Member

    Jan 26, 2010
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    Hi Bill,
    I thought I had deleted these messages. I was trying on each occasion to upload image files that were not part of the forum format. jpeg works!

    Yes I have disconnected the input voltage and have tried reversing the connections. no change. I am thinking that your comment on polarity is important. By having two negative rails, I might be dampening the output somehow. I am afraid my knowledge on this part of electronic is limited. And, especially as Sure electronics do no reply to requests for further information on manufacture techniques.

    Regards
     
  16. Wendy

    Moderator

    Mar 24, 2008
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    An experiment you could connect two SSRs back to back with their polarities reversed.
     
  17. retched

    AAC Fanatic!

    Dec 5, 2009
    5,201
    312
    Check the amperage through battery cables also. See if they are in fact over 3A. am interested to see if it is over amping the transistor. Even though the base is isolated on the switch side, you can still cause the transistor to stick from the emitter and collector. Do you have a few 9v batteries you can try this with just to see if the transistor will switch properly at the rated load?
     
  18. bobbie

    Thread Starter New Member

    Jan 26, 2010
    22
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    Hi guys,
    very interesting from here on in!

    First connecting the two SSR's as per Bill's request. Terminals + - - + gives open circuit, no switch output. Terminals - + + - gives connection, switching as per expectation.

    Second, as per retched's request. With ampmeter in series with -++- get a load of 2 amps! but, no switching!! What am I reading? A mosfet gate value?

    I cannot make head or tail of this at the moment without some real description of SSR operation for these units.

    regards

    edit: just had a reply of the SSR from the supplier.
    "This kind of SSR uses
    mosfet output.
    Feel free to contact us if you have any questions.
    Best "
     
    Last edited: Jan 28, 2010
  19. Wendy

    Moderator

    Mar 24, 2008
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    It sounds like we have partially identified part of the problem as a polarity issue. This particular SSR has a circuit inside, and if you connect the voltage backwards the circuitry can't handle it.

    It may be possible to rearrange the circuit to do what you want and bias the SSR correctly.

    Sooo, why are you feeding two motors from 3 batteries, if I might ask. I'll be thinking of alternate ways to do the same thing (more or less).

    With my work schedule it will likely be around Sunday before I can focus on it, there is a very good chance someone will step in and come up with something better. Basically we need to keep the dialog going.
     
  20. bobbie

    Thread Starter New Member

    Jan 26, 2010
    22
    0
    Hi Bill,
    Thanks for the reply. I am not feeding two motors with three batteries, I am using two batteries, I am swapping Battery 2 from series to parallel across battery 1. I have just drawn the diagram in operational mode. Thinking it might be easier to see!!

    This is a learning experiment for me. Also, I would like to collect information to see if by swapping batteries with this type of method, I can improve the power ratio supplied to these motors by judicial use of the PWM. With all the hype over Electric Vehicles, this seemed as good a place to start to try something different and maybe contribute something useful to this area.

    regards
     
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