Solid State Relay - Discharging the gate

Discussion in 'General Electronics Chat' started by Management, May 6, 2009.

  1. Management

    Thread Starter Active Member

    Sep 18, 2007
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    How should one go about do this?

    I built this in the lab and looks ok but not sure about the current on the gate. Pspice says there is 64 nA when switch is on (+15V) and 680 nA when switch is off (-15V).

    What other issues should I be aware of?

    In the lab I used an AC source of 10V @ 1 kHz. What I did in PSpice was just to have it simulate.

    [​IMG]
     
  2. nanovate

    Distinguished Member

    May 7, 2007
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    You need to ensure that the FETs are always turned completely on in the real circuit. In the simulation you have 15V which is fine but what happens when the user inputs a voltage lower than the threshold voltage of the FETs?
     
  3. Wendy

    Moderator

    Mar 24, 2008
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    Have you ever played with real SSRs? They are plenty nifty devices, high power out but can be turned on with watch cells. Not too expensive either, I've bought used for $5. Of course, they are AC devices, not DC.
     
  4. t_n_k

    AAC Fanatic!

    Mar 6, 2009
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  5. Wendy

    Moderator

    Mar 24, 2008
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    Look further down on that page, and you will see what I'm talking about.

    Thing is, almost any transistor can be used to switch DC. Most SSRs don't switch DC on the output, but AC. One of the key features I would look for is the isolation of the input from the output, with optocoupling.

    Read the specs on the devices, they aren't what the OP is talking about. 5V 15ma input, 75-264 VAC output, 3A continuous.

    Question for Management, where is the load (singular)?

    Looking even further down that list, I found what you might be talking about (a true DC/DC SSR)...

    http://www.futurlec.com/Relays/SSRDC100V40A.shtml

    Note that is still optocoupled. That was actually the first time I've seen those.
     
  6. Management

    Thread Starter Active Member

    Sep 18, 2007
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    See what your saying. Well it is a depletion mode FET so it is normally on at voltages left than the threshold correct? So it would be conducting a little as you move away from the threshold and more positive. But only care about cutoff and saturation.
     
  7. Management

    Thread Starter Active Member

    Sep 18, 2007
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    That AC voltage and what it is riding on is the load but rather not get into that. I asked this in another thread but is it absolutely impossible to come up with a circuit that will both switch AC on AND OFF. I know on is possible but when you try to switch it off then the diode in the FET will only block on direction of current and not both.

    And funny 1 FET topolgy that would be able to solve this problem. If not I understand. I couldn't think of one.
     
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