SSR vs SCR vs Transistor vs FET vs HELP?!

Discussion in 'General Electronics Chat' started by lordratner, Jul 30, 2012.

  1. lordratner

    Thread Starter New Member

    Jul 30, 2012
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    Hi guys and gals,

    I would normally use a relay for this, but I need to it be silent:

    8 VDC to switch on and off a 12 VDC load. The load would never go over 1A, most likely never even above .25A.

    I'm using an SSR to pass an AC load with a DC current switching it on and off, but I have no idea what to use for DC to DC.

    The current is low, so I figure it should only cost a few bucks for whatever I need, and the DC/DC SSRs are rather pricey, so I must be looking in the wrong place. All help is appreciated. Thanks!

    Seth
     
  2. lordratner

    Thread Starter New Member

    Jul 30, 2012
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    Here's what it's going in for reference. Anything with screw or blade terminals is a huge plus, as I'm trying to minimize soldering.

    Thanks again!
     
  3. tracecom

    AAC Fanatic!

    Apr 16, 2010
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    I would use a MOSFET such as the IRLZ44N, but it does require soldering.
     
  4. lordratner

    Thread Starter New Member

    Jul 30, 2012
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    Ok, so how do I wire that thing? 8VDC from the controller goes to the center pin, then the load from one outer pin to the other? That it, or do I need some sort of resistor in there? The PID SSR says it provides 8VDC; short circuit @ 40mA, which I assume means it can provide up to 40mA.

    I'm assuming I have to ground the metal tab to the 8VDC ground, correct?

    And this thing will handle up to 1A? Thanks so much!
     
  5. SgtWookie

    Expert

    Jul 17, 2007
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    No, the tab is one of the Drain terminals.
    The center terminal is the other Drain terminal.
    The left terminal is the Gate, the right terminal is Source.
    http://www.irf.com/product-info/datasheets/data/irlz44n.pdf
    See the Package Outline drawing near the end of the document.

    The Source gets connected to ground (0v)
    The Drain gets connected to the ground side of your load.
    When the Gate is 4.5v to 10v with respect to the Source terminal, the Drain and Source terminals are connected together; the MOSFET is considered to be fully turned ON.
    When the Gate is 0v to 1v with respect to the Source terminal, the MOSFET is considered to be fully turned OFF.
    If the gate voltage exceeds ±20v, the MOSFET will be destroyed. It is sensitive to static electricity. If there is any possibility that the gate might be left "floating" or disconnected, you should connect a 10k resistor from the gate to the source.
     
  6. strantor

    AAC Fanatic!

    Oct 3, 2010
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    You should be able to crimp lugs onto the legs of the mosfet. It's not best practice, but should be OK. You shouldn't need to head sink it if it's going to primarily be on or off like a relay. But if you're going to be doing any really fast switching you might want to use a heat sink. You shouldn't need a resistor, assuming same as before, you aren't doing fast switching, just operating as a rely. Your +12 should go to the load, then coming out of the load, go to the Drain pin, and the Source pin should be connected to ground. make sure this ground is also connected with your controller ground. Then your signal from the controller will go to the Gate pin.
     
  7. lordratner

    Thread Starter New Member

    Jul 30, 2012
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    EDIT: Wrote this before Strantor's response. Following-up

    Head ---> Explode.

    SO the MOSFET has to go after the load (ground side), not before?

    Are there any options that allow me to switch from the supply side (before the load) rather than after?

    Followup question: since there are only 3 pins, am I correct in assuming that the gate grounds out through the same ground the load uses? Is so, will this work with the PID controller 8VDC is on a completely different circuit than the 12VDC transformer supplying the load?

    Thanks!
     
  8. lordratner

    Thread Starter New Member

    Jul 30, 2012
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    Thanks for the info. It seems the MOSFET may not work for me.

    The controller is an AC powered PID that has an internal SSR that outputs 8VDC from a + and - terminal on the back. The pump (item to be controlled, with some other lights and an SSR. Probably no more than 15 watts peak) is powered by a 12VDC transformer I ripped out of an old wall wart. The transformer and PID are powered from the same 120 VAC source, however the 12 VDC circuit and 8 VDC circuit from the PID are entirely separate. Their only connection is the 120VAC source.

    I need a relay-like ability to send 12VDC power to the pump/lights/SSR when the 8VDC circuit is live from the controller. I say relay-like because I dont know/care what the component ends up being, I just need it to be silent.

    It will cycle on and off at a rate no greater than 2 seconds per on/off cycle.

    I'd like it to be on the supply side of the 12 VDC, since it will be replacing a noisy relay on that side, and the wiring changes will be minimal. Thanks!
     
  9. tracecom

    AAC Fanatic!

    Apr 16, 2010
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    If the 8 VDC and 12 VDC can have a common ground, I think a MOSFET will work as shown in the attachment. The circuit works for me, but you be the judge for your application.
     
    Last edited: Jul 30, 2012
  10. lordratner

    Thread Starter New Member

    Jul 30, 2012
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    Ok, so this is where my ignorance come out in full force.

    Can I ground the 8VDC from one power source to the ground for a separate 12VDC source?

    Or do I just send all the various DC currents to a single terminal strip with both negative lines from the power supplies connected to it and the electrons will just know where to go?

    Or do they make some sort of 4 pin variant where the gate input has its own isolated output?

    Or is there a DC-DC version of this: http://www.amazon.com/Omron-G3MC-101PL-Thin-Profile-Phototriac-Isolation/dp/B005T8790C
    so I can just throw 5 dollars at it and make it go away?

    I wish I knew what I was doing, lol, but I really appreciate you taking the time to help me.
     
  11. strantor

    AAC Fanatic!

    Oct 3, 2010
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    the thing you linked to is for AC only. If you apply DC to it, it will turn on but never turn off.
    I looked for a photodiode optocoupler for you, but I can't find one that you could get away without soldering.

    You *should* be able to connect the grounds of the 8v and the 12V supplies. Just to make sure though, with your setup all ready to go, (step1) measure voltage from the ground of your 12V supply to the +8v of your 8V supply and (step2) measure voltage from the ground of you 8V supply to the +12V. If you measure 8V in step 1 and 12V in step 2 then your grounds are already connected. if you measure no voltage in both steps then both your supplies are isolated and it *should* be ok to connect them. If you measure something other than 8V & 12V then it's not ok to connect them.
     
  12. strantor

    AAC Fanatic!

    Oct 3, 2010
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    hang on, I just looked at your pic again; are you using a 120VAC/12VDC converter to get the +12V? (looks like a small power supply in the bottom left)
    or are you using a battery?
     
  13. lordratner

    Thread Starter New Member

    Jul 30, 2012
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    I'll take a look at measuring the voltages.

    It's a converter ripped out of a wall wart. Powered by the wall socket. The Controller that produces the 8VDC is plugged into the same wall socket.

    I can solder if need be, but I'd prefer to do it on something around the size of a sugar cube or larger, because I am not good at it.

    Who ever heard of wishing electronics were bigger?
     
  14. strantor

    AAC Fanatic!

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    Well then in that case why don't you just use the ssr that you've already got, to switch the ac side of the wall wart instead of the dc side?
     
  15. lordratner

    Thread Starter New Member

    Jul 30, 2012
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    I would, but the 12VDC powers lights and the pump which are sometimes controlled by the 8VDC controller (which is primarily running a heating element), and other times switched on independent of the heater.

    Thanks though. So a MOSFET or photodiode optocoupler?Either of those come in a four pin variant to keep the circuits separate?
     
  16. praondevou

    AAC Fanatic!

    Jul 9, 2011
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    Your "not soldering" requirement makes it more difficult to find a cheap solution. There are plenty of SMD SSR's on digikey. $3 to $4 for 1.2A.
     
  17. lordratner

    Thread Starter New Member

    Jul 30, 2012
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    I can't avoid it, so I'll solder. Just search for SMD SSR on there?

    I found a bunch with the output type of AC/DC so I assume that is what I want. It looks like most of them have input voltages of 1.2 ish, so will I need to put a resistor between the 8VDC and the SSR to drop the voltage to 1.2?

    http://www.digikey.com/product-detail/en/G3VM-41GR7/G3VM-41GR7-ND/2755204

    That one seems to be what I'm looking for. Anything I'm missing?

    You guys have been awesome. This will be the most over engineered Sous Vide control box ever!

    Seth
     
  18. praondevou

    AAC Fanatic!

    Jul 9, 2011
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    The one you picked doesn't have the necessary current rating. Try this one: http://www.futureelectronics.com/en...relays/Pages/3048428-ASSR-1611-301E.aspx?IM=0

    There a lot more options to choose from. For most of them a 680Ohm resistor in series with the input should be ok. (It limits the current through the LED)
     
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  19. lordratner

    Thread Starter New Member

    Jul 30, 2012
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    Thanks, but that one is out of stock. The one I listed said it was good up to 1A, correct? The 12VDC transformer I'm using only goes to an amp, so shouldn't that work, or do you want a buffer?

    This one is good to 1.2A and in stock. Will it work?:

    http://www.digikey.com/product-detail/en/CPC1020NTR/CLA318CT-ND/2004615

    It has a 1.2 VDC input, so the same 6800Ohm resistor like this one?:

    http://www.digikey.com/scripts/DKSearch/dksus.dll?Detail&itemSeq=117784709&uq=634793608725868824

    Thanks!
     
  20. praondevou

    AAC Fanatic!

    Jul 9, 2011
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    Sorry I didn`t see it was out of stock. The one you posted before had a continuous current rating of 120mA at 25C.

    The CLA318 should do it, put a resistor value between 470 and 1,2k in series.

    I would only use it if your load current goes "occasionally" up to 1A. The maximum current rating is good for 25C, it will be less at higher package temperature (0,58 A at 85C)
     
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