Relay question- D2W203F

c131frdave

I just recieved two shiney new D2W203F solid state relays from Parallax today, but they aren't working- I think.

I'm using them to turn on and off a DC motor, but when I wrote a very simple program to test it, the motor was on all the time.

I have the coil connections ground to ground(vss), and positive to 220 ohm resitor to pin 7.

The program is:

Do

High 7

pause 2000

Low 7

Pause 5000

Loop


When I tested it, the motor just ran, so I disconnected the motor and hooked up my Fluke to test resistance.· It basically just goes from 12 mohms to 16 mohms in a loop.

So does that mean this relay doesn't fully open?· I noticed the relay says 3A 240 VAC on it.· Don't tell me I got a relay that won't work on DC.·

I need a solid state relay that will switch on and off 12DC.· Did I get the wrong stuff?· If so, what's the right stuff?

Thanks.

Phil Pilgrim (PhiPi)

Solid state relays are used for AC. They incorporate a triac, which can only turn off at zero crossings on the AC line. So if your source is DC, it can never turn off.

You need a DC output device like the Crydom CMX Series, which DigiKey carries.

-Phil

Post Edited (Phil Pilgrim (PhiPi)) : 11/9/2007 6:47:13 PM GMT

Mike Green

You have a solid state relay that's designed only for switching AC. It uses a triac for switching which, when triggered, stays on as long as there's power applied (which is what you're seeing). With AC, the power goes off 120 times a second (when the AC waveform goes to zero volts). There are solid state relays for DC which use an MOSFET for switching. You'll need to look at a catalog like Jameco's or Mouser's or Digikey's for DC solid state relays.

Easier would just be to use an MOSFET directly. A logic level MOSFET like the IRL511 will work. You can also use a conventional junction power transistor. There are examples in the Stampworks manual and in the What's a Microcontroller? tutorial shown switching a relay. The same idea works for DC motors. You need a reverse polarity diode across the motor and, for a junction transistor, you need a series resistor in the base lead that will limit the base current to something like 20mA (like 220 ohms).

c131frdave

I was afraid of that.

First, anyone wat to buy two brand new SSR's?

Second, what about this one:

http://www.crydom.com/products/productFamily.aspx?id=141

Phil Pilgrim (PhiPi)

Yes, as my addendum above indicates, the CMX series would work and will plug into the same socket as the ones you bought.

-Phil

c131frdave

Mike Green said...

Easier would just be to use an MOSFET directly. A logic level MOSFET like the IRL511 will work. You can also use a conventional junction power transistor. There are examples in the Stampworks manual and in the What's a Microcontroller? tutorial shown switching a relay. The same idea works for DC motors. You need a reverse polarity diode across the motor and, for a junction transistor, you need a series resistor in the base lead that will limit the base current to something like 20mA (like 220 ohms).

Sorry, that made my head spin.· I have the What's a Microcontroller book in my hand, but don't see anything in it about switching on and off DC.· Can you point me to a specific example so I can read?

Also, I need to reverse direction of the motor for a split second (for a brake) which is why I purchased two relays.· Can I do that with what you are describing?

Phil Pilgrim (PhiPi)

You can brake a motor by opening the relay that powers it and shorting the motor leads through an appropriately sized resistor with another relay. You don't need to run it in reverse. But make sure you don't activate both relays at once! Doing so will short your motor's power supply. A safer option would be to use a SPDT dry-contact relay that toggles the motor "high" lead between power and a resistor to ground.

-Phil

c131frdave

Ok. thanks guys. that helps a lot, though I can't stop crying. I waited 5 days for delivery of these AC relays and now I'll have to wait another 5 days.. Plus the CMX relays are bloody expensive. I'd try the MOSFET IRL511 SDFGEERRGSAFD thing, but I have no idea what that is. Relays seem to be pretty simple. If I had a clue how a transitor worked, I'd do that, but alas, I am a super noob. [noparse]:([/noparse]