Shop OBEX P1 Docs P2 Docs Learn Events
I swear my head is going to explode — Parallax Forums

I swear my head is going to explode

c131frdavec131frdave Posts: 38
edited 2007-11-13 22:38 in BASIC Stamp
Hello again. Newbie here.

In my previous post I explained that I am teaching electronics to myself, and I need some help again.

I want to turn on a DC motor.· Moreover, I want to be able to turn it on with the BS to run in either direction.

So, I purchased two Crydom DM0063 SSR's, and they both test out perfectly.· When I send a high singnal to the coil, I get zero resistance in load circuit, and when it is low, I get close to infiniate resistance (I know, there's no such thing as infinate, but you know what I mean).

So they work for DC current- sweet.

Now comes the hard part for me- it's probly super simple for you all.· I wired it up and the motor immediatly spun in one direction only.· I fired up the Fluke, and it is reading either 11 vDC or 12VDC depending on which relay is on- but no negative.· The way I had it wired is below:

cicuit.jpg


After review, obviously this won't work- and it explains why I was getting positive voltage regardless on the fluke (I put the fluke betweeen the motor and the relays.· Obviously you can not·connect the motor directly to the battery and expect the relays to change polarity.

So I'm trying to draw the circuit out on a piece of paper, and I'm completely lost!· I just can't wrap my brain around this.· No matter how I draw it, I end up with a short circuit.· So obviously some diodes will have to be used, but even then I'm not sure how to do it.· Can anyone point to a schematic that can help me out here?· I'm going to keep working on it and hopefully will figure this out, but it would be nice to hear from some of you.

Thanks. smile.gif
·

Comments

  • c131frdavec131frdave Posts: 38
    edited 2007-11-13 20:35
    Oops, wrong forum. Can somebody move this to Basic Stamp please?

    Told you I was new... [noparse]:([/noparse]
  • Mike GreenMike Green Posts: 23,101
    edited 2007-11-13 20:37
    Do a websearch on "wiki H-bridge".
    To turn a motor on and off, almost any kind of switch will do like your SSR or a power transistor or a simple relay.
    To be able to reverse a motor too, you need to use a DPDT relay (and something to turn the power on and off) or
    an H-bridge.
  • c131frdavec131frdave Posts: 38
    edited 2007-11-13 20:56
    Thank you. That makes sense- unfortunately.

    Now for the DPDT relay. They sell this at the local Radio Shack-

    http://www.radioshack.com/product/index.jsp?productId=2049720&cp=&sr=1&origkw=dpdt+relay&kw=dpdt+relay&parentPage=search

    Will this work? It says it has a 12VDC coil, but the BS only puts out 5. They guy at Radio Shack says it will, but he's just a kid. If not, what will? What do I need to look for?
  • Mike GreenMike Green Posts: 23,101
    edited 2007-11-13 21:22
    The 12V coil relay probably won't work. It needs a certain amount of current to pull in the relay contacts and, at 5V, the coil resistance is too high for enough current to flow (try Ohm's law on the coil resistance and 12V/5V and see how the current compares to the spec).

    You need a relay with a 5V coil or you need a source of 12V and a switching transistor to allow the Stamp to turn on the relay without being damaged by the excess voltage and current. The Stamp Manual, Stamps in Class tutorial and the StampWorks tutorial all give examples of using a switching transistor to control a relay or small motor. You could even use your SSR to control the 12V relay (running off of 12V).
  • DgswanerDgswaner Posts: 795
    edited 2007-11-13 21:42
    I'm currently driving my motors with relays, they are 120V 5A relays with a 12V or 9V coil. I'm driving the relays with a ULN2003 (I think it's obsolete, and a ULN2008 is the replacement) and my stamp drives the uln2003 transistor array. It's totally possible to get direction control forward reverse left right etc. with 4 SPST relays . the wiring is a little complex. I can provide you with a schematic if you want. HOWEVER! the price you pay for relays will almost pay for an average H-bridge circuit (if you build it). the H-bridge will give you the option of speed control and PWM. where relays won't. in the end you'll be happier with a H-bridge setup.


    Added the blue text

    ▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
    A complex design is the sign of an inferior designer. - Jamie Hyneman, Myth Buster

    Post Edited (Dgswaner) : 11/13/2007 10:58:18 PM GMT
  • Paul BakerPaul Baker Posts: 6,351
    edited 2007-11-13 22:38
    This thread is being moved to the Basic Stamp forum

    ▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
    Paul Baker
    Propeller Applications Engineer

    Parallax, Inc.
Sign In or Register to comment.