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Recommend an SSR for this motor? — Parallax Forums

Recommend an SSR for this motor?

T ChapT Chap Posts: 4,223
edited 2013-09-18 14:59 in General Discussion
I need to rig up a 12VDC car lock motor for a gadget that must be extremely reliable. It will get used possibly anywhere from 10-30 times a day in short 1 second duration for many years to come. I have ruled out a relay since the arcing may degrade the contacts over time. I also prefer to just rig up something from off the shelf parts versus designing/making a board with a traditional hbridge high side and low driver driver/mosfet design. The plan is to drive the switch from a Propeller, one pin will turn on the 12VDC and GND for the motor to turn one direction, and another to reverse the motor. I was thinking about using SSRs for both high side and low side, but wanted to get your opinions on a robust way to do this. I believe the motor is pulling less than 1 amp.

This is an idea, but I would have to bump up the Prop output to turn it on:

http://www.skycraftsurplus.com/relaysolidstatedccontact.aspx?gclid=CLiHhMTa07kCFenm7Aod0XkAbg

or this will drive at 3 volts, assuming a single pin will provide the current for both, I need to check.

http://www.jameco.com/1/1/730-kf0604d-dc-dc-solid-state-relay-control-voltage-3-32vdc.html
ssr motor.JPG
478 x 640 - 99K

Comments

  • phatallicaphatallica Posts: 64
    edited 2013-09-17 19:05
    For the price of one of those SSRs (wild overkill), you can get an L298N board that has 2 x H-bridges. Other folks already did the design/build.
    3.3V logic is ok (VIH = 2.3V).
    Repetitive pulse max = 2.5A = much more than you need.
  • T ChapT Chap Posts: 4,223
    edited 2013-09-17 19:08
    Good call. Thanks for the info.
  • Duane DegnDuane Degn Posts: 10,588
    edited 2013-09-17 21:12
    I like these red L298N boards more than the green version.

    Whichever version you get make sure and order a couple extra. I often have to redo some of the solder joints on these boards and more than a few haven't worked.

    The L298N will lower your voltage by about 2V so make sure the lower voltage is okay.

    I know a lot of people dislike the L298N for motor control. I suggest using a low PWM frequency if you do use PWM (I think I'm using 100Hz on my robots).

    I haven't had any problem with driving the L298N with the Propeller's 3.3V logic which as already mentioned is above the required 2.3V.
  • T ChapT Chap Posts: 4,223
    edited 2013-09-18 07:15
    http://www.radioshack.com/product/index.jsp?productId=13351656#

    This looks like the same thing basically, L298N available a mile away at the shack.
  • Mike CookMike Cook Posts: 829
    edited 2013-09-18 07:38
    This one might save a few bucks, its on clearance, Arduino version: http://www.radioshack.com/product/index.jsp?productId=12333768#tabsetBasic
  • T ChapT Chap Posts: 4,223
    edited 2013-09-18 08:49
    I just picked up the version I linked as that was the only one they had.

    This is a first experiment with the L298. Can anyone suggest if it is possible to output some method of detection of measuring a higher current when the door lock motor has reached it's limit and stalled? It would be ideal if there was a method of feedback to check if the motor had reached the fully extended mode so that the Prop code can know that the lock had moved the gadget out of the way. I see some current sensing outputs, so maybe there is a way to rig up something to detect the higher current when the motor stalls.
  • Duane DegnDuane Degn Posts: 10,588
    edited 2013-09-18 08:57
    The L298 does have two "sense" pins (#1 & #15). I haven't used them myself. They usually connected to ground when not being used to sense current.

    To use them to sense current, I believe you need some low resistance high power resistors (not wire wound) so you can use an ADC to monitor the voltage drop across the resistors.

    SparkFun has a datasheet on its L298N page.
  • Duane DegnDuane Degn Posts: 10,588
    edited 2013-09-18 09:00
    Canakit sells a board with sense resistors on it.
  • T ChapT Chap Posts: 4,223
    edited 2013-09-18 09:30
    I may just try to hack this board. I could add a .1 sense resistor on it's output to GND, put a sensing amp across it, output the sense voltage to a comparator and set the threshold with a trim pot. Output the comparator to the Prop to monitor for the stall. The other alternative is to find some reliable method to check if the motor has reached it's destination. A push button switch would be questionable over many years, and it would not be easy to reach to replace. Whereas the sensing could be done on the driver board which is accessible. I hope is that I can use the motor stall as the detection of "move complete".
  • Duane DegnDuane Degn Posts: 10,588
    edited 2013-09-18 09:42
    T Chap wrote: »
    The other alternative is to find some reliable method to check if the motor has reached it's destination. A push button switch would be questionable over many years, and it would not be easy to reach to replace.

    What about some non-moving sensor? A magnet and Hall-effect sensor? Optical?
  • T ChapT Chap Posts: 4,223
    edited 2013-09-18 14:22
    Do you have an easy to hack together magnetic idea or ir?
  • Duane DegnDuane Degn Posts: 10,588
    edited 2013-09-18 14:32
    T Chap wrote: »
    Do you have an easy to hack together magnetic idea or ir?

    I think I've played with a Hall-effect sensor I purchased from Parallax once but I haven't done much with it.

    I thought this part looked interesting.

    I wonder if one of those photo interrupt sensors would work?
  • T ChapT Chap Posts: 4,223
    edited 2013-09-18 14:59
    That LED sensor looks perfect. Thanks,
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