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Where to place the relay- SSR. — Parallax Forums

Where to place the relay- SSR.

SiriSiri Posts: 220
edited 2009-11-27 21:26 in General Discussion
I am working on a project where I need to control the number of strokes of a air pump.Initially I was gong to use a solenoid pump but changing the frequency of the stroke was not possible.

Now I am planning using a DC motor driven pump. 12VDC @ 10Amps.

I am considering usinga DC motor regulator with PWM which can handle 12- 60VDC@ 20Amps.This regulator let me set the DC motor· from 0- 100% duty cycle/PWM ·and thereby varying the frequecy of the air pump.
To run this I have a dc power supply 12-60V @ 20 amps.

I also want to cycle the pump on/off about every 30sec.This timing derived from a prop via a SSR.Which I have it working well.The SSR is capable of handling 120VAC 5-10 amps and also 10-60VDC @ 15Amps max.

Now the question is Do I : 1.Use the SSR to on/off the 110V AC· ---> to Power Supply
··································· 2.Use the SSR· to on/off the dc· power supply ---->to the motor regulator
··································· 3.Use the SSR to on/off the the reulated poer to the dc motor itself.

·· mains AC ---> SSR·(case - 1)-->dc power supply ---> SSR (case- 2) --> motor regulator ---> SSR(case-3) -- > dc motor.

I looks like any of these positions would work but what would be the best placement.

Thank you.

Siri

·

Comments

  • whickerwhicker Posts: 749
    edited 2009-11-27 18:01
    1. Absolutely not. unless you can be sure it won't be a steady on 30 sec. off 30 sec. on 30 sec. etc.
    2. fine
    3. fine

    however, if you already have something that accepts PWM, why not just leave the PWM percentage at 0 for those times.

    I guess I'm not completely understanding:
    1) are you checking an air pressure sensor every 30 seconds to see if the motor should be on or off?
    2) are you doing some cyclic thing every time every 30 seconds?


    (added[noparse]:)[/noparse]

    If the point of the SSR is power savings, waiting something like 10-45 mins of inactivity and cutting the 120V per #1 would be fine. When starting back up just have the PWM set to zero, power up the SSR, wait a few seconds (some power supplies even have a "power good" output, still would wait a second after that even), then set the PWM to whatever the feedback system wants.

    The point is to avoid making confetti out of your electrolytic caps in the power supply.

    Post Edited (whicker) : 11/27/2009 6:26:40 PM GMT
  • SiriSiri Posts: 220
    edited 2009-11-27 18:39
    Whicker,
    What I am trying to do is to start the air pump during the inspiratory phase and off during the expiratory phase.But at the same time I don't want to pump too much air either so that is why I would like to have some control over the pumps strike rate etc.

    Thanks,

    Siri

    P.S - I have many bulit in safety valves to keep the pressures at acceptable levels.
  • kwinnkwinn Posts: 8,697
    edited 2009-11-27 19:58
    Siri, the more complicated you make something the more there is to go wrong. Why not simplify this by having a fixed power supply that provides the DC motor/PWM circuit with the maximum voltage the motor is rated for and use the PWM circuit to control the speed of the motor. At 0% there is no power to the motor so you do not need to turn the AC power on and off. The prop could handle this with ease.

    PS- An aquarium air pump can be modified to be driven by a small DC motor for pumping air or liquids at low pressures.
  • SiriSiri Posts: 220
    edited 2009-11-27 21:26
    Kwinn,

    Thanks

    I will look for a suitable PWM circuit that can be controlled by the prop.

    Siri
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