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Thread: Teensy 3.6 LED dims with solenoid operation

  1. #1

    Teensy 3.6 LED dims with solenoid operation

    I have a Teensy 3.6 plugged into a custom PCB. One of the functions of the Teensy is to drive a relay to control a solenoid valve. The solenoid is powered from a 24V supply with an isolated ground (24V ground is separate from the Teensy ground), and the relay is a solid-state optoelectronic relay. There's a TVS diode to combat any inductive kickback.
    Here's a diagram of that part of the board:
    Click image for larger version. 

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    Relay documentation
    Diode documentation
    The code for the whole project is complex, but for controlling the solenoid it's just Serial.write(ioPort, HIGH) to turn it on and Serial.write(ioPort, LOW) to turn it off.

    The problem: There are several of these boxes, each with its own Teensy and solenoid, and all seem to operate the same except one. On that one, when the solenoid is powered (digital out from the Teensy pin set to HIGH), the LED on the Teensy dims considerably and flickers. It dims maybe half a second after the signal is started, stays dimmed while the solenoid is operating, and returns to normal when it's done. The Teensy seems to operate normally otherwise, but the difference in behavior caught my eye.

    Since this problem only happens with one setup, I'm guessing some component has failed or is faulty, but I'd like to be sure it's not exacerbated by the design. Can anyone tell what the issue might be? Bad relay, bad diode? Should the grounds not be isolated?
    One other thing I just thought of: should there be a resistor (even 10 ohm) between the Teensy pin and Teensy GND? Or is a direct connection (through the relay) acceptable?

    Thanks!

  2. #2
    Just a quick clarification - the Teensy is on USB power, and there is also a 12V power supply in the circuit, but none of the 12V signals go to the Teensy (although they have a common ground).

  3. #3
    Senior Member vjmuzik's Avatar
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    Isolated ground is fine, that’s the purpose of these types of chips, being that it’s electrically isolated nothing on the other side of this chip can affect the Teensy. The only issue I can tell so far is that you don’t have a current limiting resistor between the Teensy pin and the relay led which may have had an inverse effect on the Teensy if it tried to draw too much current to the LED.

  4. #4
    I would use a scope and check the 5v supply on the USB to the Teensy. Check if it drops below 5v when you energise the solenoid.

    Also the depends on the size of the solenoid, your SSR might not be able to handle the surge current. The rated current is 2A with peak load of 3.6A. Your solenoid might excess that.

  5. #5
    Quote Originally Posted by vjmuzik View Post
    Isolated ground is fine, thatís the purpose of these types of chips, being that itís electrically isolated nothing on the other side of this chip can affect the Teensy. The only issue I can tell so far is that you donít have a current limiting resistor between the Teensy pin and the relay led which may have had an inverse effect on the Teensy if it tried to draw too much current to the LED.
    Yeah, I completely blanked on the need for a current limiting resistor. I'll add a 370 ohm resistor (If = 10 mA, Vf=1.3V).

    Quote Originally Posted by skpang View Post
    I would use a scope and check the 5v supply on the USB to the Teensy. Check if it drops below 5v when you energise the solenoid.

    Also the depends on the size of the solenoid, your SSR might not be able to handle the surge current. The rated current is 2A with peak load of 3.6A. Your solenoid might excess that.
    The solenoid I'm using is rated as using 1.15 W at 24V, which works out to much less than 1 A (47.9 mA). My understanding was the diode would handle the surge current, and I picked that relay as handling much larger than the normal operation current (just in case). The Teensy is powered directly from a PC, and since this issue only seems to affect one of the attached Teensys, I figured it was an isolated issue.

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