Met problems when converting a reed organ to be MIDI controlled

  1. #1 by Ada May on 11-16-2017
    Ada May's Avatar
    Posts: 1

    Met problems when converting a reed organ to be MIDI controlled

    I'm converting a reed organ to be MIDI controlled, and I'm using solenoids to pull down on the keys. There's 88 solenoids total, and I'm going to be powering it with 2x 40A 12V power supplies.

    The solenoid control circuit I'm using is this:
    U9QI5.png

    My problem is that each solenoid draws around 1.3A @ 12V. This results in very hot solenoids after less than a minute of being held down.

    There are clever solenoid driver circuits that use a capacitor(http://www.kynix.com/Product/36.html) to give the solenoid the full power at first, then once the capacitor is charged (therefore no longer conducting DC power), power flows through a resistor that limits the current. However, this system needs time for the capacitor to discharge before being triggered again, and the capacitor has to be of a large value which would not work in this application well (my solenoid driver board design is dense enough at the moment... I want my component count to stay as low as possible). There are more complex circuits that get around the capacitor discharge problem using transistors and even more components (how they work is besides the point here) but again, I don't want many more components since every component gets multiplied by 88 circuits.

    However, I think I have an idea for a solution, but I don't know if it would work. My idea is to PWM the raw power going into (all of) the existing circuits and then adding a single capacitor in parallel with my solenoid, like this:

    HY7Bo.png

    Thus, when the MOSFET was triggered, this capacitor would get charged to 12V before discharging into the solenoid, making it fire. Now that the solenoid is down, the PWM power (lets say its 50% duty cycle, therefore equal to 6V) can keep it held down.

    Is this idea completely flawed? If it is, any ideas? Thanks.
  2. #2 by Scott Thompson on 6 Days Ago
    Scott Thompson's Avatar
    Posts: 1
    - Ada May wrote View Post
    I'm converting a reed organ to be MIDI controlled, and I'm using solenoids to pull down on the keys. There's 88 solenoids total, and I'm going to be powering it with 2x 40A 12V power supplies.

    The solenoid control circuit I'm using is this:
    U9QI5.png

    My problem is that each solenoid draws around 1.3A @ 12V. This results in very hot solenoids after less than a minute of being held down.

    There are clever solenoid driver circuits that use a capacitor(http://www.kynix.com/Product/36.html) to give the solenoid the full power at first, then once the capacitor is charged (therefore no longer conducting DC power), power flows through a resistor that limits the current. However, this system needs time for the capacitor to discharge before being triggered again, and the capacitor has to be of a large value which would not work in this application well (my solenoid driver board design is dense enough at the moment... I want my component count to stay as low as possible). There are more complex circuits that get around the capacitor discharge problem using transistors and even more components (how they work is besides the point here) but again, I don't want many more components since every component gets multiplied by 88 circuits.

    However, I think I have an idea for a solution, but I don't know if it would work. My idea is to PWM the raw power going into (all of) the existing circuits and then adding a single capacitor in parallel with my solenoid, like this:

    HY7Bo.png

    Thus, when the MOSFET was triggered, this capacitor would get charged to 12V before discharging into the solenoid, making it fire. Now that the solenoid is down, the PWM power (lets say its 50% duty cycle, therefore equal to 6V) can keep it held down.

    Is this idea completely flawed? If it is, any ideas? Thanks.
    Hello! What an exciting project. I'm really impressed at your thoughts into solving this problem.

    My name is Scott Thompson and I'm a Systems Engineer here at TC Helicon. I'm new to the forum so can't view your images (yet) but wanted you to know that generally you're on the right track--to give you some peace of mind.

    Solenoids, like relays, will take more initial current to "pull-in" and then will have a certain amount of current required to hold them in place (typically less than the pull-in amount). So, coming up with some scheme like capacitors to supply an extra amount of current to enable the solenoid to pull-in, or even PWM, *should* work.

    The PWM approach could work by applying a 100% duty cycle to the solenoid, thereby enabling the high current needed to pull-in the unit (or push, depending on your configuration). Then, you can switch the duty cycle to < 100% in order to limit the total amount of current going into the solenoid (the average current will be Imax * Duty, so you can calculate the duty cycle by finding the maximum amount of holding current needed).

    Unfortunately, this would require 88 channels of PWM if I understand your post correctly.

    We can get clever, though, and PWM all of the solenoids with the < 100% duty cycle pulse needed to hold the solenoids. For those contacts that needed to be pressed, a wired-OR of the PWM input to the solenoid channel with a voltage of sufficient duration to enable the solenoid would override that individual PWM channel with a 100% duty cycle pulse. Once the "extra" boost pulse was removed, the solenoid would return to < 100% duty cycle.

    The caveat with PWM is that if the period is too long (frequency is too low or in the audio band) then you will hear screaming from the solenoids.

    Good luck--you're on the right track.

    --Scott