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I'm building a circuit for my project. However I'm a bit insecure about whether it is correct. (I'm from software myself).

It is the following circuit:

schematic

simulate this circuit – Schematic created using CircuitLab

A couple of side notes:

  • there are 15 Solenoid assemblies (Solenoid, Mosfet, Diode) connected in parallel.
  • there are 3 servos all connected to one 5V regulator.

Since I'm not that experienced with circuitry, and neither are the people who will use this project after me, I'd like to make this circuit almost impossible to break.

So my questions are:

  • Did I use the right parts to use solenoids and servos?
  • Am I missing any parts that could make it more unbreakable?
  • I've added a capacitor, but how many Farads would I need?
  • When testing, I noticed that the MOSFET doesn't transmit all current, and it also takes a while before it is really switched off. Why is this?
  • The solenoid needs to press a button but it only has enough force when ~19V is applied. I find it inconvenient to use this 19V supply. Could this be done in a better (=safer) way?

Thanks in advance!

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  • \$\begingroup\$ Q4, D2 should not be there. R1 need not be there. Connect GPIO directly to M1 gate. Resistors are for more demanding (faster) applications. The diode would be wrongly connected anyway in its more demanding application (equalising on and off times, with an extra reisstor across it). Q5, use a servo to press the button? \$\endgroup\$
    – Neil_UK
    Jun 2, 2017 at 16:58
  • \$\begingroup\$ Thanks for the reply, I will remove D2. I just thought it wouldn't have any effect on the circuit apart from preventing current to accidentally going to the PI. (in case I connected something wrong). I would love to use servo's instead of solenoids ;) unfortunately the design needs to be very compact. \$\endgroup\$
    – A J
    Jun 2, 2017 at 17:14
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    \$\begingroup\$ At reset your GPIO line is likely set as an input, so a pulldown to keep the MOSFET off might be needed. \$\endgroup\$
    – stark
    Jun 2, 2017 at 17:30
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    \$\begingroup\$ Also, a MOSFET with floating gate (when D2 is driven down, the gate is essentially floating) is a recipe for disaster. This is why it "takes a while" to turn OFF, since the charged gate has no path to drain other than D2 leakage. More, during this transient the half-open MOSFET might experience substantial power dissipation and burn out. \$\endgroup\$
    – Ale..chenski
    Jun 2, 2017 at 18:26
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    \$\begingroup\$ "I noticed that the MOSFET doesn't transmit all current" - it may not be getting enough Gate voltage. What is its part number? Also what are the part numbers of the servos and regulator? Specifications matter! \$\endgroup\$
    – Bruce Abbott
    Jun 2, 2017 at 19:14

1 Answer 1

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As several commentors have stated, knowing the part numbers makes for more specific responses but here are some general thoughts:

D2 and R1 are probably not required.

Use a logic level gate type FET in your design or you will need a driver circuit between the uP and the gate of the FET. These types of FETs will also ensure a full turn on (low R DS ).

Place a pull down resistor from the gate of the FET to ground. This will help it turn off. The FET datasheet will give you guidelines for the value of the resistor. It is usually in the range of a few kohms.

The size of the filter cap depends on the impedance of your 19 volt supply and the load of the solenoids and the 5 volt load. You can determine the value experimentally. Perhaps start with 220 uF and take it from there.

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