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Solenoid Valve Driver Circuit

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anthony1

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Hello,
I am "trying" to design a PCB to drive 8 solenoid valves or have 8 actuator i/p's using a ARM9 based MCU. The I/O lines of the MCU will of course be used to activate the solenoid valve, the MCU supply is 3.3V and the Solenoid valves are either 24VDC or 24VAC (haven't made a decision on that) of course the first thing that come to mind is to use a opto-isolator to trigger the 24V off the 3.3 V o/p of the I/O line.
Here is my problem I would like to use the I/O line as either a 24V supply for the solenoid in which case the MCU line will be an o/p BUT I would also like to use the same line to connect an actuator switch in which case the MCU line would be configured as an input to detect a switch closure. As far as I know optoisolators are uni-directional (have seen a bi directional optoisolator but it has only one channel I would like to have at least 8 such bidirectional I/O ports configured so I would like to have 8 channels or at a minimum 4 per chip so that the board hardware doesn't go out of control and not waste I/O lines on the MCU). Might be a really easy solution out there and I should be ashamed to call myself a EE guy for not knowing it, but if someone could suggest a solution other than the optoisolator I would appreciate it.
Thanks......
 
Hi Anthony,

many MCU tolerate I/O inputs higher than their supply voltage, but they won't tolerate an input of 24VDC. Further the solenoid valves normally require an activation current between 40 and 100mA, which the MCU is not able to supply even on one output pin.

The easiest solution would be connecting the gate-pin of an N-Channel MosFet via a low value resistor (100 to 300Ω) to the output pin of the MCU, connect the 24VDC with their positive terminal to +24V and their negative terminal to the drain pin of the MosFet. The source of the MosFet will be connected to GND.

To make it safer for the MCU in any case you might use opto-isolators as well. Use the input pins to connect to the switch indicator and use a LED in parallel to the solenoid to indicate if the solenoid is really activated.

I don't like the idea of using AC-supplied solenoids. They are wound bifilar and if one side burns up, the resistance halves overstressing the driver not activating the valve, but have it just humming. If the driver happens to be an opto-triac the AC will fire up the MCU. (experienced with an MCU controlled fan controller in countless numbers of fried Z80s.)

Boncuk
 
MCU inputs can be protected from higher voltages by 1MΩ resistors.

If you want AC then you could use an opto isolator plus a triac.

If you don't need isolation then you could drive the triac directly but be aware that the 0V rail needs to be the same as the AC neutral.
 
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MCU inputs can be protected from higher voltages by 1MΩ resistors

Hi Hero,

those were no inputs, but outputs controlling MOC3021 opto-isolators switching solenoids at 230VAC and 80mA. With every died solenoid also the Z80 passed away.

You also didn't specify the "higher voltages" on the inputs.

May be you want to suggest improvement at Woods of Colchester Ltd. for the MPU2. (if they have survived)

Hans
 
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