6 Solenoid Valve Project

Using Arduino Project Guidance
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Hello,

I originally graduated with an EE degree, but have since been removed from these kind of projects. I am trying to get back into it, and start up my new hobby. I have decided to do a project that will control 6 solenoid valves (2 at a time maximum). I have done some reading on what my circuitry should look like, and have copied what seems to be the inductive load circuit norm for my circuit.

Attached you will find a scan of my first pass at the circuit. I was hoping to gain some insight from others on if it looks like a decent design and if there are any other things that I should be mindful of.

Also, I am not sure about the resistor value and the NMOS model that I have chosen for the circuits. From what I can tell, a simple NPN BJT will not work for me, so I will need a MOSFET or a Darlington. I went with the MOSFET and basically just went with something that looked decent from TIs website. It is hard to really choose a resistor without knowing what the resistance is looking into the gate. I remember doing equations for that in college, but can not recall the formula to use to calculate the resistance. Any thoughts?

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Should I just follow the pdf above instead? Thus I would have 6 of the same circuits feeding from the same 12 volt 2 A power supply.

So I would have 6 of each component (TIP 102, 1N4004, and 1000 Ohm Resistor) and two separate power supplies (one for the arduino and one for the solenoid circuit)

I will only operate two solenoids at one time via the DO pins from the arduino. The power supply should only supply 1A normally except for when first activating solenoids. There will be a inrush current when first operating the solenoids. Anyone know how what that current will be?

Also, if not using the MOSFET then am I correct by assuming a ~.7V drop from the base to the emitter * 2 for the two BJTs? Therefore, (5-1.4)/1000 =~ 4mA for the base current coming from the arduino.

Are there any issues with using a 1N4004 diode with a breakdown current of 1A considering the power supply can supply 2A?

Anything else I am missing?

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Hi, The bipolar transistor approach should be fine, even if the FETs are more 'elegant'..

The 1N4004 would be fine (guideline for reverse diodes is current rating >= to the coil current).

I do not think there's an inrush current for a plain solenoid. In fact, the inductance should result in a more slowly rising current.

How fast do you need to turn these solenoids on and off?

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Thanks for the help.

Hmmmm I just assumed that it would have an inrush current, because most inductive motors that I have experience with have inrush currents.

I think I will go with the BJT approach, but might do the FET approach later to gain additional knowledge.

No real need or specification for how quickly the solenoids turn on and off.

Is my assumption of the base current correct?

If I was to use the FET, do you know the equations for the resistance of the gate?

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Hmmmm I just assumed that it would have an inrush current, because most inductive motors that I have experience with have inrush currents.

That is because a motor until turning at a certain RPM is not generating enough counter EMF to oppose current draw. A simple solenoid is not a motor but rather just a simple inductor and if you remember your old
"ELI the ICE man" crutch, you will recall that voltage leads current in an inductive circuit.

Lefty

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Is my assumption of the base current correct?

1000 ohms is probably OK; check the collector to ground (transistor saturation) voltage with a solenoid on. It should be .5 v or less, ideally.

You can go down to 220 ohms or so for a base resistor...

Here's an easy way to do FET power switches: http://arduino-direct.com/sunshop/index.php?l=product_detail&p=60

DISCLAIMER: Mentioned stuff from my own shop...

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