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Solenoids

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sgpope

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First, I'm REALLY new to electronics. I'm building a project that will run 6 solenoids (only 1 operating at a time). I'll be basing my schematic off the attached diagram, but my solenoids are 12v and have a maximum current of 450mA. The transistors I've got are PNP type (2N3906), rated at 350mW. Would these transistors be sufficient under these conditions? Also, are the ratings on the resistors in the diagram sufficient for my needs? How much leeway is there in the resistance values? For example, I've got 100k, 10k, 1k, and 330 ohm resistors. Would any combination of these be sufficient for what I'm doing?
 

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A few things. You mention a 2N3906 which is a PNP type transistor but only rated at a max current of about 200 mA. Not good! Also in your circuit the transistor is drawn as an NPN. The circuit would be fine as drawn using an NPN. I would use a common 2N2222 transistor (NPN) with your circuit.

Ron
 
Yes a 2N2222 is good for 800MA. You should look at the speck sheet to get your base resistor. Using combonashoins of your resistors you should get close enough to proto type.
 
You will need about 45mA of base current to fully saturate the 2N2222 and turn on the solenoid. I believe that is more current than the PIC can supply so you will likely need a Darlington transistor, or two transistor stages, or a logic-level type N-MOSFET to control the solenoid.
 
Whoops! During testing I've figured out that the solenoids are no good for what I'm attempting. The intent is to be able to pour liquid in a controlled amount from a 2 liter cola bottle. I was going to use the solenoid to control the flow, but there's not enough pressure from the bottle to push through the open solenoid. :( Is there a relatively easy way to add pressure? Otherwise, I'm going to have to change tactics. Perhaps have a servo tilt the bottle?
 
I'm thinking perhaps I should break this out into a new thread, since it's no longer about solenoid schematics.
 
We should be looking at the pressure rating. The one you have is rated for .02MPa-.8MPa. .02MPa = about 2.9 PSI to open but still may work. Off hand did you add a vent hole to the top of the bottle to brake the vacuum? Andy
 
Ya, If air can not get back in then no water will come out. To test just put a hole in the top of the bottle if it works you can plumb a vent between the bottle and the valve. Andy
 
I'm using a similar circuit, only with a IRF710PBF mosfet for the switch. I'm switching a solenoid at 28v (0.4A inrush, 0.2A hold). The mosfet controls the solenoid fine, but gets too hot to touch within 10 seconds or so. The mosfet is rated at 2A/~36W/40V, and I'm pretty sure I'm below all that.

One possible reason is that I'm not reaching saturation current on the gate, but the spec sheet for IRF710 doesn't list anything about that. How do I find the saturation current of the mosfet? If that's not the problem, how do I find out what's making it so hot?
 
One possible reason is that I'm not reaching saturation current on the gate, but the spec sheet for IRF710 doesn't list anything about that. How do I find the saturation current of the mosfet? If that's not the problem, how do I find out what's making it so hot?

FETs are a voltage device on the gate unlike a transistor. What is your gate voltage?

Ron
 
The gate voltage is about 5 volts.

I read about Rds, though I don't really understand it. The 710 spec sheet says the Rds is over 3 ohms. Would a lower Rds fet not get so hot?
 
Ya, If air can not get back in then no water will come out. To test just put a hole in the top of the bottle if it works you can plumb a vent between the bottle and the valve. Andy

Hmmm...hole in the bottle made no difference. :(
 
The gate voltage is about 5 volts.

I read about Rds, though I don't really understand it. The 710 spec sheet says the Rds is over 3 ohms. Would a lower Rds fet not get so hot?

This is your data sheet. Take a look at figures 8 & 9. The RDS of about 3.3Ω is with a gate voltage of 10 volts. With 5 volts you aren't close to saturation or using your FET as a switch. You need a minimum of about 10 to 12 volts on the gate.

If you need to run with a gate voltage of 5 volts what you want is a FET from the family of Logic Level FETs. You want a MOSFET from this family or similar logic level mosfet from another manufacturer. Lacking one of these you will need to get your 5 volt gate voltage up using an interface like a driver transistor.

That is why I believe you are getting so much heat. :)

Ron
 
Hmmm...hole in the bottle made no difference.
OK then try and pressurize it. Use a bottle without a hole, energize the solenoid and squeeze the bottle. See if that works. Andy
 
Ok, I've got it working with a single solenoid based on the schematic in the first post. Now I've got to scale it up to 6 of them (only 1 operating at a time). I've attached a new schematic (my first schematic!). Can somebody sanity-check it for me? SV is the solenoid valve (12v 450mA).

**broken link removed**

Apologies for the quality of the image. I'm working with paint here. :)

Edit: Note that 3 solenoids are shown, but you can infer how I'm planning to expand it.
 
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