Author Topic: Signal controlled valve switching circuit  (Read 762 times)

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Offline jbrookleyTopic starter

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Signal controlled valve switching circuit
« on: March 25, 2018, 04:24:31 am »
Hello everyone!

I'm currently trying to toggle a valve on and off, have a preliminary circuit but thought I'd bounce it off of you guys and see if you have a better approach. 

I have a 6.5W valve that operates at 24VDC (+/- 10%), so I'm expecting it to pull about 270 mA of current. 

I basically have a circuit that is controlled by a 5V microcontroller signal that I'm switching on and off using a transistor as a switch and a BUF634U (which is rated at 250 mA of current) to generate the necessary current.  I'm attaching the circuit I have. 

I have a few concerns:
1)  THe BUF634U is rated at 250 mA of current.  If my calculations are correct, I'll likely need about 270 mA of current.  I'm guessing I probably need to switch over to an OPA547, right?
2)  I'm using a BJT to be used as a switch.  Should I consider using a FET instead?  What are the advantages/disadvantages of each?  Is one more preferable if I'm looking to switch these on and off quickly?
3)  Do I need/should I have a flyback diode on the resistor?

Let me know if anyone has a better approach to doing this or has any suggestions.  In my head, this is the way to go but I'm nowhere near as experienced as I'd like to be and I figure there are lots of more experienced EE's in here.  Thanks for your time!
 

Offline jbrookleyTopic starter

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  • Posts: 6
  • Country: us
Re: Signal controlled valve switching circuit
« Reply #1 on: March 25, 2018, 04:40:49 am »
Why can't you just use a low side MOSFET to directly control a solenoid then add a flyback diode, just like everyone else?
Or if you prefer BJT, you can use a 30V+, 500mA+ switching BJT as well, such as 2N2222 plus a flyback diode (1N4007 or whatever).

With MOSFET, you need a gate pull down to prevent accidental triggering, and with a BJT, you need a base series resistor to limit base current.

I knew I was way overthinking it . . . I thought I needed that resistor on the transistor to protect the BJT, which had me thinking about the voltage drop at higher currents, etc.  If that's not the case though, this way looks much, much simpler.  Thanks a lot!
 


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