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Driving Solenoid 48V from 3.3V

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Hey guys i am working on this project and I will need to drive 48V from my micro controller 3.3V and so I came up with this and wanted to run it by anyone for any blatant mistakes

This works and switches fast enough for my application - but I want to make sure I'm not doing anything way wrong :)

Thanks!
 
Problems!
The H11 opto-isolator, transistor side:
>do not connect to the base
>If it was connected right it would short the +6 into ground.
>Use C and E.
MOSFET will not turn on. (If the opto could pull the G up to 6 volts then the S of the MOSFET will only pull up to about 1 or 2 volts)

Do you need the isolation? Probably not.
Can we move the solenoid to 48V and move the MOSFET to ground? Probably
 
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Thanks Ron! I get conflicting results asking the question if I need the opto isolation or not - some people do it no matter what and some people say you don't need it - I figured that i would at least figure out the proper way to do it so I might have a clue :)

I wired the schematic wrong - in application I don't have the pin 6 wired on the H11 - but thanks for catching that! I have some other problems with the schematic too - I drew it from memory so I will redraw it when I get home and apply the changes (moving the solenoid to the other side of the mosfet)

Here's a question though - you show the diode across the solenoid - but the IRF511 (and other mosfets too?) has the internal diode that protects it from back emf - or at least that's what I thought it was for? in practice do you add another diode anyway - as a better safe than sorry measure?

Thanks!
 
The solenoid is like a spring. (my schematic) If you pull the bottom end down to ground, you have 48 volts across the solenoid. That is like pulling 48 inches across a sprint. Then you open up the MOSFET, it is like letting go of the spring. Both want to shoot up. Something needs to stop the voltage from going very high. (the spring from shooting across the room) The diode will limit the voltage to 48.7 volts. (48+one diode drop)

In the solenoid driver the body diode in the MOSFET never gets used.
 
Note I said LL MOSFET. You need one that will turn on well at 3 volts on the gate.
 
yes I saw that :) I am also trying to use parts I have on hand and I have a ton of the IRF511 and H11A1 parts - which was another reason for the opto - I was using the transistor portion of that as well to drive the gate harder on the Mosfet - probably "parts I have on hand" is a weak argument - but I don't want to order parts for experimentation and learning :) - I'm still blowing things up here LOL
 
another reason for the opto - I was using the transistor portion of that as well to drive the gate harder on the Mosfet
You could try this variation of Ron's circuit. It will drive the gate at ~11V (n.b. the specified max Vgs for the IRF511 is 20V, so coupling the gate directly to 48V via the opto-transistor would NOT be a good idea!), provided the transfer ratio of the opto is >50%. Switching time is ~ 50us.
"parts I have on hand" is a weak argument
Not at all. Very eco-friendly. The challenge is then bending the circuit design to fit what you've got :D

Edit: BTW, I'd always use an opto to isolate the micro from the FET. Otherwise failure of the FET could fry the micro.
 
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small addition, blowing up parts on hand is cheaper than ordering parts to blow up :) The diode inside the fet is just part of the way it's built and I don't think it's in there on purpose and as stated, not used. The inductor always tries the keep the current flowing, and when the path is opened, the voltage rises in an attempt to keep the current flowing, so you need a diode to give the current an easy route to go as the magnetic field collapses, and it goes across the inductor, not the FET or the switch. I like the way Alec_t did it, but add the diode across the Load.
 
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careful...!
ULN2003LV can't handle more than 8V (this circuit is 48V).
ULN2003A for example tolerates up to 50V.

circuit posted by alec_t is nearly perfect, the only thing that should change is value for R1 because H11A1 is opto for 10mA, therefore 220 Ohm would be much better choice.
 
circuit posted by alec_t is nearly perfect, the only thing that should change is value for R1 because H11A1 is opto for 10mA, therefore 220 Ohm would be much better choice.
Good catch. I modelled the circuit with a 4N25 opto and was too lazy to check the H11A1 spec :).
 
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