Post by thread:[EE]: Driving a solenoid

Hello all, I need to drive a solenoid. Currently, i have it connected to a NPN transistor as a low side driver. This is all well and good, but the transistor requires aprroxamately 100ma of base current which is of course beyond the capability of the pic pin im using to control the system. Does anyone have any good ideas of how to drive this 100ma base current? i tried using a resistor to pull the base high with 100ma and a smaller NPN transistor to drive it to ground, but this seems like its wasting a lot of current. Thanks in advance, and best regards. Glenn -- http://www.piclist.com hint: To leave the PICList spam_OUTpiclist-unsubscribe-requestTakeThisOuTmitvma.mit.edu>

> Hello all, > I need to drive a solenoid. Currently, i have it connected to a NPN > transistor as a low side driver. This is all well and good, but the > transistor requires aprroxamately 100ma of base current which is of course > beyond the capability of the pic pin im using to control the system. Does > anyone have any good ideas of how to drive this 100ma base current? i > tried using a resistor to pull the base high with 100ma and a smaller NPN > transistor to drive it to ground, but this seems like its wasting a lot of > current. Thanks in advance, and best regards. Glenn > Wasting a lot of current is the truth. You may find that a FET will work considerably better as a low-side driver for this type of load than a BJT transistor. Good power FET's don't consume much current at all, due to their extremely low on resistance. Plus, they are voltage controlled controlled, as opposed to current controlled. This means that you have no steady state current flowing on your control side. Put a 3k(ish) resistor in series between the Gate of the power HEXFET, and your PIC's output pin. Inductive loads generate and unpleasant kickback voltage that tends to capacitively couple through the Gate/Drain to your driving circuit. If you drive a PIC's output pin too far below your Vss reference, you'll cause it to latchup. http://www.irf.com -d -- http://www.piclist.com hint: To leave the PICList .....piclist-unsubscribe-requestKILLspam@spam@mitvma.mit.edu>

> > Hello all, > > I need to drive a solenoid. Currently, i have it connected to a NPN > > transistor as a low side driver. This is all well and good, but the > > transistor requires aprroxamately 100ma of base current which is of course > > beyond the capability of the pic pin im using to control the system. Does > > anyone have any good ideas of how to drive this 100ma base current? i > > tried using a resistor to pull the base high with 100ma and a smaller NPN > > transistor to drive it to ground, but this seems like its wasting a lot of > > current. Thanks in advance, and best regards. Glenn > > > > Wasting a lot of current is the truth. You may find that a FET will work > considerably better as a low-side driver for this type of load than a BJT > transistor. Good power FET's don't consume much current at all, due to Did I *really* say "...consume current..."? Blech. I meant to say something to the effect that the device doesn't consume power by converting it to heat. {Quote hidden}> their extremely low on resistance. Plus, they are voltage controlled > controlled, as opposed to current controlled. This means that you have no > steady state current flowing on your control side. > > Put a 3k(ish) resistor in series between the Gate of the power HEXFET, and > your PIC's output pin. Inductive loads generate and unpleasant kickback > voltage that tends to capacitively couple through the Gate/Drain to your > driving circuit. If you drive a PIC's output pin too far below your Vss > reference, you'll cause it to latchup. > > http://www.irf.com ... is *still* your friend... -d -- http://www.piclist.com hint: To leave the PICList piclist-unsubscribe-requestKILLspammitvma.mit.edu>

> I need to drive a solenoid. Currently, i have it connected to a NPN > transistor as a low side driver. This is all well and good, but the > transistor requires aprroxamately 100ma of base current which is of course > beyond the capability of the pic pin im using to control the system. Does > anyone have any good ideas of how to drive this 100ma base current? i > tried using a resistor to pull the base high with 100ma and a smaller NPN > transistor to drive it to ground, but this seems like its wasting a lot of > current. Use a darlington configuration. That means you leave the power transistor with emitter to ground and collector to the coil low side (don't forget the flyback diode unless you like fried transistors). A darlington is really two cascaded bipolar transistors of the same type (NPN, PNP). Since the PIC can't source the 100mA the base of the first transistor needs, you insert another transistor as an emitter follower. This can be a smaller transistor because it only needs to supply the 100mA base current to the first transistor. Tie the emitter of the second transistor to the base of the first. The collector of the second in a pure darlington would be tied to the collector of the first, but in this case it would be better to tie it to +5 via a suitable resistor that allows 100mA to flow over the roughly 4.2V drop. Tie the base of the second transistor to the PIC output via a resistor. This resistor needs to allow the PIC to source 1-2 mA over about a 3.6V drop. Did I succeed in totally confusing you 8-) ***************************************************************** Olin Lathrop, embedded systems consultant in Devens Massachusetts (978) 772-3129, .....olinKILLspam.....cognivis.com, http://www.cognivis.com -- http://www.piclist.com hint: To leave the PICList EraseMEpiclist-unsubscribe-requestspam_OUTTakeThisOuTmitvma.mit.edu>