Relay / Solenoid IO Driver:

High Supply Voltage to Low(er) Voltage Coil with High Efficiency

by Richard Prosser

Operation: (comments by Russell McMahon)

• L1 is relay R7 is relay current sense resistor
   
• Q2 initially off so L1 top low.
• Q1 base on via R10 with D2 acting as a "reference" voltage diode.
   
• Q1 on turns Q2 on via R5.
• Q2 on provides current to relay.
• Relay current ramps up (I ~= V.t/L)
   
• Relay current flows in R7 (here 10 ohms)
• Voltage rising in R7 increases Q1 emitter voltage via R1 so Q1 is turned off when V_R7 rises high enough.
   
• Q1 off turns Q2 off.
• Inductor "rings" (flips voltage polarity) and current in L continues via D1 L R7 in a classic buck regulatior fashion.
   
• Top of L1 flipping down in V pulls down path via R8 R9 and lowers Vbase of Q1 to add hysteresis to turnj on again point for Q1.
• When I_L falls enough Q1again turns on anc cycle repeats.

Archive:

• PICList thread "Design Challenge - low power step down switching regulator"
  • PICList post "Design Challenge - low power step down switching regulator" by Richard Prosser
  • PICList post "Design Challenge - low power step down switching regulator" changes by Dave Dilatush.

See also:

• http://www.4qdtec.com/srd.html Really interesting little circuit for driving solenoids or relays. The schematic does not identify the components, but I believe the resistors and diodes are numbered from the top down, and the transistors are numbered from the left to the right.