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# Hammer Driver Circuit

IP.com Disclosure Number: IPCOM000079357D
Original Publication Date: 1973-Jun-01
Included in the Prior Art Database: 2005-Feb-26
Document File: 2 page(s) / 43K

IBM

## Related People

Barcomb, JG: AUTHOR [+2]

## Abstract

In a hammer driver circuit, a single zener diode Z1 is used to clamp the inductive loads L for a number of driver circuits when the different drivers turn off.

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Hammer Driver Circuit

In a hammer driver circuit, a single zener diode Z1 is used to clamp the inductive loads L for a number of driver circuits when the different drivers turn off.

An up level at the input Al turns on transistor Q1, which drives the PNP-NPN Darlington pair of transistors Q2 and Q3 into saturation so that V(01) = V(BE3) + V(EC2) (SAT.).

Q2 and Q3 provide the drive current for the load. When the input at A1 goes down, Q1 turns off, turning off Q2 and Q3 and interrupting the load current. The inductive load generates a voltage (Vc in the figure) with a polarity opposite the applied voltage. The voltage at the output is V(01) = V(2) + V(c) The voltage at the base of Q2 is V(B) = V(01) - V(EB2) The generated voltage V(01) generates a current I(B) equal to I(B) = V(B) - V(D) - V(1) over R3 = V(01) - V(EB2) - V(D) - V(1) over R3 = V(2) - V(1) + V(C) - V(EB2) - V(D) over R3 If V(2) = V(1) I(B) = Vc -V(EB2) - V(D) over R3.

A large clamp voltage (Vc) is desired in applications where a rapid decay of coil current is required. This can be obtained by zener selection. The limit on the magnitude of Vc is a function of the device (Q2 and Q3) breakdown and power dissipation. This scheme permits one or more drivers to turn on and off simultaneously, even though only one zener diode is used.

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