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# Relay Driver Circuit

IP.com Disclosure Number: IPCOM000089278D
Original Publication Date: 1977-Oct-01
Included in the Prior Art Database: 2005-Mar-04
Document File: 2 page(s) / 42K

IBM

## Related People

Garofalo, FJ: AUTHOR

## Abstract

"Open Collector Drivers", such as type SN7417, are utilized to interface between lower power control logic and the high power needed to operate an electromagnetic relay. Fig. 1 shows a typical circuit and corresponding voltage curves. Power from source 1 is effective across relay coil 4 to close contact 5 when output 2 of driver 3 is clamped to ground by driver 3 in response to a signal on input line 6. A problem can arise, however, as supply 1 falls when power is turned off. Driver 3, powered from source 1, can be caused to switch by this power transition with the result that the output at 2 goes to ground, as shown at 7, before the decay 8 of 1 is complete, resulting in a pulse 9 across 4 of sufficient duration to close contact 5.

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Relay Driver Circuit

"Open Collector Drivers", such as type SN7417, are utilized to interface between lower power control logic and the high power needed to operate an electromagnetic relay. Fig. 1 shows a typical circuit and corresponding voltage curves. Power from source 1 is effective across relay coil 4 to close contact 5 when output 2 of driver 3 is clamped to ground by driver 3 in response to a signal on input line 6. A problem can arise, however, as supply 1 falls when power is turned off. Driver 3, powered from source 1, can be caused to switch by this power transition with the result that the output at 2 goes to ground, as shown at 7, before the decay 8 of 1 is complete, resulting in a pulse 9 across 4 of sufficient duration to close contact 5.

In Fig. 2, a lower amplitude, but similarly undesirable, output at pulse 10 can result if the driver 11 is moved to the other side of the relay coil. Again, loss of power results in a decay 12 which may still have a substantial value 13 when driver 11 floats due to loss of power. This results in difference voltage 10, which may be sufficient to energize the relay.

A solution is shown in Fig. 3. A pair of drivers 14, 15 are utilized to control relay winding 16. Inverter 17 at the input of driver 14 causes drivers 14 and 15 to operate in a complimentary fashion in response to the logic input signal 18. Accordingly, during a power transition 19, drivers 14, 15 provide equal level shifts 20, 21 and 22, 23 as drivers 1...