Emulating Open-Collector operation with TTL 74LS138...

John Robertson · 2022-10-06T06:24:26+00:00

Have a circuit that needs repair. Uses a TIP125 (NPN, Darlington) emitter tied to Vbb (~20VDC). Collector to load, then to ground. 1K resistor (R1) pullup on Base to Vbb, and a second 1KR (R2) to the TTL controller.Problem is of course, that R2 puts the ~20VDC to the TTL output gate on a 74LS138.Trying to solve this without a driver transistor. The circuit is for a 1ms ~20V strobe pulse repeated every ten ms.I thought of putting a 10ufd cap in series with R2, but don't like electrolytics as they fail after a few thousand hours.Possible to use a 15V or so Zener Diode, but the Vbb is not regulated so that won't work reliably.Anyone have a single component in mind that will essentially emulate (isolate) an Open-Collector output for the 138?Thanks!John :-#)#