Fail-safe output switching device

• US 4,816,960 A
• Filed: 05/12/1987
• Issued: 03/28/1989
• Est. Priority Date: 11/13/1984
• Status: Expired due to Fees

• Current Assignee: Westinghouse Brake & Signal Holdings Limited (Siemens AG)
• Original Assignee: Westinghouse Brake & Signal Holdings Limited (Siemens AG)
• Inventors: Littler, Vincent H., Corrie, John D.
• Primary Examiner(s): Envall, Jr., Roy N.
• Assistant Examiner(s): GAFFIN, JEFFREY A
• Application Number:
  US07/049,190
• Time in Patent Office: 686 Days
• Field of Search: 323/239, 323/324, 363/131, 363/132, 363/65, 363/63, 361/93
• US Class Current: 363/131
• CPC Class Codes: B61L 1/20 Safety arrangements for preventing or indicating malfunction of the device, e.g. by leakage current, by lightning
• CPC Class Codes: H03K 17/661 connected to both load terminals
• CPC Class Codes: H03K 19/007 Fail-safe circuits

Abstract

A circuit arrangement for providing in a fail-safe manner an alternating output signal to a load, e.g. in a railway signalling system. The load is connected across a first winding of a transformer, to opposite ends of a second winding of which transformer is connected a bi-directional current switching circuit. First and second modulated signal sources are connected to the bi-directional switching circuit. Each of the signal sources modulates a carrier signal with a modulation signal, the outputs of the signal sources being out of phase with respect to each other. The bi-directional switching circuit is operative to switch current through the second winding alternately in opposite directions at the frequency of the modulation signal. The circuit arrangement is such that if any of its components fails to operate correctly, the output appearing across the first winding is no greater than a threshold level.

Claims

1. A railway signalling system including a load and a circuit arrangement for providing in a fail-safe manner an alternating output signal to the load, the circuit arrangement comprising:

(a) a transformer having a first winding, connected across the load, and a second winding;

(b) first and second modulated signal sources. each of which provides at a respective output a signal modulated in accordance with an alternating modulation signal, the modulated signal provided by the first signal source being out of phase with respect to the modulated signal provided by the second signal source; and

(c) a bi-directional current switching circuit connected to opposite ends of the said second winding to switch current in opposite directions through the second winding, the (c) bi-directional current switching circuit comprising;

(i) first and second coupling means coupled respectively with the outputs of the said first and second signal sources;

(ii) first and second controllable switching devices, each connected with one end of the said second winding, the first switching device second winding, the first switching device being connected with an output of the said first coupling means so that the said first signal source provides control current for the said first switching device via the said first coupling means; and

(iii) third and fourth controllable switching devices, each connected with the opposite end of the said second winding, the third switching device being connected with an output of the said second coupling means so that the said second signal source provides control current for the said third switching device via the said second coupling means;

(iv) the circuit arrangement being such that;

if the said first switching device is conductive in response to current from the first coupling means, the said second and third switching devices are non-conductive and the said fourth switching device is conductive, current being switched in a first direction, from the said first coupling means and via the first switching device, through the said second winding and the said fourth switching device, whereas if the said third switching device is conductive in response to current from the said second coupling means, the said fourth and first switching devices are non-conductive and the said second switching device is conductive, current being switched, in the opposite direction to the said first direction, from the said second coupling means and via the said third switching device, through the said second winding and the said second switching device, so that current through the said second winding is switched alternately in opposite directions at the frequency of the modulation signal; and

if any component of the circuit arrangement fails to operate correctly, the output appearing across the said first winding of the transformer is not greater than a threshold level.

2. A railway signalling system according to claim 1, wherein each of the said first and second modulated signal sources provides at its output a modulated signal by modulating, at the frequency of the modulation signal, an alternating signal having a frequency higher than the modulation signal.

3. A railway signalling system according to claim 1, wherein the said first and second coupling means comprise first and second demodulation means.

4. A railway signalling system according to claim 3, wherein each of the said first and second demodulation means respectively comprises capacitive means and rectifier means, the capacitive means being coupled with the output of the respective one of the said first and second modulated signal sources via the rectifier means.

5. A railway signally system according to claim 4, wherein each of said coupling means includes a transformer for coupling each of said rectifier means with the output of the respective one of the said first and second modulated signal sources.

6. A railway signalling system according to claim 1, wherein each of the said first and third switching devices comprises rspectively a diode and each of the said second and fourth switching devices comprises respectively a transistor.

7. A railway signalling system according to claim 6, wherein each of the said transistors is connected in a common collector configuration, the respective one of the said diodes being connected between its base and emitter.