Determine break in long circuit

Question

I have a circuit of switches like this. The switches are a long way from the supply and a long way from each other.

 S1 S2 S3 S4 S5 [24VDC+]----->>-o=o----o=o----o=o----o=o----o=o--+ | [input ]-----<<----------------------------------+ [24VDC-]--|

Assuming the power supply is not accessible at all — it must always be on, and you cannot get access to 24VDC reference, is there a simple way to test at each switch with a multimeter to determine between which switches there is a break in the line?

If you test with a multimeter, voltage across a closed switch is 0. Voltage across an open switch is also 0 as the input is high impedance.

Any ideas?

So you can access the switch terminals? If the 24VDC supply is grounded you might be able to measure between the terminal and a cold water pipe or ground rod. — John D
– John D, Commented Aug 25, 2014 at 0:53

CopperMaze · Accepted Answer · 2014-08-25 06:24:37Z

You will have to test the output of each switches in reference with the ground, and starting by the first switch. If your voltmeter reads 0, it means the switch is open.

Prepare some cables you can carry around, connect one side to ground (or zero volt) and the other to your voltmeter common.

Example:

You measure the output of sw1, it's ok. Then you measure the output of sw2 and you figure out it's zero. So you close sw2.

 sw1 sw2 sw3 sw4 sw5 VCC-o=o----o/o----o=o----o o----o=o- +24 0

And going on like this you should find the good position for all switch in one pass.

 sw1 sw2 sw3 sw4 sw5 VCC-o=o----o=o----o=o----o/o----o=o- +24 +24 +24 0

TDHofstetter · Accepted Answer · 2014-08-25 12:24:39Z

Inject a high-frequency AC that rides on the DC carrier, and "sniff" for that AC signal. One simple means of doing so would be to clip a small FM transmitter's antenna connection to one end of the wire (no other connections) and drive it with a simple oscillator. At each switch along the line, an FM receiver tuned to the same frequency should be able to pick up that modulated signal provided that all the upstream switches are still closed. Expect low efficiency and considerable distortion, but it should be good enough to detect.

You need to expand this answer to indicate how you'd accomplish this within the constraints described by the OP. — Dave Tweed
– Dave Tweed, Commented Aug 25, 2014 at 12:08

Olin Lathrop · Accepted Answer · 2014-08-25 13:01:44Z

Put a resistor between the INPUT and the 24V output to deliberately draw some current, just for testing. For example, at 2.4 kΩ resistor will cause 10 mA to flow in the loop. Note that it will also dissipate 240 mW, so it should be at least a "1/4 W" resistor, but "1/2 W" would be better.

Now you have a much lower impedance circuit and any ordinary multimeter should be able to see the voltage accross a open switch.

If more than one switch is open simultaneously, then this method won't work directly either. Each open switch will read 0 V since there is no current flowing. In that case, use clip leads to temporarily bypass all the switches with a short. Verify that the load resistor is seeing the full 24 V. Then check each switch by removing the bypass and checking the voltage accross it with a meter. All the open switches will read close to 24 V. Don't forget to replace the bypass around each open switch you find until you are done checking the whole chain.

The OP is looking for a means of finding line breaks between switches. — TDHofstetter
– TDHofstetter, Commented Aug 25, 2014 at 13:21

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