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5 Switches in series

Hi, I would like to know the best way of turning on an LED for each switch that is opened. The switches are normally closed. The load is a motor. When any switch opens the motor turns off. But it is not always easy to see which switch it is. A nice LED display would work good.

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You could put a resistor and a LED wired in series and together in parallel with each switch. This will mean that if a switch opens, a small current will flow through the resistor, the LED, the closed switches and then thru the motor. The problem might be that the small current (say 20mA) might still cause the motor to move a little especially if it's a small 48 volt type.

If you can live with this and the supply is DC and always polarized the same way (i.e. not an output from a H bridge) then it should work.

If it's an AC motor then an extra diode is needed in parallel with each LED to prevent reverse bias.

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  • \$\begingroup\$ +1 Diode or use bipolar LEDs. \$\endgroup\$ – Spehro Pefhany Dec 4 '14 at 22:22
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When I worked in industrial automation, we had a few projects that needed to do that. All of them had multiple contacts per switch. When the motor contact would open, an auxiliary contact would also move, which had its own power supply and was wired to a dedicated input of the PLC (computer). We would then display a graphic that told the operator or maintenance guy which one was out. Sometimes the aux contact was NO, sometimes NC, depending on the project.

Unlike the LED across the switch, this method would not power the motor at all if a safety was open, and would not care about the motor's polarity or voltage. (our loads were often 480V 3-phase) But it does require isolated multi-gang switches.

As a side-note, if these are safety switches to prevent injury or worse, then they should each have multiple contacts in series and designed so that if one them welds itself (does happen on occasion), the other(s) can still work. This is in addition to the monitoring contact(s). And you need a way to detect this working-but-compromised condition as well. Do not allow the motor to start again until the bad one is verified working. (It just might be your luck that the other one welds the next time, and then you're screwed.)

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  • \$\begingroup\$ Thanks for the reply, and concern. This is actually a simple schematic of part of a machine that I use at work. Have been working on these for 25 years. Never seen a switch fail. But should that happen, the motor would overload causing the circuit breaker to blow. I made a mistake, though. The switches are actually connected to a relay which then powers the motor. I would like this circuit for new employees that have trouble detecting which switch is open. \$\endgroup\$ – Tony Dec 5 '14 at 3:39
  • \$\begingroup\$ In that case, if the load is actually a relay coil, then LED's across the switches might work pretty well. Just make sure that the current through the LED with any one switch open is below the holding current of the relay. This is lower than the pull-in current. \$\endgroup\$ – AaronD Dec 5 '14 at 5:01
  • \$\begingroup\$ Also to consider is the flyback voltage when a switch opens. Switch contacts can be pretty tough; discrete silicon parts like LED's generally aren't. You'll want to add some more parts to manage the voltage spike when the relay turns off. \$\endgroup\$ – AaronD Dec 5 '14 at 5:06
  • \$\begingroup\$ @Tony: Actually, on further thought, you probably won't get a big voltage spike in your case. The inductance of the coil will try to force the same current to flow through it as before the switch was opened, and it will generate any voltage to make that happen. That's where the spike comes from, but if there's a path to continue that current (through the LED), then the spike is limited to what's required to push the previous current through the LED now. If it can handle that for a brief moment while the energy bleeds off, you're probably okay. \$\endgroup\$ – AaronD Dec 9 '14 at 15:32
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   R

schematic

simulate this circuit – Schematic created using CircuitLab

Replace SPST switches with SPDT switches connect the switches as shown V1 48 VOLTS R1 MOTOR LOAD.R2 10K 1W.D1 D3 D4 D5 LEDs. If any switch is off it connects corresponding LED through R2 - ve of the power source and lights up indicating that particular switch is opend.

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