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I have a led in series with my toggle switch to be an indicator to power a 30/40amp relay. The power source is a 12v battery. When I close the switch the led comes on, but the relay stays open?

schematic

simulate this circuit – Schematic created using CircuitLab

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    \$\begingroup\$ Draw us a circuit (hit the 'edit' link below your question, then hit the 'schematic' button or press Ctrl-M). But if you've wired the LED, the switch & the relay coil all in series then that'll never work. \$\endgroup\$ – brhans Feb 19 '18 at 19:24
  • \$\begingroup\$ yes that's how I have it wired, could you tell me why it doesn't work? \$\endgroup\$ – Bulldog Feb 19 '18 at 19:30
  • \$\begingroup\$ Please post a link to the relay datasheet or at least give us the coil resistance. Put all the details in your question rather than sprinkled through the comments. \$\endgroup\$ – Transistor Feb 19 '18 at 19:56
  • \$\begingroup\$ That is a poor design at best. Lucky if the LED does not blow from back-EMF. \$\endgroup\$ – Sparky256 Feb 19 '18 at 20:01
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    \$\begingroup\$ It doesn't work because the majority of the voltage is dropped across the LED indicator lamp, or else, if it is a simple LED, the voltage left across the coil is not enough to activate it. \$\endgroup\$ – Trevor_G Feb 19 '18 at 20:15
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Given that it looks like an car project I'd guess that by LED you means an indicator lamp with built-in series resistor.

schematic

simulate this circuit – Schematic created using CircuitLab

Figure 1. (Left) What you've done. (Right) what you need to do.

If there is a series resistor in the LED it will limit the current through the circuit to about 10 or 12 mA. This won't be enough to energise your relay and if you measure the voltage across the relay you probably only have a volt or so.

Instead, wire the indicator in parallel with the relay. Relays give an inductive reverse-voltage "kick" when switched off and this is enough to blow the LED. Unless you know that either the relay or the indicator has protection built in then add D3 to protect the indicator LED.

Finally, note the use of the ground or chassis symbols in my schematic and omitted from yours. You have no return path for the current so, as drawn, neither the relay or LED would work. The schematics above have a ground return and so complete the loop to allow current to flow.

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  • \$\begingroup\$ +1 from me... Only thing I would add is the switches should really be on the top side, especially if this is automotive and the relay/LED is remote from the switch. Hard to put a switch in the chassis ;D \$\endgroup\$ – Trevor_G Feb 19 '18 at 21:17
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    \$\begingroup\$ Agreed, Trevor, although there's plenty of precedence for negative side switching in auto-stuff so that's why I left it in. \$\endgroup\$ – Transistor Feb 19 '18 at 21:19
  • \$\begingroup\$ True, there are still a few booster cable/alternator burners out there :) \$\endgroup\$ – Trevor_G Feb 19 '18 at 21:21
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    \$\begingroup\$ I was thinking of oil pressure switches and some funny stuff on the alternator indicators but I thankfully haven't had to do any auto-electrics in many years. A friend, trying to fix his dad's car radio while he had his head under the dashboard and feet over the seat had a vision of the assembly line cradle coming out of the stores with a car radio around which the complete car was built. We had some chance in the days of 1/4" spades but everything is inaccessible in multi-pole connectors now. \$\endgroup\$ – Transistor Feb 19 '18 at 21:44
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You want to have your led in parallel with the coil of your relay and you want a resistor in series with your led to limit the current through it. You should always limit the current through an led with a resistor , else you will just end up blowing it up.

schematic

simulate this circuit – Schematic created using CircuitLab

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    \$\begingroup\$ Given that it looks like an car project I'd guess that by LED s/he means an indicator lamp with built-in series resistor. You've forgotten the snubber. The LED will get zapped! \$\endgroup\$ – Transistor Feb 19 '18 at 20:12
  • \$\begingroup\$ @transistor in that same vein, don't most automotive relays include the snubber? \$\endgroup\$ – Passerby Feb 19 '18 at 21:36
  • \$\begingroup\$ I can't remember pulling one apart for a few decades. Do they? \$\endgroup\$ – Transistor Feb 19 '18 at 21:51

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