I wanted to know if there is a way to protect the circuit from a relay.

Usually you add a backwards diode for protection, because when you turn off the relay, the magnetic field in the coil collapses and tries to keep the current flowing, if there is no escape path, it turns into a high-voltage pulse that can destroy anything connected to the coil, adding the diode bleeds the pulse off to the positive rail before it can do any damage.

Relay circuit

So now my problem is I have is that I can't access the relay to put a diode protecting the transistor.

The relay I have is VF4-11F13-C01, it's 12V rated at 40A.

So how can I create a circut that will be able to protect itself from this, the relay is connected to a 12V 40A battery.

Can I use a mosfet, or hexfet, or some other combination that can be used to protect the circuit.

EDIT: This is what I was thinking but some people said that it can't be done, so I asked on the forum.


Relay, I removed the case, but it's still encased in something black, the wires go under and come out of a terminal so I have no idea what's inside.


  • \$\begingroup\$ That 2nd picture can be done and two people have said that is possible. However I am still confused why you do not have access to the other terminal (the power rail) \$\endgroup\$ – JonRB Feb 8 '15 at 17:10
  • \$\begingroup\$ The relay is in a water prof case, that I can't access, and it's covered with something black on top of it. Relay \$\endgroup\$ – ilijamt Feb 8 '15 at 17:12
  • \$\begingroup\$ Use another relay to drive this one? \$\endgroup\$ – copper.hat Feb 9 '15 at 1:15

Your question is a little ambiguous, but I'll take it you're asking how to protect the transistor if you only have access to ground and the low side of the relay coil, not the power supply the other side of the coil is connected to.

Here are two possibilities:

  1. Use a zener diode. Connect the cathode to the collector and the anode to ground. This will conduct when the bottom side of the relay reaches the zener voltage. This voltage can be substantially higher than the relay supply voltage. It only needs to be below the maximum C-E voltage the transistor can handle.

    This method has the advantage of turning off the relay faster compared to when using a typical flyback diode as shown in your question. This happens when the zener voltage is substantially higher than the relay coil supply voltage.

    The zener max current must be rated for whatever the relay coil current it.

  2. Use a snubber. That's a resistor and capacitor in series between the collector and ground. The relay coil current time the resistor must not exceed the transistor C-E voltage capability. The capacitor has to be sized so that the total charge dumped on it from the relay coil flyback current doesn't cause more voltage than the transistor can handle.

  • \$\begingroup\$ Check the schematic above, this is what I was thinking originally, but I was told it won't work, that it won't protect the transistor. Is that what you thought or is there another approach. The max current running through the relay that I could measure was 0.2A, on the other side of the relay it can go up to 40A. \$\endgroup\$ – ilijamt Feb 8 '15 at 17:02
  • \$\begingroup\$ @ilij: If that zener can handle 200 mA, at least for a short while, its voltage is above 12 but below what the transistor can handle, then it should work. Many zeners can't handle that kind of current, so you have to make sure you get one that can, or use it with a transistor to make a sortof "amplified zener". \$\endgroup\$ – Olin Lathrop Feb 8 '15 at 17:25
  • \$\begingroup\$ Well according to the datasheet Vz = 21V, Vf = 1.2V@If=200mA. Any recommendations then on what zener diode and mosfet I can use, I was thinking IRF540, and for a zener have no idea \$\endgroup\$ – ilijamt Feb 8 '15 at 17:40
  • \$\begingroup\$ Look into Silicon TVS instead of a Zener. \$\endgroup\$ – JonRB Feb 8 '15 at 17:43
  • \$\begingroup\$ @JohnRB, Any recommendation, as to which one I would I can use for this case. Never used TVS diodes, I assume I will need to connect it the same way as I would a Zener diode, and I would probably use a uni-directional one instead of bi-directional one. \$\endgroup\$ – ilijamt Feb 8 '15 at 18:03

You mention in a comment that you have only access to one wire, a white one, which when connected to ground operates the relay.

This is good, and means you can connect that to a low side switch, either a BJT or an N-channel MOSFET to operate the relay as shown in your first schematic diagram.

If the wire you have access to operates the relay when grounded, this must mean the other wire (that you can't get to) is tied to +12v. Otherwise the relay wouldn't operate like it does.

So connect the anode of the diode to either the collector of the BJT or the drain of the MOSFET, and connect the cathode to +12v. You don't need direct access to the other terminal.


While there are other methods possible (as mentioned by Olin Lathrop: snubber or Zener across the switch) I am confused why you do not thing you can place the diode across the relay

The relay cited is the typical 4wire type with direct access to the coil.

While you might not be able to place the diode across the coil in close proximity, you can still provide a freewheel diode


simulate this circuit – Schematic created using CircuitLab

As long as there is a freewheel path the immediate problem of allowing the inductors current to flow is mitigated. Depending on the length you may have concerns with regards to transmission line but for a 1m or so and a FET with a reasonable voltage you should be fine

  • \$\begingroup\$ I can't access the relay so I can connect it like this, so this won't work for me. \$\endgroup\$ – ilijamt Feb 8 '15 at 16:59
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    \$\begingroup\$ but do you have access to the relays terminals? you must do to include the FET and the PSU. That is all you need. \$\endgroup\$ – JonRB Feb 8 '15 at 17:05
  • \$\begingroup\$ No I don't have access to the relay terminals, if I had it would have been easy, I wrote above that I can't access the relay, I only have access to one of the wires coming out of the board, and the board is in a waterproof case, so I can't touch anything inside. So I only have 1 wire that I can access, WHITE one, and if I connect the WHITE to GND to relay triggers. \$\endgroup\$ – ilijamt Feb 8 '15 at 17:09
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    \$\begingroup\$ if you only have access to the LOWER relay terminal then you can use the ZENER across the FET method. Since this must be sharing a common 0V, is there something specific that is limiting you gaining access to the same rail? \$\endgroup\$ – JonRB Feb 8 '15 at 17:12
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    \$\begingroup\$ ok fair enough ,but there must be power going into it ... \$\endgroup\$ – JonRB Feb 8 '15 at 17:55

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