Relays
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Join Date: Mar 2008
Location: Pacific Northwest USA
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I'm adding a few relays to the car and wonder if l need to add a diode too. Its something l have never had to do in my other car.
For most typical accessory additions I'd say a diode isn't needed.
Cheers
DD
#6
Usually the relay terminal numbers 85 and 86 refer to the coil connections.
The diode across the coil is to prevent a reverse voltage spike from being applied to the wire connected to the +ve coil terminal when the 12V power source is removed from the relay coil. This voltage spike is caused by the collapse of the magnetic field created by the coil during it's powered up state. These spikes can harm sensitive electronic devices in the vehicle so I would look for the type of relay with an inbuilt diode to install for any future additions.
In the attached example, 85 would be coil -ve and 86 coil +ve for normal relay power up. In this state, the diode is not doing anything.
When the relay is powered down, the diode conducts to snub out any reverse voltage produced by the collapsing coil magnetic fields.
Clear as mud?
The diode across the coil is to prevent a reverse voltage spike from being applied to the wire connected to the +ve coil terminal when the 12V power source is removed from the relay coil. This voltage spike is caused by the collapse of the magnetic field created by the coil during it's powered up state. These spikes can harm sensitive electronic devices in the vehicle so I would look for the type of relay with an inbuilt diode to install for any future additions.
In the attached example, 85 would be coil -ve and 86 coil +ve for normal relay power up. In this state, the diode is not doing anything.
When the relay is powered down, the diode conducts to snub out any reverse voltage produced by the collapsing coil magnetic fields.
Clear as mud?
Last edited by EXEF; 10-31-2017 at 10:14 PM.
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#8
The 85 and 86 are the low power contacts from the switch. The current to the device being activated flows though the other two (87 30), as EXEF's diagram shows in post no. 6.
#9
So it is important that the + and - go to the correct terminals on the relay. If you get + and - incorrect the diode will conduct and at best prevent the relay from operating.
The coil in a relay acts the same as an ignition coil, so when the relay is switched off the coil "generates" high voltage and can cause an arc at the switch which significantly reduces the life of the switch. It can also effect other electronics such as ECU's.
#10
Wow thanks guys, useful info. I am using the relays to operate the rear windows with a separate switch. As noted the original set do have the diode as they go to the roof ecu. so l guess l should do the same.
Anyone know what diode l need ?
Anyone breaking a convertible ??
Anyone know what diode l need ?
Anyone breaking a convertible ??
Last edited by malc4d; 11-01-2017 at 08:16 AM.
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#12
I had the idea to use the roof switch to operate the rear windows. As the ecu controls the neg side of the roof/window relays. I have a push on contact switch next to the roof switch, this switch controls a relay in the trunk that changes the roof switch wires over from the roof control through 2 changeover relays to up down window control.
I have unplugged the control wires from the ecu plug and using new wire extended these to go through switching relays then back to the connector. These then feed the neg side of the window relays and up/down they go, as this is all Neg controlled, this also means that the relay will send neg. feed to override the shift lock and handbrake lock so I can use the windows whilst driving.
So while I push the switch, the roof switch will now control the windows, then when l let go it controls the roof.
If you can work out what l mean.....looks better in my head than on here. :-)
That's the theory anyway. .......And can all be reset back to original with little work.
Oh and l have the rear seat conversion so all the roof stuff is in the back. Hardest bit so far is routing the push switch wires to the boot/trunk. What a pain in the **** that's being. Unlike my Stag, the XJS isn't open over the wheel arch.
I have unplugged the control wires from the ecu plug and using new wire extended these to go through switching relays then back to the connector. These then feed the neg side of the window relays and up/down they go, as this is all Neg controlled, this also means that the relay will send neg. feed to override the shift lock and handbrake lock so I can use the windows whilst driving.
So while I push the switch, the roof switch will now control the windows, then when l let go it controls the roof.
If you can work out what l mean.....looks better in my head than on here. :-)
That's the theory anyway. .......And can all be reset back to original with little work.
Oh and l have the rear seat conversion so all the roof stuff is in the back. Hardest bit so far is routing the push switch wires to the boot/trunk. What a pain in the **** that's being. Unlike my Stag, the XJS isn't open over the wheel arch.
Last edited by malc4d; 11-02-2017 at 06:27 AM.
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