Megasquirt Support Forum (MSEXTRA)

Hi Guys, I've just come across something I cant crack on my own, please help!

I am using a single relay on my Fuel pump output to switch a ground that is common to my other accessory relays that I want switched with the F.P. (one of which is the F.P. itself)

with 2 relays on the common ground the array worked fine, but I have recently added 2 more and the relays are now staying on, I have tried disconnecting them and checking resistance, they are all 130-150k then connect 1 at a time, 2 is fine, the third I can JUST feel toggle then number 4 goes "clunk".
Is there for some reason a limit to the amount of relays that can share a ground?
(edit)Actually I should clarify that all the relays do not share a common +12v supply, I just tapped-into a supply and at least one shares the supply for the output with the supply for the relay. Could this be a feedback issue?
So it actually looks more like this... Anyone have some input on this?

thanks in advance!

To prevent relays interacting, fit a diode in series with the coil. 1N4001 is a suitable type. If might be possible to buy them with one built in.

BTW, I doubt the coils measure at 140,000 ohms.

(edited first post)

DaveEFI wrote:To prevent relays interacting, fit a diode in series with the coil. 1N4001 is a suitable type. If might be possible to buy them with one built in.

BTW, I doubt the coils measure at 140,000 ohms.

...thanks so, make it more like this? Are the diodes better-off on the ground side or the power side?
and yeah, I rechecked and the coils measure around 80ohms

Yes- that's the way. May not actually need a diode for every coil, but won't do any harm.

It sounds like you may have stumbled on creating a latching relay, by connecting the pins in such a way to self trigger, or rather are looped between contacts and coil.

If you could draw a complete diagram of all connections, that would help sort out what's going on.

A test though would be to disconnect the load side of the new relays and make sure that the relays will switch on and off correctly.

It's also possible that you have internally suppressed relays (diodes across the coil internally) that could be causing the issue.

Using a standard Bosch style 30/40 relay you would be able to run about 90 relays off one main control relay. A typical Bosch style draws around 300 mA, and the continuous current capability of the contacts of a typical Bosch style relay is 30A.

I think you created a loop between a switched 12V and your switched (relay) grounds.
Make sure that the connected grounds of the relay array isn't connected to anything else.