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User outputs a gnd with power off
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Trying to control a cooling fan with user out 1 set to trigger from aux (go to ground) at 88 degrees. I've wired as per the diagram in the instructions but have noticed that when no power is there to the mjlj the outputs go to ground which switches my fan on permanently. Is this correct? Currently looking at having to fit a relay to isolate output 1 when ignition is off as I need the fan to be able to be run with ignition off occasionally.

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I think I've sussed it... I've wired a relay as per the instruction in the installation guide so that user out 1 is controlling a relay that has 12V permanently to the other side if the coil. Unfortunately, this seems to feed UO1 with 12Volts and actually powers on the mjlj.

Hopefully the addition of a 1N4001 diode in the line from UO1 will sort it.

My next question is this... is damage likely to have occurred to the mjlj by allowing 12V to UO1 in this way? Both my V3 units behave the same.

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Glad you figured it out. Unsure of the damage risk, if you see anything just report back here.


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Brent Picasso
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Having an absolute nightmare with this. With the ignition off and no power to the mjlj at all, the feed through the relay to user out 1 is switching the mjlj on. With a diode in circuit it stops this, but user out 1 will then not switch on the relay. How do I stop this happening? If I switch the 12V feed to the relay with the ignition I then no longer have the functionality of the fan without ignition being on.

Surely this has been noticed before, or is nobody else in the world switching a relay with the user outputs and was this scenario not tested at development?

Both my V3 units do the same thing, so it must be the design allowing it somehow though the ULN2003 IC.

Please advise as I no longer have a cooling fan on the car that works independently.

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As you know, we're using the ULN2003 in the standard way, nothing special about the implementation.

If you post a schematic of your entire setup, that might help all of us help you out.

Thanks!


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Brent Picasso
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It's wired exactly as the diagram in the installation pages with user out 1 connected to a relay coil with the other side of the relay coil to a permanent 12V

https://wiki.autosportlabs.com/File:MJLJ_User_output_examples.png

This puts 12V on user out 1 and powers up the unit even though there is no power to the main 12V in

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I'm running an experiment on my desk with my spare V3 unit. Currently I have only 2 wires to it connected to Gnd and User Out 1.

User Out 1 is connected to pin 85 of a standard automotive relay. Pin 86 is connected to 12V as per the diagram listed above. The coil has a resistance of circa 75 ohms. The megajolt is alive and talking quite nicely with my pc.

Can the ULN2003 somehow shunt the 12V back down its Vcc pin thus powering the unit?

To recap, the ONLY connections to the Molex are Gnd and User Out 1.

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I've been looking at the data sheet for the ULN2003. The internal freewheeling diodes will allow 12V through from the outputs to the 'safe 12V' line of the mjlj which I think is powering it up.

My next trial will be to remove pin 9 from the circuit and add the freewheeling diode at the relay instead and see what happens.

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And the verdict is... Success.

This was all because my relay has a permanent live feed to allow manual override and operation with the ignition off after a long run...

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Here's what I have found.

There is a slight error when using a relay off any of the user outputs if that relay has a permanent feed as it will power up the MJLJ unit.

This is due to the way the 12V line from Pin9 of the ULN2003A is connected to the input of the Voltage regulator IC.

See attached diagram.

The simple solution is to have the relay switched off with the ignition.

This however, is not ideal when running an electric fan as, like I have done, it has an override switch to allow it to be run independently of the ignition / MJLJ.

In this situation the solution is to disconnect pin 9 of the ULN2003A and fit a freewheeling diode across the relay terminals instead, which offers the same protection and is quite common in automotive installations like the old relay type immobilisers from the 90's Moss alarms etc.




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