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power supply protection for car projects


SparkyLabs

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I building a powermanagement circuit for a friends vintage car. It all works fine but for the fact that it has a life of about 15 minutes after which succumbs to ignition spike destruction. the 78L05 reg in particular packs up. Any recomendations on filtering out the spikes from the ignition circuit to protect my circuit ?

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Yes vintage cars can be especially bad at suppressing ignition noise.

How much current are you drawing?

What's the typical input voltage? I assume this is a 12V system and not a 6V system.

Connect a 15R to 18R 0.5W resistor in series with the regulator.

Connect an 18V 1W zener and 10μF capacitor in parallel with the regulator's input.

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What do you want the inductor for?

To connect to the 3A load?

I don't see what adding an inductor to the low current regulator would achieve.

The resistor just limits the surge current through the zener to a safe level, it just needs to be low enough to prevent the voltage loss from being too great. As it's only a 10mA load you could use a higher value resistor, 150R to 180R.

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the dynamo feild coil is being PWM from a mosfet that has a breakdown voltage of 100 V, This not only has to work on paper but in real harsh life conditions as its going on his car and if it breaks on the other side of the country I won't exactly be able to help !

I/We are considering taking something from the original design in which the field coil is control by a (type of) relay with a resistor over it's switch, this means that the coil always get some power although not always all of the power as the switch opens and closes. May this would help stop spiking in the coil ? apart from the fact we think it may eliminate some of the ripple we are getting

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the dynamo feild coil is being PWM from a mosfet that has a breakdown voltage of 100 V, This not only has to work on paper but in real harsh life conditions as its going on his car and if it breaks on the other side of the country I won't exactly be able to help !

I/We are considering taking something from the original design in which the field coil is control by a (type of) relay with a resistor over it's switch, this means that the coil always get some power although not always all of the power as the switch opens and closes. May this would help stop spiking in the coil ? apart from the fact we think it may eliminate some of the ripple we are getting


Do you have a diode in reverse parallel with the coil?

MOSFETs can be pretty tough, most can happily breakdown and absorb a small amount of energy. without being destroyed.

The MOSFET is rated for over 8 times the supply voltage so it should be fine as long as you have the free-wheeling diode correctly installed across the inductor.
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yes I have the diode on the coil. again I suspect spikes from the ignition system.

I am wondering if it was damaged via the gate its +-20 V max and full on at -4 V, perhaps there have been high voltages hitting it (the TL084 used to drive it is connected straight to the "12" volt supply from the battery not via the regulator. I'm considering a 1/1K voltage divider.

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I thought the gate was driven from a PIC connected to the regulated 5V supply.?

Please post a schematic, there's no way I can help you unless you tell me the full story.

Yes the gate is the most sensitive part of the MOSFET and it's also highly likely that the TL084 will be killed by the high voltage spikes.

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