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By moose4621
lethe wrote:The way you connected D4 it will short to ground, you need to switch it around.

Doh! It was late. I was tired. I was drunk. (I wasn't drunk but I should have been). :lol:

Thanks for the recomendation on the buck but as Rudy has already pointed out, it needs to be 24v system tolerant. 28v is a bit too low.
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By moose4621
rudy wrote: Schottky diodes are great but they have a lower reverse breakdown voltage. I use a general purpose silicon diode. A transorb/transil is great but you want to have some impedance between it and the electrical system. A "polyswitch" resettable fuse is worth looking at. You can't clamp the electrical system, you just need to prevent spikes and surges from getting into your electronics.

I have selected a 60v 1.5 amp (750mah cont) resettable fuse. I measured less than 200 mah so I am assuming this device is suitable.

rudy wrote:I also try and include a series inductor to keep high frequency transients out. Basically you want to pass nominal dc voltages and block what you can and then clamp what you couldn't block.

I have no idea how to select a suitable inductor. Do I select a current just above the maximum expected? How do I calculate the inductance?

rudy wrote:In one of my designs I included a higher voltage transistor in series with the circuit. Normally it is on and passes power to the voltage regulator. But if the voltage gets above 20 volts it turns off and keeps the regulator from seeing that surge.

Hmmm. That's a great idea.

rudy wrote:You need to protect the stepper driver as well.

The 5v side of the stepper is supplied via this same power supply circuit.
The stepper motor power side is supplied via a buck booster to 40vdc.
The stepper is a nema34 drawing 5A per phase. The driver drives to 48vdc at 5 amps but it is limited to 4.5 amp.

Heading off on a tangent here. I was browsing buck converter chips and came across a LT1766-5 5.5V to 60V 1.5A, 200kHz
Step-Down Switching Regulator with a 5v fixed output suitable for automotive use. What I found really interesting is the relatively simple circuit shown under the "typical application" heading in the linked data sheet. It seems almost simple enough for me to do. ;) Is this the type of thing you had in mind? And could this one be suitable?

Thanks so much for your input. A great help. 8-)