Power circuit review

[DTJ]

Anyone got any thoughts on this circuit. I've been built and seems to work reliably but now its time for some PCBs to be ordered.....


This circuit powers a micro and a few other bits.
It's located on the end of a 25m long shielded cable - most probably located outdoors in an agricultural environment.
The circuitry it supplies is isolated from ground - the only electrical connections are back via the powering cable.
The input voltage is nominally 12Vdc, but could perhaps be as high as 16V.
The combined maximum load drawn from the 5V & 3V supplies is around 10mA.



Should I add a low value series R, say 100ohms for additional input protection?

For the surge suppressor I'm considering a unit with Vworking = 18V, Vbreakdown = 20V and Vclamping=29V. (1SMB18CAT3G).


Thoughts?


Cheers
DTJ

[danadak]

I am thinking the zener should attach between the polyfuse and the
R. Limit zener current, allow very short transients to not blow fuse but still
protect regulator while limiting Iz and protecting it as well.

polyfuse >> R >> regulator in, zener to ground between R and Regulator in.

You might also consider diodes across regulators to protect them if their
inputs get shorted to ground with a full charge in output cap.

Regards, Dana.

[capt bullshot]

You might consider to use the LM2940 5V regulator for U3 - it contains all the protection for an automotive environment (including reverse polarity and 60V load dump)
And you should place the fuse at the starting point of your cable (so it's able to protect the cable). No fuse required at the end of the cable.

[Circlotron]

I'd ditch the surge suppressor and instead put a 20v Zener right at the input to the voltage reg. Also put the low value res at the input like you say. Any spikes presented to the Zener will be relatively slow because of the r and c and so it will catch them nicely. Also any reverse voltage will also be clamped at 0.6 v or so. The original surge suppressor will not clamp reverse voltage to a low value.

Also, with the added 100R resistor, watch how quickly the regulated output rails come up. If they are too slow some micros won't power-on reset properly if their reset cap is not big enough.

For the 100R use something with a bit of meat in it like a 1 or 2 watt. I've had 1/4 watt ones go open cct in an automotive environment.

[DTJ]

Thanks for the suggestions guys...

I'd ditch the surge suppressor and instead put a 20v Zener right at the input to the voltage reg. Also put the low value res at the input like you say. Any spikes presented to the Zener will be relatively slow because of the r and c and so it will catch them nicely. Also any reverse voltage will also be clamped at 0.6 v or so. The original surge suppressor will not clamp reverse voltage to a low value.

Also, with the added 100R resistor, watch how quickly the regulated output rails come up. If they are too slow some micros won't power-on reset properly if their reset cap is not big enough.

For the 100R use something with a bit of meat in it like a 1 or 2 watt. I've had 1/4 watt ones go open cct in an automotive environment.

Done. I was tossing up whether to move the diode cathode to the right of the resistor. This is for use in an electrically clean environment so I'm not too sure on the resistor power rating. At the moment they are well and truly sufficient in case someone inadvertently connects 24Vdc as a supply instead of the expected 12V.

You might consider to use the LM2940 5V regulator for U3 - it contains all the protection for an automotive environment (including reverse polarity and 60V load dump)
And you should place the fuse at the starting point of your cable (so it's able to protect the cable). No fuse required at the end of the cable.

I looked at the LM2950, it's a bit heavy for this circuit. It's quiescent current is greater than my total load. Your suggestion did inspire me to check a few things and I've changed the 5V regulator for one with a higher input rating and an internal protection diodes.

I've got a fuse in the unit to protect against overload should the device fail s/c. There should be one at the start of the cable but I have no control over what the end user puts there.

I am thinking the zener should attach between the polyfuse and the R. Limit zener current, allow very short transients to not blow fuse but still
protect regulator while limiting Iz and protecting it as well.

polyfuse >> R >> regulator in, zener to ground between R and Regulator in.

You might also consider diodes across regulators to protect them if their inputs get shorted to ground with a full charge in output cap.

Regards, Dana.

Yep - made some changes.
I changed the 5V reg and its got an internal protection diode.



The amended circuit is below. How does it look now?
Thanks.
DTJ.