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Does a shunt regulator like 431L (datasheet), wired like this enter image description here

(VKA is not connected to anything) help protect other electronics supplied from INPUT net (battery connection)?

INPUT net also supplies 4V directly to 4 small dc motors, which I believe generate quite large voltage/current spikes at start, stop, or from sudden stop (due to propeller hitting an objetc for example). INPUT net also powers an LDO that powers MCU, sensors, etc.

I am confused, because it is trying to keep VKA constant, then when the spike comes, VKA will change, then 431L should sink the spike fast to keep VKA at 2.5V, but there are two facts that go versus this: the 431L can only sink 100mA, that looks little, so maybe it burns? And there is 1K resistor before the 431L, so maybe the spike just ignores the 431L and goes directly to the LDO and next electronics, generating noise, maybe burning them? So the 431L would protect nothing?

Or maybe the 4V motor spikes are not so dangerous and I should forget about protection?

Please help me understand this, or point me to some reading about this. Thank you very much for your time.

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  • \$\begingroup\$ Don't you want to put a reverse diode in parallel to the motor? That is unless you're gonna run the motor two ways. \$\endgroup\$ – Bradman175 May 19 '16 at 13:06
  • \$\begingroup\$ 2 of the motors are single rotation direction, and the other 2 are two ways rotation. For the single rotation I actually have 100nF parallel capacitors, and for the two ways, I have a 100nF capacitor from each end of the motor to gnd. I still have to investigate this matter. \$\endgroup\$ – LuisOn May 19 '16 at 15:21
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The shunt regulator being isolated from the VIN node via a resistor is not going to help you with spikes, surges and sags in the VIN rail. It is a proposed misapplication of such a part.

Instead you want to look at two things. For over voltage spikes you want to use clamping devices such as tranzorbs to turn on somewhere above your operating voltage and absorb the extra energy in the spikes and surges. For voltage sagging you want to apply capacitors as temporary energy storage on the VIN when the load demands extra energy for short periods of time.

You can also use inductors and/or ferrite beads to isolate noisy circuits from ones that need to be quiet in conjunction with the above techniques.

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  • \$\begingroup\$ Thank you Michael. I already have some capacitors. I will look into campling devices, inductors and ferrite beads. I have been told that even if the 431L is not protecting vs spikes, the fact of having it there creating a constant voltage point, helps with noise on the gnd plane, any clue about this? Is it like this? \$\endgroup\$ – LuisOn May 19 '16 at 15:16
  • \$\begingroup\$ @LuisOn - I think whatever you heard about the shunt regulator helping the GND noise is doubtful at best. You will do a lot more with regard to noise by using proper design techniques with good PC board layout, full GND and PWR planes, proper power decoupling at the loads, isolation between critical sections, limiting noise at its sources, eliminating GND loops, using shortest connections and study study study. \$\endgroup\$ – Michael Karas May 19 '16 at 22:40
  • \$\begingroup\$ Thank you, very helpful. I will remove the shunt regulator and study the other commented options for spikes and noise. :) \$\endgroup\$ – LuisOn May 20 '16 at 5:24

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