I'm trying to wrap my head around transient voltage protection and how to implement it when also trying to rectify a AC source to DC then step it down to 3.3V for a microcontroller. I understand that MOVs are great for high voltage and AC, where as Zener diodes are good for low power DC overvoltage protection.

My problem is that I have an 24V AC source which tends to be rather noisy and can spike due to solenoids and motors sharing the same rail. This is unfortunately out of my control. Obviously, surge protection is required, but how to implement it escapes me. I haven't been able to measure how high these surges get but I assume they are higher then the 40V max of the LM2596.

My current thinking, as illustrated by the diagram, is to take the 24V AC in then have a MOV to take any major spikes out. Once this is done then rectify the AC current to DC and have a second transient voltage suppressor in the form of a Zener diode. This diode has a reverse voltage of 39V which is lower then the max input of the LM2596 which is 40V. I have read that I might need to add a resistor to this Zener to reduce the load, but what load would be on the diode in a surge event? I know my max load on the 3.3V side of the regulator is 1A, however nominally around 400mA.

My real question is then, is my thinking correct or have I miss understood how to protect the circuit? If so, what do I need to look at/change to achieve my goal?

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  • \$\begingroup\$ You could solve many of your worries with an opto-isolator. They're available with back-to-back LEDs so you don't even need the rectifier. Would you consider that solution? \$\endgroup\$
    – Transistor
    Apr 22 at 15:27
  • \$\begingroup\$ Are you trying to make a power supply for the micro, or as Transistor implies, have the micro detect whether 24VAC is present or offline? \$\endgroup\$
    – bobflux
    Apr 22 at 15:29
  • 1
    \$\begingroup\$ Analyse the threats. Analyse the potential victims. Work out whether protection is required and what job it should do. Then design it. There's nothing concrete in your question that can make an answer. \$\endgroup\$
    – Andy aka
    Apr 22 at 18:45
  • \$\begingroup\$ I'm attempting to make a power supply for the micro not to detect the presents of 24VAC \$\endgroup\$
    – Ezzy C
    Apr 23 at 0:46

24VAC only has a peak voltage of 33.9V (24*1.414) So I the specified MOV looks reasonable.

You get two diode drops with the bridge (-1.4) so 33.9 becomes 32.5Vdc.

Instead of a zener here, I would pick a TVS diode (Transient Voltage Suppressor). They are fast and able to take large transients and have finer voltage resolution choices when compared to zener diodes. You can easily get one that is designed to stand off 33V all day long and starts conducting around 36V.

Also, if you have the space, I would use parallel input caps instead of one large one. Each one has ESR (series resistance) and so the more you parallel them the lower the overall ESR becomes. And that enables the input cap to also slow down/prevent noise as well. Recall from physics that you can not change the voltage on a capacitor instantaneously, instead it must charge up over time.

Don't worry about anything downstream of the power supply. With the two/three protection devices in place, then the regulator will do its job and take anything from up to 40V and turn it into 3.3V.

  • \$\begingroup\$ If you can afford a small resistance in series with the fuse then the current drawn during a surge may be significantly reduced, which would be kinder to all the components concerned. An inductor would be better still. \$\endgroup\$
    – Frog
    Apr 22 at 23:50
  • \$\begingroup\$ During a surge event, couldn't the peak voltage be much higher then 33.9V, in the hundreds of volts? A lot of the TVS diodes I have seen do have the specs you mention but their max clamping voltage tends to be in the 50V range. So the circuit could still experience voltages beyond the maximum of the LM2596, or do I misunderstand the difference between breakdown voltage and clamping voltage? Your thinking on the parrallel caps makes a lot of sense, I was basing that portion off the TI datasheet. But a bypass capacitor setup could be useful for noise. \$\endgroup\$
    – Ezzy C
    Apr 23 at 0:55
  • \$\begingroup\$ @Frog Doesn't the current vary widely due to the LM2596 drawing different amounts of current to keep the 3.3v output? So restricting the current initially could possibly choke the circuit? \$\endgroup\$
    – Ezzy C
    Apr 23 at 0:59
  • \$\begingroup\$ A 33V TVS diode like SMAJ33CAQ-13-F should work just fine. Yes, the transient voltage can be very high. The MOV will cut that down significantly, then anything energetic enough to get the caps to go over the TVS breakdown will get trimmed by the TVS diode which turns on between 36-40V. \$\endgroup\$
    – Aaron
    Apr 23 at 1:05
  • \$\begingroup\$ @Ezzy C yes certainly that’s a possibility. For non-initiates: switch-mode converters draw more current as the supply voltage drops, so if there’s a series resistance they can get into a state where the switcher can’t get enough power to satisfy the load and it ends up drawing as much current as the resistor will allow to flow, but there isn’t enough voltage to provide the required power to the load. Intuitively this will happen where the switcher attempts to draw a quarter of the power that the resistor would consume if it were connected directly across the supply. \$\endgroup\$
    – Frog
    Apr 23 at 1:11

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