I have to say that this circuit is mounted over a protoboard. This is a probe scenario.

I have a logical circuit where Vin = 12V and sometimes this control signals must switch to 0V and from 0V must switch to 12V. This Vin nodes are the logic gates inputs (AND, OR, NOR, etc..) from Texas Instruments families: CD40xxB.

This is not about my doubt but I explain this for having a general scope: The logic gates outputs will connect to a driver and this one drives current to relay's coil. Driver has its own internal flyback diodes, etc.


simulate this circuit – Schematic created using CircuitLab

During switching, I have a transient from 12V to 16V at every point of the circuit where any control signal switches from 12V or from 0V to the opposite value. Logic gate inputs and outputs are getting this transient:

enter image description here

I think 12V to 16V maybe is not a danger neither a too much big spike. But I'm trying to minimize it as much as I can. Because I don't think this spike will be good for gates inputs that have input protection diodes that can be forward biased during transients.

I'm not totally sure if it could happen, even if there could be something wrong.

  • Is it normal?
  • How can it be avoid?
  • Have you seen something wrong?
  • 3
    \$\begingroup\$ Can you post your circuit? It sounds to me like your flyback diodes aren't working properly. Maybe all you need is a small change in how they are connected. \$\endgroup\$
    – JRE
    Oct 17, 2019 at 11:23
  • \$\begingroup\$ Thanks @JRE I doesn't put the circuit scheme because my question is about how to choose the TVS according to the needs (values) I posted and the 3 commented manufacturer voltage values. But my first intention with my question is to know how must I select a TVS according to my requirements: 12V (steady state), maximum spike value = 16V, desired max transient value = 13V (if it is possible - as short as better-), etc.. ? This is the first point. \$\endgroup\$
    – Suvi_Eu
    Oct 17, 2019 at 13:13
  • \$\begingroup\$ @JRE The second one is that even if I'm not connecting the driver stage to the logic gates circuit I am having the same transient at the same pins. Give me some time for editing my post and share with all of you the transient and circuit pictures. \$\endgroup\$
    – Suvi_Eu
    Oct 17, 2019 at 13:13
  • \$\begingroup\$ I asked for the circuit because you seem to be trying to solve the wrong problem. You shouldn't be getting spikes on your supply if your flybacks are properly catching the kickback from the relay coils. You are trying to select a TVS to catch something that shouldn't be there. \$\endgroup\$
    – JRE
    Oct 17, 2019 at 14:29
  • \$\begingroup\$ If the same spikes are occuring even without the relay coils connected, then there's something really wrong, and the schematic diagrams become doubly important. \$\endgroup\$
    – JRE
    Oct 17, 2019 at 14:31

1 Answer 1


If these spikes are near or < 30ns, they are false probe errors from long inductive ground connectors and probe ground length.

$$ V=L dI/dt $$ and transistor turn off (dt) times can be very fast then air coupled RF inductive crosstalk begins.

Turn on DSO 20MHz to suppress or fix with short connections 1cm with twisted pairs if long.


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