Hi All,

I need to protect a voltage regulator and a comparator circuit from a powerful motor inductive back current. the regulator connected to a battery and a motor - both are outside the regulator and the comparator circuit.

As I cannot block the path with a diode as it needs to be connected to the battery, I am trying to find a simple solution, maybe to use a tvs diode... please advice if you have any idea?

Attached the relevant schematic.

Thank you in advance,

Barg

 

The "back current" creates a negative voltage which will be blocked D1.
If necessary you could add another diode to the left of D1 across the motor (anode to ground).

 

Hi Crutschow,

Thank you for your reply.

Please clarify for the negative voltage, is it in all reverse current levels? do you mean that leaving D1 as it is sufficient? what about spikes? assuming D1 is 5A Schotky.

I also Added D3 as you suggested, can you please clarify ho it can stop current spikes/reverse currents?

thanks

 

The motor is inductive.
Inductors try to keep the current flowing through them in the same direction when power is removed.
This generates a negative voltage at the inductor (if the original voltage driving the inductor was positive), so that's what you need to protect against.
The diode you added should do that.

 

I note a 1 µF capacitor at the input to the regulator. If this is a ceramic cap and you are expecting fast transients on the input connections to the regulator then a bulk capacitor should be used in parallel. A ceramic capacitor can form a high-Q resonant circuit with wiring inductance, resulting in very large amplitude ringing. A bulk capacitor will both shift the resonant frequency way down and also dampen the ringing by introducing some loss at high frequency. The combination of a a bulk cap and a ceramic cap will go a long way toward "eating" short transients. Sometimes a small resistor can be used in series with another ceramic capacitor to dampen ringing and can be useful if there is a desire to avoid having to find space for an electrolytic cap.

 

Thank you folks,

for your advice, do you mean a 1uF Radial in parallel with the 1UF or can be also instead the 1uf ceramic?

Crutschow, Regarding the D3 diode, if I will put instead the D3 a BIDIRECTIONAL tvs diode, would that be better?

In Addition, the battery voltage is 12v, the regulator can work at 8v, if I will add a voltage divider (appx 20k+- each to reach 8v), would that prevent also spikes?

 

Crutschow, Regarding the D3 diode, if I will put instead the D3 a BIDIRECTIONAL tvs diode, would that be better?

No.
A TVS diode will cause the reverse spike to be equal to the TVS breakdown voltage instead of <1V from the standard diode.

In Addition, the battery voltage is 12v, the regulator can work at 8v, if I will add a voltage divider (appx 20k+- each to reach 8v), would that prevent also spikes?

A 20kΩ voltage divider can only tolerate a load of a fraction of a mA, but no, reducing the voltage won't prevent the spikes.
Use a standard diode.

 

Thanks for the reply.

Ebp, Regarding the bulk radial capacitor in parallel with the 1uf ceramic, a value of 1uf is enough (considering the motor consumption is 500w, 12v)?