Please guide me what is purpose of some components?

What is purpose of these components in following CIRCLES in attached images. What are risks if I don’t use these components, please explain it along their proposes.

Image C1

C1 - 470 ohm resistors and VCC (+5V)

Image C 2-3-4

C2 - 200 ohm resistor and diode
C3 - 1.0uf capacitor and 2.2k resistor
C4 - Diode


The resistors attached to the outputs of the 7809 in the C1 image are because the 7409 is an open collector device and it can only pull an output to ground (LOW). When they are not doing that... the output pins float. Since what they are attached to expects 2 voltage levels... High and Low... the resistors are used to create the HIGH signal when the gates are not asserted on the 7409.

So... Not optional.

Heck, the designer could have used a 7408 instead of a 7409 and then they would not be needed... but they didn't... so they are there. Maybe that was all he had in the parts bin. Who knows?

In fact... I'm going to stop here...

Components in a design are not optional unless YOU yourself can determine they are not needed. In general, all components in a design are there for a reason (the designer chose to use them) and are not for you to leave out because they are inconvenient or you don't understand why they are there.

Thanks for reply.

C2, C3 C4 are remaining. All are important for me to know. Please give me some expert words about them.


The pull-up resistors on the 7809 gates are basically for speed I think - TTL chips can't pull up very hard, but a low value 470 ohm resistor on an open collector wlll provide more pull up current for faster switching.

The better solution would be to use 74HC chips, not old TTL.

The diode in C2 means the gate will discharge fast (so the MOSFET switches off in time, ie before its counterpart starts to conduct - this is to prevent shoot-through which would destroy all the MOSFETs).

C10 decouples the power rail to reduce EMI being radiated from it. It will probably need a good ripple current and ESR rating to avoid overheating.

C4/R34 protects the current sensing chip from undervoltage spikes induced from the large dI/dt on the main MOSFETs. With lots of current being switched really fast, significant voltage spikes are induced on nearby wiring. The fact that the current sense wire is grounded by 0.1 ohms will do nothing to protect against magnetically induced voltages like this, which are created directly on the wiring itself.