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power supply circuit for PIC

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grekson

New Member
I am thinking to build my power supply circuit based on L78M05CV ic.
Does this ic has any protection agains harmful voltage spikes (normal thing in car electrical environment) and I just need to add 0.1uF and 0.33uF ceramic capacitors, or do I need to build more complex circuit with big electrolytic capacitor on the input side and possibly a 15V reversed biased zener? If I need to add extra capacitor and zener will the below circuit be fine?

Also what should be the value of capacitor near the MCUs vcc and gnd pins? Someone suggested me to use 100nF, but those capacitors are to big to be solder below the MCU (on the 28DIL socket).
 
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Oznog

Active Member
The size of input and output caps are specified in the datasheet.
What is the Absolute Maximum Vin in the datasheet? 35V is common. In this case you can- and should- use a much higher zener. Zeners do not actually conduct zero current beneath the zener voltage, and actual zener voltage. So a zener rated for 15V @ 1A might draw significant current with the system at 14.6v when running. Might even burn the zener up. A 25v or 30v zener will draw insignificant current at 14.6v, and still protect the regulator from spikes. BTW, a "transorb" zener is much better than regular zeners. The surge ratings are very good.
 

MikeMl

Well-Known Member
Most Helpful Member
What is the Zener trying to protect against? If the automobile battery is reversed, then the Zener will vaporize before the fuse will blow. If the there is a positive spike from the automobile side, the Zener will still likely vaporize before the fuse blows. You need to have some series resistance between the fuse and the cathode of the Zener. Pick a resistance that has only ~1V of drop during normal operation, but provides current limiting while blowing (heating) the fuse...
 

DirtyLude

Well-Known Member
If you are putting this in a car, you should really try to use a automotive spec'ed regulator. These regulators have protection built in and are spec'ed for these voltage spikes and reverse voltage.

Saying that, though, I have made a few one off devices that I've just put standard regulators in.
 

grekson

New Member
what should be the value of capacitor that is connected directly to vcc and gnd pins of PIC.
I wish to mount it underneeth the MCU so keep the size in mind.
 

smanches

New Member
100nF sounds about right to me. They don't have to be under the device, just as close to the supply pins as possible. Preferably inline with the trace; that is don't make a special trace for it, but put it right in the middle of the existing trace. This will allow it to be the best "speed bump" possible for transients.

I would also say use an automotive spec'd regulator and an actual transient supressor like a Transorb, instead of a regular zener.

Welcome to STMicroelectronics makes a lot of automotive spec'd devices like these. It would be a place to get familiar with them at least.
 
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colin55

Well-Known Member
What is the actual current you require on the 5v rail? If it is only 100mA, you can put 22R (or even higher) on the input as a safety resistor.
 
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grekson

New Member
100nF sounds about right to me.
100n or 0.1u is to big cpacitor to be mounted beneath the mcu. All the boards I saw have this capacitor mounted in the middle of the mcu socket, so it must be smaller that 100n, and works wery well. I want to mount it as I said because lines on the PCB won't cross (no need for double side pcb).
 
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smanches

New Member
100n or 0.1u is to big cpacitor to be mounted beneath the mcu. All the boards I saw have this capacitor mounted in the middle of the mcu socket, so it must be smaller that 100n, and works wery well. I want to mount it as I said because lines on the PCB won't cross (no need for double side pcb).
There is no reason it needs to be under the MCU socket. That's just pure coincidence. If you're V+ trace goes under the socket before it gets to the chip, maybe you should re-route it so it doesn't. You can't just dictate a physical size in all cases. The circuit requires a cap of a particular rating, and you have to make the concession on size.
 
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