Does the 7809 need an input bypass capacitor if it has large output capacitors in parallel?

Question

I am using RFID aliexpresslink. RFID is suggesting this schematic:

The 7809 has this schematic in the data sheet:

1. Why is RFID power design suggesting big capacitors with low resistors? Does it mean that if big capacitors are connected parallel, they don't need input capacitor?
2. Why do they not add an input capacitor?
3. Will this design work without problems? (1000uF capacitor design.) I didn't see anything like this schematic on the internet.
4. If I use only input and output capacitor, will it affect the reading distance? I mean, i I use only 0.33 uF and 0.1 uF, will my read distance be lower?
5. I read in this forum that the 7809's output capacitor is 0.1 uF because in 1970, capacitors were really expensive. What is the best output and input values in these days? (I searched but only I found this schematic 0.33uF and 0.1uF.)

Current schematic of mine.

For freewheeling diode:

Answer 1

Most regulators will get unstable if the impedance of the input power supply is too inductive, due to long wires or traces. So an input capacitor is added.

You can use whatever value of cap you already have that's equal or greater to what the manufacturer says in the datasheet. If the input supply bulk caps are close, then it's probably not necessary, but a 1µF SMD caps only costs a few cents...

Why is RFID power design suggesting big capacitors with low resistors?

Probably because their board has low power supply rejection and needs clean power. To be honest, it looks a bit overkill.

Personally I'd use a snubber on the transformer to remove rectifier noise at the source, and a large 1000µF output cap because RFID draws pulsed current.

Answer 2

In general, the presence of an input bypass capacitor does not depend on how much capacitance there is on the output. It depends on many other factors, such as for example how long wires there are to the input bulk capacitor. If 1 cm, most likely a separate bypass is not needed, if 1 meter, most likely a separate bypass is needed.

Bear in mind that we don't know why the circuit is like that, ask the one who provided the circuit. Internet is full of circuits that look random and made by hobbyists and tinkerers instead of engineers.

1. The regulator may omit the drawing of the input bypass capacitor, if it is expected to be near a power supply bulk capacitance which is not drawn in the circuit. In general, it is best to have the 330nF input capacitor that to omit it. The fact that there is a lot of output bulk capacitance does not mean you can omit the input bypass cap. Take the circuit as a block diagram type of example circuit, not as exact design.

2. They may assume or expect that the power supply with its bulk capacitors is within short distance from the regulator input, and it is just an example picture, because it might be different depending on which regulator you use. It may not resemble the exact way how you should do things.

3. It might work. Or it might have problems. It is impossible to say just by looking this circuit in isolation. If you have a random 12V power supply, there is no way to prove it works with it or it won't. And there must be better ways to do it than with three high value caps. The first problem might be that the inrush current is so large that a power supply might hit a current limit and shut down or blows a fuse on the supply output. So it may require a power supply with large output current, even if the circuit consumption is very small.

4. How would we know how it affects it. If you remove bulk caps and add input bypass, it might even work better depending on your power supply which is unknown. The point is, if there is no specs what the RF device needs from the power supply and regulator, it is impossible to know what power supply and regulator is good enough for the RF device.

5. No the values are not 330nF and 100nF because capacitors were expensive. The manufacturer just guarantees that these values of high frequency bypass caps have proper impedance range to keep the regulator stable. The user of the regulator is responsible for using larger bulk capacitors on input and output if necessary depending on the circuit.

Answer 3

You may want to check that 78xx schematic. To the best of my knowledge, pin 2 is GND and pin 3 is OUT. I stumbled upon this looking up the pinout on google and it might confuse some folks, causing premature release of The Magic Smoke. If you want a relatively stable and quiet power supply with one of these, simply throw lots of capapcitance at it, say a 470uF plus a 100nF in parallel on the input and a 1000uF plus 100nF in parallel on the output. The more the better, depending on the current draw of the application and surge-load capability of the supply. For good measure use ferrite chokes on the in and out, a donut type coil one on input and a rf blocking barrel on the output. Any rf pickup by the circuit or its wires will make it into your device, consider using a clip-on ferrite close to the device on the supply wire. That will kill any remaining junk right at the device side.