Linear voltage regulator off the PCB.

[davelectronic]

Really just a general question about linear voltage regulators like the 7812 positive voltage regulator and the LM 317 variable voltage regulator. If you have the regulator some distance from the main PCB, do you add capacitors on the remote regulator PCB board ? In the case of the LM 317 I know the voltage set resistors must be close to the regulator. But do I need extra capacitors on the regulator board. It would be from 8" to 10" away from the PSU main board, on its own small PCB board. Thanks for reading any thoughts appreciated.

[Jeroen3]

Yes, any regulator requires capacitance to provide bypassing.

• TI recommends that the input terminal be bypassed to ground with a bypass capacitor.
• The optimum placement is closest to the input terminal of the device and the system GND. Take care to
minimize the loop area formed by the bypass-capacitor connection, the input terminal, and the system GND.

Another example:
https://www.onsemi.com/pdf/datasheet/lm317-d.pdf

[davelectronic]

Thank you for your reply, I knew the regulator would need bypass capacitors. But I meant as the voltage regulator is 6" or more away from the bridge rectifier and filter capacitors. I suppose I meant voltage drops or transients etc. probably didn't word my first post with the best explanation. But I was asking in addition to bypass capacitors do I need additional capacitance closer to the voltage regulator, being as it's some distance from the PSU main PCB board.

[PGPG]

The answer is simple for low-drop regulators that need capacitor at output for their stability. If you add extra distance between IC and capacitor you are adding inductance in serie with capacitance making IC "don't see" capacitor at its output.

For example a description of Output capacityor from TI:LM1117 datasheet:
------------------
9.2.2.1.3 Output Capacitor
The output capacitor is critical in maintaining regulator stability, and must meet the required conditions for both
minimum amount of capacitance and equivalent series resistance (ESR). The minimum output capacitance
required by the LM1117 is 10 µF, if a tantalum capacitor is used. Any increase of the output capacitance will
merely improve the loop stability and transient response. The ESR of the output capacitor should range between
0.3 Ω to 22 Ω. In the case of the adjustable regulator, when the CADJ is used, a larger output capacitance (22-µF
tantalum) is required.
------------------

Mentioned by you standard regulators don't have stability problems. Even we know this I would use C as specified in datasheet close to regulator.
You can also do some experiments. If you design fast switching load and observe IC output voltage when load changes abruptly you can form an opinion if IC is completely stable or close to instability.

Going the path of 'as specified in datasheet'...  In TI:LM317 datasheet you read about output capacitor:
-----------------
• CO improves transient response, but is not needed for stability
-----------------
So as they write 'not needed' then 'not needed'.

[aeg]

One traditional technique was to put the regulator IC on the PSU main board but put a pass transistor off board. These days it usually makes more sense to use a switching regulator.

[peter-h]

6" should be fine; just make the caps bigger than the DS asks for

[davelectronic]

I only asked, as the regulator was some 6" or more away from the main PCB board. In as much as having the regulator off the PSU board might cause voltage drops under sudden load, and a lower output voltage than the regulator is rated for. So if I put the linear regulator on a remote small board, would I need I larger electrolytic capacitor on that small regulator board on the regulators output. Say 25 volts 1000uf or similar, or is such a capacitor unnecessary on the remote regulator board. It was just to avoid voltage line drops as the regulator was some distance from the main board. Thank you for your replies.