Linear voltage regulator off the PCB. - Page 1
Forum section: electronics / projects

author: davelectronic
author_rank: Frequent Contributor
timestamp: August 19, 2024, 10:41:40 am
content: 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.

author: Jeroen3
author_rank: Super Contributor
timestamp: August 19, 2024, 12:06:59 pm
content: Yes, any regulator requires capacitance to provide bypassing.Quote from:https://www.ti.com/lit/ds/slvs044y/slvs044y.pdf?ts=1724047230771Another example:https://www.onsemi.com/pdf/datasheet/lm317-d.pdf

author: davelectronic
author_rank: Frequent Contributor
timestamp: August 19, 2024, 04:16:19 pm
content: 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.

author: PGPG
author_rank: Frequent Contributor
timestamp: August 19, 2024, 05:06:58 pm
content: 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 CapacitorThe output capacitoris critical in maintaining regulator stability, and must meet the required conditions for bothminimum amount of capacitance and equivalent series resistance (ESR). The minimum output capacitancerequired by the LM1117 is 10 µF, if a tantalum capacitor is used. Any increase of the output capacitance willmerely improve the loop stability and transient response. The ESR of the output capacitor should range between0.3 Ω to 22 Ω. In the case of the adjustable regulator, when the CADJ is used, a larger output capacitance (22-µFtantalum) 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'.

author: aeg
author_rank: Regular Contributor
timestamp: August 20, 2024, 06:20:58 am
content: 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.

author: peter-h
author_rank: Super Contributor
timestamp: August 20, 2024, 11:28:36 am
content: 6" should be fine; just make the caps bigger than the DS asks for

author: davelectronic
author_rank: Frequent Contributor
timestamp: August 20, 2024, 11:39:54 am
content: 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.