Simple regulator does not cooperate. - Page 1
Forum section: electronics / projects

author: tubes3000
author_rank: Newbie
timestamp: October 20, 2022, 01:56:26 pm
content: I have built this classic regulator circuit to get 6.3DC from 6.3AC but no matter what i do i'll get that oscillation on the output of the opamp as shown on the yellow trace.It stops when there is no load but as soon as a tiny current is demanded it starts and stays unchanged from around 100mA upwards.What have i tried:Opamp, reference and cap, global and local feedback caps and resistors meaning in and out of circuit as well as different values, load capacitors, rectifier capacitors, passelement from MOS to NPN, Zener, bypass caps.I took rectification and local supplies out of the circuit. Gate resistor value and position where changed. All to no change.I have checked the PCB and i have used this circuit years before and on the other very different PCB it does the same but wasn't detected back then.The sharp edges make unwanted harmonics and noise on the output.The bigger square pulses come at 113.8HZAnd the oscillation itself sits at around 1000Hz. I've put the thing on a breadboard and there it works fine.What do i miss here???

author: tubes3000
author_rank: Newbie
timestamp: October 20, 2022, 03:17:31 pm
content: The LM317 has a too high dropout voltage and too low current capability. Of course the LP and the DCDC were my first suspects too but they are both stable as very little current is drawn from then.I took the LP and the DCDC out of circuit for testing and no change.

author: wraper
author_rank: Unknown
timestamp: October 20, 2022, 03:35:03 pm
content: Quote from: tubes3000 on October 20, 2022, 03:17:31 pmI don't see a purpose of using DC/DC converter other than creating noise. LP2950 requires at least 1uF cap and with ESR in specific range of 0.1 Ω to 2 Ω, ceramic caps without workaround to add ESR do not fit there. DC/DC converter requires more input and output capacitance.

author: wraper
author_rank: Unknown
timestamp: October 20, 2022, 03:44:20 pm
content: Quote from: edavid on October 20, 2022, 03:37:17 pmShould have worded better but it's a waste of parts for such purpose and with inferior result. Either do all of the circuit to work like any normal LDO, or use simple boost converter without needing LP2950.

author: mikerj
author_rank: Super Contributor
timestamp: October 20, 2022, 04:32:28 pm
content: Quote from: tubes3000 on October 20, 2022, 03:17:31 pmSlightly OT, but be very careful using these isolated DC-DC modules. An often overlooked specification is the minimum load current, which for the Murata parts is typicaly 10% of full load. Too little load causes them to drop out of regulation (which is pretty poor anyway) and the output voltage can rise dramatically. Minimum load for the part you are using is 8.5mA, which I doubt you are meeting. They are also electrically noisy devices.

author: tubes3000
author_rank: Newbie
timestamp: October 20, 2022, 08:59:22 pm
content: Thanks i am aware of that. R6 plus the reference and the opamp give a load of >10mA.

author: temperance
author_rank: Super Contributor
timestamp: October 20, 2022, 09:19:09 pm
content: A rather complicated circuit for something simple. You can achieve the same with a PNP transistor (or a MOSFET) and an op amp.Something like this by Jay_Diddy_Bhttps://www.eevblog.com/forum/beginners/pnp-based-linear-regulator/

author: tubes3000
author_rank: Newbie
timestamp: October 24, 2022, 10:00:11 am
content: It was the electronic loadThe circuit works fine.