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How about it? Experiments of the third kind , take 999999.

How about it? Experiments of the third kind , take 999999. Update opamp and PIC power supply design: part 1

PNP --------------------------- e c--------------------------------------------------------------- + 3.5V +/- 1mV @ 20mA noise free to PIC and EEPROMS. | | b Q1 | | | |

+4 to + 8 V | | | | | | | | | R1 | | | [ ] 40k | | [ ] 100k | | | | | | | | | | + | +2.5V | === 470u | +2.1V c c / |------|

--------- | --- |------ b NPN NPN b-----< | | +2.5V | MCP1525 | | | | e Q2 e Q3 \ |------)-------------| |-----| | --- | | 1/4 | | | | | | | \ /-> ------ TCL274| | |

--------- | | --- LED | | | === | === | | green [ ] 1k | [ ] | 1u | --- 470u | | | R2 | | 100k | | |

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Transistors: PNP BC557B, NPN BC548B. The 'Tim' capacitor from the opamp output to its - input is missing, even a small one ruins step load recovery. There is LF noise from the the CMOS opamp on its output, looks like audio, but is really just noise in the audio range, and it does not bother me a bit. The rather large output caps maybe help too. The MCP1525 is a Microchip 2.5V precision reference.

Some people here do not like PNP low drop out regulators or so I have read. Reason ? Dunno. I never had stability problems with those. This circuit drops out at about 3.80 V input, it will be fed from 4 alkaline AA batteries in series, those I consider 'flat' at 1V, makes >=4 V input. IF those are 'fresh' and 1.8V we have 7.2V input maximum. I tested up to 15 V input and the output stays within the accuracy of my meters (1mV here)>

And 100 % stable. So this will be used to power the opamp (it is shown here how it is on the regulated supply it is helping to make itself, and the PIC (ADC reference is taken from internal PIC reference). All this should assure some stability. I have not draw the connection to the battery, as there is more in there, like a big IRLZ34N MOSFET switch that automatically switches in the battery when the input voltage from the wallwart AC/ DC adaptor fails (mains failure detector). The load on the opamp is about beta x R2, say 300k, not a big deal heating the opamp, so it should not affect the other 3 amplifiers in the opamp.

The LED is used as 2.1V (or there about) reference for the opamp to work against. That voltage, minus .7V for Vbe, divided by R2 sets the maximum base current in Q1 (about 1.4mA), The circuit is not short circuit proof, and does not need to be, Q1 will fail during a short circuit as it cannot do 350mA and the associated power. Battery will be fused, in case of a short the experiment ends early anyways, no more valid data, but what was collected will be in EEPROM.

A lot of circuit for 3.5V @ 20mA... :-)

Reply to
Jan Panteltje
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How about it? Experiments of the third kind , take 999999. Update opamp and PIC power supply design: part 1

PNP --------------------------- e c--------------------------------------------------------------- + 3.5V +/- 1mV @ 20mA noise free to PIC and EEPROMS. | | b Q1 | | | |

+4 to + 8 V | | | | | | | | | R1 | | | [ ] 40k | | [ ] 100k | | | | | | | | | | + | +2.5V | === 470u | +2.1V c c / |------|

--------- | --- |------ b NPN NPN b-----< | | +2.5V | MCP1525 | | | | e Q2 e Q3 \ |------)-------------| |-----| | --- | | 1/4 | | | | | | | \ /-> ------ TCL274| | |

--------- | | --- LED | | | === | === | | green [ ] 1k | [ ] | 1u | --- 470u | | | R2 | | 100k | | |

-------------------------------------------------------------------------------------------------

Transistors: PNP BC557B, NPN BC548B. The 'Tim' capacitor from the opamp output to its - input is missing, even a small one ruins step load recovery. There is LF noise from the the CMOS opamp on its output, looks like audio, but is really just noise in the audio range, and it does not bother me a bit. The rather large output caps maybe help too. The MCP1525 is a Microchip 2.5V precision reference.

I did some impulse response test, and, as the opamp is basically open loop here, needed to know the stability of the control loop, and how the system handles impulses. The PIC will cause supply current variations, but especially the EEPROMs will cause 5mA or bigger current impulses during a write. Here a scope screenshot:

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This is a 100 x per seconds repeated 20mA (resistor switched on output with a BJT). The response is very nice, opamp output goes 100mV negative during load, (negative disables Q3, and then Q2 delivers more base current to Q1 to compensate for any drop in output voltage. I have been unable to detect any drop in output voltage with my scope at 10mV / division AC, no surprise due to the huge loop gain. The 'noise' is CMOS opamp low frequency noise as far as I know[1]. With this this part of the supply has passed design review...

Looks so much like 'audio' maybe it is trying to say something?

Reply to
Jan Panteltje

Yuck, an LDO. Don't you read Joerg at all? ;-)

It could very well be audio. Hook up a speaker and see :) The op-amp probably won't be making much, but the BJT junctions will rectify RF if it's around. I wouldn't think a little circuit like this would really be prone to anything, even with a 20' power cable hanging off it.

More likely, it's amplified residual noise from the reference. References tend to be quite noisy, even with a bypass cap. MCP1525 says 500uVpp from

10Hz-10kHz, which will easily make >> 50mV at the op-amp output. It should be heavily weighted towards low frequencies though -- it's 1/f noise to begin with (including the 1uF filter cap), and the (essentially) open loop op-amp integrates that further, making more-or-less brown noise (1/f^2).

Tim

-- Deep Friar: a very philosophical monk. Website:

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Reply to
Tim Williams

On a sunny day (Thu, 5 Apr 2012 19:47:29 -0500) it happened "Tim Williams" wrote in :

No kidding, some time ago I had a shortwave station coming in, it faded in and out...

That is a very good point, and I admit I overlooked that 500uVpp, so I will investigate it a bit more. Maybe a simple RC filter after the MCP1525 before the opamp input will decrease this, and will not affect response there. Lots of things to investigate this weekend. The EEPROMS and some later silicon versions of the PIC 18F14K22 arrived too, along with some other stuff that I will address some other time I hope. I finished the design of the switching mechanism that switches in the batteries if the input power from the wall wart fails, but still need to build it and test it. That is next.

Reply to
Jan Panteltje

How about it? Experiments of the third kind , take 999999. Update opamp and PIC power supply design: part 2

This is the switch that switches over to battery when the input from the AC/DC adaptor fails:

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----------- 1N5822 | 7805 |

1N5822 ------------------------- |>|-------------------| in out |----------------|>|----------------------- >A | | | | gnd | | | | | +9 to + 15 V === === [ ] ----------- | === [ ] R2 ||| fuse 1A from AC/DC adaptor | 10n --- |3k9 | IC1 | --- 4u7 | 100k | | | | R1 | | | | ------- /// /// | /// | /// | --- 4 x 1.5 V AA alkaline 4u7 | | | | | | | ------- | ......... | | --- --------|>|--------- | | n.c. b ----- . | ------- . | . CNY17-3 | --- . --- . opto | | . / \/ . | ------- . . | --- .. |...| .. | | |

--------------------------)----------| c | e | | | | |---|

-----------------------------------| |--------------------------- e c--------------------------------------------------------------- + 3.5V +/- 1mV @ 20mA noise free to PIC and EEPROMS. | | b Q1 | | | |

+4 to + 8 V | | | | | | | | | R1 | | | [ ] 40k | | [ ] 100k | | | | | | | | | | + | +2.5V | === 470u | +2.1V c c / |-------|

--------- | --- |------ b NPN NPN b-----< | | 10k +2.5V | MCP1525 | | | | e Q2 e Q3 \ |-------)-===---------| |-----| | --- | | 1/4 | | | | | | | | \ /-> ------ TCL274| |1u | |

--------- | | --- LED | | === | === | === | | green [ ] 1k | | [ ] | 1u | --- 470u | | | R2 | | | 100k | | |

------------------------------------------------------------------------------------------------- | ///

Transistors: PNP BC557B, NPN BC548B. The 'Tim' capacitor from the opamp output to its - input is missing, even a small one ruins step load recovery. There is LF noise from the the CMOS opamp on its output, looks like audio, but is really just noise in the audio range, and it does not bother me a bit. The rather large output caps maybe help too. The MCP1525 is a Microchip 2.5V precision reference.

Reply to
Jan Panteltje

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