Whats wrong with this circuit?

[JimmyQ]

Whats wrong with this circuit?

[CaptDon]

Other than the text being wrong and sounding like it was written by a student who didn't have good command of U.S. or U.K. english? They state when the power circuit is 'OPENED' the cap charges through D1 and thermal resistor, I don't think anything charges when the power circuit is open. Also the inrush limiter device is probably not working within its design characteristics. I have run into texts with similar wrong assumptions and explainations and strangely enough most were written by students from India. I worked with an H1B 'Professor of Electronic Technology' who supposedly taught electronics at an academy but in fact had no comprehension of the simplest of circuits and often rigged up test circuits that were either dangerous to the operator or hazardous to the equipment. When that person came to the lab to borrow equipment our first thought was 'Oh Shit, here comes another disaster'. I think the resume was a complete fraud just to get an H1B in the door and replace a Senior Fellow who had actual knowledge. Only the company, the lab and the customers suffered!!! Sounds like a good business plan??? O.K., I'm done ranting. Cheers!!

[Andy Chee]

Theoretically, an empty 1.5 farad(!) capacitor is zero resistance.  If you apply power to such a capacitor, the zero resistance may pull a massive current from the +5V power source.  The NTC thermistor prevents this massive current surge by imposing a high resistance when starting up from cold.  As the NTC slowly warms up from the current passing through it, the resistance will slowly fall, gradually increasing the capacitor charging rate until it is full.

D1 prevents the capacitor from backfeeding into any circuits when main power is lost (I assume the switch is to illustrate removal of main power, rather than being a central element in circuit operation.  Its placement somewhat confuses the entire understanding of the circuit operation).  When main power is lost, backup power from the capacitor can flow via D2, you don't want to waste backup power in the NTC so it is bypassed.

HOWEVER

In practice the supercapacitor will not be zero resistance when empty.  The supercapacitor datasheet will specify its internal resistance.  And this will dictate whether the NTC and D2 is even needed in the first place, as will the impedance of the +5V main power source.

Whilst inrush current limiting is a real phenomenon (see article below), its application here seems unwarranted.

https://www.sound-au.com/articles/inrush.htm
https://www.sound-au.com/articles/soft-start.htm

[tunk]

No experience with supercapacitors, but is there
anything that stops it from being overcharged?

[CaptDon]

On our locomotives the supercaps were used to hold up the 5 volt rail and all were rated at 5.5vdc max if I recall. We used 3 in series with balancing resistors charged with a constant current circuit (basically a current limiter) from the 12vdc rail. The supercaps fed a 5vdc 3 terminal regulator that was connected to the 5vdc power rail through a schottky diode. When a main power source failure was detected The MPU and associated circuits on the 5vdc rail had about 2.5 seconds of supercap power available to close files and do some quick housekeeping before the power sequencer I.C. shut everything down. From a dead start the supercaps took about 20 seconds to reach a full balanced charge. I took the nicad batteries out of my old Kenwood TR-7850 ham rig and replaced them with several supercaps (lifted from junk prototype boards at work). The Kenwood draws about 4ma. of current from the supercaps when the rig is unhooked from the mobile 12vdc power source. At least the supercaps don't corrode and eat everything in sight!! I get about 4 hours of backup. The batteries would have lasted around 5 days but they never did. There was always a shorted or weak corroded cell I guess from the extremes of temperature changes in a car?

[wasedadoc]

Other than the text being wrong and sounding like it was written by a student who didn't have good command of U.S. or U.K. english? They state when the power circuit is 'OPENED' the cap charges through D1 and thermal resistor, I don't think anything charges when the power circuit is open.

See my post at https://www.eevblog.com/forum/repair/help-repair-nad-l76-audio-receiver-fan-not-starting/msg5238264/#msg5238264

[sparkydog]

No experience with supercapacitors, but is there
anything that stops it from being overcharged?

...using one that's rated for more than the supply voltage (5V)? (Preferably by a comfortable margin!) I can't think of ever seeing a charge-limiter on a ("regular") capacitor; I don't know why that would be different for a "super" capacitor.

[ejeffrey]

I doubt the NTC is needed or effective.  NTCs are used as inrush limiters when you have a large steady state current that keeps them hot and for which you don't want too much resistance.  But as the supercap charges, the current will naturally drop towards zero.  That will both keep the NTC from reaching it's "hot" temperature and defeat the need for it's resistance to drop.  If inrush limiting is required a simple resistor is likely fine.

[JimmyQ]

Andy, once again, sounds reasonable.

This is of course with reference to my previous goal. I'll probably give it a try with an old psu without the extra diode an such, and see what happens 

I've looked at a lot of diagrams, and most appear to have a current-limiting resistor in series with the load. I've not seen any like this, and wondered why. I would have thought it better to put one in line with just the capacitor as shown. Thereby not limiting the current from source to load. Which is why I thought of this potential solution.

Interesting range of 'responses'.

[CaptDon]

I thought that came from a text book or something. Didn't understand the hand drawing with printed text in a .jpg format. Sorry. Others have offered good ideas and something to build on. Cheers!! For several years the most popular and available supercaps were 5.5vdc rated. often charged directly through a resistor of 47 to 100 ohms and isolated with a shottky diode that conducted to provide supercap power when main power failed or glitched. It is kind of a kludge done that way and is more useful when holding up a 5 volt rail that actually powers some type of 5 volt to 3.3 volt buck converter and all of your logic circuits are operated from 3.3 volts. In our case they were charged (several in series) to 12vdc and powered a three terminal 5vdc LDO regulator powering the 5vdc rail during power fail. We got power for around 2.5 seconds until the supercap stack discharged to about 5.3vdc. That 5vdc rail b.t.w. powered 3.3, 1.8, and 1.0 power regulators and a power sequencer I.C. to run a system based on Xilinx FPGA stuff. Xilinx stuff readily self destructs when not power sequenced correctly. Not very forgiving and easy to destroy the fabric!!

[JimmyQ]

A textbook?  My school days are long past, but if my hastily 'back of a fagpacket whilst on sat on crapper' illustration using paint.net (which allows text) could be mistaken for such, then I apologise!

So a little update (been busy programming).  I finally found the specs for this particular cap. In the nomenclature, it has a circuit design, virtually the same as my scrawl (but drawn much more effectively).

Thanks for the input, all I have to do now is attempt to work out which side is actually positive.