Solving 555 Based Switcher Challenge for Niхie Clock

[Tom.G]

From page 16 (11 of the pdf) of that old Signetics 555 datasheet

Looking at the schematic on page 2 (4 of the pdf) shows that the 1.7V lower limit is the end of the linear range of the CONtrol voltage (pin 5). No damage pulling it to GND., it's the Base of an NPN Darlington pair referenced to GND. There is also a 5K pullup to supply.

Bringing CONtrol voltage to GND locks out the TRIGger signal (pin 2) and sets the THReshold voltage (pin 6) to 1.4V (two Vbe drops), terminating any Output pulse when the timing capacitor is at or above 1.4V

Overall it looks like a safe configuration of the 555, although not usually seen.

NOTE: On the manual pages 10 and 16, the formula for Duty Cycle has a typo.
It SHOULD BE: . D = 1-{R_b/(R_A + 2xR_B)}

 

[Borek]

There is a good chance that L1 is also destroyed, so you might as well order another if you don't have a spare.

After some digging - that's actually the only element that has died. Even Q1 still works (not that I plan to use it again after the stress).

1) R10 should be 1/2W due to voltage limitation of physically smaller resistors

It wasn't. Will see into that later, atm it looks like the resistor was perfectly OK (I know it can mean I am just lucky).

2) C5 should be at least 250V

400 V it was.

3) Add a 0.47μF, 250V metallized film capacitor across C5

That's just to make it react faster to the voltage changes, yes?

At this point Q1 is obviously defunct so gently remove it from the circuit. Gently because we don't want to disturb other evidence.

That was an easy extraction). After it has been removed 555's OUT was still oscillating.

Replace Q1.

For initial testing, L1 can be replaced with a 470Ω, 1/2W resistor.

Unfortunately at this moment things went fubar. When the breadbord got melted elements got stuck, so being gentle was no longer an option. It took quite a bit of force to remove the L1, took some shaking. Once it was removed, oscillations stopped. They returned later after I moved some of the cables trying to measure voltages, so I believe it was a problem with some loose connection on the breadboard (another reason to suspect it was problem with a connection from the very beginning).

What I did at this moment was I moved part of the circuit several holes to side, replaced Q1 with a new one, switched the current on - and it turned out oscillations are OK, the circuit draws 1A all the time (limited by current protection from the power supply), but the voltage on C5 doesn't grow. That made me suspect L1 and D1 and bingo - it was L1 that was shorted somewhere inside. No idea whether it is cause or effect of the meltdown.

I will look into your other suggestions later, for now I am at least at the same point I was on Thursday.

Sadly LTSpice doesn't want to cooperate when I try to simulate the circuit with Q2 attached to the RESET pin.

 

[Tom.G]

That's just to make it react faster to the voltage changes, yes?

Sort of. C5, being a large value, is probably an electrolytic of some sort. Although some chemistries are good for higher frequencies, most have a high ESR (Equivalent Series Resistance) at higher frequencies. This limits how fast they can charge and discharge, increasing ripple, and also adds to their internal power dissipation, raising their temperature and shortening their life. In your particular usage neither are likely to cause trouble because of the extremely low load current. In fact you could replace C5 with the suggested 0.47μF and stiil have only 1V of ripple.

 

[jim hardy]

More than anybody wants to know about electrolytic capacitors here
http://www.cde.com/technical-support/engineering-technical-papers

This one's close to your application http://www.cde.com/resources/technical-papers/strobe.pdf
http://www.cde.com/resources/technical-papers/reliability.pdf
http://www.cde.com/resources/technical-papers/PFC-Tech.pdf

CDE is an old line US manufacturer. I encountered a lot of their aluminum electrolytics in the nuke plant that were still good after thirty years. So i have a warm feeling toward that company.
They have good training materials like these available free. Enjoy!

 

[Borek]

In fact you could replace C5 with the suggested 0.47μF and stiil have only 1V of ripple.

My first idea was to use 1μF. In LTSpice simulation voltage got to 200 V about ten times faster, but the ripple was 1.5 V, so I decided to use a larger cap.

 

[Svein]

(https://www.eevblog.com/forum/proje...nd-buck-converter)-switching-power-regulator/ )

 

[Tom.G]

My first idea was to use 1μF. In LTSpice simulation voltage got to 200 V about ten times faster, but the ripple was 1.5 V, so I decided to use a larger cap.

Strange. Oh well, no harm either way, the Nixie tubes don't care. LTSpice may have used actual capacitor characteristics where I took a shortcut and used an 'ideal' cap, 'ideal' voltage source and 'ideal' diode..

 

[Borek]

Argh, just lost another power supply. I was ready to do some tests of the switcher when my Korad 3005 died

It is still under warranty, but I doubt I will see it back in November. Sigh.

 

[dlgoff]

... my Korad 3005 died ...

It is still under warranty, ...

If it wasn't under warranty, I'd say, it's a chance for improving your troubleshooting skills.
Sorry for your loss though.

 

[Borek]

chance for improving your troubleshooting skills

Judging from audio-olfactory signals it sent locating the problem shouldn't be too challenging. Digital part (display and control knobs) works OK, just the output is dead.