Is the electrolyt capacitor really shown in the right direction?

[maystorm]

Hi folks,

I am currently studying some of the material at talkingelectronics.com and came across this circuit:

(http://www.talkingelectronics.com/projects/FlasherCircuits/Page83FlasherCircuitsP1.html)

I am not an expert in electronics, however, I have the feeling that the E-Cap (10u) is inserted the wrong way (+ and - should be swapped). Is someone willing to enlighten me?

TIA,
Alexander

 

[Nigel Goodwin]

The LHS (-ve) is connected to the base of the NPN, so can't go higher than +0.7V, the RHS (+ve) is connected to the collector of the PNP, so can got to +6V minus the voltage drop of the LED, so about +4V, so it's the right way round. It's basically a variation on a multivibrator.

 

[maystorm]

Thanks for the reply.

However, what happens if the NPN gets damaged and doesn't turn ON anymore? Wouldn't the LHS (cathode) then be reverse-biased to +6v?

 

[maystorm]

I just made some google search and found a number of circuits which look like this:

Here, the E-cap is just the other way around. I am bit puzzled now, at least.

 

[Nigel Goodwin]

I just made some google search and found a number of circuits which look like this:

View attachment 118680

Here, the E-cap is just the other way around. I am bit puzzled now, at least.

There's lot's of incorrect circuits on the Internet, and that's another example of one - presumably someone (like yourself) saw the original circuit and decided it was the wrong way round.

 

[maystorm]

Thanks. I'd like to repeat my question:

However, what happens if the NPN gets damaged and doesn't turn ON anymore? Wouldn't the LHS (cathode) then be reverse-biased to +6v?

 

[maystorm]

I had a closer look at the PCB layout:

If I am not totally wrong, the anode (+) of the E-cap is connected to B of the NPN and not the cathode. This puzzles me even more now.

 

[Nigel Goodwin]

I had a closer look at the PCB layout:

View attachment 118683

If I am not totally wrong, the anode (+) of the E-cap is connected to B of the NPN and not the cathode. This puzzles me even more now.

No, it's clearly connected to the collector, and the right way round - the base is the middle leg in civilised pinouts

 

[maystorm]

No, it's clearly connected to the collector, and the right way round - the base is the middle leg in civilised pinouts

To the collector of WHICH transistor?? I thought (-) should be connected to B of NPN and (+) should go to C of PNP? Look at the side where the tracks are and then you'll see how the connections are done.

 

[maystorm]

No, it's clearly connected to the collector, and the right way round

I don't think so:

 

[Nigel Goodwin]

Sorry, I was obviously confused flipping the two sides together in my head

But as I've said all along, the positive goes to the collector of the PNP, as that's (by far) the most positive side.

There's no point keep posting faulty circuits and layouts you find on line, the +ve should go to the most positive point, your first one was correct.

 

[maystorm]

There's no point keep posting faulty circuits and layouts you find on line (...)

Well, I just ask questions. No reason to get rude. And don't forget: Both the layout AND the circuit come from the SAME page. This is not puzzling?

Perhaps you are still be so kind as to answer my second question:

"What happens if the NPN gets damaged and doesn't turn ON anymore? Wouldn't the LHS (cathode) then be reverse-biased to +6v?"

Will it blow up or damage the E-cap?

 

[unclejed613]

However, what happens if the NPN gets damaged and doesn't turn ON anymore?

when parts fail, all bets are off. there are amplifiers out there where an output transistor shorting will smoke the whole amplifier, all the way back to the diff amp input stage. when a component fails, expect to replace more than just that one part. such is life... over-engineering something so that all parts are fail-safe can get rather expensive and time consuming.

 

[maystorm]

when parts fail, all bets are off. there are amplifiers out there where an output transistor shorting will smoke the whole amplifier, all the way back to the diff amp input stage. when a component fails, expect to replace more than just that one part. such is life... over-engineering something so that all parts are fail-safe can get rather expensive and time consuming.

Thanks. I just wanted to get confirmed whether my understanding was right or not. And a reverse-bias of -0.7v doesn't obviously damage an E-cap. That's fine and everything I wanted to know.

 

[Nigel Goodwin]

Thanks. I just wanted to get confirmed whether my understanding was right or not. And a reverse-bias of -0.7v doesn't obviously damage an E-cap. That's fine and everything I wanted to know.

The ones with the capacitor the wrong way probably work OK as well, but they are much more likely to fail with that degree of reverse voltage, I think the fact that it's AC probably helps to keep them working?.

The last Sony VCR's, manufactured by Samsung, had an electrolytic fitted the wrong way round - and this supposedly caused them to trash the EEPROM memory. There was a modification which included replacing the capacitor, but fitting it the right way, and reprogramming the EEPROM.

However, the modification didn't prevent the memory getting corrupted in the future, so the cap the wrong way seemed to make no difference in that case.

 

[maystorm]

Thank you. I am still learning and cannot see (yet) whether something is wrong or not. Now, I learned something new and important with this thread.

 

[Ylli]

.

 

[unclejed613]

And a reverse-bias of -0.7v doesn't obviously damage an E-cap.

electrolytic caps were actually a spin-off from another electrolytic device used in the early days of radio, called an "electrolytic rectifier". i have a book from 1917 and another book from the early 1930s showing them being used for charging batteries from an AC source. the big difference was that electrolytic rectifiers actually used aluminum plates suspended in a liquid electrolyte, and the later electrolytic caps use cardboard or paper soaked in electrolyte. most electrolytic caps still have the "rectifier" characteristic, if you forward bias the "diode" it will conduct (but it's a capacitor, and what you have actually done is reverse biased the capacitor). low reverse voltages on an electrolytic cap aren't much of a problem if they are kept to about 5% or less of the cap's rated voltage (i.e. it probably won't damage the cap, but there still might be some leakage current but not enough to damage the cap).

another obscure thing about electrolytic caps: i worked at a TV shop, and the owner had a hard time getting certain values of electrolytic caps for higher voltages (200+ volts). so he had one workbench with power supplies dedicated to "reforming" lower voltage caps to higher voltages (with a proportional reduction of the capacitance). for instance if a 10uF/400V cap was needed, a 100uf/50V cap would be connected to a 400V power supply through a 10Meg resistor, and a voltmeter would be connected across the cap. basically, this created a small leakage current through the cap, that would slowly make the oxide coating on the aluminum grow thicker until the voltage rating of the cap grew to 400V, and the meter would be reading 400V across the cap and the leakage current was less than a few microamps. this process (depending on the desired voltage rating vs the original voltage rating) could take several days. this is similar to the forming process used at the factory to form the cap when it's made (except they start with no oxide on the plates).

 

[unclejed613]

i would like to find out how the leap in logic was made from electrolytic rectifier to capacitor. it's one of those things that sounds highly probable to have been discovered by accident or somebody noticing something unexpected during a test. it seems like it would make an interesting story

 

[narkeleptk]

electrolytic caps were actually a spin-off from another electrolytic device used in the early days of radio, called an "electrolytic rectifier". i have a book from 1917 and another book from the early 1930s showing them being used for charging batteries from an AC source. the big difference was that electrolytic rectifiers actually used aluminum plates suspended in a liquid electrolyte, and the later electrolytic caps use cardboard or paper soaked in electrolyte. most electrolytic caps still have the "rectifier" characteristic, if you forward bias the "diode" it will conduct (but it's a capacitor, and what you have actually done is reverse biased the capacitor). low reverse voltages on an electrolytic cap aren't much of a problem if they are kept to about 5% or less of the cap's rated voltage (i.e. it probably won't damage the cap, but there still might be some leakage current but not enough to damage the cap).

another obscure thing about electrolytic caps: i worked at a TV shop, and the owner had a hard time getting certain values of electrolytic caps for higher voltages (200+ volts). so he had one workbench with power supplies dedicated to "reforming" lower voltage caps to higher voltages (with a proportional reduction of the capacitance). for instance if a 10uF/400V cap was needed, a 100uf/50V cap would be connected to a 400V power supply through a 10Meg resistor, and a voltmeter would be connected across the cap. basically, this created a small leakage current through the cap, that would slowly make the oxide coating on the aluminum grow thicker until the voltage rating of the cap grew to 400V, and the meter would be reading 400V across the cap and the leakage current was less than a few microamps. this process (depending on the desired voltage rating vs the original voltage rating) could take several days. this is similar to the forming process used at the factory to form the cap when it's made (except they start with no oxide on the plates).

very interesting read, thank you.