DC voltage blocker - Blocking AC too?

[Kain6622]

Afternoon everyone,

I'm currently starting my journey on electronics and working through "Electronics All-In-One for dummies - Second edition" and been working on Project 14 which is a simple DC Blocker circuit which is supposed to block DC current but allow AC current to flow... I've made up the circuit as shown below in the images, It does as expected for the DC - I connect a 9v battery to the circuit, the LED lights up momentarily then goes off once the capacitor is fully charged as expected.

But when I connect the 9v AC adapter to the circuit (measuring 11.1vac output on the multi-meter so took that into consideration when choosing the resistor for the LED) the LED lights up as it should do for a while but then starts to dim until it is lit no more (just like the DC supply but only slower) and checking the flow of current of the it goes down to zero over time just like when supplying DC power.

I've look about on the internet but everywhere says that AC should flow through but this is not happening with my circuit, please could someone help me understand why this is not working the way they say it should?

The circuit components are as follows:

470Ω resister (±5%)
LED (forward voltage 3.2v, current at 20mA)
3x 100μF 50v Electrolytic capacitors (connected in parallel)
9v battery
9vac Adaptor (Input 230v 35mA 50Hz, Output AC 9V 300mA)


Thank you for any advice you can give on this.
Regards
Dave

 

[Alec_t]

Welcome to EP!
The clue is in the name Light Emitting Diode. The LED is rectifying the current, so the caps charge up like they did with the battery as power source. Let's hope you had the LED the right way round, as electrolytic caps hate to be charged with the wrong polarity.
By the way, with electronics we like to see a circuit schematic rather than a breadboard.

 

[Kain6622]

Thank you for the reply Alec_t,

It had crossed my mind with the LED as with Diodes in general, current can only flow one direction and can be used to convert AC to DC, I thought that I might of done something wrong as I expected the book's circuit to be tested and working as they said it would. I made sure the Anode(+) and Cathode(-) where connected correctly for both the LED and Capacitors (so no big pops)

Their Schematic was as follows (they where using a 9vac 1000mA adapter):


Mine was similar apart from having a 300μF capacitor instead and no switch and using the adaptor with only a 300mA output of current instead of 1000mA.

The book said that on connecting the battery we should see the LED light for a short time then go off once the capacitor is charged (which it does), and on connecting the AC supply the LED should come on and stay on (which it doesn't)... I may need to ask for some recommendation for another book to start me on my electronics journey if they have stuff wrong this early on (don't fancy super saiyan hair just yet).

Regards
Dave

 

[Alec_t]

I'd ditch that textbook, but don't have any recommendations. Other members here may have.

 

[Kain6622]

Thanks for the advice Alec_t, did a quick search on the forum and found this. The book 'The Art Of Electronics' sounds promising so may see what reviews say on it.

Regards
Dave

 

[PETERDECO]

It looks like what you're describing is what should happen. You have DC by half wave rectification and is charging the capacitor the way the battery is.

 

[bertus]

Hello,

You could also have a look at this thread for some books:
Book Reviews

Bertus

 

[Kain6622]

Hello,

You could also have a look at this thread for some books:
Book Reviews

Bertus

Thank you, Given it a read and some good suggestions on it, some sound like good investments. I've also decided not to give up on my current book 'Electronics - all-in-one for dummies sec edition' so quickly but learn from his mistakes and take notes on what went wrong and then move onto some of the books suggested once I've completed it... after all there is nothing to learn without first making mistakes, even if they are not your own.

Regards
Dave

 

[Audioguru]

The AC is destroying the LED because its maximum allowed reverse voltage is only 5VDC.

The 9V AC has a peak voltage of 9V x 1.414= 12.7V The LED might be a 2V red one then the capacitor charges to +10.7V. The AC produces negative peaks of -12.7V then the LED gets reverse voltage pulses of 12.7V + 10.7V= 22.7V. The LED will soon curl up and die.

 

[PETERDECO]

Good point Audioguru. Reverse voltage too high. Like Alec_t suggested, ditch that book.

 

[WHONOES]

I have recommended a book called "The Art of Electronics" by Horowitz and Hill several times on this forum. It is probably the best book for learning about electronics from base zero. Not cheap but worth it.

 

[duke37]

I have two versions of the Art of electronics. One review suggested that the main requirement was a strong table,
I do not use the latest version but often refer to the previous one which may be a reasonable price second hand,

 

[Ylli]

Replace what you have with this:

D2 can be wither an LED or a standard diode.

 

[Kain6622]

Replace what you have with this:
View attachment 48798
D2 can be wither an LED or a standard diode.

Hello Ylli,

thanks for the suggestion, having the second diode in reverse allows the current a easy flow path when it changes direction rather than getting blocked by the first diode, nice idea!

I have tried out the circuit still with the polarized capacitor and it works perfectly, voltage in the capacitors in the old circuit accumulated about 16v on ac power where in your suggestion only has about 3mv.

I read somewhere online that though in theory using a polarized electrolyte capacitor in such a circuit could damage the capacitor, from experiments it's not always the case and sometimes no damage is received from loads of tests. From my trials it seems to be the case from stress testing the circuit all seems to be fine, I like the idea of putting theory and practice to the test and learning from the findings, this has Sparked more enthusiasm towards electronics.

I also want to thank everyone for the suggestions towards the book 'The Art of Electronics' I shall look into investing in it in the near future.

Kind Regards
Dave

 

[Alec_t]

I read somewhere online that though in theory using a polarized electrolyte capacitor in such a circuit could damage the capacitor

Indeed it can. You can get round that by connecting two electrolytics in series, back-to-back, i.e. you connect then +--+.