Classic Toy Trains Magazine for February 2015

[webenda]

In the February 2015 Classic Toy Trains "WEEKEND WORKSHOP" on

page 69 there is an article showing a power supply for an LED light

strip. In the article C1 and C2 are called out as 47MFD, 16VDC

electrolytic capacitors. 16VDC is not a high enough working voltage

rating for these capacitors.

 

On the positive half cycle of the input voltage, capacitor C2 charges,

through rectifier D2, to a voltage of 1.414 times the input voltage

minus diode drop. On the negative half-cycle, the input voltage, in

series with the voltage of C2, charges capacitor C1 through rectifier

D1 to the desired output voltage of C2 plus 1.414 times the input

voltage minus diode drop. Capacitor C2, which aides in the charging

of a capacitor C1, sees alternating current while C1 sees only direct

current.

 

If the input voltage is 12 VAC, the voltage across capacitor C2 is

1.414 x 12 - 0.7 = 16 VDC. The voltage across C1 is

16 + 1.414 x 12 - 0.7 = 32 VDC. It is possible for this circuit to see

18 VAC, in which case C2 charges to 25 VDC and C1 charges to

43 VDC. The call out for C1 and C2 to be 16 VDC is not high enough

voltage.

 

In the article circuit, only one capacitor (C1) supplies the voltage and

current to the load while the other capacitor sees alternating current.

A better voltage doubler would be a full wave design. In a full wave

design, each capacitor sees half the output voltage of the doubler, DC

only and both capacitors share the load. I recommend the following

revision to the circuit shown in the article. Minimum rating for C1 and

C2 is 35 VDC for this circuit.

 

NOTE--Edited 20141228, added more line feeds in text to band aid an

apparent problem with CTT's new forum application. 

 

 

[webenda]

One more thing, absolute maximum rating for the input voltage of

a 7812T voltage regulator is 35 volts. I use the circuit presented

above, but my little Marx Hobby Transformer has a maximum

output of 12 VAC so I do not exceed the voltage regulator rating.

 

Other people use the circuit in the article or the one I offered

without reporting any problems, so one might assume the 35

volt maximum input rating on the 7812T is not a problem. I offer

this information so that, if the lights go out, you might have a

clue where to start your failure analysis (the regulator.)

 

NOTE--Edited more line feeds into text. 20141228

[lionelsoni]

Wayne, on my computer the right edge of your entire original post is cropped off (probably a feature of the new forum software). If you can find a way to fix that and to include the circuit as published (I don't have it yet), I would be interested to see what you have done with it.

[rtraincollector]

fine on mine might be something with some softwear you have Bob I know for a while I had a problem with Chrome and yahoo had to completely remove chrome to correct it 3 times I think in all so far this time all is fine

[rtraincollector]

 In the February 2015 Classic Toy Trains "WEEKEND WORKSHOP" on page 69 there is an  article showing a power supply for an LED light strip. In the article C1 and C2 are called out as  47MFD, 16VDC electrolytic capacitors. 16VDC is not a high enough working voltage rating for  these capacitors.

 

 On the positive half cycle of the input voltage, capacitor C2 charges, through rectifier D2, to a  voltage of 1.414 times the input voltage minus diode drop. On the negative half-cycle, the input  voltage, in series with the voltage of C2, charges capacitor C1 through rectifier D1 to the  desired output voltage of C2 plus 1.414 times the input voltage minus diode drop. Capacitor  C2, which aides in the charging of a capacitor C1, sees alternating current while C1 sees only  direct current.

 

 If the input voltage is 12 VAC, the voltage across capacitor C2 is 1.414 x 12 - 0.7 = 16 VDC.  The voltage across C1 is 16 + 1.414 x 12 - 0.7 = 32 VDC. It is possible for this circuit to see 18  VAC, in which case C2 charges to 25 VDC and C1 charges to 43 VDC. The call out for C1 and  C2 to be 16 VDC is not high enough voltage.

 

 In the article circuit, only one capacitor (C1) supplies the voltage and current to the load while  the other capacitor sees alternating current. A better voltage doubler would be a full wave  design. In a full wave design, each capacitor sees half the output voltage of the doubler, DC  only and both capacitors share the load. I recommend the following revision to the circuit  shown  in the article. Minimum rating for C1 and C2 is 35 VDC for this circuit.

 

[webenda]

Mail slow in Texas?

Bob, did rtraincollector's repost of my text help?

If not I edited more line feeds in. I can't see what I did.

Did I get all the lines?

I resized the schematic smaller and added CTT's version.

[lionelsoni]

Thanks! It looks fine on my on-line computer at work. I'll see if it improves on my home computer later today. (I use Windows XP because it's the only way I have been able to keep running Q-Basic; but the XP machine at work is off-line.)

[lionelsoni]

I've taken a look at the published circuit that Wayne was kind enough to post.  It is the 1913 Greinacher circuit, but with the final diode (D1) and capacitor (C1) interchanged.  The doubled voltage still appears across the final capacitor; but swapping those components frustrates the only good reason for using Greinacher's circuit for doubling, which is that the input and output circuits share a common return.  Whether or not that were desirable (and it often is in toy-train wiring), I certainly agree with Wayne that the voltage ratings are much too low.

Otherwise, the circuit that Wayne proposed (the Delon circuit) is clearly a better choice.

By the way, I had no idea that the inventors of these circuits were known until I looked them up on Wikipedia (http://en.wikipedia.org/wiki/Voltage_doubler and http://en.wikipedia.org/wiki/Voltage_multiplier).  Where did I think they came from?