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A Practical Problem.

Here is the problem. You are passing by a garage sale and spot a box full of parts and the price tag says $0.50. In the box is a pretty healthy power transformer rated at 250AC CT at 0.130 amps, two 100Ufd filter caps rated at 600 VDC and a 10 Henry choke rated at 150 milliamps.




You could get quite an array of voltage outputs from these components depending on what configuration you use for the rectifiers.

Three possible configurations include:

1. Full wave rectifier (2 Diodes)
2. Full wave bridge rectifier (4 Diodes) choke input
3. Full wave bridge rectifier (4 Diodes) cap input

Drum roll...

Full wave rectifier ~ 175VDC
Full wave bridge rectifier choke input ~ 225VDC
Full wave bridge rectifier cap input ~ 350VDC

That is quite a spread but while the voltage values would seem to draw our attention there are other factors which would actually directly influence your selection. Invariably many would invoke the best looking with the biggest boobs criteria (350 VDC) but fail to look at voltage sag, ripple voltage content and current availability. 

The full wave bridge with a choke input has better voltage regulation (less sag), less ripple voltage at full current versus than the Capacitor input form. So, the highest voltage configuration may not be optimum for your application. This requires you to look beyond the size of the boobs.

Not looked at is another configuration and that is the voltage doubler circuit as found in many Heathkit supplies (HP23). That 250 VAC and two caps can be made to look like 700VDC... but not at full current. 

With an eye toward those who think math the voltages in some of the forms is involved with the number 2, its square root, divided or multiplied.

In the case of the simple full wave rectifier we have the 250 VAC multiplied by 1/(2^.5) = 175 after subtracting the diode junction drops.

The full wave bridge with choke input = 250X.9 = 225. 

The full wave bridge with cap input = 250X1.414 = 350

The Voltage Doubler is 250X2.8 = 700. You can easily see gigantic boobs, terrible voltage sag, high ripple and not full current capability. But it is a cheap way to get high voltage.

So, that 50 cents could be quite a bargain. The only concern is the transformer has a short to ground, the caps are at End of Life, and the choke is open. There are no free lunches. Kind of like what the majority of Americans did last November. 

Our garage sale bargain is but another example of Flash the Cash.

Them that know can make things go.

73's
Pete N6QW

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