Re: Question on Power Supply 1.4 or 1.414 and diodes and watts

--- In CAD_CAM_EDM_DRO@yahoogroups.com, "Polaraligned"
<polaraligned@o...> wrote:

> 1.414 is the square root of 2, and is how we get the RMS value of
> a peak to peak voltage. The RMS value is also called the "effective
> voltage". Your 120vac household outlet is really 170v peak to peak.
> This RMS value is what most meters read. This value of 1.414 has
> nothing to do with the diode voltage drop. The voltage drop of a
> bridge rectifier
> happens to be 1.4v because the current passes through 2 silicon
> diodes which each have a forward voltage drop of .7v each.

As I understand, it goes AC in on the primary of the transformer. we
just call this PRIMARY

the altered voltage that comes out of the transformer is called SECONDARY.

the secondary has the same wattage, but different voltage. (don't get
hung up on 'same wattage just yet)

Then you change to DC by using 4 diodes in a bridge, or a bridge
rectifier. Depending on the parts used, the natural voltage drop of a
diode will be between about 0.5 volts on the low to over 1.5 volts on
the high end. Bridge rectifiers list this as do data sheets for diodes.

1.414 is the correct number. using 1.4 gets you close, but off, but
it really doesn't matter all that much.

12-less 1 -times 1.414 = 15.554
12 x 1.4 = 16.8 a higher voltage so if you use that to size your
caps, you built in a little safety factor.

48 less 1 times 1.414 = 66.458
48 times 1.4 = 67.2

Now, if you try to measure the secondary of the transformer, you
quickly learn that the load on the output will vary the voltage. a
24V transformer may have 30 volts on the output when there is no load.
at 24 volts, the load will also have heating, but enough to get warm,
not enough to destroy itself.

Another reason to oversize the voltage of caps by increasing the
voltage to 50% greater than you PS voltage.



as for that 'same wattage' comment.

if you have a 24V 10A transformer, you can say it is 240VA or volts
times amps. Transformers are often rated in VA. VA and Watts are
often used in place of each other in our power supply math, volts
times amps = watts so 240 VA = 240 watts.

Now you have 24volts at 240 VA, then lose the volt for the bridge and
multiply by 1.414 to get 32.5 volts.

now, you have the same watts, but different voltage.
The transformer is rated for 240 VA or 240 watts, but what is the
current at this new voltage ?
240 / 32.5 = 7.38 AMPS.

Too much math ? use watts for the motors and power supply.

Dave