120 VDC

Discussion in 'Electronic Basics' started by Chris W, May 8, 2005.

1. Chris WGuest

I was recently told that if rectify the 120VAC mains voltage and then
add a capacitor to smooth out the wave, I will then have something
around 167 VDC. Is this true? If it is true, what is the easiest way
to get a smooth 120V DC voltage?

--
Chris W

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2. John PopelishGuest

If you can find an 85 volt transformer, it would produce approximately
120 volts DC with a capacitor filter. It would also provide isolation
from the mains, so that a connection to ground would not be hit with
the full current the line could deliver. The 120 volts will not be
very smooth, however. It will have ripple at twice the line frequency
when loaded and will sag as the load current increases.

If you want a clean, accurate and stable 120 volts DC you will have to
build or buy a regulated supply. For instance, here is one that is
adjustable from zero to 120 volts DC, capable of supplying up to 1 ampere.
http://shop.store.yahoo.com/webtronics/lowcosioubep.html

3. Jonathan WesthuesGuest

Yes. The 120 VAC mains voltage is 120 VAC, RMS (root-mean-square). This
means that if you connect it to a resistor then the resistor produces as
much heat as if you connected it to 120 VDC.

The peak voltage of a 120 V (RMS) sine wave is 120*sqrt(2) = 170 VAC, minus
a couple of diode drops I suppose to get your 167. The rectifier detects the
peak value of the sine wave.
Use a transformer to turn your 120 VAC into ~85 VAC. An autotransformer
(Variac) might be easiest. What are you trying to do?

Jonathan
http://cq.cx/

4. John FieldsGuest

---
FWB
+-----+
120VAC----|~ +|----+---+----C E-------+---->120DC
| | | | B |
| | | [R] | [R]
| | | | | |
| | |+ +------+----C |
| | [C] B--+
| | | E |
| | | | [R]
| | | | |
120VAC>---|~ -|----+---------------+----+----->GND
+-----+

5. John FieldsGuest

---
Or, perhaps,:

FWB
+-----+
120VAC----|~ +|----+---+----C E----------+---->120DC
| | | | B |
| | | [R] | |
| | | | | |
| | |+ +------+----C |
| | [C] B-->[POT]
| | | E |
| | | | |
| | | | |
120VAC>---|~ -|----+---------------+-------+----->GND
+-----+

6. JamieGuest

yes.
120 * 1.414 = 169.68

to get smooth 120, use a regulator after the
fact.

oh well.

8. Don KellyGuest

Possibly he was seeing your ascii diagram in a non-fixed font where it
becomes quite garbled..