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Subject: Re: Howto: Low current PS without a transformer
Date: Thu, 14 Nov 2002 14:24:26 GMT
NNTP-Posting-Date: Thu, 14 Nov 2002 09:24:26 EST
Organization: Road Runner
Leif H wrote:
> I need a simple and cheep power supply that I can connect to 220V AC. How
> can I make this without using a large and "expensive" transformer? The
> output voltage is about 3V DC and the current is less than 50mA. Any ideas?
> Leif H
Sounds like something like what is done for electric razors. There is a
small power supply for charging the battery, however, many times these
are not isolated because it can be done cheaper that way, and the user
doesn't have access to the voltage anyway. There are some nice small
transformers available from Mouser, or other suppliers. I would consider
using one with a dual primary, and connect the two primaries in series
to handle the 220V. Like Parallax # FS10-250, or Triad Flat Pack #
FP10-250. Be sure to connect the primaries correctly! If you connect
them out of phase, the two fields will cancel each other out, and it
could blow. Anyway, these will give you two five volt outputs at .25A
each. Connect them in series or parallel, or run two isolated circuits.
My experience with these suggests to expect a bit more voltage from
these, especially without a load. If you need to cut it down to three
volts, use a regulator.
The only other alternative to this that would take less parts would be a
simple resistor (of a high enough wattage) to cut the juice down to 3v,
but this will dissipate A LOT of heat, and will not be isolated. One way
to cut the requirements of the resistor is to use a capacitor in series.
Depending on the value used, this can do most of the reduction without
all the heat, but the load capacity will be reduced quite a bit as well.
This will still not be isolated, which can be a shock hazard in the
wrong places! Here is a quick example:
----- | Bridge |
| MOV | | Rect | Output
----- | |
The values of R1 and C1 will depend on the current you need to draw. The
MOV would be about 1.2 volts above your output requirement, or around
4.2V in your case, however, I am going to guess at 4.7V instead. This is
because the actual clamping voltage will be less than the rated voltage.
If you can't get a suitable MOV, then use two zeners to clamp the
voltage. Start out with a low capacitance and high resistance to get an
idea of how it's working. Like .1MF and 22K @ 3W for example. The
capacitor should be something like Vishay type 715P, or other type
capable of putting up with higher current (non-polarized also!). Watch
the MOV for heat dissipation! Have fun, and remember, this is NOT isolated!