6V to 3V without a regulator

I want to "wall-wart" power a 3V battery operated device (2 C cells). The
device is decorative LED display controlled by a "blob" I think it is going
to be difficult to find a 3V regulator and it is also probably overkill.

What is the best way to do this and please provide a simple schematic,
recommend supply voltage, etc for a newbee.

Thanks

 

Go to Goodwill, and select a 3VDC wall wart. They're $1 apiece.

Michael

 

SUrely 2 C cells will last a fair amount of time. It's the smaller AA and
AAA that don't,, or for that matter the gadgets using button cells (which
have the further disadvantage of being costly to replace). If life
is really an issue, run it off two external D cells.

Or, put 4 or 5 silicon diodes in series, and with the cathodes facing
towards ground. Then figure a dropping resistor to limit current. It
will be sort of like a zener diode, and the about .7v voltage drop on
the diodes will add up to 3V. Or just use the didoes in series between
the adaptor and the display.

Michael

 

Thanks

 

For the future, when you *do* want a 3V
regulator, here's a couple that are easy:

---- D1 D2 D3
+6 ---+---|7805|---+--->|--->|--->|--- Vout ~2.9V
| ---- |
[.33uf] | [.1uf]
| | |
Gnd --+------+-----+----------------- Gnd


For more precision:

-----
+6 ---+---|LM317|---+------+------ Vout 3V
| ----- | |
| Adj [240R] |
| | | |
| +------+ |
| | |+
[.1uF] \ [1uF]
| / 500 |
| \<-+ |
| / | |
| | | |
Gnd --+------+--+----------+------ Gnd

You adjust the 500 ohm pot for 3 volts at Vout.
Alternatively, you could replace the pot with
2 resistors in series - a 330 ohm and a 6 ohm.
Or just use a 330 ohm resistor in place of the
500 ohm pot to get ~2.96 volts.

Either of the above circuits can be used with
a higher supply voltage - up to 35 volts, but
it is best to use a lower voltage like 12 or 6.
The higher the supply voltage, the hotter the
7805 or 317 will get.

Ed

 

Thanks

 

I don't know about overkill, but it's not hard to find them- Digi-key
lists about 700, some between 50 cents and $1.50.

 

Why don't you just use a resistor dropper? This only works if the
current drawn is sesibly constant. If you are driving the same number
of LED's at all times then you could use this approach. Failing that
then you could just use a high power zener diode, and a resistor. If
all you're driving is LED's then this should be manageable.

You can 3.3v zener diodes quite easily, or you could use a chain of 5
rectifiers, such that 5 * 0.6 = 3.

Could it be easier?

Rob.