3.8v BBoard PS

Archive New Product Ideas
1

I have several of the Sparkfun breadboard power supplies that switch between 5 and 3.3v. However, a lot of devices for cellular use 3.8V. I would like:

a) a SFE bboard power supply that switches to 3.8V

or

b) directions on how I could modify the existing 3.3/5v BBoard power supply to render 3.8V instead of 3.3V

thanks.

2

Two silicon diodes in series will drop 1.2v making 5v become 3.8v.

3

Also, you could modify the 5V / 3.3V BB PS as follows:

Replace R2 with a 470 Ohm resistor, this will get you 3.7V when the switch is in the 3.3V position. If 3.7V isn’t enough, then you can try a 510 Ohm resistor instead which will get you about 3.9V. The optimum value is 490 Ohms if you can find some series / parallel combination, but it wont be as elegant as just a single resistor.

Then its just a matter of finding the right R3 resistor to get you whatever other voltage you want (must be at least 2V less than the input voltage). To get back to 5V with the 470 Ohm R2, use a 270 Ohm R3 (this will be 5.1V).

You get the idea.

Hope this helps!

-Nate

4

Thanks for both suggestions. I’ll probably do the diode trick now, and buy one more BB PS to change the R2 value.

At the moment, I have them all built so the R2 value label is covered up - I can’t tell which of the 4 resistors on the board is R2. Do you know off-hand, if the board is oriented with the DC jack on the left, the voltage regulator is on the right, which of the 4 is R2?

Thanks again for the suggestions.

5

Sure, R2 is the middle resistor of the three right next to each other., the one that has the silkscreen label of 390.

R3 is the resistor that is closest to the DC Jack of the same three. Its labeled 330 on the silkscreen, but there are two that are labeled 330, its the one next to the 390.