6vdc - 3vdc Converter Circuit

[randyv]

I need to convert 6vdc to 3vdc. I am using 4 "D" cell batteries in series and will be using the 3vdc to drive a portable lazer alignment device. The 3vdc needs to be not over 3vdc but could be just under (2.9vdc may be acceptable). The input is obviously a nomnal voltage as with 1.5 vdc cells, the voltage will vary from full to drained cells. The current the lazer circuit will draw is about 100mA so power consumption is extremely low.

I have looked for POL converters but can't seem to find any that can be adjusted to 3vdc out rather than 3.3vdc out. The closest I have came is a Texas Instruments TPS54110 evaluation board but the resistor I need to change to adjust the output voltage is surface mount and quite difficult to change.

I am looking for a circuit that is simple, cheap easy to build and that does not consume a great deal of energy (an efficient circuit).

 

[jrz126]

The most efficient and easiest solution is to just tap into the middle of the 4 batteries. what kind of load are you putting on the 6V portion? maybe if you put enough of a load on the batteries, they will drop enough for a 2.9V output.

Does the laser really need to be 2.9V exactly. I wouldn't see how 0.1V could break it.


OR

you could put a 1 ohm 1/8 Watt resistor in series with it so that if it were running at 100mA, you would get a .1V voltage drop across the resistor, leaving you with 2.9 at the laser. This is assuming that there is exactly 3V going into the resistor/laser and that the laser is drawing exacly 100mA

 

[Nigel Goodwin]

Does the laser really need to be 2.9V exactly. I wouldnt see how 0.1V could break it.

Assuming this is feeding a laser diode directly, they generally require feeding within a precisely controlled feedback circuit - operation only occurs within a fairly narrow band, and destruction is only a very small voltage away.

 

[randyv]

The rest of the story:
I intend to use a 4 cell MagLite flashlight, Machine a new "head" for it and contain the DC/DC converter and laser in the new head. I will be connecting to the flashlights voltage where the light bulb would normally be installed. The laser is being driven through it's own circuit and when I exceed 3vdc, it will cook quickly (thus do not want to exceed 3vdc but a slight undervoltage will not cause harm, just loss of laser output power). Actual current draw by the laser curcuit will be anywhere from 75mA to possibly 130mA.

It will not be possible to tap the center of the 4 batteries as they are stacked in the flashlight tube. The reason for utilizing 4 "D" cells is battery life. I am taking a standard off the shelf pen laser (but higher power than <5mW) which runs on 2 "AAA" batteries but only has a battery life of less than 1/2 hr and extending the run time with the 4 "D" cells. My goal is to come up with a DC/DC converter that does not consume a great deal of (or any) energy itself. It would be self defeating to have a DC/DC converter that draws 250mA or more to drive a 100mA device (you get my drift).

Any help would be greatly appreciated.

 

[Oznog]

There are 3 or 4 options.
One obvious option would be to replace 2 batteries with dummy cells so the pack is only 3v total.

The other would be a 3v regulator. 100mA won't make too much heat. However, it's only burning up the extra 3v so your 4 batteries will run out at exactly the same time as 2. Or you could just add a 3v zener or several normal series diodes to burn off the extra voltage.

You can use a capacitive voltage divider/doubler/inverter ICL7660 (MAX1044). This will actually halve the current drain from the batteries so the 4 batteries will last twice as long. The current capacity is limited but I believe you can parallel them (or perhaps there is a higher current part?). The current drain to run the divider itself is extremely low.

There is also an inductor-based step-down "buck" converter which will lower the current drain from the batteries. You might be able to find a voltage regulator which operates in this mode with an external inductor. It is probably too complicated for your needs though.

 

[spuffock]

Try looking at the maxim-ic website. They have a large range of converter chips, and they'll send you one free for the asking.

 

[plot]

**broken link removed**
try that site out, might come up with something usefull.


you could also make some sort of step down circuit using diodes or an opamp if you can't find a satisfactory regulator.

 

[williB]

ok you have four batteries available..
problem is they are in series giving twice the reguired voltage..
why not rewire them giving two 3v in parallel with the other two.
solution : a little creative wiring..

see picture..