voltage regulator help!

This link has a couple of circuits that will allow you to use a three- terminal voltage regulator and use a few added components to increase the current capability.

The second and third entries of the webpage show a couple of ways to do it. The circuit with a single pnp pass transistor is the simpler. You can use a smaller resistor in the emitter of the pass transistor than the one shown, for a higher current limit.

This may be a good starting place.

It doesn't get much simpler than the "Simple Switcher" if your goal is efficency.

Mike

"The scientist is possessed by the sense of universal causation...His religious feeling takes the form of rapturous amazement at the harmony of natural law, which reveals the intelligence of such superiority that, compared with it, systematic thinking and acting of human beings is an utterly insignificant reflection." Albert Einstein (theoretical physicist)

When faced with something like this, I often just use several dropping diodes in series sized to handle the current. This provides unregulated voltage, but it's close enough for servos. Each diode drops the voltage by about 0.7V, so you would would need five 5A diodes in series for this. Note that diode drop voltage varies between diode types, so you have to check the specs.

BRW

If you sincerely want a switching supply for this, and I can understand wanting that, you will probably need to use an IC. (You can do it using discrete parts, too, but I don't have the time to try that now.)

In case a linear, discrete design may help, I've this example below which I simplified just a little bit from other existing designs posted recently in the group. It might be useful to put a current limit, but that would add a couple more parts and I'm assuming fewer parts is better for now.

I haven't given C1 much thought, by the way. Also, I think Q2 is going to need to dissipate close to a watt. Which probably too high for a TO-92 or TO-18 device. Might be able to get away with it, especially if you can place a heat sink around the TO-18, but I'm open to suggestions as to alternatives. (I think there are heat sinks for TO-92, too, but I've never used one.)

There also might be a better choice for Q1, that would have much higher beta at high currents, can dissipate the heat, and won't need Q2 to help boost things. That could save a part. Suggestions?

Jon

I was suggested this in another forum and it sounds like a simple and cheap solution and I do love the KISS principle. A couple followup questions...

1) Is there any heat concern? 17.5 watts max dissipation according to my math, should I use a heatsink somehow? How would I accomplish that? 2) Do I just wire the three diodes in series inbetween the power source and the component on the positive lead? A multimeter should verify that its 8.4v alright? 3) I may also do this to run a 6v radio receiver off the 12v battery. Same deal? 9 .7v diodes (of smaller current) will do the trick or some similar configuration thereof?

Many thanks! I don't use Usenet often but it's where I go when I've exhausted all my other options cuz I always know I'll get the answers I'm looking for!

Carsyn

See page 17 of the datasheet at:

It shows a 5A regulator that should work for you. You can simplify it if you don't want adjustable current, like this:

MJ4502 +12 ---+------- -------------+ | e\\ /c | | --- [.2R] 5Watt | | | +--[33R]--+---[LM317]-----+---> Vout 8.41V | | | | +-[240R]-+ |+ | |+ [1uF] [1375R] [10uF] | | | Gnd ---+----------------+--------+---> Gnd

You can make the 1375 ohms with resistors in series; the 240 is a standard value. Put the IC & xsistor on heat sinks.

Ed