"Fred Bloggs" wrote in message news: snipped-for-privacy@nospam.com... : : Fred Bloggs wrote: : >
: > Watson A.Name - Watt Sun, Dark Remover wrote: : >
: >> I needed to drop 1V from a 6VDC 200 mA regulated wall wart, so I tried : >> a 3 amp rectifier, but it varied by more than .2V over a range of : >> loads. So I tried this: (view with courier font) : >>
: >> + From : >> wall : >> wart >--+ | : >> +---+------+ : >> | | : >> | | : >> 400 \ / 2SC2334 or TIP31 : >> ohm / |/ NPN power TO-220 : >> WW \> pot / |\ : >> | E\ | | : >> | | : >> +----+-----+ : >> | | : >> +------> + output : >> to load : >> - >--------------> - : >> From wall wart : >>
: >> This has some advantages and disadvantages. It's simple and cheap, : >> and keeps the output at 5V within a tenth of a volt over a current : >> range. But it has a minimum current below which it loses regulation : >> and the output starts to go up to 6V, because the transistor is not : >> conducting and the current is being supplied thru the ww pot. This : >> circuit is sometimes used in the bias circuit for the output : >> transistors in high powered amplifiers. Also Win Hill showed us here : >> how to use a similar circuit to maintain the voltage steady for a : >> current regulator circuit used on four AA cell rechargeable batteries. : >>
: >> I'm thinking about putting a 5.1V zener on the output so that if the : >> voltage climbs above that, it just shunts the excess current. Oh, : >> yeah, I set the pot to various values to see what the output voltage : >> was with various loads. The two resistances were 120 ohms for the : >> upper and 280 ohms for the lower. I suppose the 400 ohms total could : >> be raised to a higher value, but the transistor needs enough base : >> current to do its job. There's only 1V available minus the .6V E-B : >> voltage, so even at 400 ohms, that's not a lot of current. : >>
: >>
: >
: > You almost had it- put the Vbe multiplier inside the feedback loop and : > buffer like so: : >
: > Please view in a fixed-width font such as Courier. : >
: >
: > 6V >---+------+---------+ : > | | | : > | / | : > | 22 | : > | / | : > | \ c : > | | |/ : > | +-------| TIP31 : > | | |\ : > | | e----+--> 5V : > | | | : > === | +----+ : > | c | | : > | \| / | : > | |-----> \ | : > | /| / === : > | e \ 1000U : > | | | | : > | | | | : > | | | | : > GND>--+------+---------+----+--> GND : >
: : Then you can work a common 5.1V zener into the equation like so: : Use a 2N3906 for the pnp for Vbe,max=6V rating. : : Please view in a fixed-width font such as Courier. : : : : : : 6V >---+--------+----------+-------------+ : | | | | : | / / | : | 22 51 | : | / / | : | \ \ c : | | | |/ : | +----------|-----------| TIP31 : | | | |\ : | | | e----+--> 5V : | c | | : | \| | pnp | : | npn |--+-----|-------- c e---+ : | /| | | \ / | : | e | | ---- | : === | | | 180 | | : | | | +----/\/\---+ | : | | | | | | : | | | | | | : | | / _/ / === : | | 1K /^ 5.1v 1.2K 1000U : | | / - / | : | | \ | \ | : | | | | | | : GND>--+--------+----+-----+-----------+------+--> GND
It's getting bigger. Soon he'll need a 2nd breadboard to build and test this thing. ;-)
Bill @ GarberStreet Enterprizez };-) Web Site -