build a simple 5v power supply for digital circuit

Some 78xx regulators oscillate a bit especially with out the proper load on them. even then, you may need to place some by pass caps on it as you did.

--
"I\'d rather have a bottle in front of me than a frontal lobotomy"
http://webpages.charter.net/jamie_5

Ok, thanks for clearing that up for me..

I still can't get that out of my head how ever, it's possible the spelling of whom I'm thinking of, may not be the same of course.

Thanks again.

--
"I'd rather have a bottle in front of me than a frontal lobotomy"
http://webpages.charter.net/jamie_5

Yeah, there's an error. The connection between the (-) output and R1 should not be there.

Best regards, Spehro Pefhany

--
"it's the network..."                          "The Journey is the reward"
speff@interlog.com             Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog  Info for designers:  http://www.speff.com

5v

voltage

through

Hi Jim thanks for the reply/hint,

I did not miss your hint, it just seems to have a depth i will need to figure out.

it seems to be a trick question and i do not want to get it wrong :)

thanks for help robb

i will take a wild guess and say the current goes to BR1 (-) and (Pin 2) ? yes ?

5v

and R1

hi, thanks Sphero,

wait a minute are you toying with me ?

well i can not figure which connection you are talking about iis it the same as the one i described ?

thanks for help, robb

Ok, I'm what they call the greater HartFord area, Windom area.

Did you ever get involved years ago when Commodore computer clubs used to be around? Just trying to piece together some old memories and names together:)

--
"I\'d rather have a bottle in front of me than a frontal lobotomy"
http://webpages.charter.net/jamie_5

Best regards, Spehro Pefhany

--
"it's the network..."                          "The Journey is the reward"
speff@interlog.com             Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog  Info for designers:  http://www.speff.com

Once you apply Spehro's correction eliminating a short, the current flows through the selected resistors below R1... R4 or R5 or R6 or R7 or R8 or R9 or pot R2

...Jim Thompson

--
|  James E.Thompson, P.E.                           |    mens     |
|  Analog Innovations, Inc.                         |     et      |
|  Analog/Mixed-Signal ASIC's and Discrete Systems  |    manus    |
|  Phoenix, Arizona            Voice:(480)460-2350  |             |
|  E-mail Address at Website     Fax:(480)460-2142  |  Brass Rat  |
|       http://www.analog-innovations.com           |    1962     |
             
         America: Land of the Free, Because of the Brave

2)

iis

not the wire i was thinking, but i was close thanks for the correction robb

so the oscillation that probably contributed to frying my first

7805 and is it typical that the oscillation shows up on the bridge out ?

thanks for reply, robb

board lay and lead run lengths plays a roll in that.

I assume you had a electrolytic cap on the bridge +/- outputs?

in any case, that does not stop the high freq you saw in there, you don't always see this problem in all 78xx, I've seen it in some depending on who made the component where is others, they work fine with out any compensation. Also, if you constructed the bridge your self instead of using a ready made unit, it's smart to put some small bypass caps across the diodes..

Other issues is when voltages on the output side exceed the voltage on the input side.. the internals of the regulator have an issue with this how ever, like I said before, it depends on who made the component. I've seen some that included the bleeder diode. In a case like that, a bleeder diode from the output to the input is a desirable component to use. For a + type, the anode would be on the output, cathode on the input. Basically, that simply causes a by pass of the regulator if the output voltage happens to be higher than the input voltage by 0.7V or more.

Hope that did something for you.

--
"I\'d rather have a bottle in front of me than a frontal lobotomy"
http://webpages.charter.net/jamie_5

The images do not show a filter cap after the bridge and before the 7805. Add a large cap (4700uF shown) across the brideg output:

-------- | +|-----+------in[7805]out--- | Bridge | | + | | | [4700uF] | | | | | | -|-----+-----------+ --------

Ed

I noticed that when I looked at the schematic. I'm planning on building this one since it's a variable power supply, hell I might just build two of them because I've got a couple products in mind that use 5V for logic and 12V to drive things like steppers, etc.

If you look at the schematic the connection between the lower end of R1 and the other end of C3 should not exist.

Nice graphic! Thanks for that, I used a plain black pen to cross out the connection on my schematic.

Jim Thompson a écrit : ///

Hello nobody mentioned it but the schematic is faulty! Negative output should not be connected to the "mass" leg of the regulator. Else, output voltage cannot be different from 5V - other effects neglected pom

"pom"

** The design is purest crap.

Output regulation wil be poor, as will hum rejection - since Iq is a highly variable quantity.

Bound to be HF unstable at some settings and loads too.

That ain't no way to treat a 7805.

Just sub in a LM317 and nearly all the troubles go.

......... Phil

It's actually fairly stable with output current, less so with input voltage and temperature. Probably better than most switchable AC adapters wot consist primarily of a badly made multi-tap transformer and a crappy slide switch with PCB contacts. But it could be That ain't no way to treat a 7805.

It should probably have a larger output cap and a diode eg. 1N5403 across the 7805 and probably across the output too since we don't know what will get hooked up to it.

Yup. Of course you have to recalculate the resistor values, taking into account that you should draw a minimum of 5mA from the '317 to keep it in regulation. Eg. 237R and 715R for 5V.

Another potential problem with the original circuit is that the output voltage will spike as high as Vin - 1.xV during the switch transitions unless it's a shorting type rotary switch.

That's typically considered an undesirable characteristic in a power supply.

Best regards, Spehro Pefhany

--
"it\'s the network..."                          "The Journey is the reward"
speff@interlog.com             Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog  Info for designers:  http://www.speff.com

"Spehro Pefhany" "Phil Allison"

** Natsemi data shows Iq may change by up to 0.5 mA in the range of normal output currents up to 1 amp, by up to 1mA over the nominal input voltage range & by 1 mA over the allowed temp range.

Adds up to 2.5 mA.

2.5 mA flowing in 1.5 kohms = 3.75 volts.

That IS craaaaap !!

** A comment about you and other circuits.

Waste of type.

** There is effectively no output stability cap.

That 0.01uF hanging off the + output finds it way back to the 7805's ground terminal via a crippling amount of series resistance.

** Just follow the data sheet for the LM317 for the needed R values.

The only way that horribly crude idea of converting a LM7805 into a variable reg can be even half satisfactory is * IF * the input voltage is itself regulated and the load current fixed.

...... Phil