Beginner help building 5V power supply

Only 5mA and you have "several" LEDs? Maybe it's 50mA?

No, it's not ok. It's possibly oscillating.

The 24VAC is RMS, the peak to peak is 1.4 * 24 = 33.6V peak. This means the regulator has to drop quite a bit of voltage, about 29V to be precise. At

If the leads going from your filter caps (3 1000uF) to the regulator is more than a few inches, then put a .33uF cap on the input pin of the regulator (real close to the regulator of course) to help prevent oscillations.

If your transformer has a center tap on the output, then use it to extract

AACircuit v1.28 beta

Alternating current is measured RMS (root mean squared);

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Thanks for your reply.

Just double checked, draw is .005A One of two LED is always on, but never both.

pin of the regulator Entire circuit is only 1 inch, everything is close togheter.

Any ideas on improving this circuit and why it goes through these cycles?

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Must be using fairly low current on the LEDs then. Good. :-) I don't think that .15W should make it very hot, unless it only gets hot when you press your finger on it. Putting your finger on the case decreases the ability of the regulator to dissipate heat, hence it will get hotter from touching it. If this is what is happening, then everything is probably fine.

If you have a scope, you can look for oscillation on the output. I'd still add the .33uF on the input and also try something in the range of a few uF on the output (parallel to the .1uF on the output to ground).

Just the ones I gave you. It's either dissipating too much power, or it's oscillating. Have you looked at the current draw on the input to see if it changes radically when the device heats up?

Arghhhhhhhhhhh......... pedant kids...... ;-) (just kidding :-)

Don't you mean 24V right here? ;-) I had a senior moment and was wondering where you came up with the 2Vf stuff. Then I recovered and went and looked at the Vishay datasheet for GBL005. Looks like maximum Vf is 1V per leg at

http://www.ortodoxism.ro/datasheets/vishay/gbl005.pdf'

Having recently done 24vac->5vdc for a prototype, i can tell you a few of the things I tried. None worked well, I eventually got the switcher I ordered (acon CX) and used that instead. There are less expensive switchers, but the CX is (1) vertical mount, and (2) isolated. My raw DC was closer to 37 V, with four 820uF caps, and about a 0.5A draw on the 5v side.

Try a 12v zener on the gnd leg to turn a 35max->5 regulator into a

Try a 12v zener and a power transistor emitter follower to pre-regulate to 12v for the 7805.

A resistor divider between the output and gnd provides a "floating" ground reference to increase a 7805's output, check the specs for details.

Use a 7824 to pre-regulate - it has a Vmax of 40V.

Use a different regulator ;-)

Use a heatsink.

Don't forget to include a small cap *after* the regulator, too.

I think you need to try and troubleshoot some more. Try and find out exactly when its getting hot. It will only get hot if to much current is flowing through the regulator or if it is dropping to high of a voltage. Since you are taking ~34VDC and dropping it down to 5VDC there is a constant voltage drop of about ~30VDC. The power dissipated is then 30VDC*I where I is the current that is going through the regulator. If its only 5ma like you say then thats only 150mW. I'm not sure which 7805(Is it a 7805 or 78M05 or what?) but this should be well within reason and you shouldn't need a heat sink. But what is happening is that your power dissipation is 30*I and I is not 5mA.

What I would do is first completely take away the pick and just add a one LED(with a current limiting resistor) and see what happens. See if it does the same thing. You can even play around with the load here go get some idea of the performance/efficiency of your regulator. Dropping that 30VDC is kinda excessive and if your drawing 1A then thats 30W and I don't think the

There are several solutions to make your regulator dissipate less heat or be able to handle more dissipation. I think though one needs to find the main issue here though. My guess is that your just pulling more current than you think and this is causing it to over heat. I'm not sure what your doing with your pic but it surely will require more than 5mA to function.

Because AC is usually given in RMS which is sort of an average. When you rectify and then filter it, the filtering "works off" of the peak. So you have to convert the RMS value which is 24VAC into peak which is 34VAC peak. Rectifying this gives 34VDC but its not constant. Filtering then converts this to approximately constant 34VDC by "filling in" the gaps.

Note that 212F is the boiling point of water. No one I ever have met can stick there finger in boiling water for over a second without burning themselfs... yet this is only 100C. Most silicon devices have a maximum working temperature of 125C or so. So just cause it does get hot doesn't mean its a huge deal... except when it shouldn't be getting hot.

Here's one way to draw an ascii schematic without a program to do it for you:

Here's an example, using your description.

--------- --------- ---- | XFORMER |-| Bridge +|---+----+----+---|7805|---+---> +5 | 24 VAC | | | |+ |+ |+ ---- | | | | | [C1] [C2] [C3] | [C4] | | | | | | | | | | |-| -|---+----+----+------+-----+---> Gnd --------- ---------

When you use polarized components, you should label them to indicate the polarity if you draw them as boxes. See the bridge and C1-C3 as examples.

Regarding other points (which have been answered), you should add an input cap (.33 uF was mentioned,AIRC) physicaly close to the 7805. That would add C5 to the above schematic, from the Vin pin of the 7805 to ground. Also, your higher than 24V DC was explained, but I'll add that a higher voltage rated filter capacitor would be better, as you are very close to the 35V rating. It is good practice to use electrolytics rated well above the voltage they will "see". And, for a 5 mA load, you do not need 3000 uF of filter capacitance.

I may have missed it in the responses, but why not use a lower input voltage? Wall wart DC supplies are dirt cheap and often free - you could use one as the input to your 7805.

Ed

Another option is to use LTSpice, and supply the ASC text. It is also very informative if you run the simulation and look at voltages and waveforms.

A good rule of thumb is 2000 uF per amp. The capacitor only needs to hold the voltage to about 2/3 of the peak, from one peak to the next. Another way to approximate the capacitor value is using a time constant at the line frequency. For 60 Hz:

C(uF) = 8300(uSec)/RL. So a 5V 0.005A load is 1000 ohms, and C=8.3 uF.