yes i googled and found lots of different mods to the basic 5v
*7805* regulator power supply that is {AC in, bridge, 7805, 5v out}
my questions are concerning the various mods/enhance such as ;
filter caps - between bridge and regulator, Is this load stabilizer also ? How to calculate proper size ? osc stability caps - .1 uF between V5+ and V0 load stabilizer - caps between V5+ and V0 , How to estimate size ? back emf diodes - diode between V5+ amd V0 , how to choose diode ? short circuit protection diodes - is tis same as back emf diode ? bleeder resistors - where, what and why ? decoupling caps - same as osc stability caps ? ? name descrp ? - diode inline/series on V+ out of rectifier , What is purpose ? soft start - inductor caps and resistor, When to use ?
Are any of these mods incompatible with one another or complicates other mods values ?
which of these enhancements have most bang for trouble ?
a good link that explains thes econcepts and how to estimate values etc would be great too .
From this, you can deduce that the voltage ripple is roughly T*I/C, where T is the period of the AC waveform. For a 60Hz supply which has been full-wave rectified, T=1/120s = ~8.3ms. So, if you are drawing 1A and can tolerate at most 3V ripple, you would need ~2800uF.
Using a higher secondary voltage will allow you to tolerate more ripple, but will increase the power dissipation of the regulator.
Also, a larger capacitor will result in a smaller conduction angle and increased RMS current (and thus power dissipation) in the transformer and rectifier.
It has a marked effect on the *peak* current, however.
--
"Electricity is of two kinds, positive and negative. The difference
is, I presume, that one comes a little more expensive, but is more
durable; the other is a cheaper thing, but the moths get into it."
(Stephen Leacock)
You can't go wrong by making 2piFCR >= 25 or so. R is an estimate of equivalent load resistance which in your case should be about 8V/Imax, gives a conservative C, Imax is maximum load on the 5V output, F is
120Hz for a full wave.
Yes- a ceramic 0.1u would be a standard for high frequency bypass...and this is in parallel with your electrolytic.
A conservative estimate is to select an electrolytic that prevents the output from going out of regulation when stressed with a no load to full load step increase in current. 5% of 5V is 0.25V so to be conservative you would want the e-cap to hold up a full load step increase for 500us without drooping more than that. Example, if full load is 1A then you have C,min>= (0.5u/0.25V)*1A=2uF, use a 4.7uF, e-cap values change a lot with temp and initial tolerance. This is in addition to your 0.1u high frequency bypass...
Yeah- that would not hurt- you mean a diode from the regulator output pin to its input pin with anode on the output. A 4001 would be good for this.
The back emf diode is the same as the short circuit protection diode- prevents blowing the regulator if the bridge filter cap sputters or goes short for some other reason.
Bleeders allow the high energy capacitors to discharge when the circuit is powered down- it would not hurt to put these at least on the bridge filter capacitor- usually a 1 MegaOhm or so...
Yep...
Not sure- that is just another form of back emf diode, and it would make sense if a second circuit was loading the bridge filter circuit in addition to the 7805...drawback is it reduces headroom, which is the differential voltage across the regulator circuit input and output...
You don't need it for a 7805 circuit...
Sure- there are tons of complicated mods you could do. Are you compiling a compendium, or is it you're not confused enough?
Real smart question, now you're starting to sound like a lamebrain..
The 7805 or LM340 series datasheets would be a good start. Most of the manufacturers now link to applicable application notes for the parts. It was so hard for people to figure things out for themselves...
And don't forget about the stepdown transformer rating, this could be important. To make a long story short, you need an 8VAC with RMS current rating at 2x the numerical DC maximum loading current. Then there's all that rigmarole about heat sinking the IC, max working voltages and ripple currents for the caps, and other stuff computed for worst case conditions which typically range over +/-20% nominal and some other things like fusing. Anything less gets you a pos that flakes out. Come to think of it, just buy a surplus open frame...or use a plug-in...
Fred, You sound like some one I may have run into over the years, your last name is ringing a bell and I just can't seem to bring it to the top of my head.
If you wouldn't mind, could you tell me a brief location and history? I'm in CT , USA btw.
--
"I\'d rather have a bottle in front of me than a frontal lobotomy"
http://webpages.charter.net/jamie_5
I came across this universal psu design while searching for 5V psu info ... is there a problem with the universal psu schematic (page 2) shown in the following document ? more specifically with the selectable voltage part of the schematic
formatting link
also i am trying to understand how that PSU can produce higher voltage than 5V while using a 7805 5V regulator and no path for the bridge rectifier (+) voltage to bypass the 5V regulator ?
Hint: The 7805 forces 5V across R1. Where does the current through R1 go?
...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
i built and experimenting with the 5v DC psu i came across a strange (to me) oscope waveform.
using most basic psu {18vac in, bridge, 7805, 5v out} with a (470 Ohm + standard LED) power indicator on the 5v side
1st i oscope the (bridge +) and (7805 out) and i see typical expected waves... that is bridge + positive 16 v sine peaks squashed together and 7805 out is 5v DC but with small dips at intervals cooresponding to positive sine peak dips
2nd (**problem here **) i add .1uF cap between 7805 out and ground (high frequency decoupling ) and the bridge + output changes to some high frequency fuzz on the tops of positive sine peaks.
When i zoom in on the ?fuzz? it is a series of about 1 uSec wide spikes that are space about 10 uSec apart and the magnitude follows shape of the sine wave ? (see pic in a.b.s.e) thats the best this amateur can dexcribe
3rd ( ? problem fix ? ) all of these things by themselves will make fuzz go away....
--
"Electricity is of two kinds, positive and negative. The difference
is, I presume, that one comes a little more expensive, but is more
durable; the other is a cheaper thing, but the moths get into it."
(Stephen Leacock)
your correct, i originally only read the circuit description so i went back and read entire article and really the only part relevant to my questions was the circuit description.
if there was an error in the schematic i think they would fix it before printing ? and i am sure the circuit description answers the question of how the circuit works for those familiar with divider junctions and regulator potential driving ? but my request is to understand how this variable voltage works.
AFAIK the (Pin 2) is 0V ground and the (Pin 3) is 5V out and the job of the 7805 is to maintain that relationship however in this application it talks about changing the voltage of (Pin 2) ground. i did not see where that was possible in the datasheet for a LM7805.
but more imporatant is the schematic (? error ?) that confuses me
i thought electricity takes path of least resistance ? that is if i take a 0 Ohm wire wire and i put a 1k Ohm resistor in parrallel with thta wire then the resistance would be 0 Ohms for tha t parralell circuit ?
in the schematic there is a piece (a segment) of wire in the bottom right corner of the schematic that ultimately connects the (-) output of the bridge rectifier to the (Pin 2) wire of the 7805 and connected along that wire is the ? resistor divider stuff ? it is the wire that goes by and around word "VARIABLE" to the right side?
Now i do not see how it is that any resistance value selected in the divider stuff is going to change the resistance between BR1 (-) out and the IC1 (G - Pin 2) ? because all the divide stuff appears to be ? shunted ? shorted by the lower right wire segment ?
any ways even if it was an error i do not see how any differential in voltage at (Pin 2) is generated using the BR1 (-) line out as that represents (0 V) to me as well ?
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