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magnetomotive

Wed Jul 15 2009, 10:48PM

Registered Member #267 Joined: Mon Feb 27 2006, 09:44PM
Location:
Posts: 46

I am going to use an LM78XX, but I am not sure about the selection of the input and output capacitors. The datasheet says to use 0.33 uF for the input and 0.1 uF for the output. However, I notice that some people here use values larger than that for the capacitors. What is the reason for this? Should I be ok using the values specified in the data sheet?

Bjørn

Wed Jul 15 2009, 11:30PM

Registered Member #27 Joined: Fri Feb 03 2006, 02:20AM
Location: Hyperborea
Posts: 2058

Use the values that the datasheets lists unelss you have a good reason for using something else, it will save you some trouble in the long run. In general those regulators work with just about any capacitor for uncritcal applications. If you use large capacitors with low ESR there might be some chance of killing the regulator if it is shorted. I know I have manage to get one to explode.

Sulaiman

Thu Jul 16 2009, 12:08PM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140

The value of bypass capacitors for input and output are not critical, anything >= 100nF,
the TYPE of capacitor is more important, use ceramic capacitors for reliable results.
(e.g. plastic/mylar caps are mostly useless)
IF your circuit has a lot of capacitance on the output side (e.g.due to your circuit)
then add a diode such as 1N400x from output to input (Cathode/Band to input)
this protects the regulator at turn-off.

Proud Mary

Thu Jul 16 2009, 12:13PM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

Go with the data sheet values - the regulators are tested thousands and thousands of times for optisimation before commercial release.

magnetomotive

Thu Jul 16 2009, 06:57PM

Registered Member #267 Joined: Mon Feb 27 2006, 09:44PM
Location:
Posts: 46

Sulaiman wrote ...

The value of bypass capacitors for input and output are not critical, anything >= 100nF,
the TYPE of capacitor is more important, use ceramic capacitors for reliable results.
(e.g. plastic/mylar caps are mostly useless)
IF your circuit has a lot of capacitance on the output side (e.g.due to your circuit)
then add a diode such as 1N400x from output to input (Cathode/Band to input)
this protects the regulator at turn-off.

I'm not sure if my circuit will have a lot of capacitance on the output yet, but it shouldn't hurt to add that diode anyway, right?

Does the diode prevent the capacitors from discharging into the regulator?

MinorityCarrier

Fri Jul 17 2009, 05:19AM

Registered Member #2123 Joined: Sat May 16 2009, 03:10AM
Location: Bend, Oregon
Posts: 312

The diode protects the regulator from reverse-bias, should the regulator input suddenly go to zero when there is a large stored charge on the regulator output, such as with numerous de-coupling capacitors etc.

There are lots of application notes available on-line for the LM78XX, you may find them very enlightening.

cduma

Wed Jul 22 2009, 08:06PM

Registered Member #1822 Joined: Fri Nov 21 2008, 08:04PM
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Posts: 300

When I use the 78XX regulators I only use a single largish capacitor on the output to handle peak currents.

MinorityCarrier

Thu Jul 23 2009, 09:42PM

Registered Member #2123 Joined: Sat May 16 2009, 03:10AM
Location: Bend, Oregon
Posts: 312

For some IC's, like high speed comparators it is advised to have a 10uF aluminum electrolytic paralleled with .01 MLC capacitor as close to the power pin of the IC as possible to decouple switching transients and prevent oscillation. 555 timers should have a .1 uf decoupling capacitor close by also. enough of these capacitors on the power rail may blow a reversed regulator, so the diode shunt is recommended. It's just a wise practice.

thedatastream

Fri Jul 24 2009, 08:05AM

Registered Member #505 Joined: Sun Nov 19 2006, 06:42PM
Location: Yorkshire!
Posts: 329

Some regulators rely on the ESR of an electrolytic on the output to maintain stability of output voltage. I would always err on the side of caution and use an electrolytic.

Size of capacitance will also affect the noise ripple voltage that you see on your supply rails if pulling pulses of current from your supply.

Finn Hammer

Fri Jul 24 2009, 08:22AM

Registered Member #205 Joined: Sat Feb 18 2006, 11:59AM
Location: SkÃ¸rping, Denmark
Posts: 741

MinorityCarrier wrote ...

enough of these capacitors on the power rail may blow a reversed regulator, so the diode shunt is recommended. It's just a wise practice.

The datasheet recommends a very small input capacitor, .33uF.
I think this is based on the assumption that the supply is already smoothed. So the bigger caps you see in most circuits, 330uF and up, are the smoothing caps, really.
However, since the circuit boards most of us display here, are small ones, the distance from smoothing cap to regulator is so small that it would appear silly to include the .33uF cap.

Cheeers, Finn Hammer

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