Which Capacitors After LM78xx/LM79xx?

I'm wanting to build a simple supply for a op-amp circuit using the LM7812/7912 regulators, but I'm not sure what size and what type of electrolytic capacitor to use at their outputs.

I read in another thread that low ESR caps like I normally use(Panasonic FM, FR) are not recommended because of rising impedance with the LM regulators under a light load.
Also, I see people using anything from 47uF up to 470uF on their outputs.

So what is the preferred series of caps to use and what size should it be?

Is it a good idea to use .1uF film caps after these electrolytics?

Thanks...
 
What does the datasheet suggest?

The datasheet shows a whopping 1uF. Sounds too small to me, but I'm no engineer.

Perhaps someone who have used these extensively can comment.
 

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Hi
There were/are concerns about the unsound mining of tantalum
exploiting those involved, as well as environmental impacts
https://en.wikipedia.org/wiki/Coltan_mining_and_ethics

If you are replacing with electro types datasheet suggests 25uf

With a bit of time you could design a capacitance multiplier circuit
for each polarity, on the regulators outputs, and use far lower values
or gain advantage of a cap multipliers ability of providing
typically 200x the used capacitance value. The figure of merit
being the transistors hfe figure.
 

rayma

Member
2011-04-29 8:37 pm
if these caps are for stability purposes, why would you want to multiply the capacitance by a figure of 200 times?

A capacitance multiplier would not be appropriate to replace the regulator's output capacitor to ground.
Certainly, you could use one after the output capacitor.

The regulator's output capacitor only needs to be large enough to ensure the regulator's stability.
Larger than that slows down the regulator's transient response unnecessarily.
 
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The regulator's output capacitor only needs to be large enough to ensure the regulator's stability.
Larger than that slows down the regulator's transient response unnecessarily.

Looking at this 7815/7915 board on eBay, it uses a LF353 op amp. What does the op amp do?

The board uses 10uF caps like you suggested, but I also see 220uF caps near the outputs.
So the large 220uF caps will slow down the regulators' transient response?
 

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It's difficult to tell what the actual circuit is. The op amp power pins do seem to run off the regulator outputs.
Do you have a schematic?

The values of capacitance suggested already, ensure the regulator
remains stable. You should build one of Walt Jungs regulators.
they will teach you far more about actual requirements placed
on regulators.

A usual op amp based regulator will have some reference voltage
with which it compares the incoming voltage to, then providing usually a replica
of that reference. Compare the board you have found to the components
used here: References & Regulators | Walt's Blog 2014

My conclusion is the op amp does nothing, other than possibly adding noise.

Cheers / Chris
 
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Hi Chris,

I looked over Walt's Blog before you posted. I believe I got the same link after looking at the "Super Regulator" board offered by the DIY Store.
The store photo shows DIP-8 op amps, while the actual board seems to use SOIC-8 op amps.

Jan Didden also has a "simple" shunt regulator design in another thread. Looks as though he's not selling(or taking orders) for any more boards, so I PM him asking for the layout file or Gerbers so I can purchase a few boards for myself.

Would a shunt supply offer audibly better performance than a LM-based supply?
 
Hi Chris,

I looked over Walt's Blog before you posted. I believe I got the same link after looking at the "Super Regulator" board offered by the DIY Store.
The store photo shows DIP-8 op amps, while the actual board seems to use SOIC-8 op amps.

Jan Didden also has a "simple" shunt regulator design in another thread. Looks as though he's not selling(or taking orders) for any more boards, so I PM him asking for the layout file or Gerbers so I can purchase a few boards for myself.

Would a shunt supply offer audibly better performance than a LM-based supply?

You are on the right path, yes a shunt supply will be audibly much better,
Something which is usually not addressed in more exotic designs is short circuit protection. They design for go, but not stop. A fuse or circuit breaker though
remains essential to use with all power supplies.

Cheers / Chris