Hey *!
Made this little page long ago, seems quite relevant now though…
http://www.avishowtech.com/mbhp/ps/
I would wikify it but it uses javascript for the wave display. :-\
Best
Smash
Nicely done! One comment: wouldn’t the voltage level on the output side of the 7805 be lower than the input voltage coming into the 7805?
Nicely done! One comment: wouldn’t the voltage level on the output side of the 7805 be lower than the input voltage coming into the 7805?
It IS? Can’t You see it from the scale?
Moebius ;D
wouldn’t the voltage level on the output side of the 7805 be lower than the input voltage coming into the 7805?
I guess my photoshop o-scope was off calibration that day. ;D
Best
Smash
Sorry to cpr this old thread, but what determines what kind of cap and how many to smooth the voltage before and after the regulator? I have been researching various linear PSU designs and it seems that everyones is very similar (rectifier -> smoothing caps -> regulator -> more smoothing caps) but the number, type, and values of the caps all vary considerably. Ray Wilsons uses 4 polar 4700 uF with a 1uF polar on the end. Yours (Smash) has one lytic and one non-polar one each side of the regulator while Seb Francis’ bench PSU has one polar and one non-polar plus a load resistor and a reverse protecting diode.
So I guess my question is what exactly is the theory behind the different cap types?
My other related question is a little more specific to what I’m working on which is a +12/-12/+5 PSU from a 2 amp center tapped 25VAC transformer. Would it make sense to use a second rectifier behind the first to drop the voltage further before going into the 5V regulator or just driving it with the output of the main rectifier (12.5 VAC) directly the way to go?
You ask many good questions.
I’ll try my best to answer them.
What kind of caps and how many, before and after the regulator:
As you can see from the cool graphics that are at the start of this thread,
the job of the “smoothing” capacitors is to average the voltage across the
cap. To “smooth out” the peaks and valleys. The two most important specs
of a capacitor are the capacitance and the voltage rating. The capacitance tells
how much smoothing you’ll get (more is generally better) and the voltage rating is
a maximum for that cap that you must stay below.
Bigger capacitors are usually Electrolytic, these are usually polarized, and so they
should have markings to show which leg is + or -. Another high capacity type
is “tantalum”. They have high capacity in a small package, and usually don’t
degrade over time like Electrolytics, but on the downside they are more expensive,
and they are not tolerant of voltage spikes.
How much capacitance do you need? That is math beyond my ability. It depends
on the current demand of your device, the properties of your transfomer, and how
smooth you need the voltage to be. Especially with electrolytics it’s a good idea
to go a bit over the minimum requirement, as they may vary by 20% right at the start,
and will degrade with time.
As long as there is load on the supply, there will always be some ripple after the first
capacitors. Each regulator requires a certain voltage to do it’s job. A typical 7805 may
require 7 volts to assure a good 5 volt output. If the ripple (at it’s worst under full load)
ever falls below that minimum, the 5 volt supply will suffer dropouts. See the datasheet
for your regulators to learn about the minimum input voltage requirements. The Texas
Instruments sheet I just read states that the input must be 2 volts over the output
for a 7805, 2 and a half for most other voltages. So if you’re feeding a 7812, you’ll need
no less than 14.5 volts at the input under worst case conditions.
The total capacitance needed may require a bigger cap than you have room for. In the case
of the example given above by Ray Wilson, there are 4 4700 uF caps. These 4 together are
less tall than one much bigger cap. I expect he’s planning for a low profile case, like a 1U rack
case or something.
So if it’s about “total capacitance”, why include a dinky little 1uf or 0.1 uf ceramic? These
smaller caps are more effective at smoothing higher frequencies. The main ripple in the supply
will be at 100Hz or 120Hz, depending on your country, but much higher frequencies can be stopped
much better with little ceramic caps.
OK, all that for the “smoothing Caps”, now on to the regulator. The popular 78xx and 79xx regulators
are “linear” devices. They require a bit more voltage on their inputs in order to provide a smooth, clean
regulated output voltage. As an example, let’s say we have about 15Volts DC, and we want regulated
+5 and +12 volts. We supply the mostly smoothed +15 volts to the input of the 7805. The output should
show something very close to +5 volts. We do the same with the 7812 regulator, and we get 12 volts out.
It all tests fine, so we connect our toys, and after a few minutes, it shuts down with a burning smell.
What happened? These “linear” regulators have to “burn off” the difference in voltage between the input
and the output. The greater that difference, and/or the greater the current, the more heat is generated.
In our example, we might be drawing 50ma at 5 volts. The difference between +15 volts in and +5 volts
out is 10 volts. that 10 volt difference at 50ma = 500 mw of heat (1/2 watt) That’s a bit warm.
Now try 500 ma at +5 volts, and suddenly you’re dissipating 5 watts through your little 7805 regulator.
This is why heat sinks are needed. They are radiators that help to dissipate that heat into the air.
Here’s a tip. the positive 78xx regulators happen to have their mounting tab at ground. So you can mount
them directly to your metal project case, which should be grounded anyway. This lets you use your case
as a big, free heatsink. Don’t try that with the negative regulators though, their tab is not grounded.
Get a bit of “thermal paste” or “thermal grease” and put a dab between the tab and heatsink. It’ll help
carry the heat across to the heatsink.
After the regulator, there are a few more caps, though not usually as big as the previous ones. These help
to smooth the load being placed on the regulator. It is also a common practice for smaller ceramic caps to
also be placed at each chip. These help to reduce spikes generated by (especially digital) signals being
created.
You asked about the “extra” diodes and resistors in Seb Francis’ designs. These may be needed
for some systems. Here’s what they are about.
Some regulators require a minimum load to regulate well. Many designs have a very light load, especially on
the negative supplies. Adding a load resistor may help the regulator to do a better job.
In some systems, when the device is turned off, the input voltage may fall faster than the output voltage.
This is a bad thing for the regulator. If your input voltage is gone, and you still have a bit of charge in the
caps after the regulator, the regulator may be damaged. Adding a diode from the regulator output to the
regulator input prevents this problem. Another diode that is sometimes added is from the output voltage to
ground, so that if ground ever rises above VOut, the diode will shunt that voltage.
You can see examples of all these things if you download the data sheet for a TI 78XX regulator.
Finally, extra rectifiers to drop the voltage? You’ll drop between 0.7 and 1.4 volts per diode, but it all adds up
to the same amount of heat. You’re just moving some of it from the regulator to the diode. That works. You can
also get high wattage resistors to drop the voltage. Just calculate for the maximum current you’ll be drawing,
and make sure that the result never falls below the input voltage required for your regulator.
I know these answers are for an old question, but I think power supply basics are useful for a lot of people here.
I hope this response helps someone.
LyleHaze
1 Like
I found the following link extremely useful, perhaps others will too:
http://www.eleinmec.com/article.asp?16
James
That means for me if I dont have a 100nF cap I could also use a bigger one like 330nF ? And why are there 2 caps everytime and not just one ? I see in both pairs one is polarized and one is unpolarized.
dj3nk,
Yes, for power supply circuits, you can usually substitute a bigger cap for a smaller one, as long as the voltage rating is high enough.
The reason you usually see two caps. One polarized and the other not, is this: The bigger polarized cap is reducing ripple at the power supply frequency (100 or 120 Hz, usually) and the smaller ceramic cap (not polarized) is absorbing high frequency noise, like you might get from a digital circuit running at 40 megahertz.
LyleHaze
I think this deserves a sticky in the SID section if it all possible.
Out of the 6? months ive being on here, this was my first time noticing this great Diagram.
Would be an asset to all the noobs if it was in there, or even a link in the MB Sid manual.
Just some honest advice.
Thanks smash, that really simplifies things.
Seconded.
Link to it in the wiki. Editable by all. Yay 
Wow I’m glad this got dug out. Great thread.
Luke
Dug out? It’s a sticky thread, at the top of the page! ;D
Link to it in the wiki. Editable by all. Yay
That damn wiki! Its a jungle of informantion in there, and i always seem to forget searching in there! grrrr!
Id like to retract my previous post, for some reason i had mistaken it for the optimized SID circuit connected to the core module,
not the Core modules rectification circuit itself! :-[
My bad.
Good explanations!
AFAIK the small capacitors are there to stop the 7xxx regulator from oscillating. If building something on breadboard, these caps should always be close to the regulator in order to minimize that risk.
Nicely done! One comment: wouldn’t the voltage level on the output side of the 7805 be lower than the input voltage coming into the 7805?
Sorry folks, I just cannot see any voltage coming into the 7805, where is it please? Can someone else not see it too? 
cheers Johnh
My other related question is a little more specific to what I’m working on which is a +12/-12/+5 PSU from a 2 amp center tapped 25VAC transformer.
Have you finished that PSU? Would be nice for Core and CV…
m working on which is a +12/-12/+5 PSU from a 2 amp center tapped 25VAC transformer
Selectronic.fr sells one at 4,90€ with 5V/1A and +/-12V at 250mA, ELPAC trade
http://www.selectronic.fr/article.asp?article_ref_entier=10.4418-9999
definitly the good stuff to get, and everything is included, you plug it on DIN5 socket !