# Connecting multiple capacitors in a greatz circuit

#### Marin Sebesic

Joined Dec 14, 2017
15
Hello folks,

so, I'm planning to build a greatz circuit using a 2200 uF electrolytic capacitor, but when I got the parts delievered, the seller sent me 5 electrolytic capacitors for some reason. Anyways, would it do something better if I would connect all the capacitors I've got, or should I just leave the one with the highest capacity? Thanks in advance!

#### wayneh

Joined Sep 9, 2010
16,399
Hello folks,

so, I'm planning to build a greatz circuit using a 2200 uF electrolytic capacitor, but when I got the parts delievered, the seller sent me 5 electrolytic capacitors for some reason. Anyways, would it do something better if I would connect all the capacitors I've got, or should I just leave the one with the highest capacity? Thanks in advance!
Are you referring to a bridge rectifier with a filter capacitor on the DC side? If so, what is the current load?

#### WBahn

Joined Mar 31, 2012
26,156
I've actually never heard the term Greatz circuit -- I've always known it simply as a full-wave bridge rectifier. Learn something new every day.

You didn't tell us what the values of the capacitors are that they sent you -- sounds like they are all different values.

Presumably there's some reason why you decided on 2200 μF. In general, for DC filtering of this type, it's safer to have too much capacitance than not enough -- but there's a limit in all things. If you put too much capacitance on there you will start having problems with inrush current, leakage current, physical size, and perhaps other issues.

#### Janis59

Joined Aug 21, 2017
1,231
Probably he mean the Graetz circuit
https://en.wikipedia.org/wiki/Rectifier#/media/File:Gratz.rectifier.en.svg
Graetz is the surname not a tick with a finger there sth be Great.

Inrush current makes an explosive character (ya ya, just blow up!) starting from hundred or few hundreds of microfarads at network voltage, and from thousands it is violent. The million Amps in the network will destroy Your diodes in nanoseconds, beat out the district fuses in microseconds, and blow out the local fuses in miliseconds. Guess who will pay the energocompany for repair.

Therefore, if really the large battery is essential, there are some standard solutions for two complex problems:

1) Problem that "normal wires" are not able to carry the "million" Amperes, for reason the diameter smallness of lamelles, for resistive component what linearly grows from nearest to most far localization, so the impartment becomes diminishing with the distance of entrance, and inductive component ca 10 uH/cm.
The standard solutions:
a)Mount battery in parallel lines in both x and y directions square-network aka chess-desk. That means every cap is connected by 4 parallel wire ends, and all has equal long shortest and equal long longest - so impartment % are roughly equal.
b)Better solution - the triangle combs of one layer going horizontally, other +30 degrees to the north north east, and last -30 degrees to north north west.
c) the ultimatively best solution but damn expensive - two parallel copper plates with textollite between it with drillings. Lamelles are going through drillings to be soldered straight to plates.
d) solution if wire for outbond connection is unavoidable - use the sendwich wires - the parallel copper restangular bars with thin (ca 1/2 mm) insulation between them, glued together with no gap.
2) Problem of shorted network at startup moment. Standard solutions:
a) The varistor in serie with Graetz. However there are no existing varistors for power over a small few hundreds of Watts.
b) The delayed relay or mag-switcher parallel to few Ohms powerful resistor. In the first second all current flows via this resistor what later is shorted. The method is not working over some 40-60 Amps because mag switchers at such amperage are expensiver as all Your circuit.
c) The thyristor power regulator with slow start regime before Graetz. Universal solution but caprice-ful, and too complicated.
d) Method what are rarely used but very effective. I announced it few years ago in one of articles: the Graetz bridge is composed of thyristors in the diode regime, but on/off permit is coming from optos. Brute parallel to it stays small diode Graetz (1N4007) via the 1 kOhm or so. In the few seconds the cap bat will be mildly charged up until some 3/4 of net voltage and THEN some circ will detect it and give a permanent permit for main Graetz (thyristor Graetz) to work on. Circuit is simple, cheap, and very stable.

#### Marin Sebesic

Joined Dec 14, 2017
15
I'm sorry that I dind't express mysef properly.
I'm actually making a ~230V AC to = 12V DC bridge rectifier using 4 diodes, and I'm using the capacitor at the DC side. The amps output will be around 8.5A and I will mostly charge 12V batteries with it.
I planned to use just a 2200uF electrolytic capacitor, but these are the others: 220 uF 2x, 100uF 2x, 4.7uF 2x, 2.2uF(lol)., connecting them of course in parallel with the load.

#### LesJones

Joined Jan 8, 2017
2,698
You can't just rectify 230 volts AC to get 12 volts DC You need a transformer to step down the 230 volts AC to around 12 volts AC before rectifying it. If you plan to draw 8.5 amps from the power supply you need a larger capacitor than 2200 uF. If you are planning just to make the very crude old fashioned type of charger to charge 12 volt lead acid batteries then you don't need a capacitor.

Les.

#### -live wire-

Joined Dec 22, 2017
912
You could step the AC down, rectify it, then use smoothing caps with a few milliFarad capacity. This should get you an ~12V output. HOWEVER, mains voltage can be lethal, so unless you really know what you are doing, please go with something like this 12V 7A power supply:
Buying one is probably less expensive and much less dangerous. Also, the output is probably regulated better, too.

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#### Alec_t

Joined Sep 17, 2013
11,651
If you end up with 12V DC it won't be a high enough voltage to charge lead acid batteries properly.

#### -live wire-

Joined Dec 22, 2017
912
If you need easily adjustable cc or cV, try getting a bench power supply like this.

It is definitely a worthwhile investment, and will be useful for many other projects, too. It is a necessity if you plan to make a lot of circuits. While there are often many advantages to DIY, you sometimes should just go with the commercially available circuit, especially when dealing with mains voltage.

#### wayneh

Joined Sep 9, 2010
16,399
I planned to use just a 2200uF electrolytic capacitor, but these are the others: 220 uF 2x, 100uF 2x, 4.7uF 2x, 2.2uF(lol)., connecting them of course in parallel with the load.
If all you're doing is charging batteries (and you have a transformer as already noted), you don't really need any filtering capacitors. You can think of the battery itself as a capacitor, and it has a thousands-fold higher capacity than any capacitor you might connect.

That said, I'd probably use the 2200µF cap and save the others for another day. It's a waste of time to add them in parallel since they will make so little difference, even for something that needs the filtering, which the battery does not.

#### Marin Sebesic

Joined Dec 14, 2017
15
You can't just rectify 230 volts AC to get 12 volts DC You need a transformer to step down the 230 volts AC to around 12 volts AC before rectifying it. If you plan to draw 8.5 amps from the power supply you need a larger capacitor than 2200 uF. If you are planning just to make the very crude old fashioned type of charger to charge 12 volt lead acid batteries then you don't need a capacitor.

Les.
Again...
Sorry that I didn't metion that I will use a 230V AC to 12V AC transformer... And yes the main purpose will *mostly be charging 12V batteries, also, do you say that it's "bad" to put a capacitor, or just it has no purpose?

#### Marin Sebesic

Joined Dec 14, 2017
15
You could step the AC down, rectify it, then use smoothing caps with a few milliFarad capacity. This should get you an ~12V output. HOWEVER, mains voltage can be lethal, so unless you really know what you are doing, please go with something like this 12V 7A power supply:
Buying one is probably less expensive and much less dangerous. Also, the output is probably regulated better, too.
I know I could buy something already done and ready to use, but I like doing DIY projects, also, this whole project costs me around 9€.

#### Marin Sebesic

Joined Dec 14, 2017
15
If all you're doing is charging batteries (and you have a transformer as already noted), you don't really need any filtering capacitors. You can think of the battery itself as a capacitor, and it has a thousands-fold higher capacity than any capacitor you might connect.

That said, I'd probably use the 2200µF cap and save the others for another day. It's a waste of time to add them in parallel since they will make so little difference, even for something that needs the filtering, which the battery does not.
Makes sense, thanks!

#### Marin Sebesic

Joined Dec 14, 2017
15
If you end up with 12V DC it won't be a high enough voltage to charge lead acid batteries properly.
Aren't all 12V battery chargers equipped with 12V?

#### WBahn

Joined Mar 31, 2012
26,156
Aren't all 12V battery chargers equipped with 12V?
No. You need something higher than the potential of the battery in order to get current to flow into the battery. Plus, the fully-charged terminal voltage of a 12 V lead-acid battery is higher than the 12 V nominal voltage.

For maximum life, you need to charge the battery to more than 14 V to deal with the sulfation.

Here's a good article on charging lead-acid batteries:

#### -live wire-

Joined Dec 22, 2017
912
I am really not a fan of lead-acid batteries. The main reason is that they contain known neurotoxins, and increase the amount of heavy metals in the environment. There are many viable alternatives, but due to cost, lead-acid batteries are far more common. I try to avoid non RoHS-compliant components whenever possible. You should also take this into consideration.

#### wayneh

Joined Sep 9, 2010
16,399
Aren't all 12V battery chargers equipped with 12V?
A dedicated battery charger will show about 13.8V while attached to a nearly charged battery, with little more than a trickle current flowing. That may vary slightly with temperature if the charger is smart enough to compensate for temperature. I think most are not.

There are a couple things you need to know about your plan. A wall-wart transformer-based power supply specified at 12V DC and some current, say 1A, will often show as much as 18V when not loaded. An increasing load will bring down the voltage and it may be down to ~12V when loaded to the specified 1A current.

Another issue is the transformer. If it's a 10:1 intended to for instance, drop 120VAC to 12VAC, the rectified and filtered voltage will be over 15V without load. This is because, with no load, the voltage is not the rms voltage, its the peak of the sine wave voltage which is 1.41 the rms value that's commonly quoted. You lose about 1.4V in the diodes of the bridge, but the net effect is still a DC voltage after the bridge that is well over the 12V AC nominal voltage.

What all this means is that if you want to charge a battery, you should use a regulator. Otherwise the trickle current might be too high when the battery is already fully charged. A LM317 voltage regulator (set to 13.8V) is a very common solution for a do-it-yourself 12V battery charger.

#### Janis59

Joined Aug 21, 2017
1,231
R(load.equiv)=12V / 8,5A = 1,41 Ohm. Tau must be >10 msec if full wave rectifier and 20 ms if half wave. Thus Tau=R*C or 10E-3=1,41*C what gives C=10E-3/1,41=7,1E-3 Farads or 7100 microfarads. It means You need all three capacitors, but if they have different sizes, means have different ESR, so the load will fall on them un-equally. One will make a main job and other will only help a slight. Probably in the case of lading the bat, it is not so very wrong. May go on. Be aware if her majesty Practice push too much a heat on that cap, what one best working. If it is over 40...50C then.... can it be anything dangerous, you are secure.... fly away anybody right now.... booom!
Actually, I understand WHY You try to cure the bad transformer by means of caps. But in the result You get a twice as big box and doubled price. Better instead is to wind additional few turns with appropriate wire and capacitors may wait in the box the right time for another project. The wire You need is i=A*J=pi()*d^2/4*J where J=3,5 A/mm2, I=8,5A thus the d^2=8,5/3,14*4/3,5=3,1 so the d=sqrt(3,1)=1,77 mm. For sure the nearest standard size 1,7 mm will be good enough, or 1,8 mm.
How much to wind - measure the one turn voltage and produce the proportion. The winding of so few turns may be processed even without of proper decomposing the trafo, probably. Just squeeze in the gap and try to not injure the lack layer.

#### Marin Sebesic

Joined Dec 14, 2017
15
A dedicated battery charger will show about 13.8V while attached to a nearly charged battery, with little more than a trickle current flowing. That may vary slightly with temperature if the charger is smart enough to compensate for temperature. I think most are not.

There are a couple things you need to know about your plan. A wall-wart transformer-based power supply specified at 12V DC and some current, say 1A, will often show as much as 18V when not loaded. An increasing load will bring down the voltage and it may be down to ~12V when loaded to the specified 1A current.

Another issue is the transformer. If it's a 10:1 intended to for instance, drop 120VAC to 12VAC, the rectified and filtered voltage will be over 15V without load. This is because, with no load, the voltage is not the rms voltage, its the peak of the sine wave voltage which is 1.41 the rms value that's commonly quoted. You lose about 1.4V in the diodes of the bridge, but the net effect is still a DC voltage after the bridge that is well over the 12V AC nominal voltage.

What all this means is that if you want to charge a battery, you should use a regulator. Otherwise the trickle current might be too high when the battery is already fully charged. A LM317 voltage regulator (set to 13.8V) is a very common solution for a do-it-yourself 12V battery charger.
It says that the LM317 has a max. output of 1.5A, but my transformer will give 8.5A.