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Using Arduino => Project Guidance => Topic started by: DryRun on Jun 18, 2019, 11:09 am

Title: Why 2 electrolytic capacitors and Schottky rectifier at power supply?
Post by: DryRun on Jun 18, 2019, 11:09 am
Hello,

I am trying to design and improve the power input part of my circuit to supply power to a motor driver that controls 2 DC motors. I did some research and i found this circuit but i cannot fully understand why these components have been used. I am using DC batteries to supply 12 V at VS and VIN goes to the VIN input pin of the Arduino UNO.

(https://forum.arduino.cc/index.php?action=dlattach;topic=622049.0;attach=313079)

I read about power supply decoupling capacitors; electrolytic capacitors filters out low frequency signals (high-pass filter) and ceramic capacitors act as a low pass filter.

I have the following questions:
1. Why use 2 electrolytic capacitors instead of just one?
2. Why no ceramic capacitors has been used? Has this been omitted intentionally?
3. Why is a high value of 470uF used? I searched and found that many power supply decoupling capacitors are less than 100uF for my application where the supply is only about 12 V DC.
4. The Schottky diode is the most intriguing to me. It is a rectifier, so is it also helping to smooth out the DC signal?

Thanks for any help. :)
Title: Re: Why 2 electrolytic capacitors and Schottky rectifier at power supply?
Post by: slipstick on Jun 18, 2019, 11:59 am
Where did you find that circuit? A little context for it might be useful.

Steve
Title: Re: Why 2 electrolytic capacitors and Schottky rectifier at power supply?
Post by: DryRun on Jun 18, 2019, 12:25 pm
Where did you find that circuit? A little context for it might be useful.

Steve
I can't find the source. It's the voltage input circuit section for an Arduino UNO which powers a dual H-bridge motor driver to control 2 DC motors at 12 V.
Title: Re: Why 2 electrolytic capacitors and Schottky rectifier at power supply?
Post by: Paul_KD7HB on Jun 18, 2019, 06:07 pm
"1. Why use 2 electrolytic capacitors instead of just one?".

Many times the reason for the two capacitors is height limitations. One single 1k mf is much taller.

Paul
Title: Re: Why 2 electrolytic capacitors and Schottky rectifier at power supply?
Post by: jremington on Jun 18, 2019, 06:18 pm
Quote
I can't find the source.
Why bother, in that case? The "circuit snippet" means nothing, taken out of context.
Title: Re: Why 2 electrolytic capacitors and Schottky rectifier at power supply?
Post by: hammy on Jun 18, 2019, 07:42 pm
As a general rule you don't need to add capacitors to a battery power supply - capacitors at this position are for smoothing a power supply.
Capacitors are usually placed physically close to sources of electrical noise ( motors etc) or   Close to processors etc to absorb high frequency noise they might make ( usually ceramic ones as electrolytic are not effective at high frequencies ) . Capacitors can also be used to absorb spikes in  current which may occur , eg in switching - without which a sudden dip in supply voltage may occur.
Title: Re: Why 2 electrolytic capacitors and Schottky rectifier at power supply?
Post by: DrDiettrich on Jun 18, 2019, 09:12 pm
Can anybody explain the use for the diode? If the power source is connected in wrong polarity, the diode can't do anything but burn out.
Title: Re: Why 2 electrolytic capacitors and Schottky rectifier at power supply?
Post by: hammy on Jun 18, 2019, 09:20 pm
No idea, the bottom end is not connected anywhere - this is a circuit pulled from the ether , don't get too hung up on it .

It's probable it's intended for transient overvoltage ( spikes) supression .
Title: Re: Why 2 electrolytic capacitors and Schottky rectifier at power supply?
Post by: DrDiettrich on Jun 18, 2019, 09:30 pm
Hmm, if the unlabeld "GND" style connector means the external power input, then the diode would prevent wrong polarity on that pin.

Apart from that I agree - the circuit is of no use for the OP. It makes more sense to add some LEDs or other gadgets to his project ;-)
Title: Re: Why 2 electrolytic capacitors and Schottky rectifier at power supply?
Post by: GoForSmoke on Jun 18, 2019, 09:39 pm
Somebody expects the motor driver to kick back hard on the power supply?

Could an inductive surge going to ground somehow cause VS to go negative and suck charge through that diode? I won't be too surprised after reading Secrets of the H-Bridge.


Title: Re: Why 2 electrolytic capacitors and Schottky rectifier at power supply?
Post by: Wawa on Jun 19, 2019, 05:46 am
The schottky diode is probably a crude reverse voltage protection, in case the power supply can be connected the wrong way. Should have a fuse then too.
Caps have a max ripple current rating, and an ESR rating. It is common to use multiple caps for high peak current demands.
Leo..
Title: Re: Why 2 electrolytic capacitors and Schottky rectifier at power supply?
Post by: larryd on Jun 19, 2019, 06:10 am
View this Youtube:

Why Electrolytic Capacitors are connected in parallel.

https://youtu.be/wwANKw36Mjw      (https://youtu.be/wwANKw36Mjw)







Title: Re: Why 2 electrolytic capacitors and Schottky rectifier at power supply?
Post by: wvmarle on Jun 19, 2019, 11:38 am
3. Why is a high value of 470uF used? I searched and found that many power supply decoupling capacitors are less than 100uF for my application where the supply is only about 12 V DC.
Depends on the rest of the circuit, and the required stability. Very large capacitors are routinely used in combination with bridge rectifiers in power supplies, for example. Well, nowadays most power supplies are switch mode, but 12V power supplies that have a small transformer are still sold as well.
Title: Re: Why 2 electrolytic capacitors and Schottky rectifier at power supply?
Post by: Romonaga on Jun 19, 2019, 02:47 pm
Can anybody explain the use for the diode? If the power source is connected in wrong polarity, the diode can't do anything but burn out.
My guess is reverse polarity protection.
Title: Re: Why 2 electrolytic capacitors and Schottky rectifier at power supply?
Post by: wvmarle on Jun 19, 2019, 03:27 pm
My guess is reverse polarity protection.
It will just burn out in the first few seconds and lose its function. Reverse polarity protection is a bit more complex.
Title: Re: Why 2 electrolytic capacitors and Schottky rectifier at power supply?
Post by: Wawa on Jun 19, 2019, 11:34 pm
It will just burn out in the first few seconds and lose its function. Reverse polarity protection is a bit more complex.
As said, a fuse could also be also needed, unless the supply is weak.
This sort of reverse protection is common in guitar pedals powered by a 9volt block battery.
The diode can easily sustain the fault current until the user discovers it that the clip doesn't fit the battery the wrong way.

Burned parts could also make it easier for the manufacturer to reject false warranty claims.
Leo..
Title: Re: Why 2 electrolytic capacitors and Schottky rectifier at power supply?
Post by: larryd on Jun 20, 2019, 12:57 am
Quote
Burned parts could also make it easier for the manufacturer to reject false warranty claims.
OMG

Would they do that?    :smiley-cry:


I remember where a series 1 ohm resistor was used as a fuse then connected to a circuit like that.

The resistor made a nice smell when the power supply was reversed :) .










Title: Re: Why 2 electrolytic capacitors and Schottky rectifier at power supply?
Post by: TomGeorge on Jun 20, 2019, 01:20 am
Hi,
Diode spec;
https://www.diodes.com/assets/Datasheets/ds23001.pdf (https://www.diodes.com/assets/Datasheets/ds23001.pdf)

Read features paragraph.

Tom.. :)
Title: Re: Why 2 electrolytic capacitors and Schottky rectifier at power supply?
Post by: larryd on Jun 20, 2019, 01:25 am
Non-Repetitive Peak Forward Surge Current 8.3ms single half sine-wave superimposed on rated load IFSM 25 amps

That would blow a 1Ω 1/4 watt resistor.


Title: Re: Why 2 electrolytic capacitors and Schottky rectifier at power supply?
Post by: TomGeorge on Jun 20, 2019, 01:28 am
(https://ui-ex.com/images250_/electrification-clipart-cartoon.png)
Title: Re: Why 2 electrolytic capacitors and Schottky rectifier at power supply?
Post by: DryRun on Jun 20, 2019, 11:17 am
I did some more research and i realised that the capacitor setup in my first post is similar to that of a Passive Low Pass Filter.

I found this website which explains signal filtering: https://www.arrow.com/en/research-and-events/articles/using-capacitors-to-filter-electrical-noise

And to make the calculations: https://www.electronics-tutorials.ws/filter/filter_2.html

It seems a bit complicated to me... But maybe it could be another reason as to why 2 x 470 uF capacitors were used in that configuration and their specific values can be explained through these calculations.

Based on my new finding, it seems like the capacitors act as decoupling to protect against voltage spikes AND filter noise at the same time?
Title: Re: Why 2 electrolytic capacitors and Schottky rectifier at power supply?
Post by: wvmarle on Jun 20, 2019, 11:32 am
As long as the context of that original snippet (i.e. the rest of the circuit) is unknown, it's all guesswork.
Title: Re: Why 2 electrolytic capacitors and Schottky rectifier at power supply?
Post by: DryRun on Jun 20, 2019, 11:56 am
I saw this circuit in an Arduino motor and sensor shield schematic and its power supply intrigued me. I have not been able to find the exact shield to get the rest of the circuit but i found some similar design from other Arduino shield which uses similar components in the same configurations.

https://www.dfrobot.com/product-378.html
Schematic: http://image.dfrobot.com/image/data/DFR0105/Power%20shield%20V1.1SCH.pdf
Title: Re: Why 2 electrolytic capacitors and Schottky rectifier at power supply?
Post by: Smajdalf on Jun 20, 2019, 03:03 pm
Another possible diode function: protect the caps from the motor. If supply is disconnected and motors are moved by some external force they will generate back EMF. If it is in the "right polarity", it will only charge the caps. But in the other direction it would damage the caps (or other parts of the circuit). The diode prevents this.
Title: Re: Why 2 electrolytic capacitors and Schottky rectifier at power supply?
Post by: TomGeorge on Jun 20, 2019, 04:30 pm
Another possible diode function: protect the caps from the motor. If supply is disconnected and motors are moved by some external force they will generate back EMF. If it is in the "right polarity", it will only charge the caps. But in the other direction it would damage the caps (or other parts of the circuit). The diode prevents this.
And any electronic components sharing the power supply,  good thought.
The circuit shows two caps, probably, as mentioned earlier, they are positioned at different points of the PCB or circuit to help reduce noise, current surges.
Tom... :)
Title: Re: Why 2 electrolytic capacitors and Schottky rectifier at power supply?
Post by: markba633csi on Jun 21, 2019, 05:52 pm
The diode is a 1 amp, 40 volt unit - will prevent the supply from going negative more than about 0.4 v if input is reversed as long as input supply is limited to under 1 amp otherwise diode would be overheated as has been mentioned above
The capacitors could serve several purposes: Filtering, ripple reduction, and decoupling (prevents signals impressed on supply by load causing unwanted oscillation)
If supply is batteries then the diode would protect the circuit against reverse connection but a fuse would be required to protect diode- schottky diode has lower forward voltage drop than a conventional rectifier diode but both types are used as protection diodes
Mark
Title: Re: Why 2 electrolytic capacitors and Schottky rectifier at power supply?
Post by: DryRun on Jun 24, 2019, 01:51 am
Thanks for all the feedback.

The schottky diode is probably a crude reverse voltage protection, in case the power supply can be connected the wrong way. Should have a fuse then too.
Caps have a max ripple current rating, and an ESR rating. It is common to use multiple caps for high peak current demands.
Leo..
What value of fuse is recommended in this case? My understanding is that i need a fuse which limits the current to a value which won't damage the batteries but also allow enough current to flow such that it can supply the circuit so that it can operate without any issues.

I am powering 2 gearbox DC motors and a few sensors. I have not yet decided if the motors will be 6 V rated or 12 V rated DC motors. Does it make any difference?

I am considering to install a Fast Blow Cartridge Fuse but i need to know the current rating to use:
(https://forum.arduino.cc/index.php?action=dlattach;topic=622049.0;attach=313809)
I already have a couple in my electronics box but they are rated at 250 V, 0.5 A. Should i get a lower current rated one?

Another possible diode function: protect the caps from the motor. If supply is disconnected and motors are moved by some external force they will generate back EMF. If it is in the "right polarity", it will only charge the caps. But in the other direction it would damage the caps (or other parts of the circuit). The diode prevents this.
Nice suggestion. I didn't think of back EMF. :)