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Topic: Why 2 electrolytic capacitors and Schottky rectifier at power supply? (Read 674 times) previous topic - next topic

DryRun

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.



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. :)

slipstick

Where did you find that circuit? A little context for it might be useful.

Steve

DryRun

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.

Paul_KD7HB

"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

jremington

Quote
I can't find the source.
Why bother, in that case? The "circuit snippet" means nothing, taken out of context.

hammy

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.

DrDiettrich

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.

hammy

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 .

DrDiettrich

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 ;-)

GoForSmoke

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.


1) http://gammon.com.au/blink  <-- tasking Arduino 1-2-3
2) http://gammon.com.au/serial <-- techniques howto
3) http://gammon.com.au/interrupts
Your sketch can sense ongoing process events in time.
Your sketch can make events to control it over time.

Wawa

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..

larryd

View this Youtube:

Why Electrolytic Capacitors are connected in parallel.

https://youtu.be/wwANKw36Mjw     







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wvmarle

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.
Quality of answers is related to the quality of questions. Good questions will get good answers. Useless answers are a sign of a poor question.

Romonaga

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.

wvmarle

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.
Quality of answers is related to the quality of questions. Good questions will get good answers. Useless answers are a sign of a poor question.

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