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Topic: Why 2 electrolytic capacitors and Schottky rectifier at power supply? (Read 682 times) previous topic - next topic

Wawa

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..

larryd

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 :) .










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TomGeorge

Everything runs on smoke, let the smoke out, it stops running....

larryd

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.


No technical PMs.
If you are asked a question, please respond with an answer.
If you are asked for more information, please supply it.
If you need clarification, ask for help.

TomGeorge

Everything runs on smoke, let the smoke out, it stops running....

DryRun

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?

wvmarle

As long as the context of that original snippet (i.e. the rest of the circuit) is unknown, it's all guesswork.
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.

DryRun

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

Smajdalf

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.
How to insert images: https://forum.arduino.cc/index.php?topic=519037.0

TomGeorge

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... :)
Everything runs on smoke, let the smoke out, it stops running....

markba633csi

#25
Jun 21, 2019, 05:52 pm Last Edit: Jun 21, 2019, 06:03 pm by markba633csi Reason: embellish answer
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

DryRun

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:

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. :)

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