I'm learning about power supplies at university and I have created 2 or 3 PCBs of this stuff, but recently I saw this schematic:
I have no idea of what exactly these diodes do in this circuit. Can someone explain it, please?
Electronic – What’s the purpose of a diode across capacitor after voltage regulator
capacitordiodespower supply
Related Solutions
The main things that you need to look at when selecting capacitors for a power supply is the voltage rating of the capacitor, if it needs to be polarized, and the equivalent series resistance.
The datasheet of the IC that you are using will specify what value of capacitor you need and many times will specify the actual type of capacitor required.
Electrolytic Caps are polarized which doesn't work well for AC signal, but is the cheapest way to get a high value capacitance, which is why they are used a lot in power supplies.
So, in short, yes, any cap should work OK as long as you are in the allowable voltage range.
The schematic has a note right on it:
So the fuse and diode are intended for protection (overcurrent and reverse voltage respectively), as others have correctly noted.
Note also that the diode drops about 0.65V or about half of the 1.7V that is required to get from 5V to 3.3V. At the maximum 500mA out, the power dissipation in the regulator will thus be split between the diode and the regulator at about 325mW each. That might be significant in allowing more current or higher ambient temperature, depending on what package that Sparkfun used.
If you build the circuit yourself, the diode is optional if you don't want the protection and are okay with the increased power dissipation. If you use a diode, a reasonable type to choose would be rated at 1A and 200V or more. Very common and cheap- a 1N4004 (1A/400V) would be a popular through-hole part.
Related Topic
- Electronic – Very low power voltage regulator – diode
- Electronic – How to calculate voltage using the complete diode method
- Electrical – How does connecting a capacitor across a DC power supply with varying voltage produce a constant voltage
- Electrical – Rectenna – What’s the Purpose of the Capacitors
- Electronic – What’s this diode symbol
Best Answer
Those two diodes perform a useful function when the external load is connected between the power supply outputs (this configuration doubles the voltage potential of the positive rail).
In this case, if the load becomes short circuit then the two outputs will fight each other, one will become dominant and in this situation the diodes prevent one of the outputs (Capacitors) becoming reverse biased.
In addition, at power supply switch on, one of the rails will "come up" faster than the other one and in this situation the diodes prevent the "slow to rise rail" becoming reverse biased. I think the same situation can occur at power down.
To summarise, the diodes are reverse bias protection.
These two diodes should be rated for the full output current of the power supply.