Power Circuit Design

 
Power circuits are those types of circuits that carry electricity to operate a load such as a furnace or a light bulb etc. Although this seems simplistic, it is the only logical way to separate these circuits from control circuits.

A great way to diagrammatically show a power circuit is to have an outlet at the source of the circuit with two wires showing the path. It also includes a switch for control and a motor that represents the load that the electrical power is supplied to. When the circuit is closed, the flow of electrons (better known as the current) directly powers the motor windings, which effectively turn the motor.

These circuits are also used in Micro SD cards and other storage devices.
 

Micro SD Card interfaces for Electronics

Designers often use power circuits in media storage devices like SD cards, and they need to keep the costs down while they balance a simplistic design and don’t incur additional costs for designing them. They have to keep things like “in-rush current” and “hot insertion” in check to make sure that they have a prototype that is built for functionality and doesn’t compromise on aesthetics.

The best way to make sure that the design for the power circuits of these cards is perfected is to keep it simple and let go of components that are unnecessary and superficial. But keeping this aspect of design simple can be perplexing at times.
 

The Problem

A cursory glance at a micro SD or SD card pin-out shows that it is really simple to figure out. Even an entry level engineer can do it despite a lack of experience. But there’s a tricky part to it all.

Normally, a Micro SD card uses 100mA at 3.3V. So, connecting a 3.3V supply to it wouldn’t be an issue. But when you consider the power circuit “in-rush” current this 3.3V supply can also cause problems. When you have a prototype and you insert the SD card into it, it may cause your micro controller to reset if you don’t take the “in-rush” current into account.

This issue will happen every time you with your microcontroller, whether it’s an SD or microSD card.
 

A Possible Solution

So just how do you solve this issue? What alterations should you make to the board of a microSD card? The solution is fairly simple. Micro SD connectors have a card detection pin that the microcontroller in the power circuit can use for “seeing” if there’s a microSD card inserted or not.

This allows the power circuit of the system to turn on only when the microSD card is inserted the correct way and swiftly turn off the power when it’s removed. An additional measure you can take is have a decoupling capacitor adjacent to the power pin of the microSD card. This will make the power circuit more stable and take care of the “in-rush” current problem.

If you choose to manually solder the capacitor, it may disrupt the design a little, but it’s the only way you can get the power circuit system to work without nagging problems like the in-rush current.

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