Supplying Current Question

[LiquidOrb24]

Hello All,

I have an easy question to ask, but yet I do not know the answer.

When you build a circuit lets just say for argument a microcontroller that runs off of 5 volts and maybe a 555 timer circuit connected to it with a few LED's that flash from the 555 timer and flash from the microcontroller. If I want to run this circuit from the wall outlet and decide to use a general transformer that steps 120 down to 5 volts and say 1A at the output and I hook it up to my circuit will it fry my circuit?

The 5 volts would be fine I believe but since the transformer has 1A at the output does that mean that no matter what 1A will go through my microcontroller and 555 timer, or does it mean that it has the capability to supply 1A but if my circuit doesn't draw that much current it will only draw the necessary current to make my circuit run.

On that note would it be better to get a transformer that has higher current at the output or less?

Thanks Everyone.

 

[dknguyen]

THe transformer will change 120VAC to 5VAC. Your circuit needs DC. WHich means you need a at least a diode rectifier (or a bunch of diodes wired up to be a rectifier) and capacitors to change the voltage at the output of the transformer to DC. INductors can also be added if you want (you can take entire courses on this). Then you will also need a regulator IC (like a linear regulator) to regulate the voltage before it gets to your chips since the wall voltage is poorly regulated and all the transformer does is steo down the voltage a set amount- it doesn't try and make the output constant so if the input varies (which it will) so will the output.

Also, because of the way a diode rectifier works and the waveforms of AC and DC (and all that math stuff) the peak voltage of the AC waveform at the output of the transformer must be higher than the DC voltage you want to produce at the output of the rectifier. This voltage will also be very noisy (it has a lot of ripple and AC components in it along with the DC component that you want). So you need a linear regulator to make sure the signal is a clean steady 5VDC. Linear regulators always require a higher input voltage than what they can output so the output voltage of the rectifier also has to be higher than 5V.


THis is bang on:
" or does it mean that it has the capability to supply 1A but if my circuit doesn't draw that much current it will only draw the necessary current to make my circuit run."

You can only control the voltage you put into an electronic device (for the most part). The electronic device/circuit will try and draw whatever current it needs.

 

[Gayan Soyza]

The 5 volts would be fine I believe but since the transformer has 1A at the output does that mean that no matter what 1A will go through my microcontroller and 555 timer,

Wrong

or does it mean that it has the capability to supply 1A but if my circuit doesn't draw that much current it will only draw the necessary current to make my circuit run.

Correct

 

[LiquidOrb24]

Wow thanks for the fast responses. So basically, if I understand what you guys said, I can buy one of those enclosed transformers (like a car charger) and use that for my circuit just as long as the voltage is what the circuit IC's require. Then by doing this the current will regulate itself depending on how many LED's and components I have in my circuit.

 

[dknguyen]

If a car charger is something that changes the wall output to something that can charge your car battery, then yeah. You can also just use a wallwart (a giant black block that plugs into the wall).

Those "wall-wart" giant brick thingies have the transfomer, diode rectifier, and capacitor inside them for you (and some have a regulator). Just make sure it's regulated output (there are both regulated and unregulated output versions). YOu can use an unregulated one if your circuit has it's own regulator (it should have one anyways), just make sure the output voltage is within the range of the regulator in your circuit (higher than 5V by enough so the linear regulator can do it's job, but not so high that the regulat cannot handle it).

 

[Gayan Soyza]

Those "wall-wart" thingies have the transfomer, diode rectifier, and capacitor inside them for you (and some have a regulator).

Yep.For PIC projects there should a 5V regulated supply.If the wall-wart doesn't have a regulator & the two filtering caps the trainer board must have them.

I have a separate 5V / 500mA power supply for PIC projects.It is very usefull all the time.