Can someone explain to me in simple terms what the difference between a voltage regulator and a buck converter and basic examples of when each component might be used? Thanks
The typical linear voltage regulator (an analog device) would be the 78xx series (7805 5V regulator, 7812 12V
regulator etc etc). The 79xx series are negative voltage versions (seldom used today). Another common
regulator series these days are the AP1117-xx ones - they have a lower voltage drop-out than the 78xx
series. Linear regulators work by dissipating extra voltage (ie they waste (Vin - Vout) x I as heat), negative feedback and amplification is used to adjust the output from tiny differences in voltage between the output and a voltage reference.
A buck regulator is a switch-mode device, using capacitors and inductors to store energy while transfering it
from a higher voltage rail to a lower voltage rail without resistance. In theory they waste no power, in practice
they are 85% efficient or so. On eBay you will see lots of cheap LM2596 buck converter modules. The ferrite
body of the inductor is clearly visible in the pictures.
If you had a 12V supply and a circuit needing 5V at 1A, a linear regulator would take 1A from the incoming
supply and dissipate 7W of heat. A buck converter might take about 0.5A from the 12V supply and dissipate
about 900mW in the chip, the inductor and the schottky diode.
The linear regulator would output a clean 5V with very little noise (measured in microvolts), the buck
regulator would produce a frankly filthy 5V with switching noise around 50mV or more at lots of frequencies
being multiples of the main switching frequency. This is fine for powering digital circuits, but many sensitive
analog circuits require cleaner power to perform properly (microphone amplifier for instance).
So the linear regulator is not as energy effiecent, but produces a better( cleaner) 5v signal?
Yes. A clean 5V is not actually "better" for a digital circuit, which doesn't care (and injects more
noise onto the rail anyway). Any analog though, you have to be careful. Higher frequency switch-mode
regulators tend to be less of an issue for audio circuits, but can really degrade RF circuit performance
by orders of magnitude - RF signals are often in the nanovolt range.
, but can really degrade RF circuit performance
by orders of magnitude - RF signals are often in the nanovolt range
I can confirm this. Having designed the rf bits of mobile phones, switchmode devices were a no-no for this very reason. Every time some clever spark wanted them to increase battery life we resisted hard!
Allan
edit : there's enough racket around already in a mobile with all the processor stuff rattling away - hard enough to clean up without also haviing dirty supplies. The ETSI specs permit a small number of 'deaf' channels because certain oscillator harmonics - even up at a few GHz - are almost impossible to get rid of.
thank you guys seem very knowledgeable. I had a second question that I can't seem to get answered on the forum.
I know that you can hook up and dim and led with just a potentiometer but if I connect the middle pin of the potentiometer to the gate pin of an n channel MOSFET then connect in my led in the cicuit, I can also control the brightness of the led with the potentiometer. What is the difference between using or not using the MOSFET ( why can't I just use a variable resistor)
What is the difference between using or not using the MOSFET ( why can't I just use a variable resistor)
You can, but have to be very careful about doing so, otherwise you can easily allow too much current to the led and send it to led heaven. It also means it's an analog circuit.
Using the mosfet, you use it like an on/off switch. You can have it fully on by allowing the proper gate voltage, or dim the led using PWM, in which there are 254 steps in dimming. Using the mosfet can be either digital OR analog, but with analog the led would be either on or off, as using the mosfet in the linear region is avoided.
Both have uses, along with simply driving the led with pwm straight from the arduino pins. You just need to ensure the led doesn't use more than the 40mA max (20mA preferred)
good explanation but cant I kinda of mimic using the Arduino and a PWM pin by hooking the middle pin of the potentiometer to the gate pin of the mosfet and control it manually and not have to worry about putting too much current since the pot is hooked up to the MOSFET?
tjones9163:
good explanation but cant I kinda of mimic using the Arduino and a PWM pin by hooking the middle pin of the potentiometer to the gate pin of the mosfet and control it manually and not have to worry about putting too much current since the pot is hooked up to the MOSFET?
I think you are getting confused. The gate is what controls the mosfet. The source/drain is the actual power circuit. On the gate, you don't want a variable voltage. You either want it ON or OFF, with as little time in the in-between area, as this will produce heat with power circuits.
Using a potentiometer on the source circuit with a led would be baffling. If you are doing this by analog means, you don't even need the mosfet to drive an led. The appropriate resistor and a switch would do just fine.