Rectifiers, What do they do? How do they work?

Firstly they convert AC into DC using diodes. A diode is a device that will let current flow through it in one direction but not in the other direction, kind of a one way valve for electricity.

Have a look at the wave form below, it is AC. You can see that it varies from 0v to +V back down to 0v then to -V and back up to 0v again. What we need to do with our rectifier is to get rid of the negative components from this wave form.

And this is exactly what a half wave rectifier does, it just discards the negative cycle of the wave form to leave us with this,

As you can see it doesn’t look very DC. The voltage rises from 0v until it reaches +V it then decreases at the same rate, it waits for one half cycle then repeats. Believe it or not this is technically DC. Now what we can do to improve the situation is to add a big capacitor to the output to smooth it somewhat, the result is,

Ahhh, that looks allot better! What is happening hear is that on the + cycle the capacitor charges up but on the missing cycle the capacitor discharges apparently filling in the gap! BUT we can do better, lets remove the capacitor for now and move to a full wave bridge rectifier. What this does instead of just removing the negative cycle it reverses it (i.e. takes it from the bottom and puts it on the top!) so that we don’t have a missing cycle anymore.

As you can see the gaps that we have are half the size we had before so all that is left to do is to put the capacitor back in and we have a near perfect DC. Obviously the bigger the capacitor the longer it will take to discharge and therefore the less ripple there will be at the output.

The Field Effect Transistor,

Various types of FET exist, N channel, P Channel, Enrichment and Depletion types are available all of which are basically controllable resistors. In operation they are quite similar to the old type valve (that big glowing thing in your old TV), as with the valve only one voltage (applied to the Gate) controls the current flow. Unlike the bipolar transistor no current flows in the Gate. This means that the FET has a high impedance and it does not load the device that is driving it..

The type of FET that we are most interested in is the Metal Oxide Semiconductor FET or MOSFET for short, the main reason for this is the huge amounts of current that they can control are ideal for motor drivers. It is worth mentioning here that we tend to use the N channel Enrichment type as it has been developed for this type of use.

How do they work? I hear you say! Below is a design example,

There are two simple circuits here to consider, both using a power MOSFET (you will also notice that these devices also have a built in diode) as a switch and a bulb. We will look at the lowside switch first, named because the MOSFET’s Source is connected to GND. As a general rule with MOSFET’s (logic level excluded), to turn the device hard on (lowest Rds) then we need to supply the Gate with 10v with reference to its Source. This is easy to do when the Source is connected to GND, all we have to do is put 10v on the Gate and it will switch on. (and light the bulb) To turn it off then the Gate has to be connected to GND, if we leave it floating (i.e. remove the 10v) the device will stay on! Now for the High Side Switch. As you can see the Source is NOT connected to GND. So lets say for arguments sake that the MOSFET is switched ON. If you measure the voltage between Source and GND or the top of the bulb and GND (both electrically the same place) then you will see 12v (this is the Source voltage), this is because when the MOSFET is on it has about as much resistance as a piece of wire and the bulb could be anything up to 300R (depending on wattage) Therefore to switch the MOSFET hard on (as we require) then the Gate must be supplied with 12v + the previously mentioned 10v(above Source) and this gives you a switch on voltage of 22v! And yes you’ve guessed it, we only have a 12v battery. (I bet you cant wait for me to do a bit on voltage inverters!)

Op-Amps

An Op-Amp (or Operational Amplifier) can be characterised by the following properties. It is a very high gain, high input impeadance, DC coupled difference amplifier with a low impeadance single ended output. Well that’s what the text books say about it so what use is it to us? Well there are two things it can do for us, the first is that it can amplify very small voltages into larger voltages and the second is that it can compare 2 voltages and give us the differance. The main use that we have for it is in the current limiter, if you look at the diagram you will see that it uses 2 Op Amps, the first one (U5:A) is used as an Amplifier, the circuit is re drawn below.

This arrangement is known as a Difference Amplifier, and what it does is to amplify the difference between V1 and V2. In this circuit both R1’s have to be the same value and so do the R2’s. The gain of this circuit is calculated by R2/R1, so lets say R1 = 1k and R2 = 10k, then 10/1= 10. The gain is 10. Obviously it is easier to leave R1 as 1k for calculation purposes. Now an Op Amp works from 2 supply rails normally a +12 and a -12 so the device cannot amplify a signal above these supplies i.e. you cannot amplify a 1v difference to give a 400v output! A 741GP should be a good candidate for this type of circuit. Now to make things a bit easier for the next circuit I opted to use a single supply rail to rail Op Amp, this Op Amp works from +5v (no -5v) and its output can is in the range of 0v to 5v (oh look, logic levels!), this makes this device ideal for a comparitor.

In this circuit RV1 enables us to vary the voltage in the inverting input (+) from 0v to 5v, lets say we have set it to 2.5v for this example, this is our Reference voltage. R2 in this example is 560k and will therefore supply little feedback, having a small feedback will cause the amplifier output to swing from 0v to +5v (or Amplify beyond its supply rails) when the Input voltage exceeds the reference voltage.

I have found 2 uses for this circuit, one is in the Current Limiter and the other is in PWM generation.