Power Supplies


A power supply converts 230V (125V) AC mains into a low and stable DC voltage. A simple power supply consists of a transformer, a rectifier, a capacitor and a simple stabilizer.
For complex medical equipment, better power supplies with a more sophisticated stabilization are needed. Often these power supplies deliver two, three and more different voltages.
Many failures of medical (and electronic) equipment due to power supply defects. Therefore a lot of devices can be repaired just with the knowledge of the functioning of power supplies.



Types


Power supplies are 'voltage sources'. That means, that the output voltage is stable even if the output current fluctuates.
Battery charger for example are 'current sources'. They deliver a stable current and the voltage changes depending on the state of charge of the battery.

A stable voltage is very important in electronics. A (biomedical) measurement equipment will run out of range with a poor stabilization and will deliver wrong diagnostic results.

Nowadays power supplies with expensive and heavy transformers are replaced by cheaper switched-mode power supplies. Switched-mode power supplies have smaller transformers but therefore more electronics. As a result of the smaller transformers they are cheaper but also more difficult to repair.



The parts of a simple power supply


In different stages the high AC voltage is converted into a stable DC low-voltage.
First the mains voltage has to be reduced (transformer), then converted to DC (rectifier), filtered (capacitor) and finally stabilized (zener diode and transistor or voltage stablizer IC).


The transformer transforms the mains AC voltage (230V) to low AC voltage.
This has always to be done at first because transformers only can transform AC.


The smaller AC voltage gets to a rectifier. The rectifier converts the negative part of the wave into a positive signal.


A (small) capacitor is added. The ability of voltage storage of the capacitor makes the signal smoother.


Is the capacity high enough the output signal is completely flat. We have created a DC voltage.


Under a bigger load the DC voltage breaks down. Instead of a open-circuit voltage of e.g. 20V we now have less. This is unacceptable because a voltage fluctuation has big influence to the connected stages. It is very important to stabilize the output voltage now.


The output voltage now is the voltage across the zener diode. That means: absolutely stable within the range of the diode's specifications. A series resistor is always needed where the (unstable) voltage difference can drop.

With this little circuit we produce a very clean and stable DC voltage. But unfortunately only a small current can be taken from this zener diode circuit. For running electronic applications it is not enough.
For a practical usage, this current has to be amplified. This is the job for a transistor. The stable voltage now controls just the input of a transistor and the transistor makes sure that a much higher current can be taken from the circuit.



Power Supply with Transistor


Transistor Principles



The three pins of a transistor: Base, Emitter and Collector.

A transistor is a three-terminal semiconductor device. The three pins are named: Base, Emitter and Collector. Transistors are used to switch or to amplify signals, voltages or currents.
The three terminals are used for input, output and for the common connection. Which terminal is what depends on the wiring. Three variations are possible.
In general the Base of a transistor is the input lead. The input current flows from Base to Emitter. When a current flows the voltage drop across BE is like the voltage drop of a diode, always 0.7V. That also means that always a base resistor is needed which limits the base current and let drop the excessive voltage.
This Base current now controls the CE path of the transistor which means a much higher Collector current. The transistor acts as an amplifier: A small Base current causes a big Collector current. For example a Base current of 10mA can control a load current of 1A.
In principle the Base current controls the CE path. The CE path opens or closes depending on the Base current. The higher the base current, the smaller the CE path (CE voltage drop) and the higher the Collector current will be.
With the maximum base current the transistor is fully controlled, the current is maximum and the CE voltage is minimum. The transistor acts like a switch or relay.


A small Base current controls a much bigger Collector current.
The higher the Base current, the higher the Collector current.
The higher the Collector current, the smaller the CE voltage drop.



Function of a power supply with a transistor


For a power supply the transistor is used as a current amplifier. The right transistor mode for this operation is called common-collector mode. This means the Base is used as control input, the Collector as power supply input and Emitter as the controlled output.

The stabilized voltage of the zener diode is used to control the transistor. The zener-voltage is connected to Base. This is possible because the needed Base current is low enough not to effect the zener voltage.
An additional base resistor is not needed because the series resistor of the zener diode also acts as a series resistor for the transistor.


Collector: Unstable input voltage
Base: Stable control voltage
Emitter: Controlled (stable) output voltage

This Base current now controls the much bigger load current C to E. In our case a stable voltage at the base keeps the output voltage stable or more precise controls the CE voltage until the Emitter-to-ground voltage is stable. The output voltage has to be stable because the BE voltage drop is always fixed to 0.7V and it is in series with the also fixed zener voltage (12V for example). If both voltages are fixed the resulting voltage must be also fixed. The resulting output voltage is the zener voltage minus the BE voltage:

12V - 0.7V = 11.3V.

or

Vout = VZ-diode - VBE


The output voltage is stable because the zener voltage and the BE voltage are stable.
Both voltages are in series.

What ever the input voltage is, if it is drifting up or down, the output voltage is always 11.3V. What changes is the CE voltage across the transistor. This is of course the difference of input voltage and output voltage.

Vout = Vin - VCE


When the input voltage changes only the CE voltage of the transistor changes because the Base voltage is fixed. The Emitter voltage (output voltage) is also fixed because it depend on the fixed Base voltage minus the fixed 0.7V Base-Emitter voltage.

Now the power supply is stabilized or regulated. The output current can be much higher because it now depends on the specifications of the transistor and not any more on the small zener diode.


In practice an additional capacitor is always connected to the output in order to buffer the voltage against fast current peaks which could courses fast voltage drops.
The only thing which is missing now, is a mains switch and a fuse. Then the power supply is complete.


The current through the transistor now is stabilized and high enough to supply small electronic applications.



More power


In the above shown circuit the limiting device now is the transistor. The parameters of the transistor defines the output voltage (or more precise the maximum EC voltage) and the maximum current which can be taken. Important is always the situation between Collector and Emitter. Here the high load current flows and together with the CE voltage drop the heat loss of the transistor is created.
If the power supply has to deliver a higher output current or the difference between input and output voltage is too big (VCE) a bigger transistor is needed. Unfortunately a bigger transistor also needs a bigger base current which again stresses the zener diode and thus the stabilization. What we need in this case: An additional transistor. A transistor which controls the main transistor. Two transistors in series. One controls the other.


Now the smaller transistor takes the zener voltage and gives this stable voltage (minus 0.7V) to the bigger output transistor. The Base current for the bigger one now flows through CE of the lower one and does not effect the zener diode.
The upper transistor is always much bigger than the other one, because the main load flows through this transistor, while the lower transistor only has to deliver the small base current for the big one. Such a power supply can deliver some Amps. But note, together with the CE voltage drop this high current creates a big power loss, which means heat. The load transistor always has to be mounted on a heat sink.

Again a look at the voltages:

- Zener voltage is fixed at 12V
- Voltage drop BE of the first (smaller) transistor is also fixed at 0.7V
- Voltage at E: (12V - 0.7V)= 11.3V
- Voltage drop BE of the second (bigger) transistor is also fixed at 0.7V
- Voltage at E which is the output voltage: (11.3V - 0.7V) = 10.6V
- The output voltage is stable but only 10.6V
- Or the other way round: If we need a 12V output voltage the zener diode has to be one for
  13.4V (12V + 0.7V + 0.7V)



Power loss


Now a look at the power loss:
The current through the transistor together with the voltage drop between C and E makes the power loss. In case of the upper load transistor there can be several watts of power loss, which means heat. The transistor gets hot. That is why the load transistor of a power supply is always mounted on a heat sink or directly to the metal housing of the equipment. The rule of thumb is: Every semiconductor with a power loss bigger than 1W needs a heat sink.
The power loss or heat is the product of the VCE voltage drop and the load current through the transistor ICE

P = Iload × VCE



Negative Voltage


Now something confusing.
Power supplies can also generate negative voltages. The technology is the same as for positive voltages. It is just a matter of grounding or where the reference point for our measurement is.
Negative voltage means, that the output voltage is more negative against the ground.
Is the positive terminal of a battery connected to the ground, then the negative terminal is more negative than the ground. The output voltage is negative.


Imagine two 9V-batteries in series.
First we connect the minus connection of the lower battery (and our measuring cable) to the ground. In the center we would measure 9V at the top 18V.
Now we put the center point to the ground (and also our measuring cable). On top we would measure 9V and at the minus connector of the lower battery -9V.
We get two voltages, a positive and a negative one.


In the same way a power supply for a positive and a negative voltage works.
The + connection is more positive and the - connection more negative compare to the ground.



Power Supplies with Stabilizer-IC


Beside voltage stabilization, often a short circuit protection and an overhead protection for power supplies are demanded. Nevertheless the circuit should be as simple, small and cheap as possible.
The solution is a special IC (Integrated Circuit), which contains all these functions. The most common stabilizer is the 78xx series. This IC contains the whole stabilization and all the safety circuits.



Positive stabilizer 78xx


The IC has three pins and is build into a transistor housing. The output voltage is fixed. Different types for different voltages are available.


It looks like a transistors but it is complex integrated circuit.
The 78xx type (left) is a stabilizer for up to 1A and the smaller 78Lxx (right) for up to 100mA.

The IC is available for different output voltages. The output voltage is expressed by the name. An 7812 is a 12V stabilizer for a positive voltage.

Output voltage Stabilizer
5V 7805
6V 7806
8V 7808
10V 7810
12V 7812
15V 7815
18V 7818
24V 7824

78xx for these voltages exist.

The pin connection depends on the case type. Good to know that the metal part of 78xx is ground. The IC can mounted directly to a heat sink without any isolation.


The pin connection for the positive 78xx type.
The most common type is the 1A type in the TO-220 case.

The connection pins are:

left - in
center - ground
right - out

The application is simple. Only a input capacitor and a small output capacitor is needed for a fully stabilized power supply. The power supply is short circuit protected and delivers up to 1A.




Negative stabilizer 79xx


Beside the positive 78xx also a stabilizers for negative voltages exist. It is the 79xx series. The stabilizer look similar but the connecting pins are different.


Here the pin connection for the negative 79xx type.
The most common type is the 1A type in the TO-220 case.

The connection pins are:

left - ground
center - in
right - out

Important: This time the metal case is NOT ground.

Also the negative stabilizer is available for different output voltages. A 7912 is a -12V stabilizer.

Output voltage Stabilizer
-5V 7905
-6V 7906
-8V 7908
-10V 7910
-12V 7912
-15V 7915
-18V 7918
-24V 7924

79xx for these negative voltages exist.


The following power supply circuit from an oxygen concentrator combines two power supplies, one for a positive and one for the negative voltage.

The upper part provides the positive voltage (+5V), the lower part the negative voltage (-5V). Note that the rectifier is drawn inverted. The positive lead is down, negative up. Also the following capacitor is upside down. The input voltage of the IC is negative (more negative than ground). After stabilization the two reference potentials are put together to ground.

Below a similar power supply of a spectrometer.


The transformer is drawn somewhere else, but anyway the AC voltage gets to the points AC-15-2-15V and AC-15-2-0V, which obviously means 15V AC (at the diodes) against the ground. The rectification is done by just two diodes (D5,D6). The ground is now drawn in the middle, the upper part shows the part for the positive voltage, the lower part is for the negative voltage.
(By the way, there is a fault in the circuit. Look at the voltages specially in the negative part...)



Troubleshooting and common problems


Reasons for defects in electronic circuits in general are always high currents, high voltages and power losses with the development of big heat. All this applies to power supplies. That is why troubleshooting in electronic equipment should always start with checking the voltage(s) of the power supply.

In theory voltage regulators should never fail because they are protected against short circuit and over heating. But in practice they sometimes gets broken. (Why? - I don't know.)

A function check has to be done under power. Even if stabilizers look like transistors, they are ICs. You can not check a stabilizer with an ohm-meter!
A voltage check is very easy:
- Connect your multimeter to the ground (metal housing, minus of biggest capacitor...)
- Left pin is input voltage (up to 30 V), center is ground (0 V) and the right pin is the
  stabilized output (for the common 78xx type)
- The pin connection of negative regulators (79xx) is different (ground - input - output).

Think about the short circuit protection when there is no output voltage. No output voltage can mean, that the stabilizer is defective and delivers no voltage. But it also can mean, that there is a short circuit after the power supply and the integrated protection pulls the voltage down. Therefore, always disconnect the load from the stabilizer if there is no voltage. Just take off the cables to the connected stages or cut the output leg of the IC with a small cutter. Now you can check the output voltage directly at the IC. Later you can solder it again.


This is the power supply of a spectrometer. Clearly to see the big charging capacitors at the left and in the center, two rectifiers in between and three stabilizer mounted on small heat sinks.
First step for checking the board is: Connecting minus of the voltmeter to the ground (minus of the capacitors, the largest conductor track or the center pin of the stabilizer 78xx). The unstable input voltage is at pin 1, the stable output voltage is at pin 3.
Remember, negative 79xx stabilizer have different pin connection.


Here again the pin connections of a 78xx and a 79xx

If a stabilizer is defective and the right one is not available, maybe another one can be taken. The trick is to take a stabilizer for a lower voltage and putting up the ground by a zener diode. Zener voltage and stabilizer voltage make the output voltage.


A 9V-output voltage (8.9V) can be created by a common 5V-stabilizer and a 3.9V-Zener-diode.



Do-it-yourself power supply with stabilizer IC


A power supply is often needed. A battery powered equipment shall run on mains voltage or the defective external power supply of e.g. a microscope is not repairable. In this case a power supply can be build by yourself. But for the construction some experiences are needed and some calculations have to be done.
Here some universal hints to calculate the values of the needed parts:

Transformer: Output voltage should be 3-5V higher than the needed (unstable) DC voltage. The output current should be 10-20% higher than the needed DC current.

Rectifier: The proof voltage must be at least 1.4 x transformer output voltage.

Capacitor 1: Charging Capacitor as big as possible. 470F per 100mA is perfect. Proof voltage at least 1.4 x UTransformer

Stabilizer: Power loss bigger than 1W always with heat sink. P = (Uout - Uin) x I

Capacitor 2: Output capacitor. For Audio applications 220F, for all others 10F. Proof voltage at least 1.4 x UStabilizer


Often you find two small bipolar capacitors C2,C3 in the input and output path of the stabilizer. Their task is to suppress unwanted oscillating of the IC. They should be mounted close to the stabilizer. The values are not critical. 0.1F are common.



Prices


Stabilizer ICs are cheap and standard electronics spare parts. Some types specially the 7805 and the 7812 should be present in every workshop.

78xx, 79xx (TO-220) 0.30 €
78Lxx, 79Lxx (TO-92) 0.20 €
78xxK, 79xxK (TO-3) 1.50 €



Sources and additional information


Power supply: http://en.wikipedia.org/wiki/Power_supply
Switched-mode power supply: http://en.wikipedia.org/wiki/Switched-mode_power_supply
Voltage stabilizer: http://en.wikipedia.org/wiki/Voltage_stabilizer
Data sheets: http://frankshospitalworkshop.com/electronics/ic_analog.html