In this post we discuss how an effective and efficient, yet very cheap, and stabilized bench power supply can be designed by any electronic hobbyist for safely testing all types electronic projects and prototypes.
The main features that a bench power supply must have are:
- Should be built with cheap and easily available components
- Should be flexible with its voltage and current ranges, or simply must include the facility of a variable voltage and variable current outputs.
- Should be over-current and over-load protected.
- Should be easily reparable, in case a problem arises.
- Should be reasonably efficient with its power output.
- Should facilitate easily customization as per a desired specification.
The majority of the power supply designs thus far incorporate a linear series stabilizer. This design uses a pass transistor which works like a variable resistor, regulated by a Zener diode.
The series power supply system is the more popular, possibly due to the fact that it is a lot more efficient. Except of some minor loss in the Zener and feed resistor, noticeable loss only happens in the series pass transistor during the period it is supplying current to the load.
However, one disadvantage of the series power supply system is that these do not provide any kind of output load short-circuit. Meaning, during output fault conditions the pass transistor may allow a large current to flow through it, eventually destroying itself and possibly the connected load also.
That said, adding a short circuit protection to a series pass bench power supply can be quickly implemented through another transistors configured as a current controller stage.
The variable voltage controller is achieved through a simple transistor, potentiometer feedback.
The above two additions enables a series pass bench power supply highly versatile, rugged, cheap, universal, and virtually indestructible.
In the following paragraphs we will briefly learn the designing of the various stages involved in a standard stabilized bench power supply.
Easiest Transistor Voltage Regulator
A quick way of getting an adjustable output voltage is to hook up the base of the pass transistor with a potentiometer and Zener diode as shown in the figure below.
In this circuit the T1 is rigged as an emitter-follower BJT, where its base voltage VB decides its emitter side voltage VE. Both VE and VB will precisely correspond with each other, and will be almost equal, deducting its forward drop.
The forward drop voltage of any BJT is typically 0.7 V, which implies that the emitter side voltage will be:
VE = VB - 0.7
Using a Feedback Loop
Although the above design is easy to build and very cheap, this type of approach doesn't offer great regulation of power at the the lower voltage levels.
This is exactly why a feedback type control is normally employed to get an improved regulation throughout the entire voltage range, as demonstrated in the figure below.
In this configuration, the base voltage of T1, and therefore output voltage, is controlled by the voltage drop across R1, mainly due to the current pulled by T2.
When the slider arm of the pot VR1 is at the ground side extreme end, T2 becomes cut off since now its base becomes grounded, allowing the only only voltage drop across R1 caused by the base current of T1. In this situation the output voltage at the T1 emitter is going to be almost same as the collector voltage, and can be given as:
VE = Vin - 0.7, here VE is the emitter side voltage of T1, and 0.7 is the standard forward voltage drop value for BJT T1 base/emitter leads.
So if the input supply is 15 V, the output can be expected to be:
VE = 15 - 0.7 = 14.3 V
Now, when the pot VR1 slider arm is moved to the upper positive end, will cause T2 to access the whole emitter side voltage of T1, which will cause T2 to conduct very hard. This action will directly connect the zener diode D1 with R1. Meaning, now the base voltage VB of the T1 will be simply equal to the zener voltage Vz. So the output will be:
VE = Vz - 0.7
Therefore, if the D1 value is 6 V, the output voltage can be expected to be just:
VE = 6 - 0.7 = 5.3 V, so the zener voltage decides the minimum possible output voltage that could be obtained from this series pass power supply when the pot is rotated at its lowest setting.
Although the above is easy and effective for making a bench power supply, it has a major disadvantage of not being short circuit proof. That means, if the output terminals of the circuit is accidentally short circuited, or an over load current is applied, the T1 will quickly heat up and burn.
To avoid this situation, the design could be a simply upgraded by adding a current control feature as explained in the following section.
Adding Overload Short Circuit Protection
A simple inclusion of T3 and R2 enables the bench power supply circuit design to be 100% short-circuit proof and current controlled. With this design even an intentional shorting at the output will not cause any harm to the T1.
The working of this stage could be understood as follows:
As soon as the output current tends to go beyond the set safe value, a proportional amount of potential difference across R2 is developed, enough to switch ON transistor T3 hard.
With T3 switched ON causes T1 base to be joined with its emitter line, which instantly disables the T1 conduction, and this situation is maintained until the output short or overload is removed. In this way T1 is safeguarded from any undesired output situation.
Adding a Variable Current Feature
In the above design, the current sensor resistor R2 can be a fixed value if the output is required to be a constant current output. However, a good bench power supply is supposed to have a variable range for both voltage and current. Considering this demand, the current limiter could be made adjustable simply by adding a variable resistor with the base of T3, as shown below:
VR2 divides the voltage drop across R2 and thus allows the T3 to switch ON at a specific desired output current.
Calculating the Part Values
Let's start with the resistors, R1 can be calculated with the following formula:
R1 = (Vin - MaxVE)hFE / Output Current
Here, since MaxVE = Vin - 0.7
Therefore, we an simplify the first equation as R1 = 0.7hFE / Output Current
VR1 can be a 10 k pot for voltages up to 60 V
Current limiter R2 can be calculated as given below:
R2 = 0.7 / Max Output Current
Max output current should be selected 5 times lower than T1 maximum Id, if T1 is required to work without an heatsink. With a large heatsink installed on T1, the output current can be 3/4th of T1 Id.
VR2 can be simply a 1k pot or preset.
T1 should be selected as per the output current requirement. T1 Id rating should be 5 times more than the required output current, if it is to be operated without a heatsink. With a large heatsink installed, the T1 Id rating should be at least 1.33 times more than the required output current.
The maximum collector/emitter or VCE for T1 should be ideally twice the value of the maximum output voltage specification.
The value of zener diode D1 can be selected depending on the lowest or the minimum voltage output requirement from the bench power supply.
The T2 rating will depend on the R1 value. Since the voltage across R1 will be always 0.7 V, the VCE of T2 becomes immaterial, and can be any minimum value. The Id of T2 should be such it is able to handle the base current of T1, as determined by the value of R1
The same rules apply for T3 also.
In general T2, and T3 can be any small signal general purpose transistor such as BC547 or maybe a 2N2222.
Having understood all the parameters for designing a customized bench power supply, it's time to implement the data in a practical prototype, as shown below:
You may find a few additional components introduced in the design, which are simply for enhancing the regulation capability of the circuit.
C2 is introduced to clean any residual ripple at the T1, T2 bases.
The T2 along with T1 forms a Darlington pair to increase the current gain of the output.
R3 is added to improve the zener diode conduction and therefore to ensure better overall regulation.
R8 and R9 are added to enable the output voltage to be regulated across fixed range, which are not critical.
The R7 sets the maximum current that can accessed at the output, which is:
I = 0.7 / 0.47 = 1.5 amps, and this appears quite low compared to the rating of the 2N3055 transistor. Although this might keep the transistor super cool, it may possible to increase this value up to 8 amps if the 2N3055 is mounted over a large heatsink.
Decreasing Dissipation to Increase Efficiency
The biggest disadvantage with any series transistor based linear regulator is the high amount transistor dissipation. And this happens when the input/output differential is high.
Meaning, when the voltage is adjusted towards lower output voltage, the transistor has to work hard to control the excess voltage, which is then released as heat from the transistor.
For example if the load is a 3.3 V LED, and input supply to the bench power supply is 15 V, then the output voltage has to be lowered to 3.3 V which is 15 - 3.3 = 11.7 V less. And this difference is converted to heat by the transistor, which could mean a efficiency loss of more than 70%.
However, this problem can be simply solved by using a transformer with tapped voltage output winding.
For example the transformer may have taps of 5 V, 7.5 V, 10 V, 12 V, and so on.
Depending on the load the taps could be selected for feeding the regulator circuit. After this, the voltage adjustment pot of the circuit could be used for further adjusting the output level precisely to the desired value.
This technique would increase the efficiency to a very high level, allowing the heatsink to the transistor to be smaller and compact.