All you need to know to make your own regulated power supply…
3-terminal voltage regulators
The voltage regulator is one of the most common components to be added to a project. It’s the heart of what we call a “built-in” power supply. It allows the project to operate from almost any type of voltage. It can be AC or DC and any voltage (within prescribed limits). The voltage can come from batteries, a plug pack or a transformer. The only other components that need to be added are diodes, a few capacitors and electrolytics and the power-supply section of a project is complete. The voltage regulator has made the designing of a power supply a relatively simple task.
However, before we take the designing too simply, there are a number of features and facts that must be taken into consideration. Here are three pointers:
Here are the two most popular types of voltage regulator. The only difference is the current capability:
|LM317T||+1.2V to +37V||1.5A||TO-220|
The circuit for a positive 5v regulator is shown below:
A 100n monoblock
The 7805 connected to a heatsink with thermal grease.
There are four important points to remember when designing a power supply:
The diagram below shows a 7805 regulator circuit built on breadboard. This is not the same circuit as shown above as the 100n in the output has been replaced with an electrolytic. This open-type arrangement is only suitable for a very low output current as the 7805 is not heatsinked and it needs to have short leads between the output and a 100n monoblock to prevent it from oscillating internally.
A 7805 “test circuit” mounted on breadboard
The circuit above can be fitted onto a small PC board using a W04 1.5 amp bridge, a 2200u electrolytic, a 10u electrolytic and an indicator LED with 220R dropper resistor. The circuit for this is shown below.
A 7805 power supply module
The 7805 regulated supply with a LED indicator.
Here’s an example of how not to draw the diodes in the bridge:
The wonderful part of electronics is its universal nature. Imagine if every country had their own colour code for resistors! The same applies to laying out circuit diagrams. Stick to the universally accepted way of presenting “building blocks.” The diagram above takes quite a while to work out that the diodes are actually forming a standard bridge. By merely placing them on an angle (as shown in the first circuit above) you can see that all the diodes are facing the one direction. This allows you to put a single diode in the centre of the diamond to represent the bridge.
The output voltage of a supply can be increased by “jacking up” the voltage produced by the 7805. The way the 7805 works is this: It maintains a voltage of 5v between output and common terminal. If the voltage on the common terminal is increased (jacked up), the output voltage will be 5v higher. The 7805 always maintains 5v between output and common. The circuit below produces an output of 12v.
1 amp 12v Power Supply Circuit
Almost any voltage between 5v and 30v can be obtained by this method. This saves stocking the complete range of regulators.
The output voltage is determined by two resistors in VOLTAGE DIVIDER MODE. Five volts is always present across the 120R resistor and if another resistor is placed in series, it will have a proportional voltage across it. In the circuit above, 7v is developed across the 180R resistor, making a total of 12v on the output.
To increase or decrease the voltage, only one resistor has to be changed in the circuit above. The 120R is retained and the 180R is changed. If it is increased to 220R, the output voltage will be 14v, for a 330R, the output voltage will be 18v. The resistor in the common line can be a potentiometer. This will produce an adjustable output voltage. The dropper resistor for the LED will also have to be increased so that the LED is not over-driven on the higher voltages.
A meter can be placed on the output to monitor the voltage and current taken by the load. Click HERE to see how the meter is connected.
There is only one problem with an adjustable supply. The regulator must be heatsinked so it is capable of dissipating the heat for the worst condition. In addition, the input voltage must be sufficient to cater for the maximum output voltage.
The output voltage can be adjusted (varied) from 5v to 24v via a potentiometer connected to the common line of the regulator. The input voltage and heatsinking of the regulator must be sufficient for the output voltage and current. The output may not deliver more than 100mA @ 5v due to the heat produced by the regulator if the input voltage is say 24v - 36v. See above discussion.
5v to 24v Power Supply Circuit
The circuit below is dangerous! Do not use it. The output of a 3-terminal regulator will jump to full output voltage when the selector switch is changed from one voltage to another. If the input voltage is 36v (for 0-24v power supply), the output will rise to 36v or more as the selector switch is changed from one position to another. This is because one set of contacts break before the next contacts are connected and thus the common terminal will be FLOATING during this short interval of time. Any project being powered by the supply may be instantly damaged!
The layout on this board could be improved
The photo above is an example of how not to layout a PC board. The pot has been placed in the centre of the board and this makes it difficult to get to the connections, if you wish to add an external pot. In addition, the pot chosen for the project needs a small screwdriver and is difficult to adjust. Always use a pot with a small shaft, so you can adjust it with your fingers, and it can have a screwdriver slot as well. Axial electrolytics are more expensive than radial types and take up a lot more board-space. Axial electros went out with the Ark. If you want your PC board to look modern, use only electros that stand-up.
If any power supply is supplied with a voltage more than 4v above the output voltage, this is called an OVER VOLTAGE and can cause problems. The most serious will be overheating of the regulator. Most circuits do not require a steady current and thus it is impossible to give an absolute size heatsink for a particular application. The simplest way of determining the size is to be able to hold your hand on it almost constantly. If you have to let go, the regulator is getting too hot.
The heat it dissipates is simply a matter of working out the voltage across the regulator multiplied by the current flowing. In most cases the current flow will fluctuate and the value of heat dissipation will be very complex and beyond the scope of this discussion.
However, the only thing we can say is the need to keep the input voltage as low as possible (providing it is above the minimum stated above). Every volt above this minimum is wasted as heat in the regulator and if 1 amp is flowing, the waste will be approx 1 watt.
Sometimes you can connect a 12v 1 amp plug pack to the power supply, drive a particular load, and the regulator will get fairly warm. If you then connect another 12v 1 amp plug pack, the regulator will get extremely hot!
The second plug pack may actually produce a higher voltage. The 12v rating is only an arbitrary value and the actual no-load output may be 15v to 18v. The manufacturer allows the voltage to drop to 12v on full load. This is called “regulation” and applies to “transformer regulation.” This is one of the hidden problems you may have to take into account when determining the size of the heatsink. That’s why the only way to check the project is to feel the heatsink.
The 7805 has an internal thermal shutdown when the temperature reaches a high value. It is obviously important that the regulator does not reach this condition, however if the circuit shuts down, the regulator will cool and turn on again. In this case the regulator will not be damaged.
The regular also has an internal short-circuit detector. If the output is shorted, the regulator will shut down.
If you require more than 1 amp, the 7805 can be combined with other components to provide an output of up to 3 amps, with the circuit below. The current is switched through the TIP2955, so the 7805 can be run without a heatsink since it only regulates the voltage. Note the 3-amp diodes in the power supply.
For currents greater than 3 amps, additional TIP 2955 transistors can be “piggy-backed” on top of the TIP in the circuit. If the gain of each transistor is approximately the same, the transistors will current-share the load and get equally hot.
A 3amp diode (1N 5404) compared with a 1 amp diode (1N 4004).
A negative supply can be produced with a 7905 voltage regulator. Three things that have to be remembered are:
The 7905 -5v regulated supply circuit
This just about covers the intricacies of the power supply. Almost every project needs a power supply and provided you adhere to the rules above, it will be a simple task to add a supply to a corner of your project and produce a reliable design that will not need any further attention.
I can remember, “in the old days,” the power supply was always giving trouble. TV’s had electrolytics that dried out, diodes became open-circuit, voltage-doubling electros failed and switch-mode power supplies blew up for no apparent reason.
It was the Japanese, with their quest for improvement, that created transistors with improved reliability and higher voltage ratings.
Also, IC’s (in the form of voltage regulators) simplified the power supply to a single device.
That’s how things have changed.
A voltage regulator has the capability of smoothing a 3v ripple to less than 1mv. This is an improvement of 3,000:1. At the same time it is capable of delivering 1 amp.
A link to a Dual Power Supply.
Five more Power Supplies with some helpful notes from Bill Bowden.
A variable 0.7v to 24v Power Supply with adjustable current 50mA to 2 amp using discrete components - ideal as a bench supply.
A very clever circuit to convert a meter from 0-24v to 0-2amp.
An LM317 circuit. The advantage of an LM317 regulator is the supply will go down to 1.2v. If you add two diodes (in series with the output line), the voltage will go down to 0v.
The University links that were tried, did not go through or were .pdf files and were very messy to search. However this should be sufficient to cover the topic.
If it is at all possible, keep your power supply to 1amp rating. This is the cheapest and easiest to implement. Once you go over 1.5 amp, the power transformer becomes expensive and bulky and impossible to get as a “plug pack.” (A plug pack is a transformer tightly- housed in a plastic box with two or three pins mounted in the plastic so that the whole assembly can be plugged into a power point. Some of these are rather large and bulky and only allow one power-point to be used, when a dual power-point is available. However the plug pack is the safest and best way to get 12v to 15v @ 1amp).
The latest release is a switch-mode plug pack. The model I purchased was 12v @ 3amp and weighed only 1/10th the weight of a 12v 1.5amp plug pack. The cost was double the cost of a 12v 1.5 amp plug pack but when you work it out, the cost is about the same on a performance-basis and occupies only one outlet. The pack runs totally cool as it is more than 85% efficient. This is the way the plug pack is going. There is also a 5 amp version. This beats the transformer “hands down.”
The next stage in power supply design will be switch-mode. ONE MORE …There is one more amazing trick you can perform with a 3-terminal regulator. It can be wired to produce a CONSTANT CURRENT output. Click HERE for the details.
See also our discussion on the transformer