An adjustable power supply for model railways and general projects.
This handy power supply has an output of 0v to 12v at 700mA with a transformer that is rated at 1-amp (such as M-2155) or 1.4amp for a transformer that is rated at 2-amp (such as M-2156). Your local electronics shop or model railway supply will have these or equivalent transformers. The mini pot mounted on the board has a scale to show the approximate output voltage and this can be replaced with a standard pot on the front of a case. You can also include an ammeter if you wish.
HOW THE CIRCUIT WORKS
First the mains voltage is reduced to a usable level by the transformer. Two different low-cost transformers can be used. The M2155 is a 1 amp type and M2156 is a 2 amp type. This is the AC rating and when you connect any transformer to a DC power supply circuit you must de-rate the current rating by 30% to give the maximum DC current that can be delivered by the power supply. The reason for this the AC voltage is increased by 40% when it is rectified and to maintain the VA (volt-amp) rating for the transformer, the current rating must be decreased. This means a 1 amp transformer will produce a 700mA power supply and a 2 amp transformer will produce a 1.4 amp power supply. For a low-cost power supply this output is quite sufficient and is all we can get as quite a lot of heat will be developed in the diodes, transistor and transformer when 700mA is flowing and literally burn your finger when 1.4 amps flows. The BD 679 regulator transistor must be heatsinked if any more than 100-200mA is required and will certainly need a large heatsink when the full rated current flows. The heat generated in the transistor is due to two factors. One is the current flow. Obviously, as more current flows, the transistor will get hotter. But the other factor is the voltage across the transistor. If you are drawing 100mA at 12v, the transistor will rise to a certain temperature. If you reduce the output to say 6v, while still drawing 100mA, the transistor will get hotter because the voltage across it will be greater. In the first case the voltage across the transistor will be the voltage from the bridge rectifier minus the output voltage. Our figures were 22v - 12v = 10v across the transistor. In the second case the voltages are: 22v - 6v = 16v. In the first case the transistor will dissipate 1 watt and in the second case 1.6 watts will be dissipated. This is a 60% increase and is one of the hidden factors behind heat-generation in a power supply. Both transformers have a 15v AC output and the diode bridge rectifies this voltage. The output of the bridge is termed unsmoothed DC and if used to power an amplifier, for example, it will produce a very loud buzz from the speaker.
This unsmoothed DC is then fed to an electrolytic. The function of the electrolytic is to charge on the voltage peaks of the unsmoothed DC, then discharge into the load during the time between the peaks. This smoothes the DC. The voltage will still fluctuate a small amount during the charge/discharge cycle and if you connect it to an amplifier, a small amount of annoying background hum will be produced. This "hum" will be greatest when maximum current is required as the electrolytic is not capable of delivering enough energy during peak requirements and the waveforms becomes rippled.
The only way to improve this is to put high value electrolytics on the board or use a simple electronic regulator in the form of a transistor. The Darlington transistor in the output is capable of delivering the varying current while maintaining a constant voltage. It does this by having an additional voltage available to it from the bridge rectifier. The voltage on the output is adjustable from 0v to 12v via a 5k pot. The pot gets is voltage from a zener regulated source and you can pick off any voltage from 0v to 12v via the wiper. This stable voltage is fed to the base of the Darlington transistor. The transistor is wired as an emitter-follower. You can think of the transistor as a low resistance that automatically varies according to the current requirement of the output. If the load requires additional current, the normal effect would be for the output voltage to drop according to ohms law, across the low resistance transistor. But what happens in this case is the drop in output voltage turns ON the transistor so that it delivers the extra current from the power rail. In doing so, the voltage may fall by as much as 2 - 3 v on the power rail but the output does not see this as the regulator transistor is separating the voltage on the power rail from the output. 
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CONSTRUCTION All the components fit on to a small PC board with the transistor at the edge of the board so that it can be screwed to a heatsink. Next fit the resistor, zener diode and 100n monoblock capacitor. The zener looks like a signal diode and may have a number of different markings on it. Sometimes it is marked with the zener voltage and sometimes it has a code number. It may have 1N 5535 or 1N 5245 or IN 5861 or even another number. Zener diode numbering is very messy, but they all refer to a 15v zener. The mini pot is next to be fitted. If you want to mount the pot on the front panel, it will be best to use a standard 5k pot with shaft so you can fit a knob. The two electrolytics are next, making sure the positive lead goes down the hole marked on the overlay. Finally, the BD 679 is fitted so that the heatsink goes between the transistor and PC board (to provide the best heat transfer). See below for more details on choosing the correct heatsink and applying thermal grease to improve heat transfer. An adequate heatsink is most important when building a power supply, both to make it reliable and keep the components operating within their temperature range. When any of the parts get too hot you can introduce unwanted hum (another name for ripple) or even create premature failure of the diodes or transistor. When all the components are fitted, and the heatsink is in place, it can be placed in a suitable case, along with the power transformer.   
MAINS OPERATION
This project has two options. It can be a mains operated project, using a 2155 or 2156 transformer or it can be connected to a plug pack. We recommend it be connected to a 16v AC 1 amp plug pack as these are double insulated and provide complete safety for the constructor. As we have mentioned in the introduction, the 2 amp transformer M 2156 will not provide much more than 1.4 amps DC output so the 1.5 amp AC plug pack has nearly the same rating. By the time you buy a M 2156 transformer and power cord, the total will be the same as the cost of the plug pack so it should be one of your considerations. 
HEATSINK
The heatsink is one of the most important components in a power supply. It must provide adequate heat dissipation to protect a heat-sensitive device, from being damaged. All power supplies dissipate heat. Some are more efficient than others but whenever voltage and current are present together, heat will need to be dissipated. The heatsink in our project fits between the metal side of the transistor and PC board to get direct contact with the transistor. It is most important to get good thermal contact so that any heat generated in the transistor will be carried away by the heatsink. Since the transistor has a very small area for this heat transfer, (all the heat must pass through the side of the transistor) it is most important that the gap between the face of the transistor and the heatsink be filled with thermal grease (thermal compound). We have included some in the kit for this purpose. You only need a small amount smeared over the face of the transistor. The bolt is then passed though the transistor, heatsink and PC board and the nut tightened until a small amount of grease is squeeze out from under the transistor. This proves the nut is tight but it must not be too tight as you may damage the transistor itself by cracking the case. Don't use ordinary grease, as it doesn't have the heat transfer characteristics we need. SIZE OF HEATSINK The way to select the correct-size heatsink is to build the project and fit a heat sink about 4cm x 10cm as a trial experiment. Next you need some high wattage resistors or car lamps to load the power supply to its maximum rating (this will depend on the transformer you use). Place one finger on the transistor and another on the heatsink, about 2cm from the transistor, and monitor the temperature rise in both positions. You should be able to hold your finger for the duration of the experiment. If not, a thicker heatsink is needed, as the one you are using is not transferring the heat away from the transistor fast enough.
AMMETER
The kit comes with a "Movement." A movement is a moving coil with a pointer with no resistors connected to the terminals. The movement we supply is is actually a SIDE PANEL METER that you convert to an AMMETER. 
Any Movement can be converted to an ammeter, quite easily. To produce a 0 -1 amp reading, two 1R and one 4R7 (all in parallel) are connected across the two terminals. These are called SHUNT RESISTORS.
The three shunt resistors can be clearly seen in the photo. To produce a 0 - 2 amp reading, four 1R and one 2R2 are connected in parallel across the terminals. The kit contains a total of 6 resistors for this conversion. All you have to do is open the movement and change the scale. You can put a sticker over the scale and write 1A or 2A on the sticker.
HOUSING
The project should be housed in a small case, preferably plastic, with either a flying lead on the output or a set of terminals on the front of the case, marked with positive and negative. The power cord must be anchored inside the case. You can place a metal plate in the inside of the bottom of the case to act as a chassis and this can be bent to form part of the heatsink for the transistor. If you are going to use a plug pack, the case will be a lot smaller, and a lot cheaper. You could even leave the project out of a case and use the mini pot on the board as the voltage adjustment. I prefer to leave things in their skeleton form, so I can see the "works" - that's why I hardly ever suggest fitting a project into a case. Decide carefully which arrangement you are going to choose. Either way it will be a handy addition for your model railway or workbench. |
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