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Mini-PSU Technical Details

Please begin by clicking here to open up a window for the schematic of the Mini-PSU circuit board.  Position it so that you can see it while you read the following.  Note that you will normally install fewer components than are shown and the actual values in the schematic may not apply to your design.  You can also open a window for the printed circuit board of either (or both) the Mini-PSU or the Flexi-PSU so that you can see where each of the components go.

Regulator (U1 or U2)

Your most important design choice in creating a power supply will be the regulator itself.  The Mini-PSU circuit board supports regulators in the TO220 package with one of the two pin-outs shown in the diagram at right.  Most fixed-voltage regulators have the pin-out shown in Figure 1 and must be installed as U2 on the Mini-PSU printed circuit board, that is to say, in the right-most three holes.  Adjustable-voltage regulators have the pin-out shown in Figure 2 and must be installed as U1, in the left-most three holes.  Some regulators, such as the LM1117, are available in both fixed and adjustable variations and the fixed-voltage types have the Figure 2 pinout where the ADJ pin is actually GND.  To use these regulators, install them as U1 and put a wire link in as C2.  Consult the data sheet of your regulator to make sure it is compatible with one of these pin-outs.

The regulator can be mounted standing straight up or flat against the circuit board.  In both cases, there is space for a small heatsink.  For modest power requirements, a heatsink may not be necessary.  If the regulator is mounted flat, it can be fixed to the board with a 4-40 or similar screw and nut.  Even without a heatsink, the large copper area on top of the board will help draw heat away.  This area is not electrically connected to anything else; it is only a heat spreader.  For very low power, it is quite possible to use a TO92 regulator.  Just spread out the leads and solder it into the appropriate three holes according to which pin-out it matches.

Power Input Jack (J1)

The Mini-PSU has provision for a center-positive co-axial jack that mates with the type of plug commonly found on wall-wart AC-to-DC power adapters.  The PCB footprint is suitable for jacks that mate with plugs having a 5.5mm outer diameter and either a 2.1mm or 2.5mm inner diameter.  Jacks with proper PCB pins can be a little hard to find, but with care you should be able to modify one of the more common solder-tab types to fit.  (ECROS Technology Mini-PSU parts kits include a 2.1mm jack with true PCB pins.)  If you don't wish to use a plug and jack arrangement, simply wire input power to the pads of J1.  Positive goes to the pad near the silk-screen marking "J1" with the trace across to D1.  Negative goes to the pad directly above it along the axis of the jack and connects to the ground plane on the bottom of the PCB.  The third pad, close to the anode of D1, is not used.

Polarity Protection Diode (D1)

If there is a risk that input power to the power supply be connected with the wrong polarity, install diode D1 to prevent damage to the regulator.  Remember that this diode will have a voltage drop and that this will reduce the maximum output voltage you can get for any given input voltage.  If you want to minimize this dropout voltage, that would be a reason not to use D1 and to make sure that input power is always of the correct polarity.  If you do not use D1, install a wire link instead.  A schottky diode will provide polarity protection with a smaller voltage drop.  Another reason to use this diode is if you plan to use a much higher input voltage than is really necessary for your desired output.  The voltage drop across the diode will reduce power dissipated in the regulator.

On/Off Switch (SW1) - Flexi-PSU Only

The Flexi-PSU includes a location for an On/Off slide switch, which is supplied in some kits.  If you do not have or want a switch, solder a wire link between the top two switch terminals (closer to D1).  You can also run wires from these points to an off-board switch, for example a switch on a front-panel.

Power LED (R3 and LED1) - Flexi-PSU Only

On the Flexi-PSU PCB, R3 and LED1 are intended for a power indicator LED. Install a T1 (3 mm) LED with the cathode (shorter lead) close to the bottom edge of the board.  It should also be possible to squeeze a T1¾ (5 mm) LED into this space. If you install R3 exactly as marked on the silkscreen, the LED current will come from the input side of the regulator.  This is recommended for variable output supplies, as otherwise the brightness of the LED will vary as you adjust the output voltage.  Calculate the resistor value to give the desired LED current, typically around 10 mA, at the expected input voltage.  You can also install the bottom end of R3 in the extra hole closer to LED1.  It will then be powered from the output side of the regulator.  This is recommended for fixed output supplies.  Calculate the resistor using the output voltage.  For regulators that require a minimum load current to function, such as the 7800 series, the power indicator LED can be used to draw this current when the output is not connected to a load.

Input Bypass Capacitor (C1)

Installing a ceramic capacitor in the C1 position is highly recommended.  Most regulator data sheets ask for a high-quality 0.1F component here.  The popular and inexpensive 7800-series requires 0.33F.  The purpose of this capacitor is to lower the high frequency impedance at the input of the regulator and ensure stable operation.

Output Bypass Capacitors (C3 and C4)

Output bypass capacitors are usually present for the stability of the regulator and to improve the transient response (regulation during sudden load current changes).  Consult the data sheet of your regulator to see what kind of output capacitor it needs for stable operation.  The Mini-PSU provides a location at C3 for a ceramic capacitor and at C4 for an electrolytic capacitor.  The outline of C4 is suitable for a large radial-lead capacitor with leads spaced at 0.2".  There is a second hole for the positive (long) lead of a smaller capacitor if this is your choice.

Read the data sheet carefully when choosing these capacitors.  Usually, you can freely choose the value of the electrolytic capacitor, C4.  Use a high value if you may be drawing pulse currents from the supply, as in the case of a relay or motor circuit.  Selecting C3 can be more tricky.  Some regulators actually do not want a ceramic output bypass capacitor and rely on the series resistance of an electrolytic being present.  Other regulators require the low series resistance of a ceramic (or similar) component.

Input Short Circuit Protection Diode (D2)

Regulators don't like having their output terminal at a higher voltage than their input terminal.  This can happen if the input is shorted, quickly disconnected or input power turned off and the output is held up by C4 or an external capacitor.  Usually, the regulator has an internal diode to protect it.  However, in the event of an input short circuit and with a large-valued output capacitor, this internal diode may not be able to handle the peak current.  The Mini-PSU provides a location at D2 for an external protection diode.  Consult the data sheet of your regulator to see whether it is necessary.  Note that short circuits at the power jack or earlier cannot affect the regulator if D1 is installed and in this case you are unlikely to need D2.

Output Voltage Adjustment (R1 and R2) - Adjustable Only

For adjustable voltage regulators, the output voltage of the circuit is set by R1 and R2.  The action of the regulator is to keep the voltage across R2 constant.  If R1 is zero (turned all the way down), then this will be the output of the power supply.  As R1 is increased, current flowing into it from R2 causes a voltage drop across and this is added to the output voltage.  For example, if R1 is increased to be equal in value to R2 then the output voltage will double.  If the voltage across R2, set by the design of the regulator, is VREF, then the circuit's output voltage is approximately VOUT = VREF x (1 + R1/R2).

R2 should be selected so that the current flowing in it swamps (is much larger than) the current at the ADJ terminal of the regulator.  Then, the above equation, which ignores the current at the ADJ terminal, becomes sufficiently accurate.  To take an example, the LM317M regulator has a typical current of 50A at the ADJ terminal.  A reasonable choice of current in R2 would be 100 times this amount, i.e. 5mA.  The VREF of this regulator is 1.25V, so by Ohm's law the value of R2 will be 250 ohms.  A practical value would be 240 or 220 ohms.  Note that the current in R2 comes from the regulator output and counts towards the minimum load current.  For the LM317M regulator, however, the maximum value of the minimum load current is 10mA, so it may be necessary to ensure than another 5mA is always drawn from the output to get the rated performance.

R1 should then be selected according to the maximum output voltage desired.  Transposing the equation for the output voltage, we get R1 = R2 x (VOUT/VREF - 1).  Continuing the example of the LM317M with R2 = 220 ohms and further supposing that we want a maximum output voltage of 12V, we calculate R1 using this equation to be 1892 ohms.  To allow for tolerance in VREF and the resistor values, we would select a 2kohm potentiometer.  Consult the data sheet of your regulator for more information.  Of course, when R1 is set to minimum (zero resistance), the output voltage will be VREF (1.25V).  This may not be what you want.  In this case, R1 should be a combination of a fixed and variable resistor.  The PCB of the Mini-PSU makes no provision for this, but if you are wiring R1 as a panel-mount potentiometer is isn't hard to hook a resistor in series.

To make a fixed-voltage power supply from an adjustable regulator, calculate the value of R1 and install a fixed resistor.  Mount it vertically in the center hole of R1 (with the trace across to C2) and bend the top lead over into the hole in the square pad of R1.  To improve the accuracy of the output voltage and the available selection of values, use 1% resistors.

For higher output voltages, be careful of the power dissipation in R1.  Again continuing the above example, it is tempting to reduce R2 to 120 ohms so that the minimum output current requirement of the LM317M is satisfied.  This would be fine with a 2kohm 1/4W potentiometer for R1 (it would dissipate 200mW), but using 5kohms to obtain higher maximum output voltages would result in more than 1/4W of power dissipation.

Adjustment Terminal Bypass Capacitor (C2) - Adjustable Only

With many adjustable regulators, you can improve ripple rejection performance with a capacitor in this location.  Typically, a small electrolytic capacitor, for example 10F, would be used.  If you install C2, it may then be necessary to install D3 (see below).  Consult the data sheet of your chosen regulator for more information.  If an ECROS Technology parts kit includes C2, it will also include D3 and you should install both or neither.

NOTE: On early versions of the Mini-PSU, the footprint provided for C2 was for a ceramic capacitor with pins spaced at 0.2".  To install an electrolytic capacitor, it is necessary to bend the positive (long) lead.  This must go to the narrow trace joining R1, R2 and D3.  The negative (short) lead must go to ground on the back of the PCB. 

Output Short Circuit Protection Diode (D3) - Adjustable Only

The purpose of this diode is to prevent the ADJ terminal of the regulator from having a higher (more positive) voltage than the OUT terminal in the event of an output short circuit.  It is only necessary if C2 is used and has a high enough value to hold up the voltage at the ADJ terminal.  Consult the data sheet of your regulator to see if you should use this diode.

(Use the menu on the left hand side to go back to the Mini-PSU introduction.)