Tuesday, June 12, 2012

Versatile Bench Power Supply

The power supply described here provides an output of up to 20V with current in two ranges of 100mA or 1A. Ideally a workbench supply should have an output voltage that can be adjusted right down to zero as it is occasionally useful to be able to power a circuit gradually from this when fault-finding. There should also be a fast and effective current limiting facility, again adjustable from zero as this also provides valuable protection when testing circuits.

If the supply can be set to deliver a constant output current instead of a voltage it can also be used as a charger for the many types of re-chargeable battery that are available nowadays, both alkaline and  small sealed lead-acid types. The latter are normally charged to a constant voltage but until this is reached a current limit is often required to prevent an excessive charge rate.

Preferably the supply should have two meters so that the voltage and current supplied to the load can be seen at a glance, and most users would probably prefer analogue meters to the digital type for this.

Power supplies meeting these specifications can be expensive, but this project can be constructed at reasonable cost, especially if some of the parts such as the case and meters are already to hand, or are purchased from inexpensive sources such as surplus stores or amateur radio rallies.

Design Points:
The design of a good power supply is more complex than might be supposed. It must provide a constant voltage or current whilst being able to react rapidly to any changes in the load it is supplying. This means that the circuit must have a fast response and remain stable for a wide range of output loads, voltages and currents, so the circuit is inevitably a compromise between speed and stability.

The design used for this project should be fast enough for most purposes and has good output stability.

Better Control:
An arrangement which overcomes the design difficulties is shown in Fig.1. It uses two power supplies, a “main” one of about 25V capable of supplying up to 1A of current for the output, and an auxiliary “split” supply of +12V for the controlling circuit. The “0V” rail of this is the “ground” for the control section. It operates as follows: 
simple voltage control
A reference voltage is produced using regulator IC1 and applied to resistor R1. During normal operation op.amp IC2 uses feedback to maintain equal voltages at its inputs, so a constant current flows into R1. This current also flows through resistor R2, developing a voltage across it directly  proportional to its value. If a linear variable resistor (potentiometer) is used for R2 it will provide linear control of the output.

Perhaps the easiest way to understand the action of the circuit is to consider what would happen if the voltage at the junction of R1 and R2 were to rise slightly. The output of IC2 would start to rise and thus turn on the MOSFET TR1 a little more. This would cause the 0V rail to rise with respect to the main negative supply rail so that more current would flow through R2 to bring the input to IC2 down again, restoring the balance.

The 0V rail, of course, is the positive output so the voltage developed across R2 is what is delivered to the load. Naturally, as the 0V rail rises with respect to the main supply, so do the positive and negative  12V rails, so that even when the output voltage is close to the maximum of the main supply there will still be a full 12V available for IC2 to use in controlling the gate of power MOSFET TR1.

Output Current:
It is also simple to measure the output current in this arrangement using the sensing resistor R3, which develops a current dependent voltage with respect to the control circuit “ground”. The only disadvantage of this arrangement is that the return path for the reference current from resistor R1 is via the main supply, TR1 and R3, so this current should be kept small to prevent any noticeable effect on the measured output current. This also applies to the voltmeter which is connected directly across the output.

Mains Supply:
The full circuit diagram for the Versatile Bench Power Supply is shown in Fig.2. The incoming 230V a.c. mains passes through a 2-pole isolating switch S1 to the two transformers T1 and T2, whilst the transient suppressor VDR1 removes any brief high voltage spikes that may occur. 
Versatile Bench Power Supply Schematic
 No supply fuses are fitted to the prototype, a 3A fuse in the mains plug being considered adequate.  Transformer T1 is a 20V 20VA (1A) type and, with bridge rectifier REC1 and reservoir capacitor C2, produces a no-load voltage of about 30V. Under full-load conditions this drops to about 24V which is  still sufficient to maintain the 20V output. Capacitor C1 and resistor R1 also help to eliminate transient voltage spikes.

Transformer T2 is a smaller 100mA type with a centre-tapped 9V-0V-9V output. Arranged as shown with bridge rectifier REC2, it develops both positive and negative supplies of about 12V each. A 5V  reference voltage is generated by IC1, a standard 78L05 regulator.

The earthing arrangements used in this project are slightly unusual and require some explanation. Earthing of a mains powered project is essential both for safety and because capacitive coupling  between windings in the transformer can transfer potentially damaging a.c. voltages to the output, even though from a very high impedance.

However, the output of a bench power supply is often required to have d.c. isolation from earth so that other earthed equipment, such as oscilloscopes, can be safely connected to any part of the circuit on test.

Commercially produced bench supplies often overcome this problem by using transformers with internal foil screens between the primary and secondary windings which are earthed to eliminate the capacitive coupling. These are not readily available to the home constructor, so a different method must be used.

Switch S2 allows the mains earth to be connected directly to either the positive or negative output terminal or left “floating”, where capacitively coupled a.c. voltage is grounded through capacitors C3, C4 and resistor R2. Although not ideal, this system works well in practice and reduces noise and a.c. voltage at the output to a few millivolts at most. For safety, capacitors C3 and C4 must be 250V class X2 suppression types.

Voltage Control:
The voltage controlling part of the circuit is constructed around op.amp IC2 and operates as described earlier. The resistance to which the reference voltage is applied includes the preset VR1 so that the  maximum output voltage may be set to exactly 20V.

The input to IC2 has a pair of protection diodes, D1 and D2. Capacitor C12 ensures stability but is small enough to allow a fast response to output load changes. Power MOSFET TR1 handles the full output current. Output voltage is indicated by meter ME1 with VR3 allowing preset adjustment for calibration to 20V full-scale.

Load current is sensed by the one ohm resistor R10. At full output this dissipates a watt of heat, so a 7W type is used to dissipate this safely. A potential divider across R10, consisting of R12 with R13 and R14 in parallel, allows selection of either the full potential across it or one tenth through switch S3, giving the 100mA and 1A current ranges.

The voltage from S3 is compared by op.amp IC3 with an adjustable voltage derived from the reference by divider network VR4, R16 and the current control VR5. When the S3 voltage exceeds this  secondary reference voltage, the output of IC3 goes low, pulling the gate voltage of TR1 down through diode D3 to limit the current flow through this transistor.

It also turns on transistor TR2, which illuminates light emitting diode (l.e.d.) D7 to indicate that current limiting is taking place. Preset VR4 allows the current ranges to be set precisely to 100mA and 1A,  whilst VR5 gives control from zero to the selected maximum.

Op.amp IC4 also receives the voltage from S3, and converts it into a 1-100uA drive for the current indication meter ME2. Preset VR6 allows calibration of this meter for full-scale corresponding to the 100mA and 1A signal inputs. The high value of resistor R21 prevents any possibility of meter damage through overdriving.

Double-pole switch S4 allows the output to be switched off independently and also provides instant isolation from the circuit connected to it at any time.

Zener diode D6 protects TR1 from any high voltage pulses arising from the connected load. This can happen with some types of inductive load and, whilst normally tolerant of brief high currents,  semiconductors are easily damaged by excessive voltage.

The aim of using separate op.amps for the various functions is partly to make testing and trouble shooting easier, so hopefully this will be the case.

The first test is to connect the 9V-0V-9V transformer as shown in Fig.3. Using 0V (the centre-tap connection) as a reference, the circuit should be powered up and the presence of +12V and -–12V checked at pins 7 and 4 respectively of the three d.i.l. sockets.

 The presence of +5V from regulator IC1 can also be checked at its output pin. Next, the MOSFET TR1 and VR2, the Voltage control, should be connected, along with the two leads from the main power supply, from REC1 and C2. IC2 should now be inserted into its socket.

Whilst monitoring the output with a voltmeter, the circuit should be powered up, and it should now be possible to vary the output from zero to around 20V with Voltage control VR2. If so, VR2 can be turned right up and the voltage set to exactly 20V with preset VR1. If the voltmeter ME1 is now connected this can be adjusted for an indication of 20V (full scale) with preset VR3.

Next the current range switch S3 should be connected and set to 100mA, and the current control VR5 should be connected. The current control op.amp IC3 should be inserted into its socket, and a meter set to read a current of about 100mA connected in series with a 100 ohm resistor across the output.  The circuit should again be powered and the voltage control turned right up.

Potentiometer VR5 should now control the current, from zero to around 100mA. Preset VR4 can be adjusted for a maximum output of exactly 100mA. The 1A range should automatically be correct following this. The l.e.d. D7 can be connected, this should light whenever current limiting is active.

Finally, the current meter ME2 should be connected, op.amp IC4 inserted, and preset VR6 set for full scale at 100mA. As before, the 1A range should now automatically be correct following this adjustment.
Current Limitations:
It should be noted that the current limit takes a finite time to operate so there are some applications where external limiting is needed. The output of IC3 has to come out of saturation and slew through about eight volts before limiting begins, and this will take a microsecond or two!

The limit protects the supply itself against short circuits, and in almost every practical fault situation it is fast enough to prevent damage to faulty circuits on test as these are rarely total short circuits

However, it should be remembered that the MOSFET output device can handle very high currents and has a large capacitor behind it so, for the brief time it takes for the limit to operate, many amperes can be delivered into a short! An obvious example is the testing of l.e.d.s.

The limit should never be relied upon on its own for this, a suitable series resistor should always be used.

1 comment:

  1. You should attribute this design and write-up to Everyday Practical Electronics (January 2002) where it originated.