Thursday, April 11, 2013

Simple Mini Bench Supply

Every electronics engineer is familiar with the anxiety of the moment when power is first applied to a newly-built circuit, wondering whether hours of work are about to be destroyed in a puff of smoke. A high-quality power supply with an adjustable current limit function is an excellent aid to steadying the nerves. Unfortunately power supplies with good regulation performance are expensive and homebrew construction is not always straightforward. Many of the ‘laboratory power supplies’ currently on the market are low-cost units based on switching regulators which, although certainly capable of delivering high currents, have rather poor ripple performance. Large output capacitors (which, in the case of a fault, will discharge into your circuit) and voltage over-shoot are other problems.

The power supply described here is a simple unit, easily constructed from standard components. It is only suitable for small loads but otherwise has all the characteristics of its bigger brethren. Between 18 V and 24 V is applied to the input, for example from a laptop power supply. This avoids the need for an expensive transformer and accompanying smoothing. No negative supply is needed, but the output voltage is nevertheless adjustable down to 0 V.  

A difficulty in the design of power supplies with current limiting is the shunt resistor needed to measure the output current, normally connected to a differential amplifier. Frequently in simple designs the amplifier is not powered from a regulated supply, which can lead to an unstable current regulation loop. This circuit avoids the difficulty by using a low-cost fixed voltage regulator to supply the feedback circuit with a stable voltage. This arrangement greatly simplifies current measurement and regulation. 

Mini Bench Supply Circuit Diagram
Mini Bench-Supply-Circuit Diagram
To generate this intermediate supply volt-age we use an LM7815. Its output passes through R17, which measures the output current, to MOSFET T1 which is driven by the voltage regulation opamp IC1C. Here R11 and C4 determine the bandwidth of the control loop, preventing oscillation at high frequencies. R15 ensures that capacitive loads with low effective resistance do not make the control loop unstable.

 The negative feedback of AC components of the current via R12 and C5 makes the circuit reliable even with a large capacitor at its output, and negative feedback of the DC component is via the low-pass filter formed by R14 and C6. This ensures that the volt-age drop across R15 is correctly compensated for. C7 at the output provides a low impedance source for high-frequency loads, and R16 provides for the discharge of C17 when the set voltage is reduced with no load attached. 

Current regulation is carried out by IC1D. Again to ensure stability, the bandwidth of the feedback loop is restricted by R19 and C8. If the voltage dropped across R17 exceeds the value set by P2, the current limit function comes into action and T2 begins to conduct. This in turn reduces the input voltage to the voltage regulation circuit until the desired current is reached. R7, R9 and C3 ensure that current regulation does not lead to output voltage over-shoots and that resonance does not occur with inductive loads. 

The controls of the power supply are all voltage-based. This means, for example¸ that P1 and P2 can be replaced by digital-to-analogue converters or digital potentiometers so that the whole unit can be driven by a microcontroller. IC1B acts as a buffer to ensure that the dynamic characteristics of the circuit are not affected by the setting of P1. IC1A is used as a comparator whose out-put is used to drive two LEDs that indicate whether the supply is in voltage regulation or current regulation mode. If D2 lights the supply is in constant voltage mode; if D1 lights it is in constant current mode, for example if the output has been short-circuited. The power supply thus boasts all the features of a top-class bench supply.IC1A and its surrounding circuitry can be dispensed with if the mode indication is not wanted. 

A type LM324 operational amplifier is suggested as, in contrast to many other similar devices, it operates reliably with input voltages down to 0 V. Other rail-to-rail opamps could equally well be used. The particular n-channel MOSFET devices used are not critical: a BUZ21, IRF540, IRF542 or 2SK1428 could be used for T1, for example, and a BS170 could be used in place of the 2N7002. The capacitors should all be rated for a voltage of 35 V or higher, and R15 and R17 must be at least 0.5 W types. The fixed voltage regulator and T1 must both be equipped with an adequate heatsink. If they are mounted on the same heatsink, they must be isolated from it as the tabs of the two devices are at different potentials. 

Author : Alexander Mumm - Copyright : Elektor