GB2236414A

GB2236414A - Controlled electronic load circuit

Info

Publication number: GB2236414A
Application number: GB8921504A
Authority: GB
Inventors: John-Weyers David St
Original assignee: STC PLC
Current assignee: STC PLC
Priority date: 1989-09-22
Filing date: 1989-09-22
Publication date: 1991-04-03
Also published as: GB8921504D0

Abstract

A dynamic electronic load circuit has a MOSFET (3) in series with a load resistor (4) connected across a d.c. supply under test. A differential amplifier (7) compares the voltage dropped across the load resistor with a reference voltage from a zener diode (8) and controls the MOSFET to draw a regulated current from the d.c. supply for any input voltage within a specific voltage range. The circuit is stabilised by capacitors (10, 11). A plurality of such circuits can be connected in parallel across the supply and enabled by different bias voltages to draw a range of load currents. <IMAGE>

Description

DYNAMIC ELECTRONIC LOAD CIRCUIT This invention relates to a dynamic electronic load circuit for testing of power supplies for electronic equipment.

According to the invention there is provided a dynamic electronic load circuit including a pair of terminals for connection to a d.c. supply, the terminals being shunted by a fixed value load resistor in series connection with a MOSFET, a voltage sensing circuit arranged to sense the voltage drop across the load resistor and to produce an output signal when the voltage drop exceeds a reference voltage, and control means responsive to the output signal whereby the MOSFET is maintained ON until the voltage drop across the load resistor is sensed as being greater than the reference voltage when the MOSFET is turned OFF.

Embodiments of the invention will now be described with reference to the accompanying drawings, in which: Fig. 1 is a schematic diagram of a dynamic electronic load circuit, and Fig. 2 is a schematic of a selection control for a combination of load circuits.

The dynamic electronic load circuit of Fig. 1 is intended for, inter alia, testing of d.c. power supplies and is readily set to draw a regulated current for any input voltage in a specified range. Typically, for electronic equipment power supplies the voltage range may be 3.2v to 20v.

In the circuit of Fig. 1 terminals 1 and 2 are connected to a d.c. power supply. The source/drain channel for a MOSFET 3 is connected in series with a fixed value load resistor 4 to shunt the power supply connected to terminals 1 and 2. From the junction between the MOSFET 3 and load resistor 4 a further connection is made via resistors 5 and 6, which form a potential divider to the positive voltage rail connected to terminal 1. The centre connection of the potential divider is connected to the non-inverting input of a differential amplifier 7. The inverting input of amplifier 7 connected to the non-zero (positive) rail via a Zener-diode 8. The output of the amplifier 7 is connected to the gate electrode of the MOSFET. The non-inverting input to the amplifier is also connected to an 'enable' control via diode 9.

When the circuit is enabled, and until the voltage dropped across resistor 6 becomes greater thanthat of the voltage Vref derived via the Zener diode, the non-inverting input to amplifier 7 is more positive than the inverting input. Consequently the output of the amplifier is high and this maintains the MOSFET 'ON'. When the current through, and the voltage dropped across, resistor 4 is high enough for the non-inverting input to the amplifier to become more negative than the inverting input then the output of the amplifier goes low, turning the MOSFET 'OFF'. The result of this is that the circuit tries to reduce the voltage dropped across the load resistor 4. By compensating the amplifier with a small capacitor 10 as a negative feedback and damping the MOSFET gate control with capacitor 11 equilibrium- is reached at a fixed current regardless of the input supply voltage.

A number of load circuits may be coupled together to provide a greater range of load current to be applied to a power supply. For example, 12 x 1 amp stages may be arranged. A variable potentiometer is connected between the common terminals 1 and 2 and, ignoring losses, the wiper connection can be regarded as being able to travel between two limit values representing 0% and 100% respectively. The range of each of the load circuits is assigned respectively as 8%, 16%, 248 etc., as shown in Fig. 2, and the twelve outputs so obtained are used as reference voltages for enabling the 12 respective load circuits. When the wiper voltage exceeds that of a given load circuit's output the respective amplifier output goes high, thus enabling that particular circuit. In this manner any number of circuit stages may be added, each demanding a specific range from the sweep of the potentiometer wiper. This arrangement ensures that each amplifier's input is with reference to a percentage value of the potentiometer rather than with a fixed voltage.

To indicate the number of stages that have been enabled, and thus indicate the number of amps being drawn from the d.c. supply, a light emitting diode (LED) 12 is connected across each load resistor 4 so that as voltage is dropped across the resistor so the respective LED is lit.

All power necessary for the operation of the load circuit(s) is derived from the power supply being tested, thus no additional power source is required.

Claims

1. A dynamic electronic load circuit including a pair of terminals for connection to a d.c. supply, the terminals being shunted by a fixed value load resistor in series connection with a MOSFET, a voltage sensing circuit arranged to sense the voltage drop across the load resistor and to produce an output signal when the voltage drop exceeds a reference voltage, and control means responsive to the output signal whereby the MOSFET is maintained ON until the voltage drop across the load resistor is sensed as being greater than the reference voltage when the MOSFET is turned OFF.

2. A dynamic electronic load circuit according to claim 1 wherein the voltage sensing circuit includes a differential amplifier having one input connected to the non-zero terminal of the d.c. supply via a Zener diode to provide the reference voltage, the other input to the amplifier being connected to the junction between the load resistor and the MOSFET, the output of the amplifier being applied to the gate control of the MOSFET.

3. A dynamic electronic load circuit according to claim 2 including a first capacitor in a negative feedback connection from the amplifier output to the one input and a second capacitor connected between the MOSFET gate control and ground.

4. A dynamic electronic load circuit according to any preceding claim including means for biassing the other input to the differential amplifier to provide an enabling control for the circuit.

5. A dynamic electronic load circuit arrangement including a number of load circuits as ciaimed in claim 4 having the pairs of terminals connected in parallel for connection to a d.c. supply, the biassing means of each load circuit being connected to a different tapping on a potentiometer connected across the d.c. supply terminals.

6. A dynamic electronic load circuit arrangement according to claim 5 including separate light emitting diodes (LED) for each circuit connected across the load resistor of the respective circuit to indicate when the MOSFET for that circuit is turned ON.

7. A dynamic electronic load circuit substantially as described with reference to the accompanying drawings.

8. An electronic load circuit including a pair of terminals for connection to a supply, the circuit comprising active electronic components and being so constructed and arranged that electrical power required to operate the components is driven solely from said supply, said circuit being settable to present different load values at said terminals.