Adjustable Current Sink with Integrated Clock Generator
Intelligent and easy-to-use device for testing power supplies, voltage converters and batteries that should be available in every laboratory.
Application
How do you test your power supply, no matter if it is a mains adapter, a converter or a battery? Probably you have a number of power resistors with which you load the output of the power supply. Series of measurements can be taken in this way to show the static behaviour of the regulator in relation to load change and change in input voltage. The inner resistance of the source can also be calculated from this. Unfortunately, this approach is very complicated and costs a lot of time. Much better and simpler for this purpose is the use of an adjustable current sink, which draws a constant current regardless of the applied voltage.In addition, the dynamic behaviour, i.e. the response to rapid load changes, cannot be measured with this arrangement. But this is very important, because subassemblies connected to a power supply which generates high overshoots can malfunction or even be damaged. The control loops of power supplies are often unstable and tend to oscillate at high frequencies. The effects are the same as already explained.So it makes a lot of sense if the current sink can also generate fast load changes. This is exactly what the circuit shown below does. It was tried to keep the circuit as simple as possible and not to use any exotic components. Furthermore, no additional power supply is necessary. Depending on the cooling of the semiconductors, the current sink is suitable for power dissipation up to 20 W and a maximum voltage of 30 V. Therefore, it is best suited for small and medium power. However, due to the clocking function, the dynamic behaviour of stronger power supplies can also be estimated very well.
Specification
Functional principle
A voltage regulator which delivers an adjustable and constant voltage to a fixed resistor serves as a current sink. Since the voltage at the load resistor is constant, the current through the resistor and thus through the voltage regulator is also constant, i.e. independent of the input voltage. The regulator used is an integrated voltage regulator that has extensive internal overload protection.The smallest output voltage of this regulator corresponds to the internal reference voltage of 1.25 V. In order to still be able to regulate to zero, an external reference voltage of -1.25 V is used as a reference point.Clocking of the load current is realized by means of a MOSFET, which switches the load resistor at the output of the voltage regulator. Unfortunately, the first idea of switching the setting voltage of the regulator was unsuccessful, as the voltage regulator began to oscillate and could at best be used as a short-wave transmitter.
The circuit
The good old LM317 is used as voltage regulator. The negative reference voltage for the voltage regulator is generated by the voltage inverter IC2, whose output voltage is stabilized at -1.23 V by the reference diode IC4. In order not to exceed the maximum supply voltage of IC2, it is supplied with a constant voltage via the emitter follower Q4. Two green LEDs that have a much more distinct current rise than Zener diodes with a comparable Zener voltage serve as a reference. A classic 555 timer serves as the clock generator, which switches the load current via the MOSFET Q2. By short-circuiting the oscillating capacitor via JP3, the clocking is interrupted and Q2 is permanently switched through. This part of the circuit is supplied by Q3. The combination of the adjustable reference diode IC6 with the transistors Q5, Q6 disables the timer IC3 in case of undervoltage, so that the current flow via Q2 is interrupted. IC6 is set so that a Zener current only flows when the voltage is sufficient, and so switches Q5 through.Q1 protects the circuit against polarity reversal of the input voltage. Fuse F1 blows in the event of a short circuit on the subassembly, thus preventing the risk of a fire.
The test procedure
The current sink is set to the lowest current, the clock is disabled. Then it is connected to the DUT. A voltmeter connected directly to the output of the test object measures the output voltage. If you would measure near the current sink, the voltage drop across the leads would falsify the result. In addition, you would notice voltage peaks due to the line inductance. An oscilloscope which is set to AC measuring, is connected in parallel to the voltmeter. After starting up the DUT, the output voltage is measured. By stepwise increasing the current of the current sink, a series of measurements is obtained which represent the degree of load regulation and the inner resistance of the tested unit. The measurement is very similar when testing the line regulation. The input voltage is simply varied and the output voltage is measured at the specified current.With each measurement, the output voltage is monitored with the oscilloscope in order to detect wild oscillations. For the following measurements, the oscilloscope and the current sink, now with the clock generator enabled, are required. The oscilloscope is used to measure the transient response of the DUT to fast load changes. The measurements are taken at different load currents and, if possible, at different input voltages. In addition to the transient response, this also reliably detects if the DUT tends to oscillate.While in design phase, the dynamic behaviour of the regulation loop can be optimized very well in this way.
How do you test your power supply, no matter if it is a mains adapter, a converter or a battery? Probably you have a number of power resistors with which you load the output of the power supply. Series of measurements can be taken in this way to show the static behaviour of the regulator in relation to load change and change in input voltage. The inner resistance of the source can also be calculated from this. Unfortunately, this approach is very complicated and costs a lot of time. Much better and simpler for this purpose is the use of an adjustable current sink, which draws a constant current regardless of the applied voltage.In addition, the dynamic behaviour, i.e. the response to rapid load changes, cannot be measured with this arrangement. But this is very important, because subassemblies connected to a power supply which generates high overshoots can malfunction or even be damaged. The control loops of power supplies are often unstable and tend to oscillate at high frequencies. The effects are the same as already explained.So it makes a lot of sense if the current sink can also generate fast load changes. This is exactly what the circuit shown below does. It was tried to keep the circuit as simple as possible and not to use any exotic components. Furthermore, no additional power supply is necessary. Depending on the cooling of the semiconductors, the current sink is suitable for power dissipation up to 20 W and a maximum voltage of 30 V. Therefore, it is best suited for small and medium power. However, due to the clocking function, the dynamic behaviour of stronger power supplies can also be estimated very well.
Specification
- Input voltage range : 3.3 V ... 30.0 V
- Adjustable current range: 50 mA ... 1600 mA
- Maximum power: 20 W
- Clock, can be disabled: approx. 50 Hz
- Slew rate: < 3µs
- Reverse polarity protection at input
- Thermal overload protection
- Under voltage cut-off· Supplied via the DUT
- Trigger signal to oscilloscope
- Measurement output for current measurement
Functional principle
A voltage regulator which delivers an adjustable and constant voltage to a fixed resistor serves as a current sink. Since the voltage at the load resistor is constant, the current through the resistor and thus through the voltage regulator is also constant, i.e. independent of the input voltage. The regulator used is an integrated voltage regulator that has extensive internal overload protection.The smallest output voltage of this regulator corresponds to the internal reference voltage of 1.25 V. In order to still be able to regulate to zero, an external reference voltage of -1.25 V is used as a reference point.Clocking of the load current is realized by means of a MOSFET, which switches the load resistor at the output of the voltage regulator. Unfortunately, the first idea of switching the setting voltage of the regulator was unsuccessful, as the voltage regulator began to oscillate and could at best be used as a short-wave transmitter.
The circuit
The good old LM317 is used as voltage regulator. The negative reference voltage for the voltage regulator is generated by the voltage inverter IC2, whose output voltage is stabilized at -1.23 V by the reference diode IC4. In order not to exceed the maximum supply voltage of IC2, it is supplied with a constant voltage via the emitter follower Q4. Two green LEDs that have a much more distinct current rise than Zener diodes with a comparable Zener voltage serve as a reference. A classic 555 timer serves as the clock generator, which switches the load current via the MOSFET Q2. By short-circuiting the oscillating capacitor via JP3, the clocking is interrupted and Q2 is permanently switched through. This part of the circuit is supplied by Q3. The combination of the adjustable reference diode IC6 with the transistors Q5, Q6 disables the timer IC3 in case of undervoltage, so that the current flow via Q2 is interrupted. IC6 is set so that a Zener current only flows when the voltage is sufficient, and so switches Q5 through.Q1 protects the circuit against polarity reversal of the input voltage. Fuse F1 blows in the event of a short circuit on the subassembly, thus preventing the risk of a fire.
The test procedure
The current sink is set to the lowest current, the clock is disabled. Then it is connected to the DUT. A voltmeter connected directly to the output of the test object measures the output voltage. If you would measure near the current sink, the voltage drop across the leads would falsify the result. In addition, you would notice voltage peaks due to the line inductance. An oscilloscope which is set to AC measuring, is connected in parallel to the voltmeter. After starting up the DUT, the output voltage is measured. By stepwise increasing the current of the current sink, a series of measurements is obtained which represent the degree of load regulation and the inner resistance of the tested unit. The measurement is very similar when testing the line regulation. The input voltage is simply varied and the output voltage is measured at the specified current.With each measurement, the output voltage is monitored with the oscilloscope in order to detect wild oscillations. For the following measurements, the oscilloscope and the current sink, now with the clock generator enabled, are required. The oscilloscope is used to measure the transient response of the DUT to fast load changes. The measurements are taken at different load currents and, if possible, at different input voltages. In addition to the transient response, this also reliably detects if the DUT tends to oscillate.While in design phase, the dynamic behaviour of the regulation loop can be optimized very well in this way.
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