In this episode Shahriar takes a close look at an EG&G DSP-Based Lock-In Amplifier. This instrument provides excellent sensitivity for both voltage and current inputs and is based on a fully digital architecture. The instrument’s LCD screen is dim and unreadable. Furthermore, a few lines are missing from the LCD pointing to a failed zebra strip.
The instrument’s block diagram is examined and explained. The teardown reveals the structure of the LCD screen, front-end blocks and internal PLL. The CCFL back-light inverter is located and replaced with a low-voltage DC-DC converter for use with an LED strip. The instrument’s functionality is verified through a few experiments including measuring the 3dB bandwidth of an RC filter as well as measuring the photo-current induced in an LED using a synchronized infrared light source.
In this episode Shahriar investigates a malfunctioning E3646A Dual Channel Programmable Power Supply. The power supply provides the correct output voltage on one channel. However, the second channel produces no output and continuously reads 0V and 0A.
Applying an external voltage to the malfunctioning channel produces the correct readout from the instrument display suggesting that most of the channel circuitry is functional. After a close examination of the power supply schematics, the problem is traced to a failed solder joint. The DAC output amplifier is disconnected from the output transistor and after repairing the colder joint, the power supply is fully functional.
In this episode Shahriar presents a meticulously prepared set of educational lab kits on Power Conversion by Texas Instruments. These kits include Buck, Boost, LDO and Buck-Boost power conversion courses complete with a beautiful set of lab instructions and PCBs. Each kit describes up to 7 different exercises which demonstrates various aspects of power conversion characterization and design challenge.
In particular the Buck Lab kit is examined. The PCB is populated with two different Buck DC-DC converters which are carefully described in the lab kit. The first experiment of the lab kit is performed which involves measuring system efficiency under various load conditions and switching frequency. The inductor current, MOSFET switching voltage and output waveform are examined on the oscilloscope.
In this episode Shahriar demonstrates the principle of operation of a purely passive Weston watt-meter manufactured in the 1950’s in Newark, NJ, USA. This watt-meter makes no use of active devices (tube or solid-state) and yet is able to measure real-power both at DC and AC.
The teardown of the unit shows the construction of two coils interacting to advance the needle on the display. One coil produces a magnetic flux proportional to the current input and the other coil produces a magnetic flux propositional to the voltage. The interaction of the flux moves the needle proportional to the total power which is the product of current and voltage. The instrument is then used in conjunction with a power supply and electronic DC load to verify its functionality.
In this episode Shahriar investigates the failure of a GW Instek 1080W power supply capable of providing up to 80V and 40A of programmable output voltage and current respectively. The power supply does not power on. However, relay noises can be heard inside the instrument during power on.
Teardown of the unit reveals a modular design with PCBs on all sides. The instrument comprises 6 different modules and 3 complete power supplies in parallel. The controller circuit is powered from the middle power supply module. Examination of the boards reveals three separate failed devices. The in-rush power resistor which prevents the main supply board from startup, a damaged MOS power transistor on the supply output and surface mount resistors. All components are replaced. The instrument’s performance is verified with a BK Precision 8601 electronic load.
The Signal Path (TSP) is an electrical engineering video blog for industry professionals, students and hobbyists. TSP is a non-for-profit website dedicated to provide free education spanning a wide range of electrical engineering topics. Equipment reviews, tutorials and repair videos are posted regularly.