Schematics of delabs

One of the oldest Schematic Archives on the Web. It was in the form of pdf files in the late 90s. Instrumentation and Industrial Designs by delabs and Generic Application Designs.

Search This Blog

Saturday, August 15, 2020

Power supply with battery backup - DMM

This is a simple charger circuit which will work for a light load like a DMM, the Battery can be a sealed maintenance free battery of 9V-2AH or better. The circuit will work best if the Unit is powered on many times daily on regular use, else battery will drain down.

The 555 Astable is used to generate a AC signal from which a negative voltage is generated, A 79L05 which is a low power TO92 equivalent of 7905 a negative -5 volts regulator is used as -5 volts load is less. A TO220 7805 is used for the +5V supply.

Power supply with battery backup - DMM

Many dual supplies are derived from one DC Source. A SMPS solution is the best. A Series Regulator is simple to troubleshoot in comparison to Switching Types, there is no EMI-RFI too.

Power supply with battery backup - DMM

This simple circuit is ok only for Low Current gadgets, Whereas SMPS is green and efficient. In SMPS a greater care for Product Safety is required. In a Linear Supply with Step-Down Mains-Frequency Transformer. The Transformer is the only place, where you look into safety the most. In SMPS it is the PCB, the feedback components and also The High-Frequency (200 kHz) Mains Ferrite Transformer.

Friday, August 14, 2020

Build a DMM - LM3914 Analog display

This a part of a complete Benchtop DMM that you can build to learn the internals of a Digital MultiMeter. This is oriented towards learning the measurement aspects and the protection of input circuits.

U2A opamp LF353 is used here as a constant current source, R10-D13-D14 is for protection in case you measure voltage in the ohms range. U1 4052 helps digitally select four different currents, the currents pass thru the unknown resistor to be measured and an voltage developed across the resistor is measured.

Build a DMM - LM3914 Analog display

U2B is a buffer which passes on the voltage measured to U5A for inversion of polarity as current source is a negative current (current sink). U5B amplifies to the level required for the LM3914 display circuit. U7 555 is used as a de-bouncing for switch SW1 so as to advance counter 4029 to change the range of resistance measurement.

Build a DMM - LM3914 Analog display

Use any FET input dual opamp, TLO72 will work but LM358 will cause error as it is transistor input..

D1, D12 and R8 are to ensure that the FET can be turned off, as the opamp swings from +/-3.5V only, with some FET it needs to be tweaked. LF353 is a Wide Bandwidth Dual JFET Input Operational Amplifier.

Tuesday, August 11, 2020

Monostable Multivibrator CD4538

CD4538 is a dual Monostable Multivibrator. When you trigger the chip the output sends off one single pulse or one high-low event.

Mixed Circuits Analog with Digital

The T+ pin 4 of U1a is the positive edge trigger or raising edge trigger input, the T- pin 5 is falling edge or negative edge trigger input. Now see the image of the single pulse above which shows both the edges, If this is the input pulse at pin 5 then the falling edge turns the output pin 6 from low to high, this output remains high for time T = R2 * C1 and then goes low again, The output Q at pin 6 also looks like the image of pulse above.

Monostable Multivibrator CD4538

The Output pin 7 is the complementary state of pin 6, it is the reverse state or inverted form of pin 6 output.

Now why is a slope shown in the edges, this i have exaggerated a bit so that it can be explained. But then there is a slight slope due to gate input and output capacitance.