## 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.

## Monday, July 13, 2020

### AC conversion and Continuity - DMM

U1A LF353 opamp turns on a buzzer when the voltage at Test + falls very low indicating a short between Test + and Test - or a resistor less than 5 ohms. The R9-R14 divider determines the minimum voltage that can be at Test + for buzzer to just turn on.

The Opamp here is a comparator and R10 10 Meg gives a very small hysteresis so that there is no oscillation at threshold levels, that is when both inputs at same levels.The Zener is for protection and R6 to limit current.

U2A and U1B opamps form a precision rectifier, note that this is not a true RMS rectifier circuit for that see some devices from Analog Devices. When you need to rectify a 200mV AC signal you cannot use a diode like 1N4148 as the diode turns on at 700mV so this circuit is used.

AC conversion and continuity buzzer test

The AC signal measurement is best to do with a True RMS convertor. This Type is able to quantify the Heating power of a AC voltage, includes all harmonics and Noise too.

### Battery Backup Supply

This is a 9V power supply which will work even on power failure. It uses a rechargeable battery and regulators. A transformer with 15-0-15 AC volts output is required.

From my Power Electronic Circuits

In the first regulator U1 the output is lifted up by 1.4V and in the second regulator U2 by a resistor divider. In the second regulator the voltage across resistor R3 is 5V, so the current is 5V / 1K = 5mA this adds to the quiescent current of 5mA from the regulators ground terminal and flows into the resistors R1 and R2 in parallel which form 404 ohms, 10mA thru 404 ohms is 4V. So the output will be 5 + 4 = 9V. Note that the charge and discharge paths of the battery are separated with diodes.

## Monday, July 06, 2020

### Simple Water operated relay

This was done in my early days, i have upgraded it, it ought to work, reduce the number of transistors to make it less sensitive, also a lower value in place of 10M will reduce its sensitivity, use clamping diodes to protect.

BD139 is used to drive the relay as it has good Ic. So you can even use a low ohm relay. If a Relay resistance is high its quality is higher, its power consumption is less and it needs thinner wire SWG-AWG. T2 and T3 form a darlington pair which drives T1. LED1 shows that the water level has reached the top of tank and also that the Relay is energised.

Circuits using only discrete devices

D1 a freewheeling diode. R3 10M ensures that the high gain input does not float, yet the low leakage current thru the water is not drawn away by the 10M. R2 limits base current in case water is saline.