Low Power Under- and Over-Voltage Monitor

This voltage monitor has two  threshold, VTH for undervoltage and VTH’ for overvoltage. Using the component values shown in the schematic diagram below, this circuit give 6V for VTH and 15V for VTH1. Above 6V, the LED indicator of this voltage monitor  circuit will increase the flash rate until reach 15V.  This circuit will stop flashing at voltage below 6V and above 15 volts since there will be no current flowing through C1.  At threshold  boundary, the output of LM10 will saturate to negative below VTH and saturate to positive above VTH’.

To customize circuit we can select the resistors values according to chosen VTH and VTH’ using the following formula:
Since the current consumption is very small (around 500uA), this voltage monitor will be suitable for various application  demanding low cost solution,  such as battery monitoring, small testing equipments, or power line indication.

5V FET Voltage Regulator

This voltage regulator circuit gives a stable 5V output from unregulated inputs (more than 5V). The stability of the output voltage is good enough, only change less than 0.1 volts when the load current changes about 60mA. Here is the schematic diagram of the circuit:

 The basic principle of the voltage regulation rely on the mechanism of keeping the the FET’s gate voltage at the cut-off point. The FET’s gate volage is the voltage across R2. At zero volt (when there is no current flowing through R2), the FET will be conducting, and a small current at FET (Tr1) will cause much larger current to flow through Tr3. This current will flow  through R2 and the gate voltage becomes negative. A some level the negative voltage at FET’s gate will cut-off the FETs current and  keep the output voltage stable

26V-to-5000V DC-DC Converter

This circuit can provide 5,000 VDC from 26 VDC. This circuit has ripple of under 0.01% due to Voltage-doubling capacitors. As sinusoidal oscillator, a 2N217 transistor is used. The diode and the capacitors at the output stage should be of high voltage type.  Here is the schematic diagram of the  circuit:

Transceiver Saver (Overvoltage Protector)

This is a transceiver saver circuit that protect  a transceiver device (applicable to other device as well)  from overvoltage of the power supply. This circuit is used to protect the device  by regulating the power supply,  avoiding damaging the device if overvoltage occurs.  If the transceiver transmits current of above 2A, a heatsink should be used for the transistor. The value of resistor must provide output of 12.6 V during normal operation, you can make trial and error through measurement when choosing this., you can start with value around 100R. It’s recommended to use a high wattage Zener diode. Here is the schematic diagram of the circuit:

Auto-Off 12V NiCd Battery Charger

NiCd/NiCad battery charger circuit is still needed since some application demanding high current is still rely on NiCd type, since this type is still superior in term of  high current output (low internal resistance) and low cost. This  battery charger circuit is used to charge 12V NiCd battery at  around 74 mA until battery is fully  charged. This circuit need  around 4 hours to fully recharge a totally empty/dead battery, depends on the battery capacity.  Here is the schematic diagram of the circuit

 This circuit is basically a current source with auto cut-off. The current regulation is done by maintaining a fix voltage across a 68R at the emitter of  2N2219 transistor. This voltage is stabilized by a 5.6V zener diode 1N752, which keep the voltage at 68R resistor at around 5V, giving a constant current of 74 mA.  The auto-off feature work by monitoring the output voltage (before the 1N4001 diode) relative to ground, as this voltage increases in accordance with the battery voltage which is being charged.  After the battery voltage reach the fully-charged level, the lower 1N752 zener diode will pass the current to activate the 2N222 transistor, which short the upper transistor’s base, turning off the charging process. To calibrate the shut off point, connect a 270 ohm / 2 Watt  resistor across the charge terminal and adjust the pot until the charging terminal voltage  show 15.5V level

Foldback Current Limited High Voltage Regulator

This circuit is high voltage regulator which has foldback current limiter protection. This circuit uses LM10 comparator with voltage reference, and this core integrated circuit is connected directly to high voltage circuitry. This high voltage direct connection is possible since the IC is inserted to a bias network and directly drop the applied voltage, so this IC is only suffering small voltage across its supply pins.

 The foldback current limiter is different with ordinary current limiter in the way the limiter responds to dynamic load. When we plot the regular current limiter, when the load draw a linearly increasing current, the plot of the current will be linear ramp which stops at a specified level determined by the limiter. A foldback current limiter will give same response until the current reach the maximum level, but will fold the current back to a much lower current level if the load try to further increase the current. This foldback action will prevent the final driver  transistor in the regulator from overheating

1 KW Power (Watt) Meter

This watt-meter circuit has measurement range up to 1-KW. This circuit can give the complete (X)(Y) function although uses only one transistor. Actually, this circuit is used for 117 Vac±50 Vac operation. For lower or lower voltage, this circuit can be modified easily. This circuit only measure power on negative cycles. The advantages of this circuit is this circuit does not need external power supply. This circuit measures true power that is delivered to the load. Here is the schematic diagram of the  circuit:

 At idle section, this circuit draw only 0.5W. This circuit has load current-sensing voltage of 10mV and load voltage loss of 0.01%. For linear loads, Rejection of reactive load currents is better than 100:1. When using a 50-μA meter movement, the nonlinearity of this circuit is about 1% full scale. Copper shunt can be used to give correct gain due to temperature

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