High Voltage Meter or Probe Design

Common voltmeters, digital or analog, usually range to some hundred volts maximum. Higher voltages not only cannot be indicated, but will also destroy the instrument. However, the range of any voltmeter can easily be extended using extra series resistance, as shown in the figure. Calculating the necessary resistance implies knowledge of the input impedance of the voltmeter.

Circuit and formula for constructing high voltage probes. For example, let's assume we want to extend the range of a standard digital voltmeter (input impedance 10MOhm) to 100kV. The maximum DC voltage the meter can take is 1000V. This means we need an external 1GOhm high voltage resistor in series with the meter. The total voltage ios given by the value indicatedby the meter, times 100. If we wanted to read the voltage in kV directly, we would need a resistor 1000 times as large as the input impedance of the voltmeter, i.e. 10GOhm.

Such home-brew high voltage probes are good for DC only. For AC voltages, capacitive input impedance of the meter and capacity of the probe must be matched, which is difficult to achieve because of parasitic capacitance of the resistor chain. A few pF (the capacitance of a 1cm radius metallic sphere) make a big difference, especially at higher frequencies.

Source: kronjaeger.com

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Rangkaian Transistor Tester

The circuit shown below is a simple circuit transistor tester. In some digital and analog avometer now mostly been contained this feature, but it can not hurt us a little more creative. This circuit can also be used to detect whether a transistor is NPN / PNP.

Skema Rangkaian Transistor Tester

Circuit operation is as follows. The 555 timer is set up as a multi-vibrator 12hz. The output on pin 3 drives the 4027 flip-flop. This flip-flop divides the input frequency by two and delivers complementary voltage outputs to pin 15 and 14. The outputs are connected to LED1 and LED2 through the current limiting resistor R3. The LED's are Arranged so Pls That the polarity across the circuit is one way only one LED will from light and Pls the polarity reverses the other LED light earnest, therefore Pls no transistor is connected to the tester the LED's will from alternately flash. Also The 4027 outputs are connected to resistors R4 and R5 with the junction of these two resistors connected to the base of the transistor being tested. With a good transistor connected to the tester, the transistor will of turn on and Produce a short across the LED pair. If a good NPN transistor is connected then LED1 will from flash by Itself and if a good PNP transistor is connected then LED2 will from flash by Itself. If the transistor is open both LED's will from flash and if the transistor is shorted then neither LED will from flash.

IC NE555 Pinout

IC 4027 Pinout

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Rangkaian Regulator Variable Sederhana

Regulator Variable Sederhana

A simple but less efficient method of controlling a DC voltage is to use a voltage divider and transistor emitter follower configuration. The figure below illustrates using a 1K pot to set the base voltage of a medium power NPN transistor.

Skema Rangkaian Regulator Variable Sederhana

The collector of the NPN feeds the base of a larger PNP power transistor which supplies most of the current to the load. The output voltage will be about 0.7 volts below the voltage of the wiper of the 1K pot so the output can be adjusted from 0 to the full supply voltage minus 0.7 volts. Using two transistors provides a current gain of around 1000 or more so that only a couple milliamps of current is drawn from the voltage divider to supply a couple amps of current at the output.

Note that this circuit is much less efficient than the 555 timer dimmer circuit using a variabe duty cycle switching approach. A fairly large heat sink is required to prevent the PNP power transistor from overheating. The advantage of the circuit is simplicity, and also that it doesn't generate any RF interference as a switching regulator does. The circuit can be used as a voltage regulator if the input voltage remains constant.

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Simple switch-Off Time Delay Circuit

Designing a switch off delay circuit is quite simple and will cost you no more than $5 to make. All parts can be picked up from Radio Shack or Fry's if you have them as well as Parts Express. This will cover the mechanical aspects of it - theoretical topics can come later. If you suffer from pops on your amps or any other components, this will help you eliminate it, but it does not work in all cases.

Simple switch-Off Time Delay Circuit

Designing a swictch off delay circuit is quite simple and will cost you no more than $5 to make. All parts can be picked up from Radio Shack or Fry's if you have them as well as Parts Express. This will cover the mechanical aspects of it - theoretical topics can come later. If you suffer from pops on your amps or any other components, this will help you eliminate it, but it does not work in all cases.

The two circuits di atas illustrate opening a relay contact a short time after the ignition or ligh switch is turned off. The capacitor is charged and the relay is closed when the voltage at the diode anode rises to 12 volts. The circuit on the left is a common collector or emitter follower and has the advantage of one less part since a resistor is not needed in series with the transistor base. However the voltage across the relay coil will be two diode drops less than the supply voltage, or about 11 volts for a 12.5 volt input. The common emitter configuration on the right offers the advantage of the full supply voltage across the load for most of the delay time, which makes the relay pull-in and drop-out voltages less of a concern but requires an extra resistor in series with transistor base. The common emitter (circuit on the right) is the better circuit since the series base resistor can be selected to obtain the desired delay time whereas the capacitor must be selected for the common collector (or an additional resistor used in parallel with the capacitor).

The time delay for the common emitter will be approximately 3 time constants or 3*R*C. The capacitor/resistor values can be worked out from the relay coil current and transistor gain. For example a 120 ohm relay coil will draw 100 mA at 12 volts and assumming a transistor gain of 30, the base current will be 100/30 = 3 mA. The voltage across the resistor will be the supply voltage minus two diode drops or 12-1.4 = 10.6. The resistor value will be the voltage/current = 10.6/0.003 = 3533 or about 3.6K. The capacitor value for a 15 second delay will be 15/3R = 1327 uF. We can use a standard 1000 uF capacitor and increase the resistor proportionally to get 15 seconds.

Source: bowdenshobbycircuits.info

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Simple Switch On Time Delay Circuit

This Switch On Time Delay circuit has been designed to create a lamp switch operated electronically with an option of setting a delay in the time of execution of operation to reduce one or more lamps in a stairwell or any other places where this circuit may be useful. The circuit can be useful to control various lamp or appliances that can be connected in relay contacts.

Simple Switch On Time Delay Circuit

The circuit that takes advantage of the emitter/base breakdown voltage of an ordinary bi-polar transistor. The reverse connected emitter/base junction of a 2N3904 transistor is used as an 8 volt zener diode which creates a higher turn-on voltage for the Darlington connected transistor pair. Most any bi-polar transistor may be used, but the zener voltage will vary from about 6 to 9 volts depending on the particular transistor used. Time delay is roughly 7 seconds using a 47K resistor and 100uF capacitor and can be reduced by reducing the R or C values. Longer delays can be obtained with a larger capacitor, the timing resistor probably shouldn't be increased past 47K. This Switch On Time Delay circuit should work with most any 12 volt DC relay that has a coil resistance of 75 ohms or more. The 10K resistor connected across the supply provides a discharge path for the capacitor when power is turned off and is not needed if the power supply already has a bleeder resistor.

Sumber: http://www.bowdenshobbycircuits.info

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9 Second Countdown Power-On Relay With 7 segment Display

9 Second Countdown Power-On Relay Circuit

This circuit provides a 9 second delay using a 7 segment display. When the switch is closed, the CD4010 up/down counter is preset to 9 and the 555 timer is disabled with the output held high. When the switch is opened, the timer produces an approximate 1 second clock signal, decrementing the counter until the 0 count is reached. When the zero count is reached, the 'carry out' signal at pin 7 of the counter moves low, energizing the 12 volt relay and stopping the clock with a low signal on the reset line (pin 4). The relay will remain energized until the switch is again closed, resetting the counter to 9. The 1 second clock signal from the 555 timer can be adjusted slightly longer or shorter by increasing or decreasing the resistor value at pin 3 of the timer.

Note:

• The circuit can be powered from a 9V PP3 battery or 12V DC power supply.
• The time delay can be varied by replace the resistor value at pin 3 IC555.
• The push button switch is for starting the timer.
• The appliance can be connected via contacts relay.

Source: bowdenshobbycircuits.info

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