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Circuits designed by David Johnson, P.E.
Last Updated on:
Thursday, March 22, 2012 07:52 AM
Master Category List
- Dave's Circuits
Text & Graphics
Copyright © David A. Johnson, PE - ALL Rights Reserved.
LINKING to Dave's circuits is permitted but DO NOT COPY to your WEB SITE server! |
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Relay
Circuits |
Bidirectional Solid State Relay
The circuit below can switch up to 4 amps of current with a peak voltage up to
20v. It is fully isolated using two inexpensive red LEDs. One LED forms a
red light source, which illuminates a second red LED. The circuit takes
advantage of the 1.5v produced by super bright red LEDs when illuminated with
red light. The voltage produced by one red is routed to two n-channel FETs,
configured in a bi-directional switch circuit. The two FETs selected have
a low gate-source threshold voltage, which allows them to turn on hard from the
1.5v from the red LED. The bi-directional configuration allows the circuit
to switch both AC and DC power. Using quality super bright red LEDs, the
circuit will switch on fully with only 5ma of LED current. |
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- CHARGE COUPLED BI-DIRECTIONAL
POWER MOSFET RELAY
The circuit uses an inexpensive C-MOS inverter package and a few small capacitors to
drive two power MOS transistors from a 12v to 15v supply. Since the coupling
capacitor values used to drive the FETs are small, the leakage current from the
power line into the control circuit is a tiny 4uA. Only about 1.5mA of DC is needed
to turn on and off 400 watts of AC or DC power to a load.
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- On/Off Switch
with Latching Relay
The circuit below switches AC or DC power to a load using an ultra low power 3v
latching relay with two coils. The relay can handle about 10 amps of current,
up to 250vac. Each time the small pushbutton is pressed, the circuit either
latches the relay contacts closed or unlatch them open. The 3v powered circuit
uses a single 74HC14. This device contains six Schmitt trigger inverters in the one
14 pin package.
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Reduced Power Relay Driver
Relays can handle a lot of power. However, for certain power sensitive designs
you would like to reduce the power needed to hold a relay closed. The circuit
below performs such a task. It uses a single CD4093 quad NAND gate. When
the “on” logic input signal is detected, the relay is first pulsed on for about
500ms. This is sufficient time to insure the relay is fully closed.
After that initial pulse the relay is then driven with a square wave signal, whose
duty cycle can be adjusted. The signal duty cycle can be adjusted from about
10% to 90%. In most cases a 50% duty cycle will hold the relay closed.
This reduces the average DC current required by the same factor, which means a 4:1
reduction in power. The circuit can operate over a wide 3v to 15v range.
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- SOLID STATE RELAY REQUIRES
ONLY 50uA DRIVE CURRENT
This circuit demands a control current that is 100 times smaller than that needed by
a typical optically isolated solid state relays. It is ideal for battery powered
systems. Using a combination of a high current triac and a very sensitive low
current SCR, the circuit can control about 600 watts of power to load while
providing full isolation and transient protection.
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Ultra Low Power Latching Relay Circuit
The circuit below takes advantage of some inexpensive small super capacitors.
The circuit pumps 6 volt pulses into the separate 5 volt latching and unlatching
relay coils. A short 25ms pulse is all that is needed to flip the relay
states. A third super capacitor is used to supply a higher peak current than a
small lithium cell might otherwise be able to supply.
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- Universal
Flasher using Latching Relay: This circuit is powered by a 9v battery and
controls any AC to DC load through a set of relay contacts, rated at 10 amps.
To conserve power, the circuit uses a latching relay. A variable frequency
oscillator controls the flashing speed from 0.2Hz to 2Hz. A pair of pulse
generators first latch the contacts closed then unlatches them open. The power
consumption is so low that a 9v battery will last for several months.
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