Relays--electrically actuated switches--have many uses in electronics. Applying current at or above a relay's threshold voltage causes the relay to close, which can complete a different circuit, open or close a lock, or perform many other tasks that turn electrical current into physical motion. However, a relay will not function reliably if the voltage applied is below its threshold voltage. If you have a 12V relay but only 6V available to switch it, can you use that relay? The answer is yes--with help from a type of circuit called a DC-DC voltage doubler. This design, created by Aaron Cake, is quite efficient and uses components that you can find at any electronics shop.
- Skill level:
Things you need
- 12V relay
- 6V power supply (4 D-cell batteries in series will work for testing)
- Prototyping board
- Utility knife
- Soldering iron and solder
- Wire cutter/stripper
- Ruler or calipers
- Hot glue gun
- High-temperature hot melt glue
- All resistors are 1/4 watt:
- Two 2200-ohm resistors
- Two 4700-ohm resistors
- 1000-ohm resistor
- 1500-ohm resistor
- 33K-ohm resistor
- 10K-ohm resistor
- 0.1uF ceramic capacitor
- 470uF 25V electrolytic capacitor
- Switching diode, 1N914 or similar
- Rectifier diode, 1N4004 or similar
- 12V 400 mw Zener diode
- 3 45V 100 ma general-purpose NPN transistors, BC547 or similar
- 80V 6A Darlington transistor, BD679 or similar
- 0.5mm magnet wire
- 40mm (outside diameter) toroidal ferrite core
- Binding posts
- Insulated wire
- Alligator clips (for prototyping only)
- Heat-shrink tubing
- Hair dryer or heat gun
Measure the space available for the circuit, if it will be used in an enclosed space (such as inside the dashboard of a car). Cut the prototyping board to fit. Mark the best locations for the 6V input and the 12V output on the edge of the board.
Wind the ferrite core with 80 turns of 0.5mm magnet wire. This will be inductor L1 on the 6V to 12V Converter Schematic.
Following the 6V to 12V Converter Schematic and the marks on the board, lay out the circuit on the prototyping board. (Don't solder it yet.) Make sure to place the heatsink on the Darlington transistor (Q3 in the schematic).
Test the connections of the prototyped circuit with your multimeter. Make sure that you have not created a short circuit.
Connect the 12V output of the prototyping board to one terminal of the relay. Connect the other terminal of the relay to a separate ground.
Connect the ground of the 6V power supply to the ground line on the prototyping board, using lead wire and an alligator clip.
Connect the + of the 6V power source to the 6V input on the prototyping board with lead wire and an alligator clip. The relay should close. If it doesn't, double-check for short circuits and that you have assembled the circuit correctly.
Disconnect the power supply from the prototyping board.
Solder the components together. Trim the leads with the wire cutters. Test for shorts again with the multimeter.
Reconnect the 6V power supply. The relay should close. Disconnect the power supply.
Insulate the solder joints on the back side of the prototyping board by covering them with high-temperature hot glue. This protects against shorts caused by foreign objects and provides more mechanical stability.
Install the completed voltage doubler and relay in the space you measured in Step 1.
Tips and warnings
- If possible, cover the connection between the board and the 6V power source you will be using in your application with heat-shrink tubing. Use a hair dryer or a heat gun to shrink the tubing.
- This circuit is rated for a 2A input, and provides about 800mA output. If your input current is higher, you must increase the ratings of your components appropriately. Failure to do so may result in overheating and fire.
- Never solder while the board is powered. You could create a dangerous short.
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