Monday, November 15, 2010


When a magnetic solenoid/relay is energized, a magnetic field has to be formed strong enough to pull the plunger in. When this plunger is pulled in, the power can be reduced to a much lower level to keep it in.
 This really cool circuit can do the above. If you pull a line, straight down, just before U5 and then build the circuit on the left, you will have a pulse width controlled relay driver circuit that can handle two relays with a total current of 1.5 amps. There are DRV103 IC's that can handle 3 Amps and by changing your power supply to 3 amps, can pull in really big relays/solenoids. 
These PWM relay drivers can be activated by supplying 3 to 5V on pin  8 of U2 and U3. The two red LED's  will turn on when an over current or over temperature is detected and will shut the IC's down. 
This circuit can be changed to drive 12V relays by changing R1 and R2 resistors on the power supply. 
When 3 to 5V is supplied to pin 8 of one of the IC's It will turn on fully, supplying 24V to the relay/solenoid and  22milliseconds later (C4 andC5) will start to pulse the voltage at 5000Hz (R3 and R4) and the duty cycle can be changed by adjusting R5 and R6. 

R3 and R4 can be changed to get a different frequency, C4 and C5 can be changed if the relay/solenoid needs more time to pull in before a pulsed signal is supplied, just check out the data sheets.
How to adjust:
Put a voltage of 3 to 5V on pin 8 of the DRV103 IC. When the relay pull in, adjust R5 or R6 until the relay fall out. Set it back a little and energize it again to see if it stays in. If the relay doesn't want to pull in at all then replace C4 and/or C5 with bigger capacitors.


  1. Hello Fritz, Very nice spot weld, you could
    explain theoretically how does one measure the capacitance of the capacitor and the resistance of the part to be welded. I imagine it indirectly through a relationship of pulse frequency and time of discharge at low voltage.

  2. There are a couple of methods to determine capacitance. The easiest way is to charge the capacitor to a known voltage, lets say 12V. Multiply 12V with 0.368 to get the discharge voltage. 12 X 0.368 = 4.416V. ---0.368 is a constant and never change, use it with any capacitance,resistance and/or voltage.
    Connect a known resistor, lets say 10 ohms between the positive and negative terminal, start a stop watch and wait until the voltage reach 4.416V before stopping it. Take the time in seconds and divide it by the resistance to get the answer in Farad. Example: capacitor discharge from 12V to 4.416V in 15 seconds using a 10 Ohm resistor. 15/10 = 1.5 Farad.
    The smaller the resistance is the warmer it will get and higher the resistance is the longer it will take to discharge the capacitor.

  3. 0.367 = e^(-1)

    special number for any kind of decay

  4. Hi Fritz
    Hello from Germany. I am new to the Blog and really don't Know how to navigate through the site so sorry for the questions that are already answered. I would like to know if there are any more boards available to purchase? How much do they cost and how do I pay for the products? I would also like to know where I find all they actual pictures and schematics for this project. I really think it's great and you did a super job. I would really like to build this project myself.

  5. Hobbymann, sorry I'm sold out and are not planning to get more boards. The files are not available online, I usually email it to people that bought the boards from me. I might make the files, schematics, gerber and the Hex code for the processor available for download later but I will not give any technical support or answer any questions.

  6. HI FRITZ!
    I can get it by email: okay?
    files, schematics, Gerber and Hex code for microprocessors

    1. Phong Van, You can get the files at the this link


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