fan control

Description: The MFJ-4225MV is a lightweight power supply that provides approximately 25 A output, with around 20 A suitable for continuous use. It is compatible with equipment such as the IC-706GII and K3. The fan operates quietly compared to other devices, but after upgrading to a new laptop and a quieter K3, the power supply became a noticeable source of noise in the ham shack. This project aims to apply modifications to any power supply that can operate with the fan off under low load conditions. The objective is to minimize fan noise, particularly when the fan operates intermittently. The fan is intended to run only when necessary to cool the heat sink, with a temperature threshold based on observations made at a 20 A load. Initial temperature measurements indicated that the heat sink reached significant temperatures, prompting further testing with a temperature sensor. Various fan voltage levels were tested to reduce noise, and a control circuit was devised using an op-amp to manage fan operation based on temperature readings.

The MFJ-4225MV power supply is designed to deliver a stable output of approximately 25 A, making it suitable for various amateur radio equipment. The power supply features two heat sinks, one equipped with two devices and the other with a single device. A temperature sensor, the LM34Z, is utilized for monitoring heat sink temperatures during operation. The sensor's output, which is directly proportional to temperature, simplifies the calculations for thermal management.

To control the fan operation based on temperature, an op-amp, specifically the LMC6482, is employed. This op-amp is capable of operating on a single 5 V supply and is configured to create a hysteresis effect for temperature regulation. This hysteresis is achieved using a feedback resistor network, allowing the fan to operate only when the heat sink exceeds a predetermined temperature threshold.

The fan's current draw is relatively low, allowing the use of a 2N7000 MOSFET as a switch to control the fan. To enhance reliability, two MOSFETs are connected in parallel. The fan is integrated into the circuit such that it operates in the return path to ground, ensuring efficient control. The circuit includes a 78L05 voltage regulator to maintain a constant supply voltage for the op-amp, which is critical for consistent deadband operation.

The overall design philosophy prioritizes quiet operation while ensuring adequate cooling for the power supply. The temperature monitoring and fan control system will help maintain optimal operating conditions without unnecessary noise, enhancing the user experience in the ham shack. The integration of SMT components with an adapter board facilitates ease of assembly and modification, making the design adaptable for various applications.A little lightweight MFJ-4225MV power supply. It puts out about 25 A, maybe 20 continuous, which is enough for my IC-706GII or my new K3. I also liked the fact that the fan was fairly quiet. But that was in comparison with myFT-1000 with its fairly noisy fan and my hamshack computer with two or more attention getting fans. After upgradingto a new laptop and the quietly cooled K3, the MFJ P/S became the distracting source of white noise in the ham shack. Actually, this project ought to be applicable to any P/S that can run with the fan off in low load conditions. I don`t like noisy fans. I also don`t like noisy fans that start, run for a few seconds, and stop again. If the unit is hot enough to require a fan, I want it to run long enough to cool the sink down a little.

I don`t think the fan is too critical in this P/S, so I decided to go with a temperature reached after 5 minutes at 20 amps load. In this way, it might never come on during light duty operation and just listening. If it does come on, I want a deadband of about 10 degrees at the heat sink before it goes off. If the 20 A load continues, the fan might never go off, which would be OK. First I made some temperature measurements. The P/S has two heat sinks, one with two devices mounted to it and one with just one. Using a LM34Z sensor glued to the heat sink with two devices, I measured the temperature at 20 A load for 10 minutes.

It went from 74. 5 F to 115 F at 5 minutes and 132 F at 10 minutes. But then I realized the sink toward the middle of the unit was getting hotter. So I moved the sensor there and did the test again. Ambient was 79F. In 5 minutes T was 134 F, in 10 it was 138 F. I also experimented with running the fan at lower voltage. It got very quiet at less than 11 volts, but with the cover back on, it was not so quiet. Pulling air through the perforated cover made the difference. The fan current was 127 mA at 14. 9 V and reduced to 80. 6 mA at 10 volts. So a 2N7000 with a rating of 200 mA could handle it. I was going to use a comparator (LM319A), but couldn`t find where I`d stowed them in my vast component collection. My next choice was an op-amp that could be driven rail to rail and work on a single 5 V supply. For a chip with saturation voltage (high) of 5V and low saturation voltage of 0V, the hysteresis is 5 / (N + 1) where N is the ratio of the feedback resistor to the input resistor.

I used 390 k and 10 k for a hysteresis of about 5/40 or 0. 125 volts, 12. 5 degrees F. The op-amp of choice was a LMC6482. It`s SMT, but an easy to handle S08 size. I used a little adapter board to interface it to the non-SMT components. The chip has two op-amps, so I biased the unused one to Vcc/2 to keep it from doing anything crazy. The LM34Z temperature sensor is kind of over-kill for this application, but it made calculating settings easy. Its output is directly proportional to temperature, so volts * 100 = temperature in degrees F. I considered using a Vbe multiplier as a sensor but already had the LM34Z in hand from my temperature measurements.

To maintain a constant deadband, the op-amp`s supply voltage needs to be constant, so I used a 78L05 regulator. The LM34Z will work off 5 volts, so I connect it to that bus. You could also run it off the power supply`s output voltage. Since the fan`s current draw is low, the 2N7000 works as a switch. One could handle the current OK, but I put two in parallel for conservatism. The fan goes in the return (to ground) lead. I cut the black wire to the fan and connected the end from the fan to the drains of the MOSFETs. The other end of the black wire is ground and goes to the board for its ground. I shaved the insulation from the intact red wire and tapped off another wire to go to the board`s 78L05 regulator`s Vp/s input.

Top left is the adapter board for the SMT op-amp, setpoint pot right of it. At far right is the 78L05 regulator and the two devices cr