Bench Power Supply Part 2

Now that I’ve got my specs figured out, it’s time to start some high level design. This will allow me to get the layout of the power supply set before diving into the small details. Hopefully this will make the design process more efficient. One of the biggest things that will affect this high-level design is one particular design specification lexapro 10 mg. That is, the call for a switching knock-down stage. The reason I chose to include this is efficiency. Many lab power supplies I’ve seen out there have one thing in common: Many of them use linear regulators like the LM7805 or LM317. These are good devices, but they all have very low efficiency, especially when the dropout voltage is high. Enter switching regulators. Switching regulators can have very high efficiency (upwards of 95%) which allows for higher current handling, and less heat dissipation. However, they have a drawback. Switching regulators typically have more noise on their outputs. They may be OK for some circuitry, but this inherent noise will not do for the lab power supply I intend to build. To get the best of both worlds, I plan to use both types of regulators in my design. The switching regulator will take care of most of the voltage dropout first, while leaving about 2-3 volts for the non-switching (a.k.a. linear) portion to drop second. This will reduce power dissipated in the non-switching section of the power supply, which has numerous advantages, including (hopefully) eliminating the need for a noisy fan, as I’d like to make this thing as small, quiet, and cool as possible. This would definitely not be possible without the switching section in front. Now, it’s time to make some initial part choices:

Parts List:

Linear Output transistor: P-Channel MOSFET IRF9540
Switching Regulator: LM2679-ADJ
Switching Regulator Inductor: Digikey# 553-1121-ND
Switching Regulator Capacitor: Digikey# P15372CT-ND
Current Sensor: ACS712
This should help lay the groundwork of the power supply. Next we’ll look at putting in some control circuitry, including op-amps and so on…

How to choose a power supply

astronMeterMany modern ham radios operate on low voltage DC instead of high voltage AC. This is done for several reasons. First, most of the circuits inside these radios actually run on low voltage DC, so high voltage AC isn’t needed. Second, running the radio on low voltage DC saves space inside the radio. This is because the radio can directly accept the low voltage DC it needs, and dosen’t need an internal power supply to convert from high voltage AC to low voltage DC. Finally, the last advantage is that these radios can easily be installed in a car or run on a car battery.

So, this leads to the main point. What power supply do I need for my radio if it says it requires DC? The short answer is: Many different supplies will be adequate. The two most important things to pay attention to are the following:

  • Does it produce the proper voltage?
  • Can it supply enough current?

Many DC power supplies on the market today provide 12 Volts DC (Actually 13.8 Volts is considered “nominal”). It is important to check the specifications and make sure that the output voltage on the supply is close to what your new radio calls for. A few volts difference is usually OK.

Next, check the maximum continuous current rating of the power supply. The power supply should provide at least the amount of current your radio needs plus 10%. The extra 10% is headroom to prevent your power supply from overheating. Most hams follow this extra 10% rule and some go even higher. Remember that you radio will not always need the max current that is listed on the radio’s specs. This is usually the current drawn when the radio is transmitting on full power. The current the radio requires when it’s receiving is typically much less.

So, in conclusion, you will need a power supply that matches the voltage of your radio, and can supply enough current to keep your radio running. Extra features like meters aren’t absolutely necessary, but can be nice for troubleshooting problems, however, they cost more!