Low amperage converters
Are there more efficient ways than the standard voltage regulators such as the LM7805 and the LM317 that we use in most of our experiments? The answer to this rhetorical question is of course yes, use the buck converter. This video explains how they work:
The following flashlight experiment is an example, in this case a buck converter is used to drive a bright white power led. What you end up with is the following schematic:
I’ve put in a blue transparent acrylic case (I’m beginning to like them):
The DC converter VMA404 is made by Velleman and it is a marvel of simplicity. In this case example it helps to convert the voltage from a 9V 220 mAh battery to a lower voltage (around 3 Volt) for the LED. To compore the required powers I need I1 and I2, and they are derived from voltages are measured over the resistors. In this way you can compute the efficiency of the converter; that is, if we replaced the buck converter by a resistor, then what power is lost over that resistor compared to the power now lost in the buck converter. I varied the potentiometer on the VMA404 over a considerable range, and this is what I got:
The horizontal axis shows the voltage of the white led and the left vertical axis the power in mW consumed by the flashlight. Blue is what you measure, and red is what you theoretically get when the buck converter is replaced by a traditional voltage or current limiter (LM7805 or LM317). So for this design the buck converter is always more optimal than the traditional design, there is even an optimum near 2,950 volt over the LED, but, this LED can easily handle a higher voltage. Does the VMA404 cause a lot of QRM, the answer is no because the currents are relatively small. You can detect its presence within a meter, but beyond the 1 meter my metrovna with a detection loop didn’t notice it.
High amperage DC/DC converter
Also this is something I’m not going to make myself, the principle is the same except that the conversion goes both ways, DC input voltages within a certain range are converted into one fixed output voltage. For this I tested the Dometic Group DC20 converter, 8 to 16 volts in, and 14.2 volts out up to a maximum 20 Amperes. This is enough for the FT-991 and the FT-857 to produce 100W.
This DC/DC converter is ideal for in the car or in the field when you run on batteries, a LIPO or NiMH etc or a lead battery, as long as it is between 8 and 16 volt. The DC20 can also charge a regular battery, for this please read the manual (I didn’t test it). On the HF I could not directly detect any spurious noise as long as you run it from a battery. There is some leakage of the power switching towards the battery, so if you mount the DC20 behind a regular power supply then the HF switching blurp goes in the power grid, and this will cause spurious signals on the HF, in particular when the buck converter has no load on its output terminal, and the metal case is not grounded. So, to reduce the HF noise with this converter, 1) use a battery, and 2) ground the case of the converter. I could not detect spurious signals on the HF (or VHF or UHF) when it is used in this way.
The DC20 has a battery low and an overload indicator, is supposedly fail-safe (didn’t test this), and can be put in remote switch-on switch-off mode so that you can install it in a boat, a truck, a car etc. It includes an automatic fan, and there are air inlets that must be kept free. Also, you don’t want to put converters in spaces where there are outgassing batteries, so when you mount it in a closed space the batteries need to be separated from the converter.
With the ft-857 (and the ft991) in the field I could oftentimes not make more than 30 to 50W because the voltage of a discharging battery was too low. The radios still work, but the HF power output rapidly goes down. With a DC/DC converter you can bring it back to the 100W we were used to in the shack, without carrying a linear in the field. Some transceivers such as the older icoms are quite critical about the input voltages, in this case a DC/DC converter is a must for mobile operations.
Last update: 31-Dec-2017