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SR-700 Boost Regulator
The SR-700 is a boost regulator intended to provide a regulated 13.8V output from a 10-14V source, such as a battery.
It is useful with some radios that do not work well with a low battery voltage, or if the voltage drop in the line is excessive. Also, most radios will provide less than rated power at lower supply voltages, so the SR-700 will help maintain full output power under all low line conditions.
It uses a switching boost regulator. Particularly notable characteristics of this topology are:
- no input-output isolation (not necessarily a bad thing),
- best efficiency when the input voltage is near the output voltage (great to compensate for the occasional line sag)
- the circuit only regulates up, if the input voltage is too high, so will the output voltage unless a shut down mechanism is implemented, which requires more parts and will affect efficiency.
- relatively noisy output waveform (compared to other topologies), which is harder to filter.
This particular model is rated 25A output continuous and 700W output, which I believe is a peak rating.
The brochure lists an over-voltage protection, but I am not sure if or how it works. I increased the input voltage to about 15V and the output went up, I did not see a shut down. Maybe the shut down is at a higher voltage. The brochure does not say.
I then took some plots of the output voltage ripple, using a 10:1 scope probe driving a Tek 2445 oscilloscope, and the 50 ohm output of the oscilloscope driving a Tek 494P spectrum analyzer (I could not find the current probe, and you cannot use a standard scope probe designed for a 1 megohm input to drive the 50 ohm input of a spectrum analyzer).
The measurements are not calibrated (too lazy to calibrate...), but they are useful by comparison and to show where most of the noise is.
These plots show the voltage ripple in the frequency domain (measured with the spectrum analyzer) at the output of the SR-700 in two different conditions (light load, heavier load).
I did look at the ripple on the scope and while I did not take a picture, the spikes were about 200mV p-p at about 100kHz (switching frequency), with a clear ringing around 12 MHz, which was later confirmed by the spectrum analyzer plots.
Output voltage ripple of the SR-700 while driving my FT-290 VHF multi-mode transceiver in Receive mode
Output voltage ripple of the SR-700 while driving the FT-290 while transmitting FM (25W output)
This plot is for comparison purposes. It shows the output ripple of my MFJ-4225MV switching power supply while driving the FT-290, transmitting FM (25W output). Please note the reference level is -20 dBm instead of 0 dBm.
Conclusion: The SR-700 Step-Up Regulator generates a significant amount of noise in the HF bands, particularly between 10 and 15 MHz, including significant levels all the way up to the VHF bands. How it will affect a particular transceiver will depend on the quality of the filtering within the transceiver itself, and the isolation between the SR-700 and its power leads and the antenna itself.
I would recommend this regulator only if a particular setup was such that a transceiver was not useable because the available voltage was too low and that could not be corrected by fixing the wiring.
I would not recommend it simply to restore full power to a transceiver that would otherwise work OK at lower voltages, or to compensate for line drop. In this last case, you will always be better off fixing the wiring.
For more comparisons, check the ARRL Power Supply product review from January 2000 QSt at ARRL Product Review - Power Supplies Jan 2000.pdf (you must be an ARRL member to view the article.)
Please note that the plots on the QST article are not calibrated either, making comparisons difficult.