VK7RHF low power mod
23 October, 2016 - Reading time: 5 minutes
Having removed VK7RHF's transmitter to test at a solar powered site, it was necessary to test standby current consumption. Basic current measurements were made. 750mA was measured as standby. 13A while transmitting (including three fans that run while transmitting and for 5 minutes after PTT finishes).
All repeaters are running 24/7, therefore standby current can be a problem if excessive, more so then if compared with current consumption while transmitting. For example, one hour of transmit time a day at 14 amps means we need to put those 14 amps back into the battery. In addition we have a standby current of 0.75A x 23 hours = 17.25 amps. So regardless of transmit activity, every single day we are going to take at least 17 amps out of our battery. Reducing transmitter power usually does not save a lot of current as the PA will be more inefficient. Therefore we want to reduce standby current.
A good way to do this is to switch off the power to all unnecessary bits of the repeater until needed. We need the receiver and repeater controller to be active all the time for a start, but we can turn off the exciter, PA and microcontroller from the converted MLS low band radio. I used an idea that Dion VK7DB had come up with. He used a SUP75P05-08 P channel FET as a high side switch on the power line to the PA board. These particular FETs allow 20A of continuous current without a heatsink, but were discontinued some time ago. More readily available were SUP53P06-20 from Jaycar. We want the resistance between the drain and source to be as low as possible. RDS according to the datasheet was 0.0195 ohms at VGS = - 10 V. Maximum Power Dissipation at an ambient temperature of 25 degrees is 3.1W. We know I = √P/R. 3.1/0.0195 = 158.974. The square root of that is 12.6. Therefore at 25 degrees and not on a heatsink, this device can handle 12.6 amps as an absolute maximum. With a TC=25 degrees (case temp - heatsink) it can dissipate 104.2W, or handle 73 amps. Plenty.
Here is the circuit. It's fairly self explanatory. We use the fan control in the NHRC controller as this pulls low to GND when the transmitter keys up and for 5 minutes afterward. We put a pull up resistor from +12V to the gate to make sure the FET is in a known state until triggered from our fan control. When the fan control goes low, our FET turns on and switches power on to our PA/exciter stage of the repeater.
Orange wire is our logic from the fan controller. Resistor is 10K I think. It works.
Current consumption when running the receiver only is now 140mA. A saving of 610mA or 14.64 amps over 24 hours!