With four components, and the on:off switch, Bill K9HZ provides the ultimate reverse voltage and over-current protection system for your µBITx. The fuse protects the circuit from excessive current draw – so you won’t blow your finals when you wind up the bias too far and they try to go into thermal runaway. The relay must be powered on to power your µBITx (and the switch must be turned on). With the series diode in place the relay cannot turn on unless the power is wired up correctly.
Bill Erickson asks, “Is it safe for me to use my current 13.8 v power supply on a ubitx, or do I need to pick up a 12 v 5a version?”
The answer is probably. Many constructors have been using 13.8v supplies with the µBITx and the BITx40, and quite happily over a reasonable period of time. The parts are generally rated to cope with 12v nominally, but this typically means up to around 14-16v.
There are components in the µBITx, however, that will not cope with more than 15v. Some capacitors are 16v rated. A component that is rated at a maximum voltage of 15v is the TDA2822.
Those of you with a WX version of the TDA2822 should be using a voltage regulator to reduce the voltage to 9v or less on this chip. The chip is, otherwise, bound to fail. Those with the FCI version needn’t worry so much, but may want to current limit (with a resistor) the output of the audio stage into speakers/headphones.
One of the discussions on the BITX20 IO list over recent weeks has been a variable power supply to control power out on different bands. This seemed like the wrong approach to addressing variable output of the µBITx to the editor of ubitx.net (where the problem of inadequate drive or inadequacies in the PA design should be addressed directly) until he spotted this circuit diagram drawn up by Walter W9KJO, based on earlier suggestions. It is simple and straight forward with the TIP142 device at the centre of the “buck” function. 24v input can be adjusted downwards to set the power output of the µBITx PA stage.
Walter says, “It controls voltage nicely. Really helps limit output power while working digital.”
However the TIP 142 really generates some heat. He has a heat sink on the device, but the heat sink will need to be much larger to be safe.
Bill Schmidt, K9HZ has designed a fool-proof control circuit for the uBITx for power control. This circuit prevents bad things from happening by shutting down the radio before any damage is done.
It faults on:
- reverse voltage
- over power
- High SWR
- High PA Current
- High voltage.
It provides a visual indication of WHAT fault occurred, and the individual fault LEDs begin to flicker BEFORE the trip so you can fix the problem before you hit a hard trip.
The fault conditions listed above can be expanded to any number by adding more SCR Trip components (they are set to trip at 1.8V whatever the fault is).
When initially turned on, the transistorised RS Flipflop circuit comes up in the “Operate” mode. If a trip occurs, it flips into “FAULT” and shuts down the PA.
The circuit is reset with the “RESET” button, but ONLY if the fault has been resolved. Turning the power off and on resets the circuit too.
Bill bread-boarded the circuit last week and has been using it on his radio for a while and found that it works flawlessly (yes transmitting and yanking the coax off the back of the radio shuts down the PA nicely!). The circuit and a build list can be found in the BITX20 list’s files section.
Parts List for the uBITx Power Control Circuit
|D6||12V 0.5W zener 1N759, or 1N5242, or 1N6002|
|R14||1K 10-turn POT|
|R15||10K 10-turn POT|
|R16||88K||0.125||Can just use a 100K POT set appropriately|
|R17||12K||0.125||Can just use a 100K POT set appropriately|
|R30||1K 10-turn POT||0.125|
|U1||LM339||(Make sure to connect Vdd and ground!!!!).|
|F1||1A Poly Fuse||50V|
|F2||4A Poly Fuse||50V|
Dave N4LKN has developed a potentiometer controlled version of his original zener power control. He added high limit and lo limit resistors as illustrated in the circuit diagram below:
He says, “I will be adding a simple accurate circuit add on to this to report voltage and current supplied to the output stage using 2 analog inputs to my system health display.”
Paul K0ZYV asks, “What is the maximum safe input voltage to µBITx?”
Paul has lithium ion batteries that provide up to 4.2 volt when fully charged, and he hoped to put four in a pack which could max out at 16.8 volts providing about 2300 maH to power the µBITx.
The consensus seems to be around 15v is the maximum voltage that should be applied to the µBITx.
The reason is that the audio amplifier absolute voltage limit is 15V. All the other components can handle the 15V voltage. This assumes of course that the 5V regulator on the raduino has a heatsink and better still has a series resistor to limit power dissipation.
Regulating the voltage when using Lithium Ion Batteries
VE7WQ uses a $1.45 Boost Buck DC adjustable step up down Converter XL6009 Module with a 4 cell 18650 Li-ion Rechargeable Battery pack. This has the following characteristics:
Wide input voltage 5V ~ 32V;
Wide Output Voltage 1.25V ~ 35V
Built- 4A MOSFET switches, efficiency up to 94%.
Dave WI6R showed off his aluminium chassis to the list, but this resulted in questions about how he mounted his Anderson 12v powerpole connectors.
The answer is that you can use either of these mounting systems (the second was used in Dave’s case):