There are several ways to extend the capability of your Arduino to allow it to drive higher current loads. Today we will look at a couple of them. Introduction The Arduino is a microcontroller, you probably already know that. The Arduino, or any microcontroller, is tiny in more than just size. We accomplished this by using a driver board to take the low-current Arduino control signals and drive the high-current motors.
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This N-Channel enhancement mode silicon gate power field effect transistor is an advanced power MOSFET designed, tested, and guaranteed to withstand a specified level of energy in the breakdown avalanche mode of operation. MOSFETs are voltage-controlled devices and exhibit a very high input impedance at dc, whereas bipolar transistors are current-controlled devices and have a relatively low input impedance.
The built-in self-regulating actions prevent MOSFETs from being affected by thermal runaway, but still needs some thermal protection R6. Rather than using a toroid which is excellent to match Q1 impedance to 50 Ohms, I have applied the "old school" radio valve coupling; impedance matching circuitry between the output and the antenna using a L-filter FET devices are more closely related to vacuum tubes than are bipolar transistors and because I do like to do things my way HI. Both vacuum tubes and the FET are controlled by the voltage level of the input rather than the input current.
They have three basic terminals, the gate, the source and the drain. These are related and can be compared to the vacuum tube terminals. The two most important relationships are called the transconductance and output. Built-in to the power amplifier is a sensitive Q2 T-R relay which will switch the unit in and out of the antenna line. When in receive, the amplifier is bypassed and the antenna feeds directly to the input jack, when you go to transmit, the T-R circuit detects the transmit RF power and automatically switches the power amplifier into the circuit and amplifies the applied RF power.
If you decide to run "barefoot" turning off the AMP it will disable the amplifier and your QRP transmitter will feed directly through the amplifier without any amplification. Power is supplied by any 14 to 25 volt or 2 x 12v battery DC source with a current draw of 1 to 2 amps depending upon RF power output and applied voltage. Band selection Switching beween bands could be done manually using a rotary switch.
You can build the amplifier for only one band or a combination of any other of the five available bands. Drive The input drive can be anything from 0. The output varies on the drive power, frequency and the applied voltage.
The impedance 50 Ohms match could be solved by using a toroid, or as I like to use, the "old school" radio valve coupling; impedance matching circuitry between the output and the antenna using a L-filter The N-channel mosfet has an input capacitance thats a bit on the high side and the output capacitance that varies with the cross over frequency.
Of course the main issue was the simple design to be able to use one band or even up to five bands if wanted, which always has some compromise in this type of design. This means that there is some fluctuation of the output power par band. Although the design allows you to work in a varied range of voltages, the maximum output is only guarenteed 24volts. Quality of all components, construction etc also influences the performance. I used breadboard to build my protoype and some "dead bug" work.
Dead bug prototyping and freeform electronics are a way of building working electronic circuits, by soldering the parts directly together, or through wires instead of the traditional way of using a printed circuit board PCB. Bias The power amplifier require biasing for proper RF performance.
Thermal protection R6 is a PTC resistor that allows which is used here for thermal protection. As the resistor heats up the resistance increases, which lowers the bias voltage. R6 should be placed near Q2. Filter RF purity and harmonic suppression is done here. This 4-element L-type narrow band-pass filter circuit and a 3 element low-pass Butterworth PI filter for the desired frequency removes out any remaining harmonic signals efficiently. A picture from my oscilloscope: RF-sensing The basic principle of RF-sensing using a relay is clearly drawn in the schematic and pretty much self explaining.
The on-time is to long for continues wave modulation formats. There are numerous of reasons why I implemented it in this design. It improves overall linearity, achieves some "protection" and enhances stability of the drive input being a transmitter, transceiver and Q2 gate. Use proper thermal grease and isolator. I used an old P3 heat sink, which work just fine. I mounted a Pentium 3 heatsink on the back of the alu-casing.
A square space is cut out of the back of the alu-box to allow Q2 to be screwed onto the heatsink. The heatsink is firmly mounted on the back of the chassis with thermal grease allowing the chassis as extra cooling surface. Construction considerations HAMs that are experienced in constructing RF projects will know a number of possibilities to create a good RF design.
This works when short connections are used. You can however solder them directly to the PCB. Always start with the lowest band and set the capacitors to maximum output and work are way up from there. Also set your transceiver to the middle of the each band segment.. One thing on the trimmer capacitors Ct1x and Ct2x. Do not use plastic trimmers, they will melt and perhaps burn through causing shortening and possible failure of Q2 and who knows what else.
Please use air- or ceramic based trimmers. If you do not have them, then the only way tweaking the amplifier is by trial-and-error, C16x and C17x. Use a choke or a snap-on ferrite bead at the point where the Vcc wires leave the alu-box.
Try to limit the maximum current to 2 Amp. I placed a 1 Ohm resistor R12 in series to buffer a minimum the maximal current and peak at power up. Grounding To prevent ground loops, spurious oscillations etc.
Arduino High-Current Interfacing – Transistors & MOSFETs
I have had a suitable box laying around for quite some time that was perfect for the amplifier project. I decided to go for Manhattan style construction using mainly the parts I already had in my junk-box and not order the PCBs and toroid set which are available from different sources on Ebay. In other words: a low-cost project. The first I did was to make room for the IRF mosfets and two heat sinks.
IRF520 - IRF520 N-Channel MOSFET Transistor
See notes below The IRF is not fully 5V logic compatible but it will work fine for many applications if it is derated to the specs we show here. See our evaluation results down below for more detail on that aspect of the module. These modules can be used to control motors, fans, LEDs and other devices. Module Connections The connectors can look a bit confusing at first, but hook-up is fairly straight forward. The attached schematic can help clarify things.
IRF520 MOSFET Treiber Modul