The Sangean HDR-1 is a HD tabletop radio which supports digital radio & hybrid signals and RDS functions on FM stations. Other features include dual alarm, IR remote control, multicast capability and Program Associated Data service. Overall, a pretty nice unit. But certain productions runs of this radio had a problem. The radio would quit powering up for no apparent reason. Lets see why.
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A RF dummy load is quite useful when working on transmitters. It allows you to test and adjust the transmitter without an antenna, eliminating interference to other radios on your test frequency. It also presents your transmitter with a proper 50 Ω load so as to not cause any damage to its final RF amplifier stage.
A recent project required me to modify and align twelve UHF transmitters. The transmitters had a 25 watt output and the alignment session on each would be short. Rather than buy a dummy load for this project, I decided to build my own.
The central part of the dummy load is a resistor (or resistors) with a total resistance of 50 Ω and a wattage equal to or greater than your transmitter. The resistors also must be non-inductive which eliminates all the common wire-wound power resistors. Acceptable types of resistors include carbon composition and thick film.
One of the most frequently misunderstood concepts when introducing people to the fundamental aspects of technology are those of voltage and current. While many may look at a wall wart and see that they are provided a voltage of 12V at a current of 0.5A what do those numbers actually represent? In short: it’s all a matter of electrical charge.
Electrical charge is measured in coulombs (C) and represents the electrical charge exerted by 6.25151×1018 protons. Alternately it can be represented by the electrical charge exerted by -6.25151×1018electrons.
Voltage is a measure of the amount of work energy in Joules (J) required to move a unit of electrical charge (C) between the two points – mathematically V = J/C. Therefore voltage can be considered the electrical potential difference between any two points: the two poles of an electrical outlet, the Earth and a rain cloud, or even you and your friend. This is why when you connect the negative pole voltage of one voltage source to the positive pole of a second voltage source (like ganging two AA batteries in your remote control) the total voltages are added: you’ve essentially increased the amount of work required to move an electrical charge from the positive pole of the first source to the negative pole of the second source.
Current, on the other hand, is a measure of the rate of flow of electrical charge (C) through two points- mathematically I = C/s. This explains the old saying that “It’s not the voltage that kills you. It’s the current.” You can safely keep your hand on an operational Van de Graff generator operating at thousands of volts but minimal current. However, mishandling a car battery operating at a paltry 12 volts but delivering a whopping 600 Amps of current can hurt or kill you. Simply put: charge measures the rate at which electrical charge moves between two points of differing voltage potential, and too much electrical charge passing through an electrically incapable (organic) medium will cause a very bad day for said material. As for which way current actually flows? Well, that’s a subject for another time.
Consequently this explains the fundamental equation that every person dawdling in the electrical sciences needs to know: V = I x R otherwise known as Ohms law. This inverse proportionality shows that as current passes through a resistive load (R) it essentially slows down thus bunching up the electrical charge on the entry portion of the load and creating a greater electrical potential (voltage) across the terminals of the resistive element. Alternately, as the resistive load (R) is decreased the amount of the rate of electrical charge (current) drawn upon to maintain a requested electrical potential difference (voltage) is increased. This is why we don’t like shorts in our electrical circuits. R effectively becomes 0 resulting in a request for infinite current likely resulting in a blown power supply, tripped breaker, or blown fuse.
Dangerous Prototypes, the people who have brought us such great open source hardware such as the Bus Pirate and the Flash Destroyer, have a little give-a-way going on every Sunday. Simply be one of the first people to comment on their Free PCB Sunday posts and get a free printed circuit board from one of their past, current or future projects. Past free PCBs have included nearly every Bus Pirate version, the Flash Destroyer, the USB RGB Color Changer, Web Platform and others. Don’t forget to check out their current projects while you are there!