Very well written Charles. , Jim (wd4air)
>
> K8JHR wrote:
>
> >I would like to add a pilot light to the front panel of my TenTec
> >RX-320D. I have SOME home brewing skills... having built several
> >kits, including a few TT receiver kits, and having designed some
> >small project circuits on my own. My experience suggests doing this
> >is not as easy as merely soldering a LED into the circuit, and
> >sticking it through a hole in the front panel.
>
> You need a power supply, an LED, and a current limiting
> resistor. Typically, you use a push-in plastic bezel to mount the
> LED to the panel.
>
> The schematic shows that a nominal 15 V enters the radio, is filtered
> with an RF choke and a 100 uF capacitor, and is available as the
> internal 15 V supply. I'd mount the current-limiting resistor on the
> PCB near C7 (one end to the +15 V PCB trace with a very short lead,
> one end floating). A wire (22-24 ga. stranded, insulated hook-up
> wire) would run from the floating end of the resistor to the anode of
> the LED (the one with the longer lead and without the flat on the
> diode body). Another wire (also 22-24 ga. stranded, insulated
> hook-up wire) would run from the LED cathode (shorter lead, flat on
> diode body) back to the PCB to ground (also preferably somewhere near
> C7). If you want to get fancy, make the anode wire red and the
> cathode wire black. Doing it this way (resistor on the PCB +15 V
> trace), if something goes wrong with the wiring you won't
> short-circuit the +15 V supply.
>
> LEDs have forward drops between 1.5 and 3 V, depending on technology
> and color. Assume ~2 V, so the resistor will have ~13 V across
> it. Since I = E/R, a 3.3k resistor will regulate the diode current
> to ~4 mA. Since P = EI, the resistor will dissipate ~0.05 W (50
> mW). So, a 1/4 W resistor is sufficient.
>
> Use appropriately-sized heatshrink tubing at the resistor (run it
> right down to the PCB, completely covering the resistor) and on each
> LED lead (again, run it right to the body of the LED). Route the
> wires neatly and use tie wraps as appropriate.
>
> Of course, do all the work with the radio off, and check it visually
> for correct polarity and for solder bridges or other shorts before
> you turn it on.
>
> Note that there are several regulated voltages in the 320 (+5
> digital, +5 analog, and +10). I did not suggest using them for two
> reasons: first, the lower voltages increase the uncertainty of the
> voltage across the resistor, and thus the uncertainty of the LED
> current. And second, there is no need to add a further current
> burden (with increased power dissipation) to the regulators. If you
> measure the drop of the particular diode you use, you can calculate
> the resistor for the desired current with the lower voltage, and such
> a small additional load will probably not adversely affect the
> regulator, so you could probably use a regulated voltage if you
> prefer. If you do, I'd use the +5 V digital supply.
>
> Best regards,
>
> Charles
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