How Does It Work - Ramsey Electronics FM-100 Assembly And Instruction Manual

HOW DOES IT WORK?

Here is where we get into a little circuit analysis. If you just want to plug it in and
start broadcasting, you can skip this section and read the FCC regulations
section. Otherwise there is some insight into why and how this FM-100 works.
We will use the schematic located at the center of the manual to analyze the
circuit.
We will begin with the power supply since it is the most common circuit we will
see. When you plug a working unit in, 110 VAC is applied across the primary
winding of the transformer T1. This in turn induces 10 VAC across each
secondary winding. The transformer included in your kit is rated for 500mA per
winding, but we have connected two of them in parallel, allowing us to get 1
Amp at 10 VAC. This current is necessary when the high power module is
added into the kit. The module draws a little more than 500mA when
transmitting, which left no more current for the rest of the kit. The displays draw
10mA per segment, and there are a maximum of 29 of them lit at any time. That
is another 290mA, making for a total of 790mA, not including the rest of the
circuitry. As you can see, this project requires a little power to get on the air, so
a substantial power supply is required for good operation.
The output AC of the transformer is fed to a bridge rectifier, where it is
converted to about 12 VDC, and then smoothed out with C86 and C52, both
3300uF capacitors. These big capacitors were required to reduce ripple in the
power supply to a minimum and therefore reducing 120Hz noise in the audio of
the project.
To get a negative supply for the opamps, we make a negative voltage multiplier
which taps some of the AC from the transformer. C89 passes AC, but not DC,
allowing D7 and D6 to rectify the AC through the capacitor to a negative
voltage. C55 then smoothes out the ripple.
Regulators are then used to completely smooth out any remaining ripple and
other noises present on the power supply. The +12 volts is regulated to +5 volts
by VR3, the +5 volts is then used for digital circuitry and some of the audio
circuitry. The +12 volts is also regulated to +8 volts for most of the audio
circuitry, and the major source of power for the high power module. The -12
volts is regulated to -8 volts for just the audio circuitry.
Now we will get on to the display circuit, and why we designed it this way.
Keeping low noise in mind, we wanted a display that produced as little noise as
possible. We determined that an LED display was our best choice for visibility,
but we couldn't use display drivers that multiplexed. Multiplexing means that all
of the digits are never on at the same time, but each are turned on in turn at a
high enough rate so that to your eye it looks as if all are lit at the same time.
The switching between the displays produced more noise than we wanted to
have to deal with, so we chose a method where all the displays are constantly
FM-100 Page 5