amp, use a three amp input fuse. This is because you will
need more than twice the input current. The input diode
(D3) is optional and is used to prevent damage if the input
power is accidentally reversed. It will reduce the efficiency
of the circuit. Make it about 10 times the expected output
current with a PIV (Peak Inverse Voltage) of at least twice
the maximum input voltage.
The major loss of power in the circuit is in the bridge
rectifier. There is about 1.4 volts dropped across it. With
an input voltage of 14 volts, at least 10% is lost here. With
only one amp of current, it will have to dissipate 1.4 watts
of power. For better heat control and efficiency, I recommend well oversized and separate diodes of 10-15 amps
■ FIGURE 5. The analog constant current circuit is quite
simple. A heatsink for the power MOSFET is required.
For continuous use, R9 and Q2 can be omitted and
R6 replaced with a wire.
■ FIGURE 2. The basic H-bridge circuit. The power
MOSFETS act as the DPDT switch. The control chips
provide the proper gate voltages; the timer and inverter
create the clock signals; and the voltage regulator sets
a fixed voltage for the low voltage components.
each with a heatsink (see Parts List). Using lower current
diodes causes considerable heating which further increases
the forward voltage drop. This causes even more heating
and greater forward voltage drop, and so forth and so on.
Losses can more than double if too small diodes are used.
(Note that I only used a two amp bridge because I have a
specific application that uses little power.)
■ FIGURE 3. The voltage doubler circuit connects to the basic
H-bridge (Figure 2) at four points. The output is close to a
DC signal but switching glitches will be present. The optional
filter (L2 and C3-C5) reduces the noise to about 200 mV.
■ FIGURE 4. You can use a transformer to generate
almost any voltage you want. The square wave input will
require considerable output filtering, depending
on the application. The transformer type and
switching speed also have significant effects.
February 2009 57