resistors which imply considerable heat losses. Proper
heatsinking is necessary. Nevertheless, these devices allow
you to control high power quite easily.
Figure 5 shows a simple constant current power supply.
The theory of operation is very basic. A small voltage is
developed across the 0.01 ohm sense resistor (R8) and is fed
into the inverting input of the op-amp. This positive voltage
is inverted by the op-amp, reducing the output voltage which
reduces the voltage across the sense resistor via the MOSFET.
This stabilizes the output voltage to the value found at the
non-inverting input. Any change in current through the
sense resistor causes a voltage change at the inverting input
which is exactly offset by the negative feedback. The result
is a constant current through the sense resistor and load.
Determining the reference voltage to be applied to
Capacitors the non-inverting input is just an application of Ohm's Law.
❑ C1 0.1 μF 25 volts
If there is one amp of current flowing through the 0.01
ohm sense resistor, there will be 0.01 volts across it. So,
the voltage-to-current ratio is 1:100 or 0.01 volts per amp.
The reference circuit is pretty straightforward. We want
a stable and adjustable voltage from 0.0 volts to about
0.500 volts. I used an LM336-5 five volt reference because
that is what I had on hand. Then, I reduced voltage with a
Misc: Heatsink for Q1
resistor network and used a 10 turn trimmer for fine adjustment. Other methods of generating the reference
voltage can be used, as well. (You can substitute
BASIC H-BRIDGE PARTS LIST
an analog signal for the reference voltage and
get a constant current power amplifier.)
Resistors (1/4W, 5% unless specified)
There are a few considerations for this
❑ R1 1.5K
❑ R2 200
circuit. The first is that separate power supplies
❑ R3 1K should be used for the load and circuit power.
❑ R4 20K This is because it's very easy to cause
significant power supply variations with 20
amps of current being drawn. If you choose to
use a different op-amp, be sure it can operate
with inputs very close to ground. The breadboard circuit is shown in Photo 5. The heatsink
shown (a requirement) is rated at 13 watts.
You may need a larger one for continuous use
at 20 amps, depending on the load resistance.
Remember that the MOSFET is acting like a
resistor in this circuit. Depending on the load,
its resistance can be relatively high or low and
its power dissipation can be correspondingly
high or low. Do the math to be sure. Note that
at 20 amps, the five watt sense resistor (R8)
dissipates four watts. If you plan on running this
continuously at high current, you may want to
increase R8 to seven or 10 watts.
Because of the very low sense resistance,
ordinary circuit resistance can increase this
significantly. (My circuit resistance was 30
milliohms.) The primary concern is that the
voltage at the non-inverting input will be higher
than expected (one amp per 13 mV rather than
10 mV). In most cases, this is not a problem for
the proper operation of the circuit.
Constant Current Supply
Power MOSFETs can be used in linear applications, as
well. In such cases, they are typically treated as variable
SUPPLY PARTS LIST
Resistors 1/4, 5% unless specified.
❑ R1, R9 5.6K
❑ R2 100K
❑ R3 10K 10-turn trimmer
❑ R4, R5, R6 10K
❑ R7 1K
❑ R8 0.01Ω5W (Mouser #588-15FR010E)
❑ Q1 IRF540 Power MOSFET (Jameco)
❑ Q2 2N3940 NPN transistor
❑ U1 LMC6482 Op-amp
❑ U2 LM336-5 Five-volt reference
Capacitors (25V unless specified)
❑ C1 100 μF
❑ C2, C3 0.1 μF
❑ C4 10 μF
❑ C5 0.01 μF
❑ C6 0.001 μF
❑ C7, C8 0.1 μF 150V
❑ D1, D2 1N4004
❑ D3 Optional, see text
❑ Q1-Q4 IRF540 Power MOSFET (Jameco 210518)
❑ U1 LM317 adjustable voltage regulator
❑ U2 555 timer
❑ U3 CD4069 CMOS hex inverter
❑ U4, U5 LM5109B half-bridge driver (Digi-Key)
Slow-blow fuse (see text)
Metal preferred for safety and EMI reduction (see text)
Additional parts for H-bridge Voltage Doubler (see text)
❑ C1*, C2* 1,500 μF 63 volt switching capacitor
❑ D1-D4 15 amp rectifier (Jameco #879318)
❑ C3 100 μF 150 volts (optional, see text)
❑ C4, C5 0.1 μF 150 volts (optional, see text)
Critical part, safety related, see text.