Download Power Supply for non Microcontroller Based Test Fixture

Power Supply for a Non-Microcontroller-Based Text Fixture
How do you supply power to test fixtures you design? Raw power from a basic
power supply works but survivability is a problem when we test an assembly that draws
excessive current. Wall-Wart power supplies are cheap but voltage regulation is usually
not included and they self-destruct if too much current is drawn. Bench grade power
supplies are expensive but certainly meet quality capabilities. If we are building a test
fixture based around a microcontroller or microprocessor we may want to be able to
control the power supply and monitor its operation also.
.
IC voltage regulators available at a cheap price over the counter are okay if your
needs fit their capability. Often they don’t. Often they are not on hand because they are
not part of a gaming device. If you have to build your own test fixtures one goal is to
design them around parts already in stock, or have a price that doesn’t kill your inventory
expenses.
A previous article suggested a power supply for a microprocessor or
microcontroller based test fixture. Here is a suggestion for one that does not require
connection to a controller.
Desired features
> Output Voltage is easily adjustable by the selection of a resistor.
> Output current can be monitored for over-current condition.
> Output monitored for being on-track, too high or too low.
> Made of easily and cheaply available parts.
> Power Control and Current Regulator circuits should be on the hot side of UUT power
so they do not degrade voltage regulation.
Objective
The objective is a power supply that will work on its own to supply a voltage to a
test fixture with a limited safe current. It should withstand a direct short on the output.
Since it will be connected to an assembly we expect to have a problem reliability and
survivability is a must. Parameters (Output Voltage and Current) should be set by resistor
selection.
Precision is not an objective. All parts are 5% rated. For higher precision of
design 1% values can be used.
VIN vs V OUT
VIN must be selected to be V OUT plus about six Volts or more. We can expect
to lose about 3 Volts across R1 and Q2 (Voltage Regulator). +V IN can be supplied by a
Wall-Wart power supply as long as voltage and current capability is beyond safe limits.
This circuit provides safe current limiting.
OC, indicator
An LED indicates when the power supply has popped into shutdown due to
excessive current.
HIGH, LOW and RIGHT ON indicators
These LEDs and the circuit are an option to the power supply. The power supply
will run the same without this circuit. The output is also monitored by an LED for an
easy visual indication. HIGH, LOW, RIGHT ON indicators provide a more precise
indication.
Circuits and design considerations
U1 circuit
U1 is a basic Voltage Comparator whose output turns Q4 on or off. By selecting
R5 we define the current level we will call excessive current. When the voltage across R5
exceeds the voltage across D9 U1’s output goes high turning off Q4. For instance if we
want to set the current limit to 250 mA we can put the voltage across R5 to be about 600
mV if D9 is a silicon diode (1N914 or 1N4148). As an option we can change D9 to a
Germanium or Schottky diode or even a Zener or any other voltage reference.
Q5
Q5 turns on when the output of U2 goes high indicating an over-current condition.
Also in this circuit is C3. Together with R12 C3 sets the minimum signal spike the power
supply will respond to. Under manual operation R12 times C3 (T = R x C) makes our
over-current trigger a Fast-Blow or Slow-Blow. Under software control we have the
option of turning the power supply off earlier than that set by R12 times C3.
Q3, Q4 and U4
U4 is a programmable voltage regulator used to set the output voltage of the
power supply. Normally the output is 2.5 Volts. This 2.5 Volts across R16 sets a current
value. This same current flows through R15. The output voltage of the power supply will
be the voltage across R15, plus R16, plus about 1.2 V across the Base - Emitter of Q3.
Q4 turns Q3 on and off. When Q4 is on current is supplied to the Base of Q3 as
well as U4. When Q4 turns off we remove Base current to Q3.
U3 Window Comparator
U3 is a dual Voltage Comparator that monitors the output voltage of the power
supply. R18, R19 and R20 set up a voltage divider that determines the points we will call
too high and too low. The outputs of U3 are digital open-collector design. U3A output is
Low as long as V OUT is below the maximum value and goes High when V OUT
exceeds the maximum set value. U3B output is low as long as V OUT is above the
minimum set value and goes high if V OUT goes below the minimum set value. If neither
output is low Q6 is turned on indicating that the output is within nominal range.
LED D1 gives us a visual indication of the output voltage but only tells us we do
or do not have a voltage. It does not really tell us much about specifically what voltage
we have out. U3 gives a more precise answer.
Limitations
Q3 has operational characteristics of about 100 Volts and 3 Amps. Operation
around 25 Volts should be well within safe parameters. Operation close to 1 Amp might
require heat sinks on Q3. The intent was for a design that provided around +12 or +13
Volts at around 300 mA to 400 mA to test peripheral boards for IGT or Williams games.
The design gives flexibility beyond these parameters.
Parts List
Q1 – TIP127, PNP Darlington, 100 V, 3 A, Gain = 1,000, TO-220
Q2, Q5 – 2N3904, general Purpose NPN, TO-92. Most any NPN will work.
Q3 – TIP122, NPN Darlington, 100 V, 3 A, Gain = 1,000, TO-220
Q4 – 2N3906, general Purpose PNP, TO-92. Most any NPN will work.
U1 – LF351N, general purpose op amp. Most any will work.
U2 – LM311N, Voltage Comparator. Most will work okay.
U3 – LM393N, Dual Voltage Comparator. Most will work okay.
U4 – TL431, Programmable voltage reference. TO-92 case.
D1 – LED. General purpose. Use what you have on hand.
C1 – 1,000 μF (470 μ F or higher. Voltage rating must be 50% higher than V IN.)
C2 – 10 μ F (1 μ to 22 μ will work okay.) Voltage should be 50% higher that V OUT.
C3 – 470 nF gives a spike limit of 1 ms. Making this higher sets the speed of our overcurrent shutdown and makes our circuit slow-blow or fast-blow. Or set it high and use
software control.
R1, R3 – 1,000 Ohm (all resistors are ¼ Watt 5% unless stated otherwise)
R2, R4, R13 – 4,700 Ohm
R5 – 2 Ohm sets current limit at 250 mA. Watch wattage requirements.
R6, R7, R21, R22, R23 – 10K
R8, R9 – 51K
R10, R12 – 2,000 Ohm
R11 – (selected to set current limit)
R14 – 470 Ohm
R15 – 1,200 Ohm (select for output voltage)
R16 – 240 Ohm
R17 – 1,000 Ohm (for an output voltage of 5 V to 13 V. 2K for higher voltage)
R18 – (selected)
R19 – (selected)
R20 – (selected)
These parts alone might cost about $5.00 total if you had to buy them. This makes the
circuit pretty cheap if you don’t consider your time. The circuits may be broken up and
built on separate boards if you prefer.
If you are not planning on connecting it to a microcontroller the circuit may be
simplified.
Other items
Circuit board (as desired)
Case (optional)
Connectors (optional)
Building suggestions
Build the circuit in stages and test each stage as you go along. test mount the parts
before soldering them in to see if they will all fit on the board. The design is simple and
presents no unusual layout considerations. If you use connectors set the connectors in
place where you want them then lay out major parts accordingly to the associated
connectors. Use sockets for U1, U2 and U3. If output current is over 400 mA you might
want to use a heat sink on Q1 and Q3. If V IN is considerably higher than V OUT watch
power limits of Q1 and Q3 and consider a heat sink.
Using the suggested parts none are overly sensitive to static electricity. Use casual
precautions.
Other alternatives.
LM317T
Easy to set the output voltage but not too easy to set the current limitation.
LM7805T
A good choice if you need +5 Volts at 1 Amp. It is also available in smaller
packages than the TO-220. The To-220 is rated at 1.5 Amps with a good heat sink. Or
lower current without a heat sink. Smaller devices are rated at lower current and lower
input voltages.
Variations in the part number indicate these different capabilities.
LM7812T
A 12 Volt version of the LM7805T.
Other regulators have an On / Off control. These are well worth considering.
Higher current devices are available in switching regulators and are available in gaming
devices.