Download Troubleshooting Power Supply and Voltage Regulator Sections

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Troubleshooting Power Supply and Voltage Regulator Sections
Whether it is a power supply in a monitor, computer, ticket printer, bill acceptor
or what ever there are only so many different design to power supplies. Once you
understand how they work troubleshooting them is mostly a matter of realizing what is
not working right.
Basic power supplies
Linear Regulators, Series type
Switching Regulators
Boost regulators
Buck Regulators
Shunt Regulators
This is the method used when the current required is relatively low. If the circuit
is just generating a reference voltage or regulating the voltage to one or two ICs and
current is in the range of tens on milli-Amps shunt regulators are typically found. Zener
diodes are typically used but these are not the only option. At lower voltages we may use
Stabistors or just about any device that has a predictable voltage drop that fits our needs.
If you have done the diode exercises you should realize that Zener diodes need a
level of current in the milli-Amp range to regulate properly. In line powered circuits this
isn’t a problem. In battery powered circuits milli-Amps are precious and regulators that
work on lower currents are needed. For these applications there are voltage reference IC
that work down into the micro-Amps. LM431 is such a device we see often.
In diagram 0106042701 we show a few examples of shunt regulator circuits.
042701A shows a Zener Diode used to generate a +12 Volt power source to run an
amplifier. If the current we are going to regulate is draws 5 mA the current through the
Zener must be about four times that amount, or 20 mA. The total current through R1 then
is the total of these, or 25 mA.
With the circuit powered from a 16 Volts source we need to drop the difference
between the applied voltage and our target regulating voltage across R1. (16 V – 12 V = 4
V). The value of R1 can then be calculated as 4 V / 25 mA, or 160 Ohms. Even if R1 is
burned up we can easily calculate what value it should be. No precision required here. All
we have to be is in the general range and the circuit will work.
As the circuit operates the load current U1 draws varies between maybe 1 mA and
10 mA. If the Zener diode is operating properly the voltage being delivered to U1 should
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stay constant. The current through D1 adjusts itself to keep the output at a steady voltage.
It may vary a few milli-Volts as current through D1 changes.
In the Zener diode exercise we found that we can tailor the operation of the Zener
diode’s voltage by adjusting the current through the Zener. If we have a 5% rated device
we can adjust the voltage across the Zener to a voltage within that range by controlling
the current (changing the value of R1). Keeping this in mind if the easily adjust the
circuit’s operation to within 1% tolerance with ease.
In diagram 042701B we are using the Zener diode to generate a very low current
use as a reference voltage only. The circuit compares the voltage on the battery against
our reference voltage to sense when the battery voltage is too low. The battery voltage is
applied to the Inverting input of the Voltage Comparator. The Non-Inverting side of the
Voltage Comparator is tied to a Voltage Divider circuit connected to our reference
voltage. Our Zener regulates at 6.2 Volts. Since both resistors are the same value the
voltage at the non-inverting input will be half that voltage, or 3.1 Volts. When the voltage
coming out of our 3.6 Volt battery drops to less than 3.1 Volts the output of the voltage
comparator goes high telling the rest of the circuit the battery voltage is low.
We select this trigger point to be above the value
Hints when changing
where the circuit powered by the battery will stop
batteries
performing properly. If it powers a SRAM chip that
Temporarily clip a
starts malfunctioning around 2.7 Volts we want our
good battery into the
circuit to warn us the battery is getting low well before it
circuit before removing
drops to that 2.7 Volt level. We can then change our
the old battery. That way
battery without having our bookkeeping corrupted.
the circuit never loses
The disadvantage of our battery test circuit as
power and our
shown is that in order to constantly test our battery we
bookkeeping stays intact.
must constantly draw current from it. Even if our test
In some boards this
circuit only draws micro-Amps this is about the same
standby current our SRAM draws. To get around this our feature is designed into
the board.
battery test circuit will often have a transistor between
the battery and the test circuit that only connects our test
circuit to the battery when we actually want to measure the battery.
Voltage reference chips like the LM431 can run reliably across a broader
operating range that a Zener Diode. Some may operate within 1% or better over a range
of micro-Amps to 100 mA. If you did the diode exercise you noted that a Zener diode
may change 20% of its value over such a range if it even capable of such an operating
range. 5% rated parts are only 5% within a narrow range of operating current.
The LM431, or similar devices, have a set output voltage. The output is regulated
between the Reference pin and ground. If that voltage is what we want we connect the
Reference pin to the Cathode for operation, see drawing 042701C. If we want to regulate
at a higher voltage we simply set the reference point higher by inserting a resistor
between the reference input and the Cathode. Figure 042701D shows such an example.
Our reference voltage is dropped across R9. Setting the current through R9 controls the
current through R8 also. We select the value of R8 so that the voltage drop, added to our
reference voltage, brings our operating voltage up to the level we want.
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