Power Circuit board

4824. 12V adjustable regulator to 5V1 

POWER CIRCUIT

Parts list

• Voltage regulator LM317T
• Red LED 1.8V, 30mA, 5mm
• Two 1N4001 Diodes; D13 and D12(protection), 830, 0.7 1A 100V
• Two Radial Capacitors; C16 and C15: 33µF, 25V (-40 to +105degrees Celsius)
• Zener Diode; D15, 4733AP, 5V1 400mW
• Resistors R1 120R 0.25W, R2 330R 0.5W, R3 1K 0.5W
• Veroboard (copper strips)
• Wires; bare and insulated

 What it does

This power circuit is useful for a range of uses such as household appliances like; cell phone or battery charges and anything else that requires a 5V input. This set up uses a 12V input.

How it works

A 12V power supply is connected to the input red and black wires. The reverse polarity protection diode(1N4001, D13) is active-forward bias voltage is greater than 0.7V, this allows current flow in only the forward bias direction hence protecting the circuit from being connected in reverse polarity, unless the maximum reverse voltage is exceeded(100V+).

The Zener Diode(D15) is then used to protect LM317T from over voltage from the supply, the filter capacitor(C15) is added in case the use of AC is applied to the input to smooth the AC current.

Next the voltage regulator (LM317T) (IC) converts from 12VDC to 5VDC and can be adjusted down to 1.2V and up to 36V output rating by changing the resistor R2. It is capable of supplying a c­­­urrent rating of 1.5A it is also useful in a wide variety of other applications. Since the regulator is “floating” and sees only the input-to-output differential voltage, supplies of several hundred volts can be regulated as long as the maximum input to output differential is not exceeded, i.e., avoid short-circuiting the output. Also, it makes an especially simple adjustable switching regulator, a programmable output regulator, or by connecting a fixed resistor between the adjustment pin and output, the

LM17T can be used as a precision current regulator. Supplies with electronic shutdown can be achieved by clamping the adjustment terminal to ground which programs the output to 1.2V where most loads draw little current.

  Because the voltage regulator is less than 6 inches from the input filter capacitors an input bypass is not needed, but regardless of this an optional output capacitor has been added to improve transient response.

The values of resistors (R2 & R3) can be calculated using this formula: Vout = Vref( 1 +(R3/R2)

 for example use Vout = 5V and Vref = 1.25,

Then R3/R2 = 3.75

So R2 =R3/3.75

And R3= 3.75*R2

1N4001 Diode D12 is the voltage regulator protection, in parallel across the voltage regulator,

is needed to protect the LM317T output from the external 33uF capacitor it is used to prevent the capacitor from discharging through low current points into the regulator. Most 10 μF capacitors have low enough internal series resistance to deliver 20A spikes when shorted. Although the surge is short, there is enough energy to damage parts of the IC. When an output capacitor is connected to a regulator and the input is shorted, the output capacitor will discharge into the output of the regulator. The discharge current depends on the value of the capacitor, the output voltage of the regulator, and the rate of decrease of Vin. In the LM317T, this discharge path is through a large junction that is able to sustain 15A surge with no problem. This is not true of other types of positive regulators. Discharge occurs when either the input, or the output, is shorted. Internal to the LM317T is a 50Ω resistor which limits the peak discharge current. No protection is needed for output voltages of 25V or less and 10 μF capacitance.

 The red LED (cheaper than a white or blue LED also the colour in this case-red is determined by the semiconductor material, not by the colouring of the package. It is a simple indicator to show when current is flowing in the circuit. This LED as with all LED’s is used with a resistor in series the required resistor value is calculated by the formula: R = (V_S - V_L) / I 

Lastly the output can be taken as 12V across the red and black or 5V across the yellow and black wires.

 

Test Points

The points tested with an ohm meter were on either side of the drilled holes to make sure there was no continually (OL).

Diode test was used to check that the diode was operation and had OL in reverse and 0.6 to 0.7V in forward bias

A voltmeter is used to check the input (12V)-red and black the output (12V and 5V1)-black yellow and red this confirmed the circuit was complete and working well.

Faults and problems

The zener diode supplied was a 5V6 instead of the 4733AP 5V1 required this was discovered when it was tested using a 12V supply, 100R resistor in series and another in parallel, the voltmeter across the zener diode gave a reading of 5.99V. This problem was solved by changing it for the correct 4733AP and researching the correct type in documents and on the datasheet.  When connecting to the supply a short occur and a small amount of smoke was seen, the problem was solved by solder sucking and re soldering as it was noticed that in an attempt to look professional by using the min number of holes, caused shorting across some very close proximately solder.

Conclusion

Therefore the circuit operated as it was designed to and was finished within a short time frame, the soldering skill level has improved from the last semester’s circuit board as has the placement of components being placed vertically using less space and fitting neatly flush to the veroboard.