**Variable Voltage Regulators**

**29 Jan 05** -
Heatsink requirements moved to a separate page.

This design is intended to produce a variable, regulated, voltage between
1.2V and 37V. The LM 317T regulator is guaranteed for a minimum output current
of 1.5A. The design is quite simple, the only complication being the working out
of the resistor values for R_{B}.

This design shows a switch that is used to alternate between 2 resistances,
giving 2 output voltages. If this facility is not required, R_{B1} and
the switch can be removed. The lower leg of R_{B2} (shown as the body
above) would also need to be moved up onto the negative output rail.
Alternatively, the switch and R_{B1} can be replaced by a suitable
variable resistor to give adjustment of the voltage.

__Output Voltage Calculation__

The output voltage is determined by the equation:

The error in omitting the I_{ADJ} x R_{B} value is usually
quite small (around 0.13V at a 12V output, and only 0.34V at 37V output) and it
can be therefore be ignored in most applications, which leaves:

or

In the design above shows an R_{A} value of 240R, which is the
standard value, although other values can be used.

If accuracy is really important the value of I_{ADJ} needs to taken
into account and is typically 50µA (maximum 100µA), which can be derived into
an alternate equation:

As an example I'll consider a regulator built to provide a voltage output
switchable between 5V and 12V and to provide
1.5A. It will operate in a well ventilated case, which reaches a maximum
internal temperature (T_{AMB}) of 25°C. The input voltage would need to be above the
12V
maximum and a therefore a 15V transformer is selected.

For simplicity, and at this low voltage the error is small, I'll use the
simplified equation above. For the 5V output R_{B} = 720R and for the 12V output R_{B} =
2064R

The 720R resistance can be made by putting 680R and 39R resistors in series. The
increase is resistance required is 2064R - 720R = 1344R, which can be achieved
by putting 1000R (1k) and 330R resistors in series or 2 x 2.7k resistors in
parallel (1.35k).

__Temperature Considerations__

Another thing that needs to be taken into account is ensuring that the
regulator doesn't overheat. It is fitted with an internal over temperature
cut-out, but you don't want the power dying when you trying to use the output. To determine whether you need to use a heatsink, **
click here**

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