The U257B series are driving circuits for a 5 or 10 LED bar display having a integrated current generator whoes current is adjusted to 20mA.
Light emitting diodes are connected in series in order to reduce power dissipation and to have the same current flow to the supply in every operating condition.
For red LED's the supply voltage is 12 volts wheras in case of green or yellow diodes the voltage is 16 volts.
