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ELM380P

Battery Charger Timer

Elm Electronics 

ELM380

Example Application

Figure 1 shows the ELM380 used in a typical

battery charger control circuit.

Power for this circuit is obtained from a 12V supply,

as shown. Usually there will be a voltage of about this

level available if the charger is to support 9V batteries.

The 12V has been reduced to 5V through a 78L05 low

power regulator. This is a relatively inexpensive and

convenient way to obtain a stable voltage for a circuit

such as this.

The required 120Hz for the clock input is obtained

from the full-wave bridge as shown below. A series

diode has been added to prevent back-feed from the

filter capacitors (which would put a constant high level

on the clock input), and a 47KΩ resistor is connected in

series with pin 5 to limit the current through the

protection diodes when the AC bridge voltage exceeds

the ELM380 supply levels.

After the peak of each sine wave, the series diode,

and for a time, the bridge diodes, will not be conducting.

This means that the clock input will be left floating,

which is not advisable with CMOS circuits. To provide a

+12V

Charger

Enable

12V Relay

2N3904

1N4001

2.2KΩ

+5V

1

2

3

4

ground reference during these ‘open circuit’ periods, the

100KΩ resistor was connected from this point to VSS.

The rest of the circuit is straightforward. Two

switches are provided for control. Both the momentary

action ‘start’ switch and the toggle type ‘8/14’ switch

have 300Ω series resistors for added ESD protection,

and rely on the internal pullup resistors to provide a full

logic swing when operated. The 300Ω resistors aren’t

strictly required, but are a nice addition to consider for

added protection.

Only one of the outputs is used for this circuit. Its

active high level is used to drive the NPN transistor into

conduction and energize the relay coil. Depending on

the circuit, a direct connection to the load might be

used, but the use of a relay allows for general control of

many different loads.

There are many other uses that this circuit could be

put to. It could actually be used for almost any

application that needs long time intervals… Lawn

watering controls… Auto-off lighting systems…

300Ω

8

7

300Ω

6

5

47KΩ

100KΩ

start

8/14

open for 8 hrs

closed for 14

10VAC

60Hz

Supply

+12V

+5V

+

150µF

0.1µF

78L05

0.1µF

ELM380DSB

Figure 1. Typical Charger Control Circuit

Elm Electronics – Circuits for the Hobbyist

< http://www.elmelectronics.com/ >

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