9V Battery Charger from 2 NPN Transistors
This circuit uses 2 NPN transistors to provide a constant charge rate of 11mA suitable for charging a 9V battery in 14 hours.
The LED serves 2 purposes
- It lights up when the battery is connected to the charger.
- It stops the battery from discharging when the power is turned off.
Volt and Amp Meters
The ammeter shows the charging current.The voltmeter shows the charging voltage across the battery.
A battery with a charging voltage that differs greatly from it's peers has something wrong with it.
A battery with a high charging voltage also has something wrong with it.
Open Voltage
An important measurement is the open voltage. This is the voltage reading with NO battery attached.When the charging voltage gets close to the open voltage, the charger will have difficulty supplying enough current.
Results
| A | B | C | D | E | |
| 15V Supply | 10.05 | 11.32 | NG | 11.32 | 11.31 |
| 19V Supply | 11.42 | 11.43 | NG | 11.44 | 11.43 |
Note the highlighted cell where the charger could not supply enough current to charge battery 'A' when using the 15V supply.
References
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4QD-TECH
Current sources and mirrors
http://www.4qdtec.com/csm.html
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9 Volt Battery Charger
Charge Rate
Wall Warts
Simple 1 Resistor Battery Charger
Constant Current Battery Charger from 2 LED's and a NPN Transistor
9V Battery Charger from 2 NPN Transistors
Adjustable Charge Rate Battery Charger from Op Amp
0 - 160 mA Adjustable Charge Rate Battery Charger
Test Results
741C Cheat Sheet
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