New Additions
x86 Instruction Chart
8087 FPU Instruction Chart
9 Volt Battery Chargers
Solid State Circuit Breaker (20V 650mA)
Spice BJT Model Viewer
Ohm's Law Calculator
Prime Factorization
Convert a Decimal Fraction to a Simple Fraction
Example POV-Ray Animation
Example Virtual Machine
Locating a Gunshot with an Array of Microphones
Software Defined Radio
Salt Flats & Aeromods
Audio Amp + Spice References
Image Processing
KFC 'Secret' Recipe
Neddy's Brownies
Ham and Cheese Muffins
Railroad Tracks
The Bike Shed
The Guy That Invented Fire
Mods for the TRS-80 Color Computer

9 Volt Battery Chargers

9V Battery Charger from 2 NPN Transistors

Schematic of 11mA Constant Current 9V Battery Charger from 2 NPN Transistors

9V Battery Constant Current
Charger on Breadboard

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.


15V Supply 10.05 11.32 NG 11.32 11.31
19V Supply 11.42 11.43 NG 11.44 11.43
Readings in mA.

Note the highlighted cell where the charger could not supply enough current to charge battery 'A' when using the 15V supply.

Back Next

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

Sparks and Flames


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