# 7805 Current constant for battery charger

I used to suggests a lead-acid batteries charger circuit with current constant, which applied many transistors, can use as well.

But several people may be not like them because doesn’t convenient ,too many device

It is advantage that we have IC-LM7805- 5-volts 1A positive voltage regulator, can build also the current regulation.

It is very convenient, since a one man show with a little parts. So into easily the battery charger circuit.

__I use a digital multimeter because high accurate, and use easy. It uses the 9-volts battery as main energy, when we often apply it must change so very consumption.

I so use rechargeable battery of 9 volts (8.4V) It is rechargeable of over than 200 times so worth,
But how to charging Therefore I think this circuit.

Idea circuit

Firstly, have to know the propertied battery before, as Figure 1, have more detail as follows:
1. It is a Ni-MH type have feature : not memory charging, makes don’t wait, use full end so rechargeable, high current, cheap, and popular for now.

Figure 1 the Ni-MH rechargeable battery of 8.4V

2. A output voltage around 8.4V and provided of 200mAh (applied current of 200mA within 1 hour.)
– however, just voltage only 8.4V but can replace normally 9-volt normal battery. Because it can supply current constant to enough to use on long time.
3. the label on it, charging is divided of 2 types.
-the standard charging with 20 mA for 10 to 15 hour.
-the fast charging with current of the 50 mA for 5 hours.

When we know enough let’s to see the current constant circuit or current regulator by IC-7805 better, as Figure 2 is basic circuit, in manual, will see that a resistor-R1 is connected to output and controller the output current.

Figure 2__Basic current constant or current regulator using LM7805

How to find R1-value.

By can find the current constant with simple formula:
IO = 5V / R1

Defined

IO = the current constant output
5V = Voltage of IC-7805
R1 = Resistance-R1

In this case, I want to supply the current constant of 50 mA or 0.05A. so try some new, to find out the R1 as follows:

R1 = 5V/0.05A
= 100 ohms

Figure 2 basic current constant of IC-7805

Practical applications circuit

When get the concept circuit then, test the circuit as this idea. Appear good results, should has a LED to check the connection terminal of battery

And the power to the battery, should is DC voltage, so new designed as Figure 3 when test circuit until worked well, use charges time for about 5 hour.

– Measure the charged current around 70mA, too many current rises, because some current flow trough LED1 and R2. Which battery don’t hot.

Figure 3 the real circuit of current constant battery charger by LM7805

For others device, You will well familiar F1,T1,D1-D4,C1 and C2, include is a DC power supply set. The voltage across C1 of 16V and supply it approximately 0.5A. Because of happened to I has 0.5 A transformer fitted.

Have you ever wondered why the LED is connected to the battery.

But disconnect the battery out. LED turn off.
– When try to measure the voltage drop across R1, on without charging, will none voltage at all.

But when connects the battery charging appear voltage drop across R1 will always has the 5V, even if we use any rechargeable battery type or short-Circuit.

___this feature, you also understand it like me, when have the voltage constant and resistance not change, so the current flowing through R1 also must stable. And current flow the output also same them.

How to builds and application.
__For this current constant battery charger, you make the PCB as copper-layout in Figure 4 and solder all parts on components-layout as Figure 5

Figure 4 the Actual-size,Single-sided PCB layout

Figure 5 the components-layout

__R1, Should be calculated to fit the battery and if current over than 0.3A should hold heatsink with IC1.

__Figure 6 is the current constant battery charger using LM7805
The prototypes I use the universal PCB because little devices and easy to build.

Figure 6 Application this projects

However, This circuit have disadvantage is without timer when finished for 5 hour then cut off power from battery, to protected battery to overheated.

Components list
1. IC1___LM7805
2. D1-D5___1N4007____1A 1000V Diode
3. LED1__LED as you need.
4.R1____100 ohms resistor 0.5 watts
5.R2____180 ohms “_________”
6.C1____1,000 uF 25V___Electrolytic capacitor
7.C3____100uF 25V______”________”
8. C2,C4__0.01uF 100V___Mylar Capacitor
9. T1__12V 0.5A transformer
10. Other parts,fuse,pcb,wires,etc.