wiring and diagram

How to Connect two or more transistors in parallel

While making power electronic circuits, configuring the power output stage correctly becomes very crucial.

These stages primarily may consist of power devices like the power transistors or MOSFETs.

Using power transistors are more common due to the greater ease of  configuring them. However when higher outputs are desired, it becomes necessary to add more number of these devices together.



And as per the rules, it becomes necessary to connect them in parallel. Though using transistors in electronic circuits is pretty easy, connecting them in parallel needs some attention due to the one significant drawback with transistor characteristics.

As per transistor specs, the devices needs to be operated under reasonably cooler conditions and thats why we install heatsinks on them to maintain the above criterion. Moreover, transistors have the "bad habit" of conducting proportionately rising currents through them as they get heated up.

Therefore if its case temperature tends to increase, the current through it also increases, which in turn heats it up further. The process may get aggravated until the devices become too hot to sustain and gets permanently damaged. This situation is called thermal runaway, in transistors.

When connected in parallel, due to non-consistent characteristics, the transistors in the group may dissipate varying amounts of current through them. Consequently, the transistor which passes more current through it starts getting heated up faster and pretty soon we find the device entering into the above thermal runaway situation  damaging itself and promoting the phenomenon to the remaining devices in due course of time.

The situation can be effectively tackled by adding a small value resistor at the emitter of each transistor connected in parallel. The resistor inhibits and controls the amount of current passing through the transistors and never allows it to go to dangerous levels.

The value should be appropriately calculated, as per the magnitude of the current passing through them.

How its connected? See the figure below.

I have posted many battery charger circuits in this site, some are easy to build but less efficient, while some are too sophisticated involving complex construction steps. The one posted here is possibly the easiset with its concept and also is extremely easy to build. In fact if you had all the required material you would build it within 15 minutes of time.

The concept is indeed hugely simple and therefore pretty crude with its going. This means that though this idea is too simple, would require appropriate monitoring of the charging conditions of the battery, so that it does not get over charged or damaged.

To make this simplest battery charger circuit quickly, you would require the following bill of materials:

• One rectifier diode, 1N5402

• An incandescent bulb, having voltage rating equal to the battery which needs to be charged and current rating close to 1/10th of the battery AH.

• A transformer having voltage rating equal to twice that of the battery voltage and current twice the charging rate of the battery. That means if the battery is 12V, the transformer should be 24V, and if the AH of the battery is 7.5 then dividing this by 10 gives 750mA which becomes the recommended charging rate of the battery, multiplying this by 2 gives 1.5Amps, so this becomes the required current rating of the transformer.

After you have collected all the above material, you may simply connect the above parameters together with the help of the diagram.

The functioning of the circuit may be explained in the following manner:

When the power is switched ON, the 1N5402 diode rectifies the 24V DC to produce half wave 24V DC at the output.
Though the RMS value of this voltage may appear to be 12V, the peak voltage is still 24V, therefore it cannot be applied directly to the battery.

To blunt of this peak value, we introduce a bulb in series with the circuit. The bulb absorbs the high peak values of the voltage and provides a relatively controlled output to the battery, which becomes self regulatory through the glow of the filament intensity of the bulb (varying resistance).

The voltage and current thus automatically becomes adjusted to appropriate charging levels which becomes just suitable for the battery safe charging.

The charging of the battery can be witnessed by the gradual dimming of the bulb as the threshold charging voltage of the battery is reached.

However once the battery voltage reaches close to 14.5V, the charging must be stopped, irrespective of the bulb glow condition.