Connecting Two or More Transistors in Parallel

In this post we will learn how to safely connect two or more transistors in parallel, these can be BJTs or mosfets, we will discuss both.


Why Parallel Transistor become Necessary

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 BJTs or MOSFETs.Normally, single BJTs become sufficient for getting moderate output current, However when higher output current is required, it becomes necessary to add more number of these devices together.Therefore it becomes necessary to connect theses devices in parallel. Though using single BJTs is relatively easier, connecting them in parallel needs some attention due to the one significant drawback with transistor characteristics.

What is "Thermal Runaway" in BJTs

As per their specs, transistors (BJTs) need to be operated under reasonably cooler conditions and that's why we install heatsinks on them to maintain the above criterion.

Moreover, BJTs have a negative temperature coefficient characteristic which force them to increase their rate of conduction proportionately as their case temperature increases.

As its case temperature tends to increase, the current through the transistor also increases, which forces the device to heat up further.

The process gets into a kind of chain reaction heating the device rapidly until the device becomes too hot to sustain and gets permanently damaged. This situation is called thermal runaway, in transistors.

When two or more transistors are connected in parallel, due to their slightly differing individual characteristics (Hfe), the transistors in the group may dissipate at different rates, some a little faster and others a little slower.

Consequently, the transistor which may be conducting slightly more current through it might start getting heated up faster than the neighboring devices, and soon we may find the device entering into a thermal runaway situation  damaging itself and subsequently transferring the phenomenon to the remaining devices as well, in the process.

The situation can be effectively tackled by adding a small value resistor in series with 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 it's connected? See the figure below.

how to connect transistors in parallel

How to Calculate the Emitter Current Limiting Resistor in Parallel BJTs

It is actually very simple, and could be calculated using Ohm's Law:

R = V/I,

Where V is the supply voltage used in the circuit, and "I" could be 70% of the transistor's maximum current handling capacity.

For example let's say if you used 2N3055 for the BJT, since the max current handling capacity of the device is around 15 amps, 70% of this would be around 10.5V.

Therefore, assuming the V= 12V, then

R = 12/10.5 = 1.14 Ohms

How to Avoid Emitter Resistors in Parallel BJTs

Although the use of emitter current limiter resistors looks good and technically correct, a simpler and a smarter approach could be to mount the BJTs over a common heatsink with a lot of heatsink paste applied to their contact surfaces.

This idea will allow you to get rid of the messy wire-wound emitter resistors.

Mounting over a common heatsink will ensure quick and uniform sharing of heat and eliminating the dreaded thermal runaway situation.

Moreover since the collectors of the transistors are supposed to be in parallel and joined with each other, the use of mica isolators no longer become essential and makes things much convenient as the body of the transistors get connected in parallel through their heatsink metal itself.

It's like benefiting from both ends...transistors combine in parallel through heatsink which in turn eliminates the current hogging situation and gets rid of the bulky emitter resistors.

Connecting Mosfets in Parellel

In the above section we learned how to safely connect BJTs in parallel, when it comes to mosfets the conditions become entirely the opposite, and much in favor of these devices.

Unlike the BJTs, mosfets do not have the negative temperature coefficient problems, and therefore are free from the thermal runaway situations due to overheating.

On the contrary, these devices exhibit a positive temperature coefficient characteristics, meaning the devices begin conducting less efficiently and begin blocking current as its begins to get warmer.

Therefore while connecting mosfets in parallel you do not have to worry about anything, and you may simply go ahead hooking them up in parallel, without depending on any current limiting resistors as shown below, except the gate resistor which must be included with each mosfet separately....although this is not too critical..

20 thoughts on “Connecting Two or More Transistors in Parallel

  1. Have questions? Please feel free to post them through comments! Comments will be moderated and solved ASAP.
    • swagatam sir i am free lance electronic hobbyist too. I want to know from scratch of basic electronics to build a strong foundation for my knowledge purpose.please suggest some books or upload some tutorial so that i can learn from you .

    • Hi Kathiravan,

      I am myself planning to write a related book and sell the publication at cheap rates for the hobbyists here, presently there's no such info online which would enable a new hobbyist to grasp the facts quickly within 6 months or so, if i find any will let you know for sure.

  2. hi again mr swagatam i have another question on paralleling two or more transistors lets say collector current is 30A and total power is 200 watts! my question is does both current and power dissipation raises or just one of them does? thanxs for your time! regards

    • Hi Roger,

      Suppose we connect two transistors in parallel, each having a max collector tolerance of say 15 amps. We apply 25 amps, to them thinking that the current will be equally divided to safe 12 amps between each of them.
      However if the emitter resistor is not connected, due to a slight variation with the spec of the transistors, one of them will start passing more current, say 13 amps.
      Due to this it will start heating up more than the other transistor, this will lead to passing more current through it say about 14 amps at some instant, this will again heat it up even more so that it starts passing even more current until eventually the transistor fuses.
      When this happens the second transistor is without any support and therefore it also gets damaged within seconds.
      So its power dissipation which starts the problem and that needs to be controlled first.

  3. Hi swagatam, i appreciate all your effort in making electronics easier for electronic enthusiasts. We stil can do more, i believe that. Pls sir, what is the minimum and maximum input current that can be applied to two identical parrallelly connected transistors rated say 30A each with equal current limitter resistors, and how are they going to share the current or will the same current going to flow through them following Kirchorf's junction law? And what formular could be used to calculate the minimum and maximum input current. Thanks man.

    • Thanks!

      The current through the collector to emitter of the transistors will be unequal due to the differences in their characteristics and gain.

      The resistors connected at the emitters will not allow current more than a specified safe limit, in the process, the resistor which receives more current, becomes hotter than the other, and thus a current balance is maintained across transistors….

  4. I'm currently working on a radio transmitter and the schematic calls for a MRF947T1 Low Noise, High-Frequency Transistor. I have a bunch of old electronics I'm cannibalizing so parts aren't really that much of a problem. Is there a way I can wire up a bunch of the abundant generic Transistors to do the same job? Also, wouldn't a light bulb instead of a resistor help maintain stability more efficiently due to its ability to heat up? Just like an old fashioned oscillator circuit?

    • I don't think that would be possible because RF transmitters involve highly sensitive configurations which require critical calculations and wiring. Using parallel transistors could easily affect the parameters making the circuit ineffective, so it won't work.

      bulbs might work but the modern circuits are not designed to work with them, so it won't suit them.

  5. Hi!
    The piece of information I cannot find anywhere is what the specs should be for the emitter resistor (that is resistance and max amperage).
    In my project I have two screwdrivers with controller, transistor and motor. I want to let one controller switch both transistors, put them in parallell and so regulate both motors from one potentiometer.
    I don't have the specs for the transistors, but know of course that they can handle full load from one motor each. The motors operates on 18 V and drops at max load to 15,6 V/18Amps.
    What specs should my emitter resistors have, and how does one dimension such in the general case. I cannot find this answere anywhere, and I highly suspect that i e electric car situations would require something rather different from low voltage/power circuit boards?


    • Hi, In general the resistors must be rated such that even with a short circuit condition the resistors are able to restrict the current from exceeding the BJTs breakdown limit.

      for example if the breakdown limit of a tramsistor is 15Amp, we can consider 14 amp as the safe limit and then use Ohms law for dimesnsionng the resistor as given below, ….assuming 12V as the supply voltage:

      R = 12/14 = 0.85 ohms

  6. Hello Hitman,
    Even by your standards this is an awesome post, particularly the idea of preventing thermal runaway by sharing heat sinks. I'm actually revisiting your solar tracker circuit and needed more power, so here i am. Keep up the good work,

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