I have a BD135 NPN and BD136 PNP transistor, When i try to apply 5V of power to the collector pin, The emitter pin Voltage output is much lower.

The goal i want to achieve for this circuit is to control when the motor spins by a programmable circuit board which emits power which i believe is 0.5V from pins on the board, I'm connection a pin from the board to the base pin of a transistor while connection 5V to the collector pin and attaching the emitter pin to the motor which i want to control by turning off/on the pin.

I have tried directly connecting my power source to a electric motor and it spins fast, But when i try to connect the power to a transistor and then connecting the emitter pin from the transistor to the motor it spin very slow and sometimes stalls and needs to be pushed to continue. The motor needs more voltage then the transistor is emitting even when the collector pin is getting more then enough Voltage input(5V).

I thought the problem could be because i'm applying 3V to the base pin and maybe the base pin resists the collector but i'm not quite sure. I,m applying 5V of power into the collector but i don't know the output because i don't have the right equipment to measure it, All i know is that much less power comes out the emitter then what enters the collector.

I have tried researching this but i have found no topics or questions about this. I,m sure that my transistor is working because the problem happens for both my BD135 NPN and BD136 PNP transistors.

This is how i put my circuit for testing: enter image description here

I have just discovered that the output is not 3v but 0.5v and the electric motor is also making a ticking sound.


closed as unclear what you're asking by Leon Heller, Brian Drummond, Michael Karas, PeterJ, Olin Lathrop Jul 5 '15 at 14:00

Please clarify your specific problem or add additional details to highlight exactly what you need. As it's currently written, it’s hard to tell exactly what you're asking. See the How to Ask page for help clarifying this question. If this question can be reworded to fit the rules in the help center, please edit the question.

  • \$\begingroup\$ Before you continue, I would strongly suggest your read up a bit on NPN and PNP transistors and how to operate them. There are tons of tutorials out there. \$\endgroup\$ – Golaž Jul 5 '15 at 9:36
  • \$\begingroup\$ You haven't defined what you want this transistor circuit to accomplish. Even ignoring the dead short you show across the motor, the circuit you show regulates the motor voltage at the B-E drop below the 3V supply level while taking most of the power from the 5 V supply. However, that doesn't seem to be what you want to accomplish. You also are using "power" too loosely when you apparently mean voltage or maybe current. We do engineering here, which means we work from clear specs and don't throw vague terms around. Closing this mess. \$\endgroup\$ – Olin Lathrop Jul 5 '15 at 14:00
  • \$\begingroup\$ The first sentence i put up defines what i need to accomplish. So what do you mean by i haven't defined what i want to accomplish. I want to know why the transistors output is lower then the input(5V) how can i be more specific? \$\endgroup\$ – Awesome Name Jul 5 '15 at 14:32
  • \$\begingroup\$ No, your first sentence states a observation, garbled as it is. It says nothing about what the circuit is supposed to do. Also, wanting to know something is different from wanting a circuit that performs a particular action. If you just want to power a motor, connect it to a power supply. You may be wanting to control a motor, but you haven't said that, nor what power supply you want to take the power from, what voltage the motor requires, or the signal levels for off and on. You should be able to see for yourself that much important information is missing. \$\endgroup\$ – Olin Lathrop Jul 5 '15 at 14:48

First: Without the proper tools you're doomed to fail, so at the very least do yourself a favor and get yourself a digital multimeter and learn how to use it.

Second: When you ask a question you should provide at least enough information (Like a schematic diagram of your circuit) to show how you connected all of the circuit elements together.

If you don't do that, then how are we supposed to know whether you connected the power supply backwards or made some other gross error which let the magic smoke escape from the circuit?

The short answer is, "We won't", and all we'll wind up doing is wasting a lot of time guessing at what your problem might be before your question gets closed.


The drawing below shows your circuit, which is an emitter follower, and a common emitter circuit.

enter image description here

The reason the emitter follower circuit doesn't work very well for your application is because the base-to-emitter junction looks like a diode and needs to drop about 0.7 volts higher than the emitter voltage before the collector can start supplying current into the load connected to the emitter. However, with the motor in there, as the current through the motor starts to increase, its voltage drop increases and starts pushing back on the emitter, so that the base-to-emitter voltage drops and the current into the load starts being limited. In your case, since there's only 3 volts available to drive the base and the base-to-emitter diode takes about 0.7 volts of that, the highest voltage that can be developed across the motor is 3 volts - 0.7 volts, which is 2.3 volts, and that's why your motor doesn't work very well.

In the common emitter circuit, however, the motor is located between the supply and the collector, so its voltage drop doesn't affect the emitter much, its being connected directly to the negative (ground) side of the supply.

That being the case, all that's necessary to turn the transistor on and put nearly all of the 5 volts across the motor is to put enough current into the base.

A transistor has what's called "current gain", or "beta", which means that if it has a beta of 100 and I want one ampere of collector current, what I have to do is force 10 milliamperes of current into the base and that'll turn the transistor ON enough to let 1 ampere flow from the supply through the load, then through the transistor's collector-to emitter junction, and then from the emitter back to the supply.

In switching situations, though, what we do to make sure that the transistor is fully turned on is to push enough current into the base to saturate the collector-to emitter junction by making the base current about 10% of the load current.

In the example I've given, I've assumed a load current of about 500 milliamperes for the motor and, consequently, forced 50 milliamperes into the base for what's called a "forced beta" of 10. If the load current is less than 500 milliamperes it won't matter at all because the transistor will still be fully turned on, but if it's a lot more than 500 milliamperes, then the transistor's data sheet will need to be reviewed to make sure everything's OK.

The diode across the motor is used to clamp the high voltage spike which will occur when the motor is abruptly turned off - and would otherwise destroy the transistor - to Vcc + 1 diode drop, or about 6 volts.

  • \$\begingroup\$ I just tried that circuit but the motor keeps making a rapid ticking sound and i just discovered the base is getting 0.5V not 3V. \$\endgroup\$ – Awesome Name Jul 6 '15 at 13:42
  • \$\begingroup\$ @AwesomeName: So, because you made a mistake with that 3 volt base supply you're unaccepting my answer? I'm no longer interested in helping you. \$\endgroup\$ – EM Fields Jul 6 '15 at 17:32
  • \$\begingroup\$ No i unaccepted because it doesn't solve my problem which it says accept question if it solves problem. I'm not doing it to be rude i'm just following what the website says. Though it did solve my problem if it was 3V but i discovered its actually 0.5V so now the answer doesn't solve my problem. I admit that its my fault that i thought it was 3V instead of 0.5V and i know that some people could take this offensively. but that doesn't change the fact for why the button is there to let everyone else know if it solves my problem. \$\endgroup\$ – Awesome Name Jul 7 '15 at 8:22
  • \$\begingroup\$ Your answer does mention a lot of facts about it, Which i am grateful for you putting in the time for that, it helped me greatly in my knowledge about the problem so thanks for that, but it doesn't exactly solve my problem with making the motor spin because it doesn't spin at all, i'm guessing because of the low voltage. \$\endgroup\$ – Awesome Name Jul 7 '15 at 8:30
  • \$\begingroup\$ @AwesomeName: The problem you had with the emitter follower schematic you posted was solved with the common emitter circuit I posted for you. If it didn't work for you because you changed horses in midstream doesn't negate the fact that I gave you the solution you asked for in the first place. In the same vein, If you go to a restaurant and order a steak, eat it, and then realize that all you really wanted was a glass of water, does that relieve you of the responsibility of paying for the steak? Besides, everything you need to know to solve your problem yourself is in the text I posted. \$\endgroup\$ – EM Fields Jul 7 '15 at 10:23

Bipolar transistors act as current amplifiers. Inject a small current into the Base, and you can extract a proportionally larger current from the Collector. But how much current does your motor need, and how much Base current is required to get it?

The behavior of your motor indicates that it is not getting enough current, most likely because you aren't supplying enough Base current to the transistor. "But how can this be?" you say. "The BD135 has a typical current gain of well over 100, and I am feeding 20mA into the Base. It should be good for at least 2A! (and my motor only draws 0.5A)". Of course it isn't quite that simple.

Several factors may be contributing to your problem:-

  1. The transistor's current gain decreases at high current. In the graph below you can see that the BD135's gain starts to drop off rapidly above 100mA, falling to almost nothing at 1A. It might be 'rated' for 2A maximum, but is really only good for about 800mA max.

enter image description here

  1. At startup a DC motor draws very high current, limited only by the resistance of the armature windings and commutator/brushes. As it spins faster it draws less current due to the generator effect (armature generates an opposing voltage which reduces the effective voltage across the motor's internal resistance). However if it isn't supplied with sufficient current to get up to speed then it will run very slowly, and even the slightest load may stall it. The stall current of a DC motor is typically 2-5 times it's rated operating current, so a '0.5A' motor could draw 2A at stall!

  2. With your transistor connected in 'Common Collector' configuration, the voltage at the Emitter will be ~0.6V (up to 1V at high current) less than the voltage on the Base. So if for example you are only supplying 3V to the Base, the output voltage at the Emitter will be ~2.4V. Any resistance in the Base circuit will drop the voltage even lower. You may be driving the transistor from an MCU with 20mA rated I/O, but at that current its output voltage might drop from 3V to 2V, reducing motor voltage to 1.4V or less!

Motor drive techniques that avoid these problems include:-

a. Use MOSFETs instead of bipolar transistors. MOSFETs are turned by voltage rather than current, and can switch very high currents from a low power voltage signal.

b. Don't use common Collector (MOSFET: common Drain) configuration. Use common Emitter (Source) instead, and connect the motor between Collector (Drain) and the supply.


c. Drive the Base/Gate with a higher voltage. MOSFET drivers often have a booststrap circuit for this purpose (may only work when appplying PWM).

d. Use transistors which are rated well above the stall current of the motor. A MOSFET driving a 0.5A motor might be rated at 5A or higher.


You are doing it wrong. You need to more powerful transistor, but for now yours will work. Connect the motor between collector and + of the battery. Connect 3v through 500-1000 Ohm resistor to the base and emiiter.

  • \$\begingroup\$ Why do i need a resistor? \$\endgroup\$ – Awesome Name Jul 5 '15 at 13:28
  • \$\begingroup\$ @AwesomeName Because you need to limit the base current, or you will burn the transistor; trust me you need it. \$\endgroup\$ – ilkhd Jul 5 '15 at 13:32
  • \$\begingroup\$ I'm following this circuit: learn.sparkfun.com/tutorials/… Don't worry about going through the whole thing just look at the last circuit, it's what i'm using for my circuit which is making it spin slow. \$\endgroup\$ – Awesome Name Jul 5 '15 at 13:40
  • \$\begingroup\$ What you drew in your post, and what "fritzing" shows is very different. \$\endgroup\$ – ilkhd Jul 5 '15 at 13:44
  • \$\begingroup\$ The diagram i put in the question was for testing, My real circuit is like that one in the link. \$\endgroup\$ – Awesome Name Jul 5 '15 at 13:49

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