1
\$\begingroup\$

I am using a transistor (TIP 122) to control a dc motor using arduino. Although the power source I use is measured at 6V, I only get 5V at DC motor(M1) when I use the transistor. Here is the schematic:

shematic

Is this voltage drop normal or do I miss something?

Thanks.

\$\endgroup\$
  • \$\begingroup\$ Did you even look at the datasheet for the TIP122? Also, you need to know the current that the motor will draw. \$\endgroup\$ – Olin Lathrop Aug 7 '15 at 10:56
1
\$\begingroup\$

Some voltage drop there is normal; you want to saturate the transistor to minimise the voltage drop, and to increase that saturation you need to increase the base current.

Your "transistor" is not actually a transistor but a Darlington pair according to the datasheet extract Dzarda posted, which means it has unusually high gain (hFE=1000) and they quote saturation voltages at Ic/Ib=250.

The downside of a Darlington is higher Vce than a singlo transistor, since Vce cannot fall below Vbe of the second transistor, or about 0.6 to 0.7V.

But note the Ib values : 12mA for Ic=3A, and 20mA for Ic=5A. That's quite a bit higher than the 5mA you can get from an Arduino pin through a 1k resistor. So there is some scope to reduce Vce by increasing base current (R1=470R or 330R) but if 0.6V is still too much, then either:

  • replace the Darlington with a single transistor (probably with a second transistor to increase Ib. Saturation is usually about 0.2V at Ib = Ic/10, so if the motor takes 1 Amp you need Ib=100mA)
  • use a MOSFET as Dzarda suggests
\$\endgroup\$
0
\$\begingroup\$

This voltage drop is indeed normal. BJT transistors have intrinsic drops in them originating from their construction. The datasheet says the following:

enter image description here

Also, on page 3 the graph labeled "Figure 2. Base-Emitter Saturation Voltage and Collector-Emitter Saturation Voltage"

So, depending on the operating conditions, you can get up to 4V of drop across the CE terminals (during saturation ~ the "ON" state).

Such high drop is really not suitable for 6V of supply voltage. Get a MOSFET, they behave much more like variable resistors.

\$\endgroup\$
  • \$\begingroup\$ I have used a MOSFET in the past using this wiring grobotronics.com/images/detailed/5/Arduino_Mosfet.jpg , but the voltage drop was similar. Anyway, since I am not doing anything wrong, I will use a higher power supply. Thank you for your help! \$\endgroup\$ – yiannisk Aug 7 '15 at 19:27
0
\$\begingroup\$

When the base of a transistor is made high then it should behave like a short circuit. But, unfortunately most of the transistors have a little voltage drop. So it is very normal according to your circuit. You can use L293d motor driver chip. The voltage drop will be less than 1volt.

\$\endgroup\$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.