TTL & TIP41 FOR DC MOTOR SPEED CONTROL

[labview1958]

I have a source that generates a TTL signal. I can vary the DUTY cycle. Can I use this to control the speed of a DC motor by just using ONE TIP41 power transistor and resisor with a 40 V power supply. I need to control the speed of the DC motor only one way.

[bridgen]

Yes.

[labview1958]

I have come across some robotics example using TWO TIP41's to control the speed of the motor ONE way. Why do they need TWO not ONE. The TTL signal I intend to use comes from a DAQ board. It is a 5V signal with very LOW current. Can that control the TIP41. I understand that the TIP41 acts like a SWITCH to turn on/off a much larger voltage. Is that true? IS there a BETTER way?

[rshayes]

If the current required by the motor was high, you might need two or more transistors in parallel to switch the current.<p>The base current needed by a transistor operated as a switch is about 5 to 10 percent of the collector current. You will need some additional amplification stages between the TTL logic level and the base of the motor driving transistors. There will be some power loss here, but that is usually tolerable.

[Chris Smith]

Fets are superior to transistors in that their on resistance can be one ohm and lower, like 1/10 th of an ohm, and even less,... where as a TIP might have several ohms? The watts and heat do add up.

[labview1958]

I have been trying to find an EASY tutorial on TIP41, FET etc. Basically I intend to control the speed of a DC motor from a PC using a NI-DAQ card.But I am also concerned about spoiling my PC! Anyone can HELP!

[philba]

Chris is on the right track (gasp!! I said it...). <p>Use a MOSFET like an IRL530 which is logic level triggered. The key for efficient PWM control is to get the MOSFET fully on where its on resistance (Rds) can be quite low. The parameters that you care about are Rds (basically the on resistance) and Vgth (Gate threshold). Vgth is voltage where the device switches "fully" on. I put that in quotes as there really is no point where it is fully on but close enough. Typically Rds is listed along with a gate/source voltage (Vgs). For example the IRL530A datasheet shows an Rds of .12 Ohms at a gate voltage of 5V. Your Rds will be higher because you wont get 5V out of your TTL logic but its should be pretty good. Some MOSFET datasheets show Rds for different gate voltages. The lower the Rds, the better as the device is dissipating less energy (and thus runs cooler).

[zotdoc]

You can protect your computer with opto-coupler chips. They basically use your pc's output to light an led which switches a photo-transistor (all inside the chip)with a different power source. Thus, no direct electrical connection from the pc to your circuit.

[labview1958]

I have a source that generates a TTL signal. I can vary the DUTY cycle. Can I use this to control the speed of a DC motor by just using ONE TIP41 power transistor and resisor with a 40 V power supply. I need to control the speed of the DC motor only one way. <p>My TTL signal from the DAQ is 5V. A friend who says he knows these things says that the 5V TTL signal will only give an output from 0V to 5V. As my motor RUNS on 40V, it would only get a supply of between 0V to 5V. Thus most likely my motor would move very slowly.Is that true?

[rshayes]

The motor probably wouldn't move at all. A TTL output is only specified to be above 3 volts as a high level. The current available is limited to a few milliamps even at that level.<p>TTL gates are quite unsymmetrical. They are capable of sinking considerably more current at the low level than they can source at the high level.<p>A pull up resistor can be added to a TTL output to get a little higher voltage when the output is high. This is often done when interfacing with cmos, since the upper logic level for CMOS is greater than 3 volts.<p>You will need some circuit between the TTL output and your transistor switch. This may not have to be very complicated or expensive, just a few resistors and a small transistor or two.<p>For a motor current of 1 amp, the base current should be about 100 milliamps. The base voltage will be about .7 volts. This can be supplied by a PNP transistor, such as the 2N3906, with its emitter connected to the 5 volt supply. The collector can be connected to the base of the power transistor with a 39 ohm resistor to limit the current to a little over 100 milliamps.<p>The base current of the 2N3906 should be about 10 milliamps. In addition, a resistor should be connected between the 2N3906 base and emitter to guarentee that it will turn off. If a value of 680 ohms is used for this resistor, an additional 1 milliamp of current will be needed, for a total current of 11 milliamps. This is within the range that can be supplied by a TTL buffer, but is marginal for a TTL gate. A resistor of 330 ohms from the base of the 2NN3906 will provide enough drive. In this case, the motor will be turned on when the TTL output is low.<p>The collector of your power transistor would be connected to the negative lead of the motor. The positive motor lead would connect to the 40 volt supply. You should also connect a diode across the motor with the anode to the negative lead and the cathode to the positive lead. This will allow a path for the motor current when the transistor is switched off (the motor tends to be inductive, like a relay). Otherwise, there will be high voltage spikes that will probably destroy the transistor.<p>If the motor requires less than 1 amp, the resistors can be made higher and the power loss and transistor stress will be less.

[sofaspud]

The 5-volt TTL is your control signal. It will be applied to the transistor so that the transistor will control the 40 volt signal.
Your choice of parts depends on the operating characteristics of the circuit and what degree of performance you want. Optocoupler isolation is a good idea to protect your PC hardware.
The only "tutorial" on TIP41, FET, etc. I can think of is the datasheet for the part. Otherwise, you'd probably want to study the workings of bipolar and field effect transistors. I don't know your level of knowledge. The basic difference in the two is that bipolar devices operate by current, FETs operate by voltage. This is a big reason why FETs were suggested - your TTL source may strain to provide the current needed for bipolar, but you know you'll have 5 volts for a FET.
And I'll second Stephen's suggestion for bipolar. Go from TTL to motor drive in two smaller jumps rather than one big leap.<p>[ April 28, 2005: Message edited by: sofaspud ]</p>

[rshayes]

If you to use a logic level MOSFET, you will probably still need a buffer stage. The problem (usually not mentioned in polite company) is that MOSFETs have a high capacitance (several thousand picofarads) from drain to gate, and that this capacitance must be charged by the driving circuit. This slows down the transition times considerably.<p>TTL usually does not provide a 5 volt output. The minimum specification is 3 volts, and they usually do a little better than this if the load current is low.<p>The 74HCT series is a CMOS logic series where the input thresholds have been adjusted to match the TTL signal levels. A 74HCT04 will accept the TTL logic levels and put out a signal close to 5 volts. If all six sections are paralleled, the output impedance should be well below 100 ohms, which should give reasonable rise and fall times with a MOSFET.<p>You should also have two diodes, such as 1N4150s or similar, from the buffer output to ground and to the 5 volt supply. One diode would have the cathode connected to the 5 volt supply and the anode connected to the buffer output. The other diode would have the cathode connected to the buffer output and the anode connected to ground.<p>The purpose of these diodes is to keep transients coupled back through the MOSFET from damaging the CMOS buffer.

[labview1958]

If I apply a TTL signal with a duty cycle of 50% would I get 50% of 40V = 20V for the DC motor? If I apply a duty cycle of 25% would I get 25% of 40V = 10V? Or is the TIP41 allows a higher current only keeping the voltage same as the BASE?
I have already connected a simple circuit containing a resistor , a TIP41 , a DC motor and a 15V supply. I am running the circuit with a labview program. When I ON the program the DC motor TURNS. However when I OFF the program, the DC motor slows almost to a STOP and STARTS up to maximum with the program in still in the OFF position. It keeps on going and then comes to almost a STOP and picks up again without the labview program being ON. WHY?

[labview1958]

I am trying to connect a resistor from the base to the grounf. Hope it works!

[labview1958]

Oops! For some reason it does not work! Any suggestions?