H-bridge with TIP 122/127

Hi all

I am trying to build an H-bridge using TIP 122 npn Darlington pair transistors and TIP 127 pnp's.

The bridge got the 3V motor to turn one way but refused to reverse. The bases had 350 ohm resistors and I did not include 4 diodes since the data sheet shows that TIP's have their own protective diodes. My power source is a 6V battery pack.

When it would not reverse, I tried each transistor separately with only the motor as load across the emitter/collector. The base was biased with a 2k to infinite ohm CdS photoresistor. The transistors turned the motor on individually but got hot in about 30 seconds of use. Is this normal? Surely if one is driving something as small as a 3V DC motor, the TIP should not overheat so?

I copied the circuit found half way down the page at http://multyremotes.com/dc-motor-controller.htm

Any suggestions on these two problems would be most welcomed.

 

I "jumpered" my inputs straight from the battery + and - terminals but through 350 ohm resistors as shown on the schematic so I am sure the inputs are + and -. I am not using voltages from RL2 yet since I just want to get the H bridge to work.

I have rebuilt the circuit twice, so I think everything is connected.

Maybe the TIP needs more than 6V.

When testing the TIP's I connected the TIP (c,e), battery and motor all in series with a separate bias to the base of the tip through a photoresistor. The DC motor itself has very little resistance (7 ohm when not turning to 500 ohm turning it by hand. I presume it gathers inductive resistance as it turns, but to start it is almost a short circuit). Is it wrong the way I tested it?

Thanks for the help so far

 

I tried putting the motor on the collector side and then on the emitter side of TIP122, leaving the battery on the other side. In both cases motor turns on when light falls on photoresistor or if a 1k resistor applies a p voltage to base. In both cases TIP got hot in30s. If I left it longer, no doubt I could fry an egg on it. I did not think it makes a difference which side of the TIP one places the motor.

 

Thanks again for the feedback

I measured the current through the TIP and motor as 0.8A. Multiply this by 1.2V drop you say occurs across the TIP and one gets 1W lost across the TIP, which I guess, explains why it gets so hot. As you said, there may be a more efficient non-Darlington transistor.

The induced (reverse) V when turning the DC motor slowly by hand, was measured as 0.08V, which would rise as the motor speed increases.

After many months of struggle I managed to build a solar tracker using a comparator and a NJM2670D2 dual H-bridge motor driver chip. It works well and does not overheat but is a bit pricey. I was hoping to use TIP's in place of the NJM. So far I have not had much luck with TIP's. But I guess the TIPs have had worse luck when, in the recent past I blew a few of these by forgetting to include base resistors.

Thanks again.

 

Harald - I made the circuit you recommended using Tip122s and 127s and it works a treat. The motor flies both ways and the TIPs seems to stay reasonably cool. You made my day.

I still don't understand why Vce is less (and efficiency much greater) when emitters are connected to the power supply rather than when collectors are attached to the power supply, and why the motor works better drawing current from collectors rather than emitters.

 

Thanks for the graphs which illustrate how the same transistor can act as a switch or an amplifier. I must say that it is not intuitive how this works (meaning very difficult to understand) but having seen the result, I am trying to catch up on the theory.

 

Thanks for taking the trouble to send these plots. I certainly hope to come to grips with this soon.

I downloaded LTSPICE and spent time on a site called circuitlab.com which allows one to design common emiter and common collector circuits and then see changes in V etc. I am from the school of fiddle with components till the circuit works (kinda), but hope to join those of you who are not surprised when circuits work first time.