Strange problem with transistors flipping current

[TaylorG]

Using an arduino uno and 4 transistors as shown in the schematic I would like to flip the current direction across a coil of wire depicted by the solenoid here. The current also has to be fairly strong (a few amps). The transistors are all TIP3055's. http://www.onsemi.com/pub_link/Collateral/TIP3055-D.PDF

The circuit seems to do perform ok up to about one amp. Above about 1.3 amps something changes and i'm not sure about what it is. The current seems to prefer one path to the other. I took some measurements while keeping the DC supply settings constant. Here is what I found: whenever the current flips the current changes between 1.3 and 1.5 amps. At 1.3 amps the supply also measures a much higher voltage.

When Q1 and Q2 are "on" (current passes through them) and Q3 and Q4 are off, 1.5 amps is measured. When Q3 and Q4 are on and Q1 and Q2 are off, 1.3 amps are measured. When I try to increase the current using the supply Q1 and Q2 allow more current through but Q3 and Q4 still only allow 1.3 amps.

I measured the collector-emitter voltages of each transistor as the circuit was running and here is what I found:

On Off

Q1 .94 1.61

Q2 .42 2.08

Q3 .69 2.25

Q4 1.18 1.59


I've made sure that each pin on the arduino is supplying the same voltage and I don't think there any inductance effects going on. And the base-emitter/ base-collector voltages remain the same as well. Could this be due to faulty wires or a bad breadboard? Or is there an obvious problem with the circuit?

 

[Rleo6965]

You may want to replace your TIP3055 into MTP3055.

 

[KrisBlueNZ]

Using an arduino uno and 4 transistors as shown in the schematic ...

I don't see a schematic... Also can you post a photo of your construction.

 

[TaylorG]

 

[TaylorG]

Sorry about that. I'll try to get a photo of it tomorrow

 

[Bluejets]

I'd be using relays or something like attached.

 

[KrisBlueNZ]

OK, that circuit is called an H-bridge. It's normally drawn like the letter H for clarity - see https://www.google.com/search?q=h-bridge&tbm=isch

The circuit you have there won't work properly because the two transistors with their collectors connected to the +9V rail are connected in the "common collector" aka "emitter follower" configuration and will not turn ON fully. Their emitters will only pull up to about 0.7V less than the Arduino's output voltage, i.e. about 4.3V if the Arduino is powered from 5V. They will also dissipate a lot more power than the other two transistors.

You also need to consider the current gain of the transistors. The Arduino can only supply a few milliamps, and a typical (non-Darlington) power transitor has a current gain of about 50. How many amps will the load draw?

There are various arrangements for H-bridges depending on your requirements. You can get H-bridge drivers, and complete H-bridge circuits including the switching devices, in IC form. Nowadays, MOSFETs are normally used instead of BJTs because they have lower losses and are easier to drive.

Google H-bridge and read some articles about H-bridge operation and design. Also download some data sheets from Digi-Key's product selector for H-bridge drivers with external switching devices: http://www.digikey.com/product-sear...rs-external-switch/2556427?stock=1&quantity=1

If you have any questions that aren't answered in the articles you find, please post them here.

 

[TaylorG]

Thank you very much.