H-Bridge Design Problem

[Que_Trac1]

tip120 h bridge

Hi everyone,
I brought a robot kit called Asuro at Jameco. I need to modify the kit with a better microcontroller and better motors. I got the kit working with the new microcontroller, but I can't seem to know how to modify the H-Bridge for the better motors to have more current. The bipolar used in the kit maximum Ic is 0.5A. There is not enough current to make the robot move. I need around 1.5A current going through it. I tried to look for bipolar transistors with a higher maximum Ic at Jameco, so I pick out MJE172 and BD237. It doesn't work. After researching more on bipolar, I know that I need to look not only at Ic max, but also hFE and Ib max, etc. The datasheet for those components doesn't tell me what I need. Without them, I can't determine the appropriate rb to use. Overwhelming with all those information and uncertain whether I am doing is correct or not, I don't know what transistor to choose and what else I need I change. May you guys help me out?

 

[IanP]

h bridge motor tip120 tip125

Instead of standard singe BJTs try to get (power) Darlingtons ..

For 1.5A you can consider the following (or similar) pairs:
BD677(NPN) and BD678(PNP),
BD681 and BD682,
TIP120(1,2) and TIP125(6,7) ..

By using Darlingtons you will not need to change anyting in the circuit ..

Regards,
IanP

 

[Que_Trac1]

2v h-bridge

Hi, IanP. Thanks for responding. How did you know that those components will work if I replace them with their corresponding BJT without changing anything? I need to know, because I would like to see if I get the parts at a different, but closer store.

I checked out the components you listed and they have already diodes built-in. Don't I need to remove the external diodes from the circuit?

 

[IanP]

bd237 motor

Power Darlingtons are designed to work with higher currents but they can be driven (Ib) by some mA rather than hundreds or thousands of mA in case of ordinary power BJTs ..
So, in your case, replacing standard BJTs with Power Darlingtons will not affect the way microcontroller drives them - a couple of mA on microcontroller pins will be enough ..

If you want to remove external diodes do it, but I would leave them just in case the type of Darlington that you can get may not have it ..

Regards,
IanP

 

[e_eja]

asuro h-bridge

i would used P-mosfet and N-mosfet rather transistor for driving motor IRFZ44 would and it's complementary will enough to drive your motor
regards

 

[Que_Trac1]

asuro h bridge

I replaced the circuit with the Power Darlingtons transistor TIP 120 and TIP 125, but the motors are still spinning at the same rate as before. If I want to increase the voltage/current across the motors, does it mean that I need to increase Ib? By doing so, do I reduce Rb to a particular value?

If so, how to do I calculate for what resistance do I need?

 

[IanP]

h bridge mosfet not enough current

With Darlingtons installed I wouldn't worry about Rbs ..
What happens if you drive motor directly of the same supplay as the bridge?
Does it rotate faster?
Maybe you just need to supply bridge of higher voltage ..

Regards,
IanP

 

[Que_Trac1]

h bridge mosfet not enoughcurrent

If I drive the motor directly through the power supply instead of through the H-bridge, it, of course, rotates faster. But I can't increase the power supply anymore than it is getting which is 5-6V. I just need 1-1.5A minimum across the motor, or I am guessing 1-2V.

 

[marko_benigno]

bjt h bridge design

I agree with e_eja. I would prefer N-channel & P channel MOSFETs. That way, you don't need to bother the current configuration of the transistor just to ensure that those transistors are in saturation. FETs are voltage controlled, so its easier for you to do the tweaking. You just need to check the Rds-on rating of the MOSFETs.

You just have to control the Vgs of every MOSFET and either you maintain the same supply across the bridge and the supply of your control circuit. It's also possible to have a different supply for the bridge for a freedom to choose on different motors with different ratings.

 

[Que_Trac1]

choosing darlington transistor in h bridge

So what parts should I get? I am not very familiar with transistors much. I have been working on this problem for quite some time. Yet, they seems so foreign to me. I just need the motor rotate faster, and I have replaced first the H-bridge with BD237 and MJE172. Then follow by TIP120 and TIP125. Still, back to where I started.

 

[marko_benigno]

h-bridge tip125

what type motor are you using?i suggest you check first the type and rating of the motor, what its rated current, voltage, and other rated conditions.

from there, you could start what type of transistors are you going to use.

With your current circuit, check first the voltage acorss the terminals of the motor. It must be close to the rated condition of the motor... in your case, it might be the supply voltage. If its below, then if its significantly lower, you might assume that the transistors in the bridge are operating in the linear region. ( try to measure the voltages on the collector-emitter of each transistor to confirm this. Those transistors should operate as switches so they should have a minimal drop across their C-E terminals.

You also try increasing the control frequency being supplied to the H-bridge. This is another way of increasing the speed of the motor aside from increasing the supply voltage.

Let me know what you'll find out.

Added after 4 minutes:

Also, check if the curent rating of the motor is lowe than the rating of the transistor. Your H-bridge transistors would break once excesssive amount of current flows into it. In your case, that did not happen.

the way I see it from your experience, teh transistors did not work as what they're expected.