why should I use a H-Bridge

I have a question why do I need to use a H-Bridge when I can use just the Arduino to drive the motor in either direction.

I think I know why but I may be wrong is it speed control?

any help would be great

thanks

The main reason is that the Arduino can only handle supplying a small amount of power and most motors would draw more power than the Arduino can supply.

When you use the H-Bridge, you supply the power for the motor(s) from an external power source so you do not put any strain on the Arduino's power supply.

thanks I have used H-Bridges before without a Microcontroller, so I guess thats why I did not see that.

No Problem. Glad to help. ;)

It is also because without an H bridge you can only control the speed of the motor in one direction, with an H bridge you can switch the direction of the current through the motor and in that way switch the direction.

without an H bridge you can only control the speed of the motor in one direction

Yes but you can do it with relays:-

http://www.thebox.myzen.co.uk/Workshop/Motors_1.html

without an H bridge you can only control the speed of the motor in one direction

Actually, on the Milton Bradley Big Trak, they used a weird "half h-bridge" design with a dual ended power supply; only two transistors used per motor.

Regarding h-bridges: on another thread, someone mentioned that using only NPN transistors for such a bridge could be a bad design; it wasn't said, but I got the gist that it was better to use NPN on one half, and PNP on the other. I have designed an h-bridge using 2n3055 NPNs (TO-3 cases), but after hearing that information, I looked into other designs - I found this:

http://www.mcmanis.com/chuck/robotics/tutorial/h-bridge/bjt-circuit.html

If I used the 2n3055 complement (MJ2955) for the other half, is this really a better design? It seems safer (ie, no possibility for SEDs/FEDs), and would allow me to implement coasting and braking (something I can't do with only NPNs?).

Am I looking at this wrong? Is there a way to use only NPNs and still get the safety from shorts? I am already using opto-couplers, and only drive one half at a time - but if both halves are high, fireworks (I imagine!) could appear...

Is an NPN only h-bridge safe, or -must- you use complementary pairs? If I have to re-design, no big deal - I am just looking for some feedback (PM me if you want).

:)

Is an NPN only h-bridge safe, or -must- you use complementary pairs? If I have to re-design, no big deal - I am just looking for some feedback (PM me if you want).

You may be able to make an all NPN h-bridge by using a boot strap setup like below for the high side MOSFETs. Supposidly this is common in switching power supplys.

Yes but this is not a half H-bridge it is just a simple high side switch and not a very good one at that. The driver circuit can't be an arduino because it does not have an open collector output. Driving a load in the source is not good because the load current backs off the gate voltage and the FET ends up operating in the liner mode. This = hot.

Actually, on the Milton Bradley Big Trak, they used a weird "half h-bridge" design with a dual ended power supply; only two transistors used per motor.

So half the batteries drive it forward and half of them backwards. It doesn't make for a very even battery usage. You end up changing them all when only half are discharged.

I looked into other designs - I found this:

Conventional enough design. It is a bit asymmetric in it's sensitivity due to the use of the transistors in the optos, one pair in common emitter and the other in common collector.

It seems safer

Can't see why you say that if all LEDs are on there is still a short through the transistors of the bridge.

Generally an NPN only bridge is less efficient (generates more heat) due to mixing common emitter and common collector drives and generally requires non overlapping drive signal to make it safe.

So half the batteries drive it forward and half of them backwards. It doesn't make for a very even battery usage. You end up changing them all when only half are discharged.

Yeah - this was a problem with it; but it shifted the extra cost to the user, away from Milton Bradley... ;D

Can't see why you say that if all LEDs are on there is still a short through the transistors of the bridge.

Ok - so how do you design a "safe" h-bridge such that no matter what the inputs are, there isn't a direct short? That is the question I would like to see answered - that, or how do I ensure that the outputs of the Arduino are always in the "safe" position? Right now, my only answer to the question is to delay the power-up of the h-bridge until I am certain the Arduino has everything set properly (via some kind of "enable" pin or something), coupled with a fuse on the power supply rail feeding the h-bridge.

Seems kinda like a hack.

:)

Yes but this is not a half H-bridge it is just a simple high side switch and not a very good one at that. The driver circuit can't be an arduino because it does not have an open collector output. Driving a load in the source is not good because the load current backs off the gate voltage and the FET ends up operating in the liner mode. This = hot.

This is not my design, so I can't comment on its quality. It is from an electronics magazine article on power supplys. My understanding is it is designed to be pulsed on/off to keep the gate voltage high, which may work in an h-bridge driven by a servo board at 50 hz. Not sure where the "not a half H-bridge" comment comes from, as it was never claimed to be such.

so how do you design a "safe" h-bridge such that no matter what the inputs are, there isn't a direct short?

The trick is not to have inputs that you can put in that way, so on a simple level this means having only one input and taking the inverse of it to drive the other half of the bridge. Look at a chip like the UC2714, the data sheet has a block diagram of the sort of thing you need.

The trick is not to have inputs that you can put in that way, so on a simple level this means having only one input and taking the inverse of it to drive the other half of the bridge. Look at a chip like the UC2714, the data sheet has a block diagram of the sort of thing you need.

So basically the two inputs become “direction” and “enable”, instead of two logic inputs, right? Guess I have a small amount of redesign to do, then - but no problems…

:slight_smile:

What about "regenerative breaking" - will this be possible with direction/enable only?

Rather than enable = on or off, make it enable = drive or brake.