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Topic: H Bridge design (Read 6445 times) previous topic - next topic


Hello all,

Although I have been playing around with the Arduino for a while now I have always used pre manufactured circuits.
I decided to break the habit with the "simple" H-Bridge.
I was wondering if someone could review it for me and maybe spot any major errors (think my cap value is wrong?).
I have attached my design schematic and design calcs which are entirely based on this article:


Many thanks in advance for you help,




Sorry it will not work. There is no way to control those top FETs. Normally you would have a P-channel FET at the top and a transistor or other FET driving it.

I haven't read you link but either it is wrong or you are not understanding things.
A 35A H-bridge is too big for a first design, it will fry.


Hi Grumpy_Mike,

Thanks for your comments, can you please clarify what will fry?

Please see this link, the Arduino is connected directly to the gate of the mosfet?


Thanks again for your comments.



H bridges need some kind of dedicated driver chip to control the Hi side Fets so that they fully saturate when turned on.
As the previous poster indicated, your circuit wont turn the hi side fets on.
Have a look at using 2 of these devices http://www.irf.com/product-info/datasheets/data/ir2104.pdf
as the Fet drivers.
They also provide the shoot thru protection needed to stop the fets frying when switching.


"Who is like unto the beast? who is able to make war with him?"
When all else fails, check your wiring!


can you please clarify what will fry?

The FETs.

Please see this link, the Arduino is connected directly to the gate of the mosfet?

Yes but there is only one FET that is sinking current. You are trying to source current with the top FETs and sink it with the bottom ones. It is not the bottom ones that are a problem it is the top ones. By only supplying it with 5V on the gate there is not enough voltage to turn it on fully. A partly on FET will present a medium resistance and that will cause the FETs to get very very hot. With a 35A load you will be able to melt solder on it before it dies very quickly.


That page is mainly concerned with the basics. The aspect of h-bridge design that
is most critical, and that people spend 98% of their time working on, is here:




Many many thanks for your help, I really appreciate it.
I will go back to the latest link and review the design.
If you don't mind I will post the revised schematic for further comments?.

Thanks again!



Hello again,

Any objections to using a L298 (http://www.sparkfun.com/datasheets/Robotics/L298_H_Bridge.pdf) to drive the the MOSFETS?
The reason I ask, is that I already have them, and getting new components where I live just takes forever!

Thanks again for your help!



Those aren't ideal for "low voltage" motors owing to the V_ce(sat) losses of their output transistors.
Any details on the motor you're planning on using?
"Who is like unto the beast? who is able to make war with him?"
When all else fails, check your wiring!


Hi Runaway Pancake,

I'n using a 12v motor (http://www.robotshop.com/ca/PDF/rbban63-banebots-DC-motor-specs.pdf)

Is V_ce(sat) the voltage loss across the output transistor?




Vce = "voltage collector-to-emitter".  sat = "at saturation".
[ I will NOT respond to personal messages, I WILL delete them, use the forum please ]


Those aren't ideal for "low voltage" motors

No I think he means can he use them to drive the H-bridge FETs.
It is not conventional but it might work, however you will need to analyse the design carefully, possibly simulate it first. As I said before designing a 35A H-Bridge is no trivial matter.


Thanks MarkT & Grumpy_Mike,

That's exactly the idea, I want to use the l298 to drive the high & low side of the H-Bridge, If its OK will you guys I will redo the schematic and repost it?

Thanks again for the help!



Go with a dedicated high side driver... The engineering has been done for you... just stick it together. making a power supply that provides about 9 - 10V above the motor Drive is a non trivial task... I've done it... I used an H Bridge for operating a bi-polar (Latching) solenoid and it's no easy task. One other small point to be aware of and that is If you bring the gate more than about 15 V above (N-Ch) or Below (P_Ch) the source lead you will send that Fet onward to it's ancestors (likely in the trash can too) An NPN- PNP pair would do the job easily... but timing is critical to avoid Shoot Through (Where both transistors on ONE side of the bridge are on at the same time). Simple circuit with a number of pitfalls that Need to be considered and Dealt with.

--> WA7EMS <--
"The solution of every problem is another problem." -Johann Wolfgang von Goethe
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