H bridge diodes ?

Discussion in 'Homework Help' started by fran1942, Jun 7, 2013.

Jul 26, 2010
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hello, I would like to know the exact purpose of diodes in an H bridge.
My understanding is that they allow current to flow in one direction only, thereby preventing short circuits if all four switching elements of an H bridge were activated at once i.e the diodes only allow current to flow through two diodes at any one time ?

Secondly, how would you modify an H bridge circuit if no diodes are used ?
That one I am not sure on at all.

Thank you kindly for any assistance.

2. Jony130 AAC Fanatic!

Feb 17, 2009
3,993
1,116
H-bridge is typical use to drive a motor. And motor act just like a inductive load.
And this is why we need a diodes that provide a path for the inductive "kick" that is generated when the motor is switched off in h-bridge circuit.

And I don't understand your second question.

3. tubeguy Well-Known Member

Nov 3, 2012
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The diodes have nothing to do with preventing short circuits if all four elements are switched on. Preventing all elements from being switched on at once is the the job of properly designed drive circuitry or using complementary mosfets/transistors on each side of the H-bridge.

Jul 26, 2010
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So I understand now that the diodes are there to prevent back EMF when the motor is switched off.

My second question in context was:
The H Bridge
What is the purpose of using the diodes in the circuit? How can this circuit be modified if no diodes are to be used ?

Can someone please help me explain how the H Bridge circuit can be modified if no diodes are to be used ?

5. panic mode Senior Member

Oct 10, 2011
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That is correct....

You have 3 second memory span? Read previous quote, one that says you understand something and purpose of diodes (EMF).

you NEED diodes for circuit to work. without diodes rest of the H-bridge gets killed at the moment motor turns off.

there are two types of elements that store energy: capacitors and inductors.

most people have no problem understanding capacitors ability to store charge because the energy can be stored statically.

inductors are different animal. they cannot store energy statically, when circuit opens, they deliver that energy regardless if you ready for it or not...

think about railroad train. it takes time to accelerate. and it takes time to stop it. it does NOT stop instantly.

inductors are like that. it takes time for current flow through it to grow, but when you turn the circuit off, current does NOT stop instantly (even though you have open circuit). inductor will keep on pushing that same current through whatever resistance that 'open circuit' is. eventually voltage builds up until there is an insulation breakdown. if the insulator is air, we can see the spark. in case of H-Bridge, sparks are inside transistors. kaput, gone, toast, smoked...

you NEED diodes in such circuit. they divert running train to an alternate track so it has time to slow down...

6. crutschow Expert

Mar 14, 2008
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MOSFETs have a built-in substrate reverse-biased drain-source diode so if you build a MOSFET H-bridge, it will already have diodes whether you add external ones or not. These substrate diodes may be sufficient to provide protection from inductive spikes.