what is the use of " LM311 " in H-bridge?

[allko]

lm311 +h bridge

hi
i made a project about H-bridge..i assemble all the components in a kit
but i only assemble them without understanding thier mechanism...

this is the shematic diagram (upper picture)

**broken link removed**

so what is the use of LM311 ?

please please dont say " it is a voltage comparator"
because i got mad from this sentece because whenever i search for its use on the web i only found this sentence with some graphs and performance specifications...


what i need is : comparator between what?
is it for amplifying current? or maybe decreasing it? or maybe amplifying voltage?

or what?

and the four BJT transistors...what for?

thx in advance

 

[Miguel Gaspar]

Well LM311 is a voltage comparator, that means it has a transistor in the output stage that works in cut or saturation, not amplification, but collector and emitter are not commited.
The collector is attached to a 470Ω, the emitter to base of Q2 and Q4.
You can think the comparator as a swith open or closed.
both comparators works in alternated way, if one is in saturation the other in cut.

Please let me know any question.

 

[SkyHigh]

The two LM311 are configured as voltage comparators for input logic 1 (less than 5V but greater than 2.5V) and input logic 0 (more than 0V but less than 2.5V) using the two 10kΩ resistors in a voltage divider between 5V and 0V.

The output logic of the LM311 is in turn used to switch on/off the darlington pairs (Q1 and Q2) or (Q3 and Q4).

To switch Q1 or Q3 on, base current must sink through 470Ω resistor, then through the open-collector output stage of LM311 (basically a NPN transistor), then down to the base of bottom darlington pair or either Q2 or Q4.

Q1, Q2, Q3 and Q4 are darlington pairs or superbeta transistor pairs for sinking large current from the 12V supply rail to the a top darlington pair, then to the DC motor, evetually down to a bottom darlington pair, before going to ground.

Darlington pair is used instead of normal transistor because it can sink large current that can kill or break down a normal transistor.

In a H-bridge, either Q1 and Q2, or Q3 and Q4, can be turned on or off, alternatively depending on the LM311 that is turned on, based on the input logic, which can be 0 or 1.

Added after 3 minutes:

The two LM311 are configured as voltage comparators for input logic 1 (less than 5V but greater than 2.5V) and input logic 0 (more than 0V but less than 2.5V) using the two 10kΩ resistors in a voltage divider between 5V and 0V.

The output logic of the LM311 is in turn used to switch on/off the darlington pairs (Q1 and Q2) or (Q3 and Q4).

To switch Q1 or Q3 on, base current must sink through 470Ω resistor, then through the open-collector output stage of LM311 (basically a NPN transistor), then down to the base of bottom darlington pair or either Q2 or Q4.

Q1, Q2, Q3 and Q4 are darlington pairs or superbeta transistor pairs for sinking large current from the 12V supply rail to the a top darlington pair, then to the DC motor, evetually down to a bottom darlington pair, before going to ground.

Darlington pair is used instead of normal transistor because it can sink large current that can kill or break down a normal transistor.

In a H-bridge, either Q1 and Q2, or Q3 and Q4, can be turned on or off, alternatively depending on the LM311 that is turned on, based on the input logic, which can be 0 or 1.

 

[allko]

first of all , thank you guys for your kindfull replies.

next : questions.
@Miguel Gaspar:

that means it has a transistor in the output stage that works in cut or saturation, not amplification, but collector and emitter are not commited.

so the LM311 has -at the output level - a transistor in cut off or sturation regions. fine but what do u mean by "collector and emitter are not commited" ?

@sky high

The two LM311 are configured as voltage comparators for input logic 1 (less than 5V but greater than 2.5V) and input logic 0 (more than 0V but less than 2.5V) using the two 10kΩ resistors in a voltage divider between 5V and 0V.

so the LM311 give us either 0 logic or 1 logic , right?
but how is the voltage divder applied here with the two 10K resistors?

To switch Q1 or Q3 on, base current must sink through 470Ω resistor, then through the open-collector output stage of LM311 (basically a NPN transistor), then down to the base of bottom darlington pair or either Q2 or Q4.

now wait a minute , first the LM311 give a logic 1 (which is 5V) so a current flows throw the 470k then it reaches the base of the darlington transistor. is it like that?

our teacher assistant talked about something like "it uses a small current so we can use it in personal computer"

now i need to know : is the output current of LM311 small or high?
is the current in the motor small or high?

one more thing...i understand y we used darlington instead of normall transistors. but y we just dont use any transistor at all ?

im sorry for my many questions....

 

[Miguel Gaspar]

About your question at me, not commited means no contected so you must conect collector and emmiter.

About current it could be small or great, depends of the resistor conected.

Example Q sat: (Vcc-Vsat)/R = collector current

 

[IanP]

Darlington pair is used instead of normal transistor because it can sink large current that can kill or break down a normal transistor.

This sentence is a lttle bit unfortunate as there is no difference, as far as "output" is concerned, between 10A darlington and 10A power transistor ..
The difference is in "input" - in the base current, that is .. "normal" power transistor will have Hfe of roughly 20-200, whereas darlingtons have Hfe of 500 to 10,000 and more ..
Therefore in this circuit darlington transistor, which need very small base current can be driven directly from a small signal comparator such as the LM311 ..

Regards,
IanP