Author Topic: h-bridge help  (Read 1773 times)

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vsching

• Jr. Member
• Posts: 10
h-bridge help
« on: April 28, 2009, 08:36:08 AM »
I can't get access to the h-bridge chip at my area so i am thinking of building my own 1 using some bjt transistor, using 2 bc237 and 2 bc227 transistor. I need to run my motor at 5v which will draw around 500mA continuously with peak current around 2A. i noticed that i need to put resistor at the base of the transistors. How do i calculate the resistor value? If the resistor value is right, the the motor should turn when A and C is connected to the positive power supply while B and D is connected to the ground. Is that right?

what if my microcontroller provide voltage of 2.7v only? Do i need to amplify the voltage to 5v so the motor turns faster? Since there are 2 type of voltage sources-5v and 2.7v(from microcontroller). Which actually provide the voltage needed by my motor?

I just build a half-h-bridge(somehow i got problem with getting a full h-bridge), since my motor only needed one direction movement, i just test with it first.I get an external 3v source to provide voltage for the base of transistors to test out the effects of different voltage. From my testing, i found out that the voltage depends more on the voltage input to the base. Applying higher voltage to this base would cause the motor to move faster. Is that correct? Btw, currently i am not using any resistor. Will add-on as long as i figure out what value to get for it.

Is there any website which sell h-bridge driver chip and can you please recommend some for my motor specs. I try mouser electronics but i get confused with the different package size. I need a standard size chip which i can breadboard it.-.-

MOtor A 5V, 0.5A-2.5A
MOtor A 12V, 0.5A-2.5A
« Last Edit: April 28, 2009, 11:18:34 AM by vsching »

Soeren

• Supreme Robot
• Posts: 4,672
Re: h-bridge help
« Reply #1 on: April 28, 2009, 02:54:12 PM »
Hi,

[...] using 2 bc237 and 2 bc227 transistor. I need [...] peak current around 2A.
Your transistors can't handle that current.

i noticed that i need to put resistor at the base of the transistors. How do i calculate the resistor value? If the resistor value is right, the the motor should turn when A and C is connected to the positive power supply while B and D is connected to the ground. Is that right?
Let's postpone the resistor calculation until you understand the operation of a transistor (Google is your friend )
And asking whether it will work when you connect *some symbolic names* is really having too much fate in out guessing abilities

what if my microcontroller provide voltage of 2.7v only? Do i need to amplify the voltage to 5v so the motor turns faster? Since there are 2 type of voltage sources-5v and 2.7v(from microcontroller). Which actually provide the voltage needed by my motor?
Once again, you need to familiarize yourself with how a transistor work (hint: it is most often used to amplify).
The voltage at the base resistor just need to be from ~0.7V and up, to control what's on the the collector.

I just build a half-h-bridge(somehow i got problem with getting a full h-bridge), since my motor only needed one direction movement, i just test with it first.
If you only need one direction, you have absolutely no use for an H-bridge and will be better off without it (each transistor will "steal" some of the available voltage).
You can't make a half H-bridge, the "H" in the name is only valid when it's complete, since it then looks like an "H", schematics-vise.
What you need is just a single transistor (that can handle the required current).

I get an external 3v source to provide voltage for the base of transistors to test out the effects of different voltage. From my testing, i found out that the voltage depends more on the voltage input to the base. Applying higher voltage to this base would cause the motor to move faster. Is that correct? Btw, currently i am not using any resistor. Will add-on as long as i figure out what value to get for it.
With 3V and no resistor on the base, you're killing your transistor. A transistor set up with the correct base resistor value (for the collector current needed) should be used.
An example:
A TIP120 transistor has a DC current gain (h_FE) of...
at least 4500 @ 2.5A collector current and temperatures at or above 25°C
at least 2000 @ 0.5A collector current and temperatures at or above 25°C
That means that going for 1mA base current will give you ample current to supply the motor what it needs.
The base emitter voltage is 2.5V (it is a Darlington transistor).
The max. allowable base current is 120mA.

You have got 2.7V from your controller, which leaves 0.2V to drop over the base resistor, which should then be 0.2V/0.001A = 200 Ohm ~ select 180 Ohm if you haven't got 200 Ohm.

Suppose you run it without a base resistor. Let's say your assembly is so poor that you got the resistance up to 1 Ohm (highly unlikely), then your 0.2V would result in 200 mA.
A BjT needs a current limiting resistor to survive!
Regards,
Søren

A rather fast and fairly heavy robot with quite large wheels needs what? A lot of power?
Engineering is based on numbers - not adjectives

vsching

• Jr. Member
• Posts: 10
Re: h-bridge help
« Reply #2 on: April 29, 2009, 01:49:55 AM »
ok, i think i got the idea. Basically i need to take note of the Hfe graph and see the dc gain for my required current and calculate my required value. Then get the 2.7v from my controller - 2.5v =0.2 , then divided it by my value to get the resistor value. However what if its not a darlington transistor. 2.7-5, wouldn't it be negative? Then i need to use pnp instead of npn?Just a random thought thou ^^

Thanks ^^

Soeren

• Supreme Robot
• Posts: 4,672
Re: h-bridge help
« Reply #3 on: April 29, 2009, 06:34:55 PM »
Hi,

Basically i need to take note of the Hfe graph and see the dc gain for my required current and calculate my required value.
Well, almost, but you got the parameter name wrong...
hFE is DC current gain.
hfe is the small signal (AC) current gain.
That is a lower case "h" followed by a subscript of either "FE" or "fe" (I'm just too lazy to hit the subscript button all the time )

Then get the 2.7v from my controller - 2.5v =0.2 , then divided it by my value to get the resistor value. However what if its not a darlington transistor. 2.7-5, wouldn't it be negative? Then i need to use pnp instead of npn?Just a random thought thou ^^
As mentioned, the 2.5V is the Ube (base-emitter voltage drop) for the TIP120 transistor which is a darlington transistor (like two transistors piggybacked to increase the hFE (by the product of each individual hFE).
If a non-Darlington silicium based BjT is used, use 0.7V for the Ube and you should come up with:
2.7V - 0.7V = 2.0V and this is the voltage drop over the base resistor.

But... I'll repeat the part about your BjTs not being very useable:
Eg. the BC237 has a peak collector current of 200mA (100mA max. normal drive) and a total power dissipation (@ Tamb <= 25°C when mounted on FR4 board) of 500 mW. and a hFE of ~250 from 100µA to 2mA, ~110 @100mA and about 30 @ 200mA (which it should only see for a few ms max.

Get yourself a handfull of TIP120's, they'll come in handy in lots of circuits, due to their extremely high gain.
Regards,
Søren

A rather fast and fairly heavy robot with quite large wheels needs what? A lot of power?
Engineering is based on numbers - not adjectives

vsching

• Jr. Member
• Posts: 10
Re: h-bridge help
« Reply #4 on: April 30, 2009, 11:23:35 PM »
I got it. So the base will be connected to the resistor then to my microcontroller pins. The the collecter and emitter will be connected to my 5v voltage source. However i got 2 questions here. Do i need a voltage regulator?

Most linear voltage regulator i saw was 1A rated but my motor is actually running at 5v with no load draw of 0.28 and 2.4A(thats my question here) when full powered. Would this current value caused problem?

My microcontroller got internal clamping diode so do i need to use an optocoupler for my base connection since it has two difference voltage sources? If so,any value i need to take note when choosing 1? Will a 4N25 IC work?
« Last Edit: April 30, 2009, 11:40:10 PM by vsching »

Soeren

• Supreme Robot
• Posts: 4,672
Re: h-bridge help
« Reply #5 on: May 01, 2009, 06:32:50 PM »
Hi,

However i got 2 questions here. Do i need a voltage regulator?
For the motors? No, not if the supply is within the range of the motors.

Most linear voltage regulator i saw was 1A rated but my motor is actually running at 5v with no load draw of 0.28 and 2.4A(thats my question here) when full powered. Would this current value caused problem?
If your (motor) power source can supply the current that the motors draw, your only real problem is that the battery life will go down when current is going up.

My microcontroller got internal clamping diode so do i need to use an optocoupler for my base connection since it has two difference voltage sources? If so,any value i need to take note when choosing 1? Will a 4N25 IC work?
That's the good thing about using the common emitter configuration - the base and the collector can have different supplies, as long as they've got a common ground (0V) line.
The controller supplies the base current which in turn controls the collector current.

The short answer: No, you don't need opto-couplers, but if, on the other hand, you'd like to use them, you can.
Regards,
Søren

A rather fast and fairly heavy robot with quite large wheels needs what? A lot of power?