h bridge help

[aamir1]

friends i have designed two h bridges and attached its photos for you guyz....my question is which one is better to use?? and the second thing which i couldn`t get is im have checked both the circuits but the problem in 4 transistor circuit is that the N.P.N transistor downwards it has a voltage drop of about 1.13V but in the other circuit based of relay the voltage drop is 0.7V although the base current is kept same and the pwm is also same why is this so plz tell me

 

[WTP Pepper]

MOSFETs are normally common in H bridge drivers mainly due to their drive current being negligable. Much better than a multiple Darlington config you posted.

 

[ronv]

In the one with all transistors there is a voltage drop across the top transistors. Since the bottom transistors are emitter followers that voltage drop is also seen in the bottom transistors.

 

[aamir1]

actually i want to control a wheel chair with buttons the problem is i need variable speed using pwm im using avr microcontroller but when the pwm is decreased means average voltage is decreased the transistors go into active region and get heated a lot so what is the solution actually please guide me

 

[ronv]

What voltage is your micro running on? What is the pwm frequency? What is your motor current and resistance when stopped?

It could be the voltage is not high enough to turn on the transistors "hard", or the pwm frequency is to high for them to switch well. or your motor current may be to high for those transistors.

You should also add flyback diodes to the bridge.

 

[aamir1]

micro controller is running on 5V and PWM frequency is 32 KHz motor max current at no load is about 1A i hve added the back e.m.f diodes in the circuit i forgot it in the picture now what should i change to make it correct

 

[ronv]

You need a FET circuit. Are you in the US where you can get parts easily?
If not search around for a 75 or 100 volt logic level N channel FET rated at 50 amps or more.
Then look for a similar rating for a PFET. It does not need to be a logic level type.
If you can't find a logic level FET we can still do it. It will just be more complicated.
Please measure just the resistance of the motor so we can be sure of the stall current.

 

[aamir1]

y i cant use b.j.t ?.............if frequency is a problem i can reduce it as much as you need i used this frequency to avoid audible range so i chose above 20KHz although i can reduce to 3Hz also is frequency a problem for B.J.T or what ?? why does this circuit fail in front of mosfet h bridge.............


and second thing i also read about L293 I.C but it can only give 1A i need atleast 4A is there any way to increase its current at output and one more thing can L293 I.C be used with PWM also ??

 

[Styx]

you can use BJT's
but if you are looking at 20kHz I really would suggest using FET's

As to the difference in Vbe, its probably downto the current flowing through them. In the 1st the BJT's are directly driving the stator while in the 2nd just a lower current coil

 

[ronv]

You can use them - maybe. It is just a big waste of power, which means heat and less power to the motor.
Change the 580 ohm resistor to 20 ohms 10 watt. Depending on the motor resistance (remember that?) you may need to add some resistance in the base of the 3055. The 2955 and 3055 will need to be on a heat sink.
20Khz would be better.
If the motor stalls you may burn them up. depends on the motor resistance.

 

[aamir1]

ive made darlington pairs in the 3055 to increase beta why should i have to add some resistance at base of 3055 ?? and what about l239

 

[aamir1]

how can i check the resistance of motor can i use meter for that ?? beacause i dont have the datasheet of the motor and one more thing you said that motor takes max current when it starts but when i put load on to it its current increases more than the current in start so at what current should we design a circuit

 

[audioguru]

A motor is stopped when it starts running then its current is the same (maximum) as when you stop it when it is running (it is stalled). As the starting motor begins spinning its current decreases faster than your meter can show the peak current.
Measure the motor resistance with an ohm-meter and do not include the resistance of the wires of the ohm-meter. The peak current is calculated using Ohm's Law: current= V/R.

 

[aamir1]

thanks a lot audio guru i checked the resistance with meter it is 4.2ohms i took it 3 ohms so at 12V current becomes 6A but if i put load on the wheel of with my hand and try to stop it the current shoots to about 9A also why is this happening? means at what current my H bridge must be designed?

 

[audioguru]

Your power supply is 12V but the emitter-follower transistors have voltage loss so the motor is probably getting only 9V or less.

If the motor draws 9A when it is almost stalled then its resistance is less than 9V/9A= 1 ohm.
You probably measured the resistance of the wires of the meter.

EDIT: maybe you shorted the power supply with your current meter.

 

[aamir1]

no i measured the resistance by removing all connections of motor and ive not included the wire resistance ive found the datasheet it mentions current and resistance but i dont get it it says 8,3 in current and 0,24 in resistance what does it means ?

 

[ronv]

When the motor is turning it acts like a generator so the current is lower than when it is not turning. This is called back emf.
But when the motor is stalled (not turning but full power supplied) the current will be the voltage (12 volts) divided by the resistance of the motor windings. (0.24 ohms) so a maximum of 50 amps. This is what you should design for. Your motor is to big for your circuits.

 

[audioguru]

Your simple circuit has voltage losses so it does not supply the 12V power supply voltage to the motor. Since the motor is only 0.24 ohms and you measured 9A then your circuit supplies it with only 0.24 ohms x 9A= 2.16V when it starts and when it is stalled. Maybe your power supply cannot supply more than a few Amps.

It might draw 8.3A when has a light load and it is running with a supply of 12V or less.

 

[aamir1]

i am using a computer power supply it is of more than 12 Amperes in normal wheel chairs they use a battery im using a very heavy power supply but as ronv said that it needs 50A i cant design a circuit for that much big currentn nor i have a supply for that much current what should i do now whats the solution to make the wheel chair running please guide in detail

 

[audioguru]

A wheel chair does not use a computer power supply. Instead it uses a lead-acid battery that can supply hundreds of Amps when needed by the motor and its drive circuit.

Your weak simple circuit has too much voltage loss so it will get very hot when feeding the powerful motor a few volts.