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shaiqbashir

Problem in an H-Bridge

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Hi Guys!

well! i have made the following H-bridge which is made up of BJTs. Im providing it with the PWM through my PIC. The problem that im facing here is this that my DC motor is just not running with this circuit. The PIC seems to be working fine but there is no motion in the motor. When i connect motor to the battery directly, it runs fine but now since i want to control it thru PWM, its not responding. I have tried to find out but couldnt find the problem out. Is there any technical problem in the following circuit of BJT???

Now below u will find three different circuits of H-Bridge. What do u think the best would be to use. I have a 24VDC motor(simple DC Motor).

If you would like to recommend me the MOSFET one then please tell me what MOSFETs should i use here. I think one should be IRF530 or IRF540 but what should be the other one?

Secondly, dont u think that the IC H-bridge would be the best one.

Thirdly, i want to give u this info that im supplying the circuit with around 17 volts DC supply. So please keep this thing in mind.

Your early reply shall be highly appreciated.

Regards

Take carez

Good Bye!

This is the BJT one (the one im using)

DC_bjt_hbridge.jpg

DC_mos_hbridge.jpg

HBridgeSchematic.gif

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Hi Shaiq,
You don't say what is the max starting current of your motor.
If it is only 1A or 2A then your circuit that uses darlington transistors should work.
The datasheet for the darlington transistors shows that your circuit's 1k input resistors might not give enough current to the darlington transistors for them to saturate well.

If the Forward input is positive then the Reverse input must be ground and vice-versa.
Set the PWM for max speed and measure the DC voltage of the Forward and Reverse inputs and the collectors of the active darlington transistors and you will quickly see why the motor doesn't run.

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Hi.

I recently "discovered" the VNH3xxx series of full integrated H-bridge chip from ST.

It might be a overkill for You, but it is a single-chip solution, that works fine with PWM.

But it is available only in surface-mount...

Anyway an interesting device and fullfills Your needs.

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hy
if you have done the problem whith your HBridge please post-it (the right solution).
i have a same problem my 2 Q on diagonal they bases dont open to let the curent/voltage flow to ground. Still in separated way they work fine themselves. 

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here
i used this circuit. 
But i have some trouble from Q1 and Q2. they are super heat up.
I put on them some  heaters/radiators  but still they are warm.
finaly i reach to conclusion that they need a fan. 
and they are steady finaly (from over-heat point of view).

i try to put some resistors on base or colector but the current flow are seriously diminish.  the motor not run.
i use BD 441  NPN-transistors and they hold out 80V and 4A.  i think this kind of transistors are much over the circuit requirement (they are very good). still they are heat.
in my opinion i think they heat because they are over-feed whith V or A  because they are first in circuit.

if you have an response to how can i exclude the over-heat from Q1 and Q2  i will be thankful. 

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Hi q12,
Your top two transistors overheat because they get hardly any base current so they don't turn on enough.
The bottom two transistors have their emitters grounded so when the computer signal goes high to about 4V, the base resistor has 4.0-0.7= 3.3V across it and makes lots of base current.
But the emitters of the top two transistors try to go up to about 3.3V when the computer signal goes high to about 4V and their base resistors hardly have any current.

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hai.....
i have same problem in designing the H-bridge 24V 6.4A, this is my circuit attach in below...
my circuit is work already but the transistor darlington are not work fully on......why?? as audioguru say that use 1k input resistors might not give enough current to the darlington transistors for them to saturate well, so how do i change it so that the transistor well work very well?? itz reduce the resistor to 470 ohm??? or more smaller?? any 1 can help me pls....... :)

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Hi Ys,
Your control signals A and B must be 0V as a low and +24V as a high. Where do they come from?

The TIP122 NPN darlington transistors have nothing to turn them off. When one is turned on with a saturation voltage of 4V then the other one gets a base voltage of 4V and it also turns on. Make a voltage divider using two resistors for each base to keep it turned off when it is supposed to be turned off.

Read the datasheets:
The TIP122 and TIP125 have a max continuous current of only 5A. Their max saturation voltage at 5A is 4V when it has 20mA of base current. The base voltage will be about 22V so your 1k base resistor will supply a base current of 22mA.
The max allowed base current is 120mA so reduce the value of the base resistors to 220 ohms if the driver allows it.

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hai audioguru..... :)

the input A & B i give it 5V and 0V to turn the TIP 125 ON at side B. or input A receive 0V and side B 5V then the TIP 125 will turn on at side A. this is my circuit H-bridge combine with the PIC controller.
%7Boption%7D
I saw at the datasheet that the Icm is 8A, itz max current that can support??
if the current flow more then that, what will be happen?? itz TIP will over heat??

how do i calcullate that which suitable resistor i should use?? and if i want to fully turn on the TIP, so i need to give the max base current at TIP and reduce the resistor?? just u mention that itz i sholud change the 1K resistor to 220 ohm for all the transistor TIP 122 and TIP 125?

from the diagram i attach, i using the relay as the switch to give input for the input A and B, i know that switch will a bit slow then using transistor, but i dont have any idea that how to use transistor as the input. cos if i direct connect the input A and B to the PIC controller, the driver will short circuit cos the input A and B receive low.

thanks lot for u answer my question.... :)

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the input A & B i give it 5V and 0V to turn the TIP 125 ON at side B. or input A receive 0V and side B 5V then the TIP 125 will turn on at side A.

The TIP127 needs a base voltage of about 22V to turn on and 24V to turn off. Your 9V battery and relay won't do it and are not needed. I modified your circuit so that the blue resistors turn on the TIP127 darlingtons and the red resistors turn them off.
The resistors and darlington transistors are too slow for PWM speed control but are fine for changing the direction of the motor.

I saw at the datasheet that the Icm is 8A, itz max current that can support??
if the current flow more then that, what will be happen?? itz TIP will over heat??

The surface of the chip will melt if the current is 8A for longer than about 30ms since you have a 24V supply. It can conduct 2.5A continuously if it has a good enough heatsink. It can conduct 5A continuously if the supply voltage is 12V or less and it has a good enough heatsink.

how do i calcullate that which suitable resistor i should use?? and if i want to fully turn on the TIP, so i need to give the max base current at TIP and reduce the resistor?? just u mention that itz i sholud change the 1K resistor to 220 ohm for all the transistor TIP 122 and TIP 125?

Calculate current with Ohm's Law. 1k gives a base current of 22V/1k= 22mA and 220 gives a base current of 22V/220= 100mA. The max allowed base current is 120mA.

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hai audioguru,
thanks for reply me...... :)

i dont really understand that u say:" The TIP127 needs a base voltage of about 22V to turn on and 24V to turn off". from the data sheet the emitter base voltage only 5V, as i know that 5V is the max to turn the TIP, am i wrong?? from the datasheet which part i have to refer to know that how many voltage to turn on and off the transistor?? cos i always refer at the base-emitter voltage.

the circuit u have modified i use the proteus proffessional 6 software to test it and it something wrong cos i didnt give any signal to microcontroller then the motor already turn on. i check back and found that itz the right 1K resistor TIP 127 that i circle have to connect to the positive 24V?? cos after i change it back that circuit are right.

why resistor for the base TIP 127 still using 1k resistor?? why didnt use the 220 ohm??
and also why u put the resistor 220 at the TIP 122 to the ground?? itz want to separate the current?? if remove it, itz can?? cos need to turn the motor with full speed.

from the circuit that u have been modified, itz that was fully turn on the TIP?? i mean that the motor will rotate with the full speed??? cos i really need the dc motor to rotate with the full speed of the motor. if not, how do i modified circuit so that motor rotate full speed?

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from the data sheet the emitter base voltage only 5V, as i know that 5V is the max to turn the TIP, am i wrong??

You are wrong. 5V is the max allowed emitter-base voltage. The emitter-base voltage is a reverse voltage when the transistor is turned off. A base-emitter voltage is a forward voltage when the transistor is turned on.

from the datasheet which part i have to refer to know that how many voltage to turn on and off the transistor?? cos i always refer at the base-emitter voltage.

The Base-Emitter ON voltage is called Vbe(on) in the datasheet. It is listed as a max of 2.5V when the collector current is 3A. A graph called Base-Emitter Saturation Voltage shows that it is typically 1.77V.

the circuit u have modified i use the proteus proffessional 6 software to test it and it something wrong cos i didnt give any signal to microcontroller then the motor already turn on. i check back and found that itz the right 1K resistor TIP 127 that i circle have to connect to the positive 24V?? cos after i change it back that circuit are right.

You are correct, I connected the resistor to the wrong place.

why resistor for the base TIP 127 still using 1k resistor?? why didnt use the 220 ohm??

Read my last reply about my schematic and learn the voltage and polarity that turns on transistors:
1) The blue 1k resistors turn off the darlingtons.
2) The red 220 ohm resistors turn on the darlingtons.

why u put the resistor 220 at the TIP 122 to the ground??

It divides the voltage from the collector of the other TIP122. The datasheet shows that the saturation voltage at the collector of a TIP122 with a collector current of 5A could high enough to turn on the other
TIP122. The voltage divider cuts the voltage in half so the wrong TIP122 does not turn on.

from the circuit that u have been modified, itz that was fully turn on the TIP?? i mean that the motor will rotate with the full speed??? cos i really need the dc motor to rotate with the full speed of the motor. if not, how do i modified circuit so that motor rotate full speed?

Typical TIP122 and TIP127 darlingtons work pretty well at 5A with the base current from 220 ohm resistors.

But there are strong transistors and there are weak transistors. If the TIP darlingtons are weak then each one will have a collector-emitter saturation voltage of 4V. Then the motor gets only 16V.
Mosfets would be much better.

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hai... :)

u say that max of 2.5V when the collector current is 3A can turn on the TIP but previous post u say that the TIP127 needs a base voltage of about 22V to turn on and 24V to turn off. which 1 are true?? im confuse?? im not understand about the meaning voltage saturation??

how do we know the TIP whether weak or not?? itz depend to the manufacturer??
itz true Posted by: bibinjohn say MOS is voltage controlled while bjt is current controlled?? if i change TIP using mosfet, how do i mmodified the circuit?? cos i not very familiar with the mosfet. IRF 530 & 9530 the VGS is +-20V, it mean that to on the Mosfet, max voltage is the 20V?? the circuit i change the TIP to mosfet itz ok cos i use the 24V to on the mosfet??

thanks a lot for replying me..... :)

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u say that max of 2.5V when the collector current is 3A can turn on the TIP but previous post u say that the TIP127 needs a base voltage of about 22V to turn on and 24V to turn off. which 1 are true?? im confuse??

The base-emitter voltage is about 2V (2.5V max for some darlingtons) to turn on.
The TIP127 has an emitter voltage of 24V so its base voltage is about 22V when it is turned on, and its base voltage is 24V when it is tured off.

im not understand about the meaning voltage saturation??

Saturation is when the transistor is turned on. The saturation voltage is the collector-emitter voltage across the transistor when it conducts current. Your motor receives less voltage because of the saturation voltages of two darlingtons.

how do we know the TIP whether weak or not?? itz depend to the manufacturer??

The datasheet shows that there is a range of specs for transistors, some are very good and some are not so good. When you buy one you don't know if it is very good or if it is not so good. Maybe a product manufacturer came before you and took thousands of the best ones. Maybe this transistor manufacturer cannot make them all very good.

I design circuits using the minimum spec's so that every circuit works. Some people design using "typical" spec's and some of their circuits don't work.

For the TIP125 and TIP127 darlington transistors:
The minimum current gain at 3A is 1000. Typical is 4000.
The maximum collector-emitter saturation voltage at 5A is 4V. Typical is 1.25V.

itz true Posted by: bibinjohn say MOS is voltage controlled while bjt is current controlled??

Yes.

if i change TIP using mosfet, how do i mmodified the circuit?? cos i not very familiar with the mosfet. IRF 530 & 9530 the VGS is +-20V, it mean that to on the Mosfet, max voltage is the 20V?? the circuit i change the TIP to mosfet itz ok cos i use the 24V to on the mosfet??

No.
The absolute maximum allowed gate-source voltage is 20V. They are spec'd with a voltage of 10V. You can use a two-resistors voltage divider to reduce the 24V down to 12V. 1k with 220 ohms makes only 4.3V. Two 10k, 1/4W resistors are fine for slow switching.

The max saturation voltage of the IRF530 at 5A is 0.8V if it is cool and 1V if it is hot.
The max saturation voltage of the IRF9530 is 1.5V if it is cool and about 2V if it is hot.
I corrected your Mosfet schematic.

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hai...... :)

The base-emitter voltage is about 2V (2.5V max for some darlingtons) to turn on.
The TIP127 has an emitter voltage of 24V so its base voltage is about 22V when it is turned on, and its base voltage is 24V when it is tured off.

u mean that if my supply is 12V, then to turn on the TIP127 is 10V and to turn off is 12V, m i right?? and for TIP122, it need 2 V to turn on, 0V to turn off?? itz true??

the mosfet is control by voltage, mean that we have to control the voltage to the gate mosfet to turn on and off?? how about the gate current ?? at datasheet which part i should refer??

for the bjt is control by current, mean that we have to control the current to the base bjt to turn on and off, but still need 2V to turn on and max current is 120mA, m i right??

for the voltage divider, if i use both 1k resistor, it still will get 12V. but the current will different and more higher. u using 10k resistor itz to reduce the current?? i dont know how many max current for the mosfet?? itz using 10k resistor is the max current for the mosfet??

thanks a lot for replying me... :)

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u mean that if my supply is 12V, then to turn on the TIP127 is 10V and to turn off is 12V, m i right?? and for TIP122, it need 2 V to turn on, 0V to turn off?? itz true??

Yes, but 2V is approximate because each transistor has a slightly different base-emitter voltage. It is the base current that turns on a transistor.

the mosfet is control by voltage, mean that we have to control the voltage to the gate mosfet to turn on and off?? how about the gate current ?? at datasheet which part i should refer??

Learn about Mosfets;
1) The Gate Threshold Voltage (2.0V to 4.0) barely turns on (250uA) an IRF530 Mosfet.
2) Static Drain-to-Source On-Resistance (with VGS=10V) is a max of 0.16 ohms when the Mosfet is cool.

The gate is a capacitor  that does not use DC current. The max leakage current is 100nA with the gate at 20V and is less with less voltage. The capacitance of the gate is high so fairly high current pulses are needed to charge and discharge the capacitance for the Mosfet to switch quickly.

for the bjt is control by current, mean that we have to control the current to the base bjt to turn on and off, but still need 2V to turn on and max current is 120mA, m i right??

Yes, in my examples the 220 ohm base resistor has about 22V across it which give a base current of
100mA.

for the voltage divider, if i use both 1k resistor, it still will get 12V. but the current will different and more higher. u using 10k resistor itz to reduce the current?? i dont know how many max current for the mosfet?? itz using 10k resistor is the max current for the mosfet??

The Mosfet does not have gate current but resistors higher than about 10k will charge and discharge the high gate capacitance slowly.
1k with 11V across it dissipates 121mW so a 1/4W resistor would be hot. I selected 10k resistors so they will not get hot.

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hai...... :)

The Mosfet does not have gate current but resistors higher than about 10k will charge and discharge the high gate capacitance slowly.

u mean that gate mosfet is the capasitor, so the resistor use to control the speed of charging and discharging the for mosfet switch, m i right?? itz use the concept first order circuit ( Source free RC circuit )? and the RC="tau". when the "tau" is low, the discharging will fast.

1k with 11V across it dissipates 121mW so a 1/4W resistor would be hot. I selected 10k resistors so they will not get hot

11V/1k = 0.011A
0.011A*11V= 121mW
1/4W=250mW

250mW-121mW=129mW

the resistor 1k still can hold for 129mW, it not yet over, why will hot??

if i use 100K resistor, itz better?? the switching will going very slow, m i right?  if i need it swithching mosfet at maximum speed, value resistor should more smaller?

how to i know the mosfet are fully on?? i need the mosfet fully on, so the my motor can receive 24V and rotate with full speed.


thanks a lot for replying me...... :)


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When Mosfets are used for pulse-width-modulation to control a motor's speed, they switch on and off at up to 100kHz. Then a gate current of up to 1A from a Mosfet driver IC is used to charge and discharge the high gate capacitance quickly. The Mosfets are heating for the time that they are not completely on or off, so the quicker they switch then the cooler they will be.

A 1/4W resistor will be extremely hot at 1/4W and will overheat unless it is in free cool air. On a pcb it is not free because the pcb and parts near it block air flow. So on a pcb its max dissipation is about only 200mW when it will be extremely hot. At half its rated power it will be hot but not too hot on a pcb but it will last a long time and parts beside it won't be damaged.

The Mosfets you selected are old and have a fairly high on-resistance which reduces the max speed and max power of your motor.

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hai...... :)

The Mosfets you selected are old and have a fairly high on-resistance which reduces the max speed and max power of your motor.

itz high on resistor have to refer datasheet at RDS(on), if more higher then the speed will go slow??

did u have any recomend using what type of MOSFET? if using IRF 1405, itz can maximum the speed motor??

thanks for replying me......... :)

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itz high on-resistance have to refer datasheet at RDS(on), if more higher then the speed will go slow??

No, on-resistance is the resistance of the Mosfet in series with the motor. The motor's current through the resistance cause a voltage drop that is reduced from the voltage for the motor. The lower is the on-resistance then the lower is the voltage loss.
The new Mosfet is more than 30 times better than the old Mosfet. The P-channel Mosfet has twice as much resistance and voltage loss as this one.
The on-resistance has nothing to do with switching speed.

did u have any recomend using what type of MOSFET? if using IRF 1405, itz can maximum the speed motor??

There are thousands of Mosfets. This one is very good but its voltage rating and its price might be too high. A P-channel Mosfet with the same spec's will be more expensive. Both Mosfets must be good for your motor to be at its fastest speed.

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hai audioguru, pls help me.....
still remember that H-bridge circuit that u have been modified for me........, i have test it and every thing is ok. But got 1 problem is when i doing the switching of the h-bridge circuit fast, the mosfet will over heat and burn the mosfet. i have put a heatsink for every mosfet but still very heat. If i just switch on the h-bridge circuit, the mosfet wont be over heat..... is there any way to reduce the heat so that the mosfet wont burn???

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The Mosfets are old and therefore have a high on-resistance. They will get hot if the motor's current is high.

Mosfets have a very high input capacitance that needs a fairly high drive current to charge and discharge quickly. The 10k resistor values are much too high for pulse-width-modulation control of motor speed because then the input capacitance of the Mosfets will slow down the switching speed so that the Mosfets get hot while switching slowly in a ramp
The 10k resistors were for the Mosfets to turn the motor on, then tuirn it off occasionally.

If you have high frequency PWM motor speed control then the h-bridge needs to have a Mosfet driver IC that has a fairly high output current.

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