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Author Topic: TIP122 + TIP127 problems  (Read 1259 times)
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I am attempting to build a H-bridge with 2xTIP122 + 2xTIP127. Before placing the four of them I started with a basic test with a single TIP122 and a TIP127.

My circuit is attached to this post.

So basically I am using a 9v battery as power source but I get readings of 3V when connecting multimeter to both collectors.

Any clues?

Thanks !


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texas
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I assume the transistor on the right is the TIP127.  To turn it on, you need to ground the base thru a limiting resistor.
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THANKS!

The voltage has now risen to 7.5V. I am not sure whether the motor might be causing such an important voltage drop (1.5V). I will keep investigating.

Thank you very much !

update: removing the dc motor causes the voltage to rise up to 8V. Measuring the battery voltage yields 9.45V.
update 2: using two 4.7k resistors causes the voltage to rise up to 8.5V. Still getting a 1V voltage drop when connecting the motor.

Any clues?
« Last Edit: February 18, 2013, 10:22:36 am by juanps90 » Logged

texas
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You will have some amount of voltage drop across each of the transistors.  Also, the 9V battery may not be able to supply enough current to the motor.  When doing your calculations, check the voltage of the battery under load and measure the voltage across each transistor and the motor.  They should add up to match the battery voltage under load.
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You will have some amount of voltage drop across each of the transistors.  Also, the 9V battery may not be able to supply enough current to the motor.  When doing your calculations, check the voltage of the battery under load and measure the voltage across each transistor and the motor.  They should add up to match the battery voltage under load.

The battery voltage remains above 9V even when on load. That means that there is a 1.5V voltage drop somewhere because my multimeter reads 7.5V between the motor wires.

Also, I think there is something wrong with the resistor values that I have chosen for the transistor bases. I tried 470, 2k2 and 4k7 and the greater the resistor was, the better the transistor performed so I think it might have something to do with that.

Thanks!
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You can use your DMM to measure between the collector and emitter of each transistor to see how much each is dropping the voltage.  I'm not sure why a larger resistor would seemingly turn the transistor on more other than it isn't wasting excess power in the base.  Two 470 ohm resistors at 9V amounts to a little more than 18mA into each base.  That's allot of base current, especially for a darlington setup like these transistors.  It's also a hefty load for a 9V battery to be dumping down the drain. I'm sure that the 4.7k resistors allow plenty of base current to flow since the minimum Hfe is 1000.
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Update: Testing the voltage with my multimeter reads 9.3V from a 9V battery and 5V from 4xAA batteries.

However, as soon as I plug the motor the voltage drops to 0V and the motor won't move.

I am going to try with lower resistor values. I am still not sure how to pick the right resistor value. I would be very grateful if you could explain that a little bit more in detail.

Thanks !
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Will the motor run when directly powered by the battery only.
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Sure.
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I am attempting to control the h bridge with a voltage of 5V and the voltage it will be driving will also be 5V. I was using a 9v battery for testing.

Expected current is under 1A.

Am i using the wrong components?
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