TTL Current Switching

[ScottyStills]

Hi there,

I'm trying to build a circuit for switching 12V, 2A power with TTL logic (+5V, 0V) voltage levels. Right now I'm using a MJ11033 PNP power transistor (rated 120V, 50A) with the collector at +12V, and the emitter connected to my load (bilge pump). For the biasing, I'm using an inverting small gain op-amp (741) setup, with the TTL logic going into the non-inverting input, and at the inverting input I have a 1k resistor to the ground, and 3k resistor going to the emitter of the power transistor. I have a 470 ohm resistor between the output of the 741 and the base of the transistor. For some reason when I put +12V power to the circuit, my pump is running continuously, regardless of the TTL level going into the input. On another note, if I disconnect the base altogether, my load will still run, which makes no sense to me considering there's no way the transistor should be in saturation if no biasing voltage has been applied. I've considered the idea of the transistor being shorted, and have used three brand new ones with the same result. Any ideas?

Thanks,
Scott

 

[ericgibbs]

Hi there,

I'm trying to build a circuit for switching 12V, 2A power with TTL logic (+5V, 0V) voltage levels. Right now I'm using a MJ11033 PNP power transistor (rated 120V, 50A) with the collector at +12V, and the emitter connected to my load (bilge pump). For the biasing, I'm using an inverting small gain op-amp (741) setup, with the TTL logic going into the non-inverting input, and at the inverting input I have a 1k resistor to the ground, and 3k resistor going to the emitter of the power transistor. I have a 470 ohm resistor between the output of the 741 and the base of the transistor. For some reason when I put +12V power to the circuit, my pump is running continuously, regardless of the TTL level going into the input. On another note, if I disconnect the base altogether, my load will still run, which makes no sense to me considering there's no way the transistor should be in saturation if no biasing voltage has been applied. I've considered the idea of the transistor being shorted, and have used three brand new ones with the same result. Any ideas?

Thanks,
Scott

hi Scott,
The way you have described the power transistor as a PNP, collector to +12v
and emitter to the motor,,,, its the wrong type of transistor polarity, should be NPN.

Post a circuit drawing.

Eric

 

[ScottyStills]

I'll scan a copy of the circuit diagram in when I get to my office in the morning. However, with an NPN transistor I'd be switching the ground on the motor, not the supply. With the NPN I'd have the +12V going to the motor, which would ground to the collector, and the emitter would going straight to ground, correct?

 

[ericgibbs]

I'll scan a copy of the circuit diagram in when I get to my office in the morning. However, with an NPN transistor I'd be switching the ground on the motor, not the supply. With the NPN I'd have the +12V going to the motor, which would ground to the collector, and the emitter would going straight to ground, correct?

hi,
With a NPN the collector would go to the +12V and the emitter to the motor.
With a PNP the collector would go to the motor and the emitter to the +12V.

I'll look at your diagram when you post, then I can be sure what to say.

Regards
Eric

 

[ScottyStills]

You know what? I'm an idiot! lol the entire time I was still thinking CBE in NPN land, and not that I should be doing EBC in PNP land. I couldn't figure out for the life of me why +5V wouldn't drive the transistor into saturation on its own, so I was using the op amp to push the full 12V from the main source into the base. Not required! Thanks for the kick anyway, I guess we all have our brain farts now and then!

 

[ericgibbs]

You know what? I'm an idiot! lol the entire time I was still thinking CBE in NPN land, and not that I should be doing EBC in PNP land. I couldn't figure out for the life of me why +5V wouldn't drive the transistor into saturation on its own, so I was using the op amp to push the full 12V from the main source into the base. Not required! Thanks for the kick anyway, I guess we all have our brain farts now and then!

Hi Scott,
That dosn't sound right to me?
A +5v/0v base voltage won't drive a high side power PNP transistor, On/Off completly.

Look forward to your drawing.

Eric

 

[Papabravo]

Connecting a TTL output to an opamp input sounds really suspicious. Have you tried this and convinced yourself that it works?

 

[audioguru]

The output voltage of an old 741 opamp doesn't go high enough to turn off a common-emitter PNP darlington, and doesn't go low enough to turn off a common-emitter NPN darlington.
A voltage divider made with two resistors must be used, then the base of the darlington won't have much current.

If you are using the darlington as an emitter-follower then the load will get hardly any voltage.

 

[ScottyStills]

Please check the circuit diagram I've attached. This one wasn't done by me, it was designed by another engineer friend of mine who actually still does electronic circuit work since university! I do data and voice networks now, I can't remember any of this detail!

At any rate, by applying a +5V to the base, it's saturating the transistor enough to run the 2A motor (with absolutely no external circuitry). If anyone has any suggestions though I'd be happy to hear them!

Thanks,
Scott

 

[audioguru]

I don't think the circuit will work:

 

[ericgibbs]

hi,
'agu' just pipped me to the post.

I agree with his conclusions, you need a rail to rail opa, the 741 has whiskers.

Eric

 

[ScottyStills]

I'm actually running the TTL at 0V/+5V, and it's driving the full +12V at the output with a high input, because it's running at a x3 gain it's essentially clipping at the +12V. For whatever reason though when I tried last night, the power transistor alone with +12V at the emitter, would power the motor when +5V high was applied to the base with no external circuitry. I don't know why it works, but it did...

 

[on1aag]

Hi Scotty,

I'm actually running the TTL at 0V/+5V, and it's driving the full +12V at the output with a high input, because it's running at a x3 gain it's essentially clipping at the +12V. For whatever reason though when I tried last night, the power transistor alone with +12V at the emitter, would power the motor when +5V high was applied to the base with no external circuitry. I don't know why it works, but it did...

Seems very logical to me, seven volts emitter/base voltage and probably
enough base current to blow the roof of your house !

on1aag.

 

[ericgibbs]

I'm actually running the TTL at 0V/+5V, and it's driving the full +12V at the output with a high input, because it's running at a x3 gain it's essentially clipping at the +12V. For whatever reason though when I tried last night, the power transistor alone with +12V at the emitter, would power the motor when +5V high was applied to the base with no external circuitry. I don't know why it works, but it did...

hi Scott,

As you say the motor will run continously, it turning OFF thats the problem.
To be sure of a firm turn OFF the base of the transistor has to be less than the Vbe voltage which about 1.4V for a Darlington. That means the output of the 741 has to reach at least +10.8V and it cant, because of the swing limitations of a 741 opa.
As a test remove the 470R from the output of the 741 and connect it to the emitter [take the 741 out of circuit] ie: between emitter and base, the motor should stop, if not the transistor maybe faulty.
Eric

 

[ScottyStills]

Which is exactly what I need to avoid! I want to run the TTL from an IO pin of a PIC, which obviously isn't going to give me any current to the base!

 

[audioguru]

A TTL signal never goes as low as 0V and nowhere near as high as 5V.

The circuit has positive feedback so its gain is not 3, the gain is infinite if the opamp was biased correctly.

If the emitter of the PNP darlington transistor is at +12V and the base is at +5V then the transistor is broken. The max base-emitter voltage of a darlington is about 1.5V, not 7V.

With a 12V supply, a PNP darlington clips at about 11V, not 12V.

 

[ScottyStills]

hi Scott,

As you say the motor will run continously, it turning OFF thats the problem.
To be sure of a firm turn OFF the base of the transistor has to be less than the Vbe voltage which about 1.4V for a Darlington. That means the output of the 741 has to reach at least +10.8V and it cant, because of the swing limitations of a 741 opa.
As a test remove the 470R from the output of the 741 and connect it to the emitter [take the 741 out of circuit] ie: between emitter and base, the motor should stop, if not the transistor maybe faulty.
Eric

Sorry we're getting confused now. The motor was running continuously last night because I had the emitter and collectors reversed, so the current was just blasting through the diode in the darlington for feedback prevention (because it was obviously wired backwards). When hooked up properly, I couldn't get it to turn on at all with the 741 circuit, so just tried pushing +5V directly to the base, which turns the motor on and off no problem. I'm concerned however with the base current, which needs to be next to nothing if it's going to be driven by a pic...

 

[ScottyStills]

A TTL signal never goes as low as 0V and nowhere near as high as 5V.

The circuit has positive feedback so its gain is not 3, the gain is infinite if the opamp was biased correctly.

If the emitter of the PNP darlington transistor is at +12V and the base is at +5V then the transistor is broken. The max base-emitter voltage of a darlington is about 1.5V, not 7V.

With a 12V supply, a PNP darlington clips at about 11V, not 12V.

Son of a.... Ok then I'm totally off track. Like I'd mentioned before, I haven't touched any of this since university, like 4 years ago. Ignore the circuit I gave you before. What I want to do, is run a 12V, 2A motor from a 12V ATV battery. I've built a +5V regulator circuit to drive my logic (pic 628), and I want to be able to current switch the motor's operation with one of the I/O pins from that chip. Suggestions?

 

[audioguru]

The max output voltage of a PIC is +5V. The base of the PNP darlington needs to be close to +12V for it to turn off.

You must use an NPN darlington with its emitter grounded. Then when its base is close to 0V it is off and when its base has current and is at about +1.5V then it is on.

I think you are mixing up NPN and PNP. You are also mixing up common-emitter and emitter-follower.
And you are connecting the transistors backwards.

 

[ericgibbs]

Which is exactly what I need to avoid! I want to run the TTL from an IO pin of a PIC, which obviously isn't going to give me any current to the base!

Scott,
Read my post again, I didnt suggest that you leave the 470R permanently connect its just to to try to convince you that the 741 is NOT suitable.
Find an OPA that is and your circuit will WORK!