Please help with TIP42 control circuit

I have a circuit wired up shown in attachment.

I can't get it to turn off. I know its an issue with resistor values just not sure where the problem lies.

I'm needing help getting the circuit to work.

The load I'm controlling is a load resistor of 6 ohms so at 12v I'm controlling around 2amps.

Thanks for any help...Also I have some p channel mosfets on the way just trying to make this tip42 tansistor work in the meantime.

Forgot value of R3 its 4.7k

Check the datasheet for the TIP42. Its drawn, labelled and connected incorrectly.

If you have indeed connected the TIP42 "upside down" with the collector to the 12V supply and the emitter to the load, then it most certainly will neither turn fully on, nor turn fully off. :astonished:

You may have blown it, note, since reverse-biasing the emitter-base junction more
than a few volts will destroy a transistor.

PNP and described as complementary for a reason:

PNP - emitter more positive than collector
NPN - emitter more negative than collector...

In a PNP device the emitter emits +ve charges, in NPN the emitter
emits -ve charges.

MarkT:
You may have blown it, note, since reverse-biasing the emitter-base junction more
than a few volts will destroy a transistor.

Only if you feed enough current in to damage it.

I can't get it to turn off. I know its an issue with resistor values just not sure where the problem lies.

... sort of implies that was attempted at least.

Only if you feed enough current in to damage it.

Once breakdown voltage occurs you will lose control of the current. I'll bet if you feed it 100V and put 100K in series with your supply to limit to 1mA you will still have a not so heavy paperweight.

Finally got my p channel mosfets.

it was indeed backwards didn't damage the tip though. now thats fixed I wan to get it working with the p mosfet.

I have attached the drawing with the changes. I can't seem to get it to work. When I measure gate voltage it goes from 8.9v down to 2.5v...never to ground so I know the problem lies there just not sure how to get it down to 0v or at least under the 2v that seems to be the min for the p fet to turn on according to the data sheet.

I also checked the resistance on the npn from collector to emitter with npn on and got .5 ohm so I feel the npn 2n4401 is working correctly.

last for the test I'm using a 1/2 watt 29k ohms as load until I get it working than transition to the 6ohm load resistor once I get it right

Still learning the world of transistors and fets...its a long road...thats just beginning

Thanks,

That circuit looks fine (although you should put 12V-In at the top so the P-channel
is drawn as everyone expects with source at the top).

You have probably damaged something given the voltages seen. Disconnect the
gate of the p-channel and measure the voltage at the junction of the 160 and 4k7
resistors, it should go between 0.4V and 12.0V if 12V-in is at 12.0V.

If it doesn't the 2n4401 is blown, if it does the p-channel is blown. Remember MOSFETs
are highly static sensitive devices without protection diodes and treat accordingly.

And double check all the resistor values and transitor orientations of course.

Hi, please relabel your circuit, 12V Positive Supply, Supply GND and Output.
This will make more sense in indicating what is connected to the 12V supply and what is the output terminal.

Can you also put on your diagram what voltages you are getting at the gate of the MOSFET with respect to GND with control of zero and 5V.
The source should always be +12v, the same as supply because that is what it disconnected to.
The drain should change with control.
The gate should be at 12V with zero control and near 0V with control on. (the 160 and 4.7K will be acting as a potential divider.)

The greater than 2V gate voltage you are trying to get, is measured with respect to the SOURCE not GND.
Read up on n-channel and p-channel MOSFETs and look at spec sheet, not matter what type it is, gate voltage is measured with respect to the source.
The transistor is a 2N4401 I hope, NPN.

Tom.... :slight_smile:
PS A picture of your project will also help.

So I have attached drawing with updated labels.

Here is the following voltages with regards to GND

Base of the 2N4401 0V = 0V 5V=.7V Also this transistor seems to work fine.

Gate of FQP27P06 transistor ON=2.5v tansistor OFF=8v

Source is 13v

There is 13v at r5..this just doesn't turn off or go to 0v because the gate voltage is too high to switch the mosfet off.

Before the 100ohm resistor I get 8v high and 0v low...after the resistor I get 8.9-9v high and 2.5v low

Should I remove the 100ohm resistor? and yes it was labeled 160ohm but its a 100ohm

Last I don't have a picture of the project as this circuit will go into a board with 10 channels

Once the circuit works I plan to test it with the 6ohm solenoid that will be used to actuate a hydraulic valve and yes I will put protection in circuit for the solenoid once this circuit is working correctly.

There is 13v at r5..this just doesn't turn off or go to 0v because the gate voltage is too high to switch the mosfet off.

I think you mean the gate voltage is too low to switch the mosfet off. It would need to be >= 11V to turn off the mosfet.
Try changing R3 to 2.2K to ensure the gate gets pulled up to >=11V when in OFF mode.

Should I remove the 100ohm resistor? and yes it was labeled 160ohm but its a 100ohm

Yes

Base of the 2N4401 0V = 0V 5V=.7V Also this transistor seems to work fine.
Gate of FQP27P06 transistor ON=2.5v tansistor OFF=8v

The base of the 2N4401 could have some noise when at 0V (signal hitting 0.3 or higher).
Try changing R1 to 470R

Measure the voltage with and without the gate connected. If the value changes the MOSFET is blown
since it should be taking no current at DC.

As I said MOSFETs are static sensitive and the symptoms suggest a blown MOSFET, which often start
to conduct through the gate if damaged.

No one new seems to learn about ESD. I've gotten in arguments about it.

Just because newer stuff is tougher than the original 4000 series CMOS chips, doesn't mean they are invulnerable.

Hi, please show us a picture of how you have everything currently connected, the MOSFET in particular.
It sounds like there is a problem with the MOSFET, possibly the way it is connected.

https://www.fairchildsemi.com/datasheets/FQ/FQP27P06.pdf

With the component facing you, the leads are left to right gate/drain/source.

As suggested earlier disconnect the 100R resistor and see if the MOSFET turns off.

Tom........ :slight_smile: