Hi

I've got a 2n2222 transistor switching power on to a circuit. I'm getting a 1.5v drop across the collector to the emitter. It's powered by 5 volts. The voltage after the emitter is only 3.5v but I want it nearer to 5v.

Any ideas?

 

Schematic with components values please.

But generally the base current needs to be increased.

 

The voltage after the emitter is only 3.5v but I want it nearer to 5v

That's interesting - 2n2222 is an NPN so normally it would be switching the collector low. You're not using it as a high-side switch?

 

That's interesting - 2n2222 is an NPN so normally it would be switching the collector low. You're not using it as a high-side switch?

I'll get you guys a schematic. Thanks

 

I'll hazard a guess he's using it as an emitter follower off a 5v MCU, with a load of several 100mA so base current is pulling MCU output down to ~4v hence 3.5v on the emitter...

 

Just to clarify, this will work:



But this will not:

 

Ok here is my circuit. I'm dropping 1.5v across Q4.

 

Ok here is my circuit. I'm dropping 1.5v across Q4. View attachment 321635

So I'll drop Q4 to low side of circuit?

 

You are lucky you are getting that much. Typically the emitter voltage will be 0.6 volts less than the base. With that LED i series, the base is getting not much more than 3 volts - I would expect the emitter to be closer to 2.5 volts. You'll have better luck with a MOSFET in that location.

 

When the Green LED is on, that current will turn on a PNP transistor. It will close very well.

 

When the Green LED is on, that current will turn on a PNP transistor. It will close very well.
View attachment 321636

I'll try it tomorrow Ty

 

You are lucky you are getting that much. Typically the emitter voltage will be 0.6 volts less than the base. With that LED i series, the base is getting not much more than 3 volts - I would expect the emitter to be closer to 2.5 volts. You'll have better luck with a MOSFET in that location.

You'll have worse luck with a MOSFET in that position, because the Vgs will be at least 1.5V, not 0.6V
It also depends on what sort of 555 you have:
A bipolar 555 would be lucky to get much more than 3.5V on the output, but a CMOS type will get to 5V, although some CMOS types have quite limited output current, especially sourcing.
But why does the relay have to be on the output side of Q4? If the relay current didn't have to go through Q4 it would probably work. You could connect the probes (and Q1's collector resistor) to the output of the 555 and eliminate Q4 altogether.

 

Ok here is my circuit. I'm dropping 1.5v across Q4. View attachment 321635

I would see that being quite normal for that device applied in that circuit.

Transistors are not trans-conductive devices. They are Trans-resistive devices. To make a transistor act like a switch, it has to have enough current gain to saturate the transistor. Where Q4 is in the circuit, it would be a series regulator and would always drop voltage even if it was a darlington transistor. Its sad that they don't teach transistor theory correctly anymore, and embraced an absurd model called "the transistor man" and just use the inverse transconductance derivative for the circuit calculation.

If you want Q4 to be a switch, use a trans-conductive device. I would use a mosfet instead of a FET because their resistance is lower across them, and they don't require a bias like a transistor would. If the input voltage was 12V or 15V The circuit could work more correctly if Q4 was set up as a series regulator and drop the voltage to 5V. But since it is a switch function you want, you want to use an enhancement mode MOSFET to switch it on with a voltage. You will have to find a mosfet that will have a low resistance like an TN0702N3-G with an 'on' resistance of 1.3 ohms across it with it in instead of a 2n2222 that has an 'on' resistance of 2.2K to 500 ohms across it depending on the voltage applied.

 

. I would use a mosfet instead of a FET because their resistance is lower across them, and they don't require a bias like a transistor would.

Nonsense. Used common drain (as in this circuit) a MOSFET needs more bias voltage than a bipolar.
2N2222 has a Vce(sat) of 0.4V @150mA, which is equivalent to Rds(on) of about 2.6Ω

 

Nonsense. Used common drain (as in this circuit) a MOSFET needs more bias voltage than a bipolar.
2N2222 has a Vce(sat) of 0.4V @150mA, which is equivalent to Rds(on) of about 2.6Ω

Hi

I've got a 2n2222 transistor switching power on to a circuit. I'm getting a 1.5v drop across the collector to the emitter. It's powered by 5 volts. The voltage after the emitter is only 3.5v but I want it nearer to 5v.

Any ideas?

I rewired my circuit to look like what boostbuck drew. It is working well. But why the huge voltage drop vs almost none in new circuit?

 

@LadySpark

To quote you, "Its sad that they don't teach transistor theory correctly anymore ... " Transistor theory! ... The only reason I graduated HS was 'cause I fixed my teachers cars in auto shop!

But seriously moving the location of the same npn 2n2222 trans changed the voltage drop Vce dramatically, no fet, no mos, no mosfets. How, why?

 

But seriously moving the location of the same npn 2n2222 trans changed the voltage drop Vce dramatically, no fet, no mos, no mosfets. How, why?

Because it increases the gain at base saturation voltage having resistance in the collector circuit.

 

For an NON tra

But seriously moving the location of the same npn 2n2222 trans changed the voltage drop Vce dramatically, no fet, no mos, no mosfets. How, why?

For a transistor to be on, the base must be about 0.7V higher than the emitter.

In the original configuration, you are putting roughly 4.2V (output of ‘555) on the base, so the emitter must be at 3.5V when the transistor is on.

In the second configuration, the emitter is at 0V, so the output if the ‘555 can easily turn it on fully.

 

Because it increases the gain at base saturation voltage having resistance in the collector circuit.

What?

 

Ulms, you should look at the datasheet of the LM555 (or NE555) to see the graph of its output voltage that does not go as high as the power supply voltage, it drops about 1.4V (at a low current).
You should also look at the datasheet of an NPN transistor that shows a 0.6V drop from base to emitter (at a low current).

Then the total 2V of voltage drops results in your output voltage of only 3V to 3.5V.