1.5v drop across collector to emitter.

Thread Starter

ulms

Joined Mar 19, 2024
56
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?
 

Ylli

Joined Nov 13, 2015
1,089
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.
 

Ian0

Joined Aug 7, 2020
10,276
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.
 

LadySpark

Joined Feb 7, 2024
194
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.
 
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Ian0

Joined Aug 7, 2020
10,276
. 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
 
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Thread Starter

ulms

Joined Mar 19, 2024
56
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?


IMG_20240505_084929.jpg
 

Thread Starter

ulms

Joined Mar 19, 2024
56
@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?
 

LadySpark

Joined Feb 7, 2024
194
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.
 

BobTPH

Joined Jun 5, 2013
9,276
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.
 

Audioguru again

Joined Oct 21, 2019
6,783
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.
 

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