# Tag Info

1 vote

### How can I drive a 5 V LED from a 3.3 V digital pin?

If only 4-5 mA LED current are needed and a LED with above 3 V forward voltage is used, it can be done this way: simulate this circuit – Schematic created using CircuitLab When the main supply ...
• 3,532

### How can I drive a 5 V LED from a 3.3 V digital pin?

You seem to already know the basic NPN circuit and just want to know how to calculate the resistor values. A transistor in saturation will have 0.6-0.7 V on its base and the collector will be 0.1-0.2 ...
• 2,232

### How can I drive a 5 V LED from a 3.3 V digital pin?

You could make a simple capacitive boost circuit that can provide enough voltage to drive the LED directly from the MCU. This example shows a 25 kHz 3 V square wave with 4.4 mA into the LED at 3.1 V. ...
• 1,131

### How can I drive a 5 V LED from a 3.3 V digital pin?

Or you could use a MOSFET with a VGS threshold under 3.3 V (like a 2N7000). But NPN will work. If you use a MOSFET, make sure to add a series resistor of a hundred ohms to limit the current on the ...

### Reference voltage to two different logic outputs

Writing your requirements in a more graphic fashion we get this: ...
• 161k

### 3 Input AND Gate with RTL Logic

In this method the output does not suffer voltage loss
Accepted

### How does T3 turn on?

Pressing the test button will allow current to flow from T3's base through T2's base-emitter junction to ground. The T3 base current will turn on T3 to power up the MCU. The button needs to be held on ...
• 161k
1 vote

### How to control high current voltage with small current voltage linearly?

Here is a simple circuit that might give you some ideas. It's very inefficient (4W dissipation during charging, as you can see in the top trace). U1 controls the charging current at 500mA based on ...
• 337k
1 vote

### Mono audio amplifier repair help (Harman Kardon HK775): resistors getting hot

You can calculate what the power dissipation should be from the voltages and resistances shown on the schematic. $$P=\frac{E^{2}}{R}$$ For example R441 and R442 are shown as 12k, 1/2 W resistors. They ...
• 5,148
1 vote

### How to improve output swing of an NMOS pixel circuit?

However, for some reason the capacitor in this configuration doesn't charge up to 5V only to about 4,2V losing almost a volt on the output range. Can someone explain what is the reason for this? This ...
• 390k

### Ab amplifier input buffer with transistors

Note that the circuit you show will thermally run away and self-destruct unless Q5, Q6, Q3, Q4, D1, D2, D3 are all well coupled to the same heatsink. Since D1, D2 and D3 are shown to be glass-packaged ...
1 vote

### Ab amplifier input buffer with transistors

what are the D1,D2,D3 diodes, the R9 resistance and the pot2 potentiometer doing? They create a "fairly" stable voltage difference between the bases of Q3 and Q4 that is sufficient to cause ...
• 390k

### Simple inverter circuit, MOSFET heating up

It worked fine until I did something. I am not sure what I did. Just concentrate your eyes on the battery: -
• 390k

### Photodiode speed limitation

As mentioned by Andy aka, a transimpedance amplifier is a good solution for the purpose. The voltage over the photodiode will stay relatively constant, reducing the effect of the photodiode ...
• 5,008

### How does a bootstrap capacitor for a MOSFET keep the MOSFET on after the source is connected to main power supply?

The bottom of the bootstrap capacitor is connected to the source, and the top is connected to VCC via a diode when the FET is off and the S is at GND, the capacitors will be charged. Next, the top of ...
• 15.5k

### How does a bootstrap capacitor for a MOSFET keep the MOSFET on after the source is connected to main power supply?

Look at the schematic from the question you linked: Note that one side of the capacitor is connected to the source of the upper MOSFET. When Q2 turn on, Cboot charges to a diode drop below the 15V ...
• 20.3k

### Photodiode speed limitation

Looks like resistors across PD are too big in all cases. PD times in datasheet are measured at 10V and 50 Ohm load. It says nothing about PD internal transient parameters like diffusion capacitance ...
• 428

### Photodiode speed limitation

Use a transimpedance amplifier; you'll get bandwidth into the MHz at pretty decent gains too: - Image from The Fundamentals of Transimpedance Amplifiers. And, in many cases (yours included I'm sure) ...
• 390k
Accepted

### Using a CPU fan to drive a MOSFET on/off

Give this a run. The 0.8 V from the fan may just be enough to turn Q1 on, which will then turn off M1. simulate this circuit – Schematic created using CircuitLab
• 3,532
1 vote

### My Tesla coil is not working

A few things, for a lower power design the smaller coil would typically be more tightly wound, and it would be located closer to or over the bottom edge of (but still well insulated from) the larger ...
• 5,839

### My Tesla coil is not working

Made with microcap v12. All FREE. NB: this circuit is very "weird". Not simple to do it "working". Here is the behavior for this configuration ... Note that "coupling" is ...
• 6,124

### How do I invert transistor output?

You are very close. You want Q1 to invert the gate signal and have that control Q2. Your mistake is that Q2 is being controlled by the original gate. Now when gate is high, Q1 turns on and brings Q2’...
• 2,232
Accepted

### How do I invert transistor output?

Here is a way (but it might be easier to just use a CMOS gate or CMOS analog switch depending on the nature of the input).
• 337k

### How do I invert transistor output?

First, understand how a transistor is used as a switch. Current flow from the Base to Emitter causes the Collector to Emitter to conduct. This is a simplifed model, there are other rules to learn also....
• 10.3k
Accepted

### Driving a transistor or CMOS gate using 50 μA of low voltage (~60 mV)?

I was able to perform a simulation with a JFET LSK170A, where a signal of 0-100 mV produces an output of 1.39V to 526mV. It is 1.07V at 60mV. That JFET has a very high Beta (transconductance ...
• 1,131

### Driving a transistor or CMOS gate using 50 μA of low voltage (~60 mV)?

I was wondering if it's possible to have a tiny input current (60 mV, 50 μA) open a gate or transistor, allowing a 9 V battery to have a line to a high-impedance microcontroller digital input. The ...
• 52.2k
1 vote

### Transistor parameter in Proteus

You have a few options here. You can either select a transistor with the desired values, or if you really want to do it with a generic BJT, double click the part to bring up the Edit Component page. ...
• 11.5k
1 vote

### Lighting an electric match with an IRF520 transistor

Firstly you should be outputting a high signal to activate the FET, the N channel FET requires a positive voltage from drain to source to allow current to flow. Secondly the fet you’ve chosen is not a ...
• 1,175
1 vote

### Which particular transistor types represent triode and pentode vacuum tubes best?

The diagram is incomplete: the direct analog of the triode is the static induction transistor. By further analogy, I suppose one could make all the other poly-odes with the same method, but a much ...
• 5,128
1 vote

### Which particular transistor types represent triode and pentode vacuum tubes best?

For your use-case of trying to get a gross understanding of a tube circuit without understanding tubes, you can think of a vacuum tube as an N-channel JFET. This applies to circuits using either ...
Accepted

### Preventing overvoltage on output

Q7 will just burn when V3 gets too high because there is nothing in your circuit that implies V3 is current limited. When ideal voltage meets silicon in a fight, the ideal voltage will win. What's ...
• 390k

### AC analysis of three-stage BJT amplifier

I haven't worked through your calculations but have you taken into account the attenuation between the stages due to the loading on each stage of the input resistance of the following stage. For ...
• 5,976
1 vote
Accepted

### MOSFET is not turning on in bidirectional power switch circuit

Two things. First, an ADC pin is an input, not an output, and cannot drive a transistor into saturation. A GPIO pin in an output mode can do this. So can a DAC pin if it is programmed correctly. ...
• 13.2k

### Why is my SCR controlled by a photodiode pulsing when supplied with high voltage

This answer is based on many assumptions, it is just an attempt. One would expect, that the thyristor keeps conducting once triggered, because there is a DC load. But the datasheet says, that the ...
• 3,532
1 vote

### Compare same signal with different voltage thresholds using op amps

You are describing a window comparator application, which typically uses comparators instead of op amps. See one of my other answers to understand the difference between a comparator and op amp, and ...
• 7,090

### IOREF default voltage reference circuit with two NPN transistors

Here is a way to avoid the voltage drop, assuming the average current draw from the Vcc_B is modest: simulate this circuit – Schematic created using CircuitLab
• 337k
1 vote

### Transistors on op-amp power rails

It has always been a great challenge for me to uncover the ideas behind the brilliant circuit solutions of the famous Bob Widlar. My assumption here is the following: The classic push-pull output ...
• 9,468

### Trouble understanding output impedance of common collector

I know this thread is old and dead but you did want a simple intuitive answer to your inquiry…and so here it is. Imagine applying a source signal directly at the emitter. Initially the source would ...
Accepted

### Strange waveform observed using push-pull configuration when connecting an inductor

The abrupt change on the output signal is about 1.2 volts (as best as I can tell) and, this is due to the fact that your push-pull transistor output stage has no biasing and you get what is commonly ...
• 390k
1 vote

### An NPN transistor without its collector-base voltage

The BE junction will behave like a diode. However current won't enter the collector? Why? Because current needs a path to flow, it cannot jump!
• 647

### An NPN transistor without its collector-base voltage

As the collector current(Ic) doesn't depend on Vcb ... This is the faulty assertion. It's a good approximation of transistor behaviour when VCB > a few volts. It fails when VCB is less than that
• 144k

### An NPN transistor without its collector-base voltage

If you disconnect (open circuit) the collector, then no collector current will flow. At any base current, the transistor will saturate (i.e. VCE will be very small), and it will basically behave as a (...
• 15.5k
1 vote
Accepted

### Lithium iron battery bank bleed circuit

Two things: Your PNP transistors are connected upside-down -- swap the collector and emitter connections. This is not immediately fatal, however -- transistors do have some gain in "reverse ...
• 165k
Accepted

### Differences between low-power and high-power bipolar transistors beyond power handling

I just realised that for a BJT at least at the fixed Ic the transconductance gm doesn't depend on a structure size. It's defined purely by the Ic itself Indeed. It only depends on current. The power ...
• 61.9k
Accepted

### How does an NMOS transistor enter the triode region?

VGD> Vtn doesn't cause Vds < Vov. However in many circuits,the loading on the drain (e.g. resistors) means that as VGS increases, VDS does decrease, and ultimately may lead to VGD > VTH.
• 15.5k

### Behaviour of Voltage on Base in a fixed BIAS Common Emitter Amplifier

Start by taking a look at what happens to collector voltage as base potential varies. We'll also need to see what's happening to base current as base voltage changes. To do this, I've modified your ...
• 9,278

### What does it mean 'leaded' near junction-to-case resistance?

Rthj-c is the thermal resistance from the junction to the component body/case and it remains unchanged irrespective of the external thermal arrangement that the designer would make. (Image source: ...
Accepted

### Turning switch state at two different voltages

The usual "jellybean" solution to these kinds of questions uses a comparator configured with positive feedback, to form a schmitt trigger with very clearly defined switching thresholds: ...
• 9,278

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