Using a DC voltage as the base for a transistor to amplify AC Voltage

Question

I'm a little new to electronics, particularly transistors, so I don't know a lot of the different types and inner-workings of them.

I have a diagram which, hopefully, will gradually increase the brightness of a light bulb over about 30 minutes. I use a DC RC circuit to slowly increase the voltage at the base of the transistor from 0 to 35V DC. The collector and emitter of the transistor are in series with the light bulb and a 120V AC power source.

I'm wondering what kinds of complications I might have with a transistor using DC as the base, and AC for the collector and emitter.

Edit: Here's my diagram

Sorry it's not the best quality, camra troubles today.

Answer 1

BJT transistors can't switch AC. They are also not suitable for switching the large currents you're planning on using them for.

I'm not an EE either, but here's my understanding:

A transistor is essentially a diode, the resistance (and thus the current flow) of which can be controlled. It limits the flow of forward current by acting like a variable resistor. It turns all the voltage that it limits into heat.

If you try to push too much voltage backwards through a transistor, you cause the diode to break down and fail. That's why it can't switch AC. The negative half of the AC sine wave tries to flow backwards through the transistor, likely destroying it in the process.

If you try to take 120V and knock it down to 30 V with a transistor, the 90 V drop will be dissipated as heat. Bad.

Transistors in linear mode (partly on, partly off) that are passing a lot of current generate a lot of heat. It's the nature of the beast.

A triac on the other hand is a circuit that is designed to switch AC. It has an input that, when you apply a control voltage, very quickly switches it on to the flow of AC current, with almost no resistance. When you remove the control voltage, the next time the voltage across the load terminals of the triac drop to zero, it switches off. When used correctly, the triac is either either fully on or fully off. (Near zero resistance, or near infinite resistance.) Ordinary dimmers work by using a triac to suddenly turn on the flow of current at some point in the sine wave of the AC power cycle. When at maximum power, the triac turns on at the beginning of each increase in voltage, and stays on. At minimum power, the triac is off for most of each +/- pulse, and turns on suddenly for the trailing edge of the sine wave. The average voltage passed through the dimmer is the area under the voltage curve (which is no longer a sine wave but a chopped up sine wave) divided by the time. Since the triac is nearly always either fully on or fully off, it doesn't generate much heat. It does, however, generate lots of really nasty harmonics in both the power output and the power lines it draws from. Thus some electronics can't deal with the output from a dimmer.

Answer 2

I'm impressed with the thought that must have gone into your circuit, but unfortunately it is fundamentally wrong.

For one: it is not the base voltage that you want to control / increase, but the voltage across base to emitter or even better the current flowing from base to emitter. The transformer and the load (lamp) prevent this.

Another thing that will potentially go wrong is that your transistor at 120V(AC) will dissipate an impressive amount of heat during 30 minutes, eventually killing it.

I can probably list several other things explaining why your circuit won't work, but the goal of this site is to help people creating electronics, rather than undermine their self confidence ;o)

The last issue I want to mention is that you force AC through the transistor where a BJT is only capable of controlling DC. As @JohnD mentioned in one of the comments, you should investigate how a TRIAC controls a lamp's brightness using phase control.

And at all times be very careful with mains power, it can seriously hurt you.