Transistor question

[zippy]

So I'm decent at basic electronics but now for a project I determined that I need to use a transistor and I understand how they work. I just don't know how to implement it.

It only has 3 leads...emitter, collector, and base. The base allows the emitter and collector power the next thing, but how do I hook up the base? In an NPN transistor the base has to be more positive than the emitter and the collector more positive than the base...so how do I implement this? Where does the negative of the base go?

It's being used at the output of a monostable 555 timing circuit. I need to use a solid state relay on the end of the timer, but 555 timers do some unusual things with relays on the output and I have determined and also read that it is advisable to use a transistor before it. Could someone explain the wiring that I would use? The output of a monostable 555 timing circuit has a positive and a negative, in my case, at ~8-9V.

Thanks!

 

[etech]

Zippy, a transistor is a current amplyfing device. There are lots of pages that will explain how to hook up a transistor.

mitedu.freeserve.co.uk

Since you are using a solid state relay you may not have as many problems as you think. What is the current requirement of the solid state relay that you are using? It may be that the 555 can drive it directly. A solid state relay should not cause problems with the operation of a 555. A regular relay being a coil can cause problems.

555 data sheet


If you need to drive the SSR because of a higher current requirement than the 555 can handle you will take the output of the 555 to the base through a current limiting resistor. The value of the current limiting resistor will be determined by the amount of current through the emitter divided by the beta of the transistor.
calculating beta

A simpler approach is to use a mos-fet. They are a voltage controlled device that can be driven by the 555. I would need a better idea of your circuit but have implemented a IRF-510 that was driven directly by a TLC555. The circuit controlled the fan speed of the 120 mm fan that I have in the front of my case.

 

[zippy]

Yeah, sorry, I guess I got confused. I equated a solid state relay with a coil relay...that isn't the case? The relay I need to use has a coil to complete the circuit.

555 timer It's the monostable design...it's pretty simple.

Problem is that the timer does some weird stuff with the relay...I think the coil stores current or something...it does some strange things (such as trigger randomly...).

What is the benefit of a mosfet over a transistor? Really I just need something that will act as a bridge between the 555 timer and the relay (and yes, I do need a relay after the transisitor or mosfet).

Thanks.

 

[etech]

A solid state relay does not have a coil, that's why they are called "solid state". They are usually a transistor or mos-fet device with low to fairly low input requirements.

555s can be a little tricky on their triggering circuits. I have used an O-scope to make sure the waveforms were correct. That probably won't help you much though.

Check your grounds, make sure the 555 grounds are returned separately from the ground of the solid state relay. A spike of noise on the ground could be triggering the 555.


I found that for me the mosfets were easier to interface than transistors. With the mosfets you don?t have to worry about the beta of the transistor and just supply the necessary voltage. I was using the mosfet as a switch in most of the circuits though they will work in a linear mode also.

You should have seen the rack we built with 128 rolls of fiber that were controlled by that type of circuit. We had to keep the tension of the fibers within a very narrow band. A 555 and mosfet controlled the speed of the unwinding with the feedback coming from a dancer arm the fiber went over. That was a fun project.

 

[zippy]

Would you be able to explain how to implement a transistor or mosfet into this? It's the monostable operation 555 timer with a coil relay on the output of it (it needs to turn on the relay for x number of seconds and then off), but I want to put a transistor (or mosfet) in between the 555 timer and coil relay because the relay is finicky and, I think, stores charge from the 555 timer and does some weird sh!t.

Thanks.

NOTE: I am a novice...so if you can explain it, don't use advanced words, visual descriptions are good. What is the beta of a transistor? The base?

 

[zippy]

woops

 

[etech]

Beta(hfe) is the gain of the transisitor. It is especially important if you are running the transistor in the linear mode. In your case you are using it as a switch so all you need to do is make sure it is in saturation.

Read this


Another problem you will have to deal with is the coil on your relay. If you have the transistor mounted below(between the relay and ground) the relay then when the transistor turns off there is a very good chance of blowing your transistor. This may also be the reason for some of the strage activity you have noted.

When you stop current flowing through a coil it doesn't immediately stop. It will keep flowing. The current flowing into a high impedance( the closed switch/transistor) will build up and produce a high voltage. There are a number of ways to deal with this.
1)Place a diode around the coil. Hook the anode up to the low side of the coil and the other to the power supply voltage.

2)Use a zener diode with a limit voltage and hook it from the low side of the coil to ground.

There are others but either one of those should suffice for your circuit. If you use the diode a 1N4001 will work. If you use a zener make sure that its wattage rating is high enough to handle the load.

 

[zippy]

<< 1)Place a diode around the coil. Hook the anode up to the low side of the coil and the other to the power supply voltage. >>

The low side of the coil? What does that mean? What would the power supply voltage be - the output from the timer? I'm using this relay or this one (I think it's this one...).

Btw, it's this timer. They don't have the input, trigger, or output leads in the picture. If you want, I can take a digital photo of the instruction sheet for the kit.

 

[etech]

"low side of the coil"

The coil lead that is closest to ground. One side of the coil will be connected to your high voltage(high side). The other side of the coil will connect to your transistors collector lead.(low side).

The output of the timer will go to the base of your transistor. If you use the first relay you show the 555 should be able to drive it directly. I would have to think about it for awhile to be sure. I haven't used a reed relay in years.

 

[Bleep]

Gee guys what does the T stand for in MOSFET = Transistor. Use a optical isolator - almost no load on the 555 instant switching also.
Bleep

 

[zippy]

<< Gee guys what does the T stand for in MOSFET = Transistor. Use a optical isolator - almost no load on the 555 instant switching also.
Bleep >>

Um, what's an optical isolator?

 

[etech]

Bleep
"Gee guys what does the T stand for in MOSFET = Transistor. Use a optical isolator - almost no load on the 555 instant switching also.

Bleep , If you can't be of some frigging help why don't you STFU instead of padding you post count with useless garbage like "gee guys" a Metal Oxide Semiconductor Field Effect Transistor has a T in it. What little paitence I had with useless post count padding BS. If you don't know what you are talking about ask a question, don't try and act like an expert in a field that is obviously so very new to you.



"Um, what's an optical isolator? "

Not something you would use or need for the application that you are looking at. There are a number of good reasons to use optical isolators, a buffer is not one of them.
An optical isolator is a light emitting diode and a light sensitive transistor that are coupled together in the same package. They probide a very high electrical and insulation factor from one circuit to the next.
example
Sometimes you need one circuit on one power supply and a different circuit on another power supply. You cannot tie the power supply grounds together but you need to send a signal from one to the other, then you need an optical isolator.



BTW, the gloves are now offically off on all nefs, you don't like it, STFU. That is all.