Silicon Controlled Rectifier

[teh_pwnerer795]

Hey i am currently building a circuit and several websites have suggested to use an SLR. My question to you guys is, What is a Silicon Controlled Rectifier? What is it's sole purpose within a circuit?

Thnxs

[Binome]

An SCR is kind of similar to a transistor, yet significantly different. An SCR acts like an electronic switch, that once a current is applied to the gate, current can flow through anode and cathode until the current is removed from the entire device, compared to a transistor where the current has to only be removed from the gate.

[EricVanWyk]

What are you making that uses an SCR? Something high(ish) voltage?

Think of an SCR as a transistor that feeds itself. This allows a tiny control signal to activate it, but it means that you can't turn it off at the gate - something else needs to cut the current off.

This means that it has a lot of transistor like properties. Most importantly, its dissipation mode is a voltage drop - this makes it good for high voltage applications.

Please let us know what you are up to, it sounds interesting.

[teh_pwnerer795]

This year our robotics team could not afford to take part in FIRST Robotics. So we divide our team into two. Our group is building a EM Gun. Its going to be a 3 stage coil gun using #23awg copper wire. Our barrel is a aluminum hallow shaft measured at 3/8 of an inch. Im still trying to work out the numbers and schematics but everything takes time and research I am going to be using voltages around 350-400 and SLR's are apparently really good at controling and switching these high voltages and ampere's.

[Al Skierkiewicz]

An SCR is a silicon controled rectifier. This particular type of device can act as a switch and can be turned on with a very low control voltage but as it's name implies (and it's schematic symbol shows) it can only control current in one direction. It's other failure is once turned on, it cannot be turned off without removing the power that it turned on. This charachteristic while it sounds bad works very well in controlling pulsed DC power or in acting as a phase controled switch on AC power. It actually makes an interesting power supply when used in a bridge configuration. It's AC version is a triac and those are sued in virtually every light dimmer in use today.

[EricVanWyk]

I thought it might be something like that.

A word of warning - be careful when paralleling these devices. Add a small resistor to the gate of each SCR. This will prevent the gates from "ringing". Gate ringing destroyed my friend's coil gun that used SCRs. To over simplify, gate ringing causes the SCRs to turn on and off very very rapidly.

Also, I'm not sure if you need to add a ballast resistance to spread the current over multiple devices. Can someone verify this?

[lukevanoort]

While SCRs can work for you application, I'd only use them in a single coil coilgun design. MOSFETs or IGBTs are probably a better choice since you can turn them off. Also, don't forget to put a diode across each coil so that you don't fry your electronics from the current generated when the field collapses.

[teh_pwnerer795]

What type of resistor would i use>? 10 ohms? or like 10 million ohms?

[Kevin Sevcik]

Originally Posted by teh_pwnerer795

This year our robotics team could not afford to take part in FIRST Robotics. So we divide our team into two. Our group is building a EM Gun. Its going to be a 3 stage coil gun using #23awg copper wire. Our barrel is a aluminum hallow shaft measured at 3/8 of an inch. Im still trying to work out the numbers and schematics but everything takes time and research I am going to be using voltages around 350-400 and SLR's are apparently really good at controling and switching these high voltages and ampere's.

As far as your barrel material, you should remember that metals conduct and thus will generate eddy current opposing the magnetic field of your coil and robbing you of some power. There a few ways around this, but I'll leave that for you guys to discover.

Originally Posted by lukevanoort

While SCRs can work for you application, I'd only use them in a single coil coilgun design. MOSFETs or IGBTs are probably a better choice since you can turn them off. Also, don't forget to put a diode across each coil so that you don't fry your electronics from the current generated when the field collapses.

You could use SCRs if you powered each coil separately, which I think you'd be doing anyways. I think starting with SCRs would be a safer bet than IGBTs since they'll stay on until they've dissipated all the energy in the caps. This would reduce your rate of fire since you'd have to recharge from zero every shot, but you wouldn't have to worry about a charged cap wanting to kill you after a firing.

Also, it's highly important to understand the purpose and placement of the extra resistor and diode you'll be adding. The coil of the gun is basically an inductor, and will be trying to keep a constant current going through it. Once your cap is spent, the constant current from the coil will act to charge it in the negative direction, which is baaad for the polarized caps you'll be using. So the idea is to re-route this current through a diode and resistor to dissapate the energy stored in the coil. The circuit would look something like:

           SCR
 + --------|>-------
               |   |
      Resistor Z   3
Cap            |   3 Coil
         Diode ^   3
               |   |
 - -----------------

The size of the resistor depends on how quickly you want to dissapate the energy, and how large a voltage differential your circuit can handle. The maximum current going through it is determined entirely by your coil's inductance and cap's size. After that, Ohm's law will tell you what the voltage drop across it will be and how many watts it will dissipate at this peak current.

[Scott L.]

This site may help with your project.
http://www.powerlabs.org/index.html

[teh_pwnerer795]

Awesome.. im gonna wrap my mind around this for a couple of days. Im not that logical so it will take me a while. I will be posting my circuits within a fews days. It would be wonderful if any of you could lend suggestions.

thnxs

keep posting if you theres an easier way to explain either the SCR or bleeder resistor.

[teh_pwnerer795]

Hey! actaully... i think i somewhat understand this stuff guys below are two photos that i scanned just recently.. let me noe if this makes any sense before i blow myself up

NOTE* You will have to save the pictures and rotate/resize them. Srry any inconvenience.

Charging Circuit:
http://server6.theimagehosting.com/i...20Circuit2.JPG Circuit2.JPG

Fire Mech:
http://server6.theimagehosting.com/i...&public_view=1 Mech.JPG

C1 and C2 are basically linkers between the two separate circuits.

My power supply, Transformers, and capacitors are a big unknown question mark...?????? so again ... as mentioned before im still working out the numbers



NOTE* ifthe images are really unreadable... then let me know i'll rewrite and scan them with a better dpi

[teh_pwnerer795]

Hey guys im back with more questions

first question. is it possible to hook up two 6A diodes in parallel to get a total diode protection of 12A?

second question. if a bridge rectifer is rated for 25A, does that mean i have to put 25A through the rectifer for it actaully work?>?

Thanxs again..

if the question seems confusing, let me noe.

[Richard Wallace]

Originally Posted by teh_pwnerer795

Hey guys im back with more questions

first question. is it possible to hook up two 6A diodes in parallel to get a total diode protection of 12A?

second question. if a bridge rectifer is rated for 25A, does that mean i have to put 25A through the rectifer for it actaully work?>?

Thanxs again..

if the question seems confusing, let me noe.

1st: two 6A diodes in parallel will not share current perfectly, so their total capacity will be less than 12A. Also, the Ampere rating of a diode is generally tied to an assumed operating temperature; if actual temperature is higher then the diode will fail at less than its nominally rated Amperes.

2nd: the 25A rating on your rectifier is a maximum (again, at the given temperature, which is usually 25 Celsius) -- you can operate the bridge at lower current.

[teh_pwnerer795]

Hello everyone.

another question for my little project.

http://www.canadiantire.ca/browse/pr...08474396672500

this inverter supplys 120 VAC @ 300 watts (2.5A)

my question is, since there are two outputs, would it be possible to hook these in parallel to acheive a 120VAC @ 600 watts (5A)?