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How do i use an SCR?

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Plasmaarc452

Mon Mar 24 2008, 05:25AM

Registered Member #1394 Joined: Sun Mar 16 2008, 06:18PM
Location:
Posts: 111

My brother bought a huge SCR for switching a multi kiljoule cannon, it is rated at 200 or 250 amps i can't remember.

I am sick on having to use spark gap switches for small 200 joule cannons because it scares the $hit out of me when i see the huge blue spark 6in away from my hand and the loud crack.

So why not put this SCR to use.....

Its looks exactly like this

My brother said something about there being the fat wire and the screw terminal as input and output and the small white wire as a trigger.

He said that it required a specific voltage and current to trigger that would require digital electronics. (Is this needed?)

I just thought it was like a relay where you can hook up a voltage source above the trigger voltage and it turns on.

Basically could someone explain how to trigger this SCR!

uzzors2k

Mon Mar 24 2008, 10:24AM

Registered Member #95 Joined: Thu Feb 09 2006, 04:57PM
Location: Norway
Posts: 1308

Your brother is in this hobby too?! Lucky. Why didn't you just get him to help you?

Anyway, use the numbers on it to get a datasheet which will give you all the info you need. The SCR is triggered by applying current to the gate which will latch it on. SCRs are like fire emergency doors, once open they can't be closed until people stop pushing through. So to stay closed the SCR needs a minimum current passing through it, and as long as this current flows it stays closed. (Not closed like a door, but like a switch.) Positive voltage to the gate, negative to the cathode.

Bjørn

Mon Mar 24 2008, 01:36PM

Registered Member #27 Joined: Fri Feb 03 2006, 02:20AM
Location: Hyperborea
Posts: 2058

Did you remember to check our wiki?

Plasmaarc452

Mon Mar 24 2008, 05:48PM

Registered Member #1394 Joined: Sun Mar 16 2008, 06:18PM
Location:
Posts: 111

Thanks for all the help, i have my scr right here.

The model # is 35 32 T7001235 86 39

I only this T7001235 is the model number but i will see in a second.

Alright, i need 150ma at 3v to trigger this thing.

Now i just need to put this into my design and get the correct resistance.

I just recently got my micro railgun to work for real, its shoots chunks of aluminum foil across the room. I can't wait to integrate this into it!

Correct me if i am wrong but i found out that i need a resistance of 360 ohms?

I have no idea if this is correct, this is what i did R=(3v-.6v) * 150ma

Someone should put an example of this in that hv wiki please!

Now from looking at the circuit i only have to connect the positive terminal of the battery to the gate? Or do i have to connect the negative terminal to somewhere on the circuit?

Its a dumb question but i don't want to break this SCR we got it for $70 but it retails for like $250 so we got a good deal but still i really don't want to break it.

Wow i am very confused as to this schematic on the wiki, it is confusing me like you would not believe.

Whoever drew the schematic connected the load to the gate? That makes no sense, if i did that i would be pumping 500v through it from a bank of capacitors at a high current. I thought this was like a relay where you apply a battery voltage that switches the larger voltage.

This is driving me nuts, Uzzors said i just need to apply a positive voltage to the gate so thats what i am going to try.

[Edit: Quadruple post]

EDY19

Mon Mar 24 2008, 06:41PM

Registered Member #105 Joined: Thu Feb 09 2006, 08:54PM
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Posts: 408

You really need to watch your quadruple posting... And for the current to the gate- Its alright to overshoot a little, the values on the datasheets are usually the threshold voltage/current, so it'd be safer to use a bit above this voltage anyways. For supplying the voltage to the gate, you want: (Vsource-.7V)/(Igate) I believe

Plasmaarc452

Mon Mar 24 2008, 07:29PM

Registered Member #1394 Joined: Sun Mar 16 2008, 06:18PM
Location:
Posts: 111

Yea but still i am confused as to how i trigger it, i can get a voltage source at 3v and 150ma but do i just connect the positive end of the voltage source to the gate and it turns on?

That is my question, i understand how to hook it up i just don't get how it is suppose to be triggered. The wiki got me really confused, it had the load connected to the gate now it says i need 3v to trigger my scr but if i connect the load to the gate it will be pumped with 500v.

Sry for the quadruple posting, i had a lot of questions and put them in 4 separate posts for no reason lol!

Bjørn

Mon Mar 24 2008, 07:42PM

Registered Member #27 Joined: Fri Feb 03 2006, 02:20AM
Location: Hyperborea
Posts: 2058

It seems prettly clear to me, there is a current limiting resistor connected to the gate. The load is clearly connected between the anode and the positive of the power supply.

Plasmaarc452

Mon Mar 24 2008, 08:40PM

Registered Member #1394 Joined: Sun Mar 16 2008, 06:18PM
Location:
Posts: 111

I see what this circuit does, but the load is connected to the gate.

This circuit uses a battery as the load meaning low voltage 3v and low current, i know that the resistor limits current but this circuit does not work in my case.

I cannot connect the load to the gate, my load is a 450v 3900uf capacitor fully charged so if i did connect it to the gate i would be pumping 450v through the gate when it was only made for 3v.

I see what this does, when you close the switch the resistor limits the current down to the trigger current and it turns on but the load is a BATTERY i am using a large CAPACITOR so if i did it this way i would need a high voltage trigger which my scr is not made for a high voltage trigger.

look at the attachment, will this rig work? or will i have to assemble this some other way. Remember i cannot connect connect the load to the gate!!!!!!!!!!!!!!!!!!!!!!

Dr. Dark Current

Mon Mar 24 2008, 09:15PM

Registered Member #152 Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384

I don't know what are you getting at. You turn on the SCR by putting a positive voltage (with a current limit) between the GATE and the CATHODE.

rp181

Mon Mar 24 2008, 09:19PM

Registered Member #1062 Joined: Tue Oct 16 2007, 02:01AM
Location:
Posts: 1529

the diaagram is correct, but the negative of the battery should be connected to the cathode.

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