Diode SCR and Diode

[shermaine]

What's the difference between Diode SCR and Diode? Does they have different function?

 

[plot]

A Diode SCR is just like a normal diode, except it's controlled by a gate. The gate is basically a third line that goes into the SCR, and for the conduction of the diode (whether it's "ON" or "OFF") is controlled by the signal applied to the gate.

 

[zachtheterrible]

The cool thing about an SCR is that if current is allowed to flow from anode to cathode when a gate signal is applied, the current will continue to flow even when teh gate signal is removed.

 

[Dr.EM]

Hmm, never knew that. so, how would you stop the current flow? By switching it off, presumably at the current source, then the SCR is reset and non-concucting once more?

 

[Nigel Goodwin]

Hmm, never knew that. so, how would you stop the current flow? By switching it off, presumably at the current source, then the SCR is reset and non-concucting once more?

Yes, to switch it OFF the current through it has to drop below it's 'holding current', the SCR than drops back to being non-conducting.

This is why SCR's are commonly used in AC circuits, as the AC voltage drops to zero the device turns OFF, if the gate is kept HIGH it will turn ON again as the supply voltage resumes.

To turn them OFF with DC is a little more tricky, what's usually done is to switch a capacitor in parallel with it - this momentarily bypasses the current through the SCR and it turns OFF. This is often done using a second SCR, with the capacitor connected between them - as one turns ON, it turns the other one OFF. DC speed control for large motors (fork lift trucks and such) used to be done this way - in fact I've got a big SCR here which came out of a dodgem car at a fairground :lol:

 

[Dr.EM]

Cool, that clears that up very nicely

I have an SCR, thought it'd be a useful part to have available, only a TO-92 one though.

 

[Nigel Goodwin]

Cool, that clears that up very nicely

I have an SCR, thought it'd be a useful part to have available, only a TO-92 one though.

Hears a picture of the one I have :lol:

For scale, it's on a sheet of A4 paper, and that's my tea mug next to it!.

 

[Optikon]

Cool, that clears that up very nicely

I have an SCR, thought it'd be a useful part to have available, only a TO-92 one though.

Hears a picture of the one I have :lol:

For scale, it's on a sheet of A4 paper, and that's my tea mug next to it!.

I once saw one as big as your mug!

 

[Styx]

Cool, that clears that up very nicely

I have an SCR, thought it'd be a useful part to have available, only a TO-92 one though.

Hears a picture of the one I have :lol:

For scale, it's on a sheet of A4 paper, and that's my tea mug next to it!.

I once saw one as big as your mug!

I wonce worked with one (well 6) hocky-puck type, ie abt an inch thick and 6" diameter, silly amps

Thyristors are great!!!
But the commutation can be an issue and you have to come with funky cct (with lots-o-caps) if you need to force-commutate.

Eqaully the gate-drive for them are easy!!! pulse transformer iw an R-C on the output to shape the pulse

 

[Someone Electro]

WOW! :shock:

That thing is huge.How much curent can it pass?

 

[Styx]

WOW! :shock:

That thing is huge.How much curent can it pass?

Oh a few thousand...
traction mainly

 

[dar2525]

The SCR conducts by triggering the gate with a substansial amount of current or voltage. Need to check spec sheet.

Once the SCR is conducting, high current in must cases, the only way it goes into nonconducting mode is if the current conducting through the SCR falls below the holding current. The holding current is on the spec sheet, about 8mA. Once the current that passes through the SCR falls below 8mA the SCR stops conducting. It conducts again by triggering the gate.

Also, once the SCR is conducting, triggering the gate cannot cause nonconduction. In other words, the SCR is independent of the gate once it is conducting.

Reply or go to this website for more info.

 

[dar2525]

Hmm, never knew that. so, how would you stop the current flow? By switching it off, presumably at the current source, then the SCR is reset and non-concucting once more?

Once the SCR is conducting, high current in must cases, the only way it goes into nonconducting mode is if the current conducting through the SCR falls below the holding current. The holding current is on the spec sheet, about 8mA. Once the current that passes through the SCR falls below 8mA the SCR stops conducting. It conducts again by triggering the gate.

 

[Styx]

Hmm, never knew that. so, how would you stop the current flow? By switching it off, presumably at the current source, then the SCR is reset and non-concucting once more?

Once the SCR is conducting, high current in must cases, the only way it goes into nonconducting mode is if the current conducting through the SCR falls below the holding current. The holding current is on the spec sheet, about 8mA. Once the current that passes through the SCR falls below 8mA the SCR stops conducting. It conducts again by triggering the gate.

There are ways to force-commutate a conducting thyristor, this does involve causing the current through the device to drop below the threshhold current.

this always involves charging up some cpacitor somewhere so it can then use the charge voltage to reduce the forward voltage across the device => reduces the current flow.

A few R and C to have a time constant. The best bit abt force-commutation is once they start, they dont stop

 

[zevon8]

Styx, I have used those puck style ones before also. On DC galvanizing tanks. Scary current involved. Most were commutated by other SCR's. those beasts need serious clamping to get them to conduct, and provide low resistance connections, in the order of hundreds of pounds clamping force.

Sometimes a large pass transistor is also used in smaller circuits, effectively bypassing the current and turning the SCR off.

Here's a pretty good link to SCR's ( one picture shows a puck device ):

**broken link removed**

Other info on BJT's, Triacs, etc is also there.

 

[Dr.EM]

I've heard of this clamping of high current SCRs before, but never knew why you had to do it?

 

[shermaine]

Anyone can help me check if https://www.electro-tech-online.com/custompdfs/2005/06/sd1700c-k.pdf

is SCR type? I'm confused

 

[Dr.EM]

I think thats just a high current rectifier. It says standard rectifier at the top. It should say silicon controlled rectifier.

 

[Styx]

yup, standard diode, you can tell via the fact it says diode ( ) and also there is no gate connection on the mach drawing or any mention of gate specs.


Yup those puk type are great!!!, you need alot of pressure on them to ensure good electrical contract. An old collegue of mine worked on the DC-link connection to Europe (uses alot of thyristors) and they have carbon between them. They had the pressure right, however while the Thyristors were switching they would flex, after a while there was a build up of carbon dust that would every now and again fall and cause a nice beeg arch between thyrstor stacks.

Eventually got around it by chaging the potting they were using - would loved to have seen that tho

 

[zevon8]

...after a while there was a build up of carbon dust that would every now and again fall and cause a nice beeg arch between thyrstor stacks...

LOL, I bet it would. In my case, everynow and then there would be an electrical "anomoly" in the galvanizing plant feed, usually some big load getting dumped by an emergency shutdown, and the control box for the thyristors would lose its mind.

A few times when this happened, a couple of the puck's got detonated. Full-on hazmat team time. Berylium-Oxide dust just about everywhere inside the switchgear panel. Darn nice ka-bang tho. You could hear it all over the plant. I heard that now they will submerse these set-ups in cooling oil to help cooling ( obviously ) and to contain the material in failures like these.