SCR question

Make sure your load is on the anode side. Here's diode OR triggering:

• ----------+ --- | +---|

I think it's working now. I hooked up a transistor between the cap/res and SCR to amplify the short 'burst'. It seems like the burst may have been extremely weak to trigger the SCR directly previously. +V |

____ |/

PWR----||----+-----|____|--K NPN 10n | 2K2 |\\ | | _ ------> SCR Gate | | | | 10M - | | GND

Any suggestions or comments regarding this are very welcome, as I'm still learning! =)

Jason.

Hi, Jason. A couple of things:

• SCRs have a spec called I(gt), the minimum gate current guaranteed to turn on the SCR. The typical I(gt) for the C106Y is 50mA, and can be up to 200mA. A 555 just can't source that much current.

• Your transistor booster circuit will work much better for the C106Y. You're depending on the current gain of the transistor, though, to limit the current going into the gate of the SCR. Not only that, but you haven't taken the leakage current of the transistor into account -- for your other SCR, it could cause false triggering (although that's not much of a problem with the C106 line). You might want to add a couple of resistors like this (view in fixed font):

| | VCC VCC VCC VCC | + + + + | | | | | | .--o---o--. | | | | | | .-. | | | | ( X ) | | |.01uF | '-' | | | 3|| ___ |/ | | | o--||-|___|-o-| | | | 555 | || R1 | |> V | | | .-. | ___ - | | | R2| | '---|___|-o/| | | | | | R3 | | | | | '-' .-.| | | | | R4| || | | | | | || | '--o------' === '-'| | | GND | | | === o-' | GND | | === | GND | (created by AACircuit v1.28.6 beta 04/19/05

If you've got a 12V supply voltage, try 1K for R1, 33K for R2, 33 ohms

Also, there are SCRs which are made to be triggered on low gate current. They're called logic level or sensitive gate SCRs. Your BT169D is one of those. The NTE equivalent data sheet says gate trigger current can be less than 1mA. You should be able to drive it from the 555 directly without the transistor, but you may want to bump up the cap to 0.1uF or 0.22uF. This gives more time for the SCR to turn on. It might not be wise to use the transistor follower to crank so much currrent into this SCR -- you are getting close to the peak gate current rating.

I hope this has been of help.

Good luck Chris

Ummm.... take a look at

And where on earth did you get the Igt for the C106Y? According to the spec sheet:

The *maximum* trigger current I(gt) is 200 *micro* amps at 25 C The *typical* I(gt) is 15 *micro* amps. Your 50 - 200 mA is way out of line.

Maybe both of those datasheets are wrong? That's why I'd love to see where you got the specs!

That point is valid and important. The gate current *must* be limited to < 200 mA.

The circuit below is not necessary. His problem is dv/dt. The 10 meg resistor has got to go. He should try his cap/resistor circuit (no transistor) at 10nf and 10 or 15K instead of 10 meg. But he hasn't posted a complete circuit, so who knows what is connected to where.

Ed

Thanks for your comments guys.

The 555 was mentioned only as an example of triggering the SCR.... instead, it could be a logic gate output, or maybe just a transistor, or push button. Sorry to confuse the situation with the 555 example and the lack of diagrams, but valid points were raised regarding the float, etc. which I'll keep in mind.

Ed, I've tried taking away the transistor and replacing the 10M with 15K and the ceramic cap and is working fine! I didn't have a 10nF ('103') like you said to use, but I only had a cap marked "151"... It's working, that is the main thing. I will be getting more caps this weekend and I'll try the 10nF like you said.

I've hooked up a 22K drop-down resistor from the SCR gate pin to ground so that it doesn't falsely trigger at power-on (thanks for that Chris.... although the 220R you suggested wasn't high enough a value to trigger). ALL GOOD GUYS.

Oh hang on, I just realised I don't need the 22K drop-down anymore now that I removed the transistor, so just ignore that paragraph. Cheers.

Jason.

Hi, Ed. Thanks for catching that. Since the C106 is no more, I went to the NTE website, crossed to NTE5452, then misread their data sheet, which does say Igt is 50uA typ. / 200uA max. Thinking about those old clunker SCRs, I misread uA as mA.

ON semi also has this data sheet for the C106D:

The rest of your observations are also correct. I just missed that one, which is odd, because I worked with the part in days past. Thanks again for the spot.

Cheers Chrs

The link you posted to Onsemi is helpful! I had to "back into" getting there when I did my "fumble-fingered" searching. And my result is a link that is convoluted. Yours is straightforward. Nice.

Ed

Hi Jason,

I really didn't think about the value of the cap - I just thought that was what you had on hand based on your diagram. The issue is dv/dt, or from another perspective, tc. dv/dt refers to the speed at which the voltage level at the gate of the scr changes. tc is the time constant of the R/C connected to the gate.

You can use a "diode or gate" to experiment with multiple cap/resistor combinations, logic drives, switches, whatever to turn the scr on. Chris made the important point about protecting the scr from too much gate current - be sure to use a series resistor to limit gate current.

Ed

(LOAD)

___

\\ /

____ -----

TRG >--||---o--|____|--o------/| C106Y

| R2 | |

R1 | |C2 |

.-. --- V

| | --- GND

| | |

--- |

| |

V V

GND GND

C1 = 10n

C2 = 2n2

R1 = 15K

R2 = 2K2

C2 prevents false triggering at power-on.

IT'S WORKING!

yup, noted. Thanks again for your help Ed and Chris.

Jason.