Problem with SCR.. or is it

A few wrong terms, the LM311 is a comparator not an op-amp and the SCR control electrode is called "gate" not base.

One possible problem is the SCR can be unlatched if pulled low by a low resistance, like the output of LM311. So the simplest cure for that is to insert a diode or small series resistor in the gate wire of perhaps

300-1000 ohm.

The random triggering could be noise. Is Vcc bypassed to ground with a capacitor? You could also try a small capacitor from SCR gate to ground of 100-1000pF.

Is there anything to unlatch the SCR or is it supposed to remain ON once triggered until supply is interrupted?

Where does the input CB come from, does it have a dc pathway to ground?

piglet

Thanks for the info,

The input is a walkie talkie, so one ground and one input I did put a electrolytic cap on the VCC and ground. Eventually there will be a NC switch in series with the LED.

K

Alright. Is the input to the lm311 straight across the CB loudspeaker? (in which case the chip sees its dc path to ground through the voice coil, which is fine) but if you have a capacitor in the way to the lm311 input then please try connecting a high value resistor (at a guess 100k) between the lm311 input and ground just in case the floating input is the cause of your random triggerings.

piglet

Ask him if he is building a trigger device for a bomb.

Captoro is over his head in electronics. Remember he's the one with the 300Amp 18 to 14.5V "regulator" task with no clue and no balls to face down the management. ...Jim Thompson

--
| James E.Thompson                                 |    mens     | 
| Analog Innovations                               |     et      | 
| Analog/Mixed-Signal ASIC's and Discrete Systems  |    manus    | 
| San Tan Valley, AZ 85142     Skype: skypeanalog  |             | 
| Voice:(480)460-2350  Fax: Available upon request |  Brass Rat  | 
| E-mail Icon at http://www.analog-innovations.com |    1962     | 
              
I love to cook with wine.     Sometimes I even put it in the food.

How short in time is the pulse on the input of the LM311 that is supposed to turn on the LED? And how were you planning to switch off the LED?

I 'guesstimate' a current of 20 mA through the LED:

9V Vcc - 1 V SCR - 2 V LED = 6 V over 0.33 kOhm = < 20 mA. The holding current varies from 5-10 mA which I find a bit close to your operating current, so for me this is a bit a 'scary design'.

You could lower that 330 Ohm resistor if your LED and application allows.

joe

So, he could use some help... ;)

joe

You don't need an SCR, latch it with the LM311 alone.

HI Guys,

By adding a pull down resistor on the gate stabilysed the gate. The input varies, depending if someone wants to talk to you LOL This project is similar to ringing sown on a phone for non seeing people.

K

Hi Jim

Just wrote in the other group. The 300 amp project is doable, just the specification concerning the heat dissipation will be a challenge. Nothing to do with management. The project will be underway at a university in Ontario to some friends of mine. I'll see how it goes in a few months.

Ken

[snip] [snip]

Must be a gov'ment project, no one is responsible.

Fry baby, fry >:-} ...Jim Thompson

--
| James E.Thompson                                 |    mens     | 
| Analog Innovations                               |     et      | 
| Analog/Mixed-Signal ASIC's and Discrete Systems  |    manus    | 
| San Tan Valley, AZ 85142     Skype: skypeanalog  |             | 
| Voice:(480)460-2350  Fax: Available upon request |  Brass Rat  | 
| E-mail Icon at http://www.analog-innovations.com |    1962     | 
              
I love to cook with wine.     Sometimes I even put it in the food.

[snip]

Bish Ray ?? ...Jim Thompson

-- | James E.Thompson | mens | | Analog Innovations | et | | Analog/Mixed-Signal ASIC's and Discrete Systems | manus | | San Tan Valley, AZ 85142 Skype: skypeanalog | | | Voice:(480)460-2350 Fax: Available upon request | Brass Rat | | E-mail Icon at

| 1962 | I love to cook with wine. Sometimes I even put it in the food.