Author Topic: Scope external trigger "box" for TTL and Mains  (Read 2430 times)

Offline PekkaNF

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Scope external trigger "box" for TTL and Mains
« on: February 05, 2016, 04:41:58 PM »
Hi,

Long time ago in school lab there were external "boxes" that connected to scope external trigger port. They were handy to trigger analog scope with digital signals and sometimes higher signals too.

Now, I have a two channel digital scope and would like to have means to use the external trigger. It should have (optical) isolation from the signal, because that would make make me feel whole lot better when measuring trigger signal on board that has optocouplers to separate processor from power signals :zap:

I tried to google economical device or DIY circuit, but I'm probably using wrong search phrases. Got some results to trigger scope with 8DI word and then this:
http://en-us.fluke.com/products/all-accessories/fluke-itp120.html

Probably very simple, like current limit + optocoupler and buffer that can drive scope BNC. Scope spec says: 1.2V for external trigger.

Ideas? Can try something on breadboard.

Pekka

Offline hopefuldave

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Re: Scope external trigger "box" for TTL and Mains
« Reply #1 on: February 14, 2016, 08:28:21 AM »
I'd treat it as two separate circuits, an input circuit to drive an opto-isolator's LED, output circuit using the opto-isolator's photo-transistor to switch the trigger voltage.

The input side probably wants a voltage divider (certainly if you're triggering from mains voltage! Consider the maximum current that could flow on e.g. 480V and size the divider resistors accordingly!), followed by a bridge rectifier (so you don't have too much worry over polarity) and a capacitor to give a transient with the change in voltage on its input if you want to look at edges (i.e. mains on and off), straight to the LED if you want to trigger from mains waveforms, hence a pulse for triggering that drives the LED (i.e. with power switch on/off Vs at a consistent point in the continuous waveform) - a Zener diode clamping the LED voltage to something it can handle is probably a good idea!

The output side will need a power source - the Fluke uses a PP3 9 volt battery, can't see anything wrong with that! The opto-transistor can be in the +ve line feeding a resistive voltage divider to get down to the 1.2v the 'scope requires and limit the current through the transistor.

Remember that attaching it to mains requires some serious attention to insulation and PCB design, several mm of air-gap between the two sides of the circuit, ideally an earthed strip between, and it's a good idea to thoroughly clean off any flux residue (which becomes conductive once it absorbs moisture from the air) and apply a conformal coating to reduce the likelihood of flash-over

An useful data sheet:
https://www.fairchildsemi.com/application-notes/AN/AN-3001.pdf - replace C1 with a bleed resistor and add a capacitor (with a switch to short it for waveform measurements - with sufficient insulation for the incoming voltage!) between the bridge rectifier and the diode to give a pulse/step with a change in input (AC or DC) voltage, perhaps add a Zener across the LED for reliability, could be worth a try?

If you want it to trigger from millivolt signals, you're going to need some kind of amplification up front on the input side, which will then need its own, separate, isolated power supply - that's when it starts getting complicated...

Hope this helps!
Dave H. (the other one)
Rules are for the obedience of fools, and the guidance of wise men.

Offline PekkaNF

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Re: Scope external trigger "box" for TTL and Mains
« Reply #2 on: February 17, 2016, 02:49:17 PM »
Thanks. I have been cheking out that info and I have concluded that most likely direct driving of opto with mains power or TTL is not the way to go.

I think that direct drive of the opto led would load some circuits too much. It needs and amplifier at front end. Output side of the opto looks whole lot more simple.

My first instict was to put differential front end to it, but that is difficult to build with gain selector and gain cnrol that has little effect on CMRR....possilble but might be a bit over my skills.

But single end front amplifier could work with a floating supply. So, this is what I'm going to check next.

Also I should find out how fast those optoisolators are. No point of using too slow circuit for trigger.

pekka

Offline PK

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Re: Scope external trigger "box" for TTL and Mains
« Reply #3 on: February 17, 2016, 05:46:50 PM »
Remember that an optocoupler is a VERY slow device. Turn on times are OK, but a 4 pin opto will take 250uS to turn off.
If you are looking to trigger of signals faster than a few 10's of kHz then either use a digital isolator, or an opto that exposes the base of the output transistor so that you can pull charge out of it.

Offline PekkaNF

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Re: Scope external trigger "box" for TTL and Mains
« Reply #4 on: February 18, 2016, 03:08:12 AM »
Remember that an optocoupler is a VERY slow device. Turn on times are OK, but a 4 pin opto will take 250uS to turn off.
If you are looking to trigger of signals faster than a few 10's of kHz then either use a digital isolator, or an opto that exposes the base of the output transistor so that you can pull charge out of it.

Thank you for reminder. I had "something" at the back of my head, but now that you put it right, it will be easier to search. I think I'll settle first with mains voltage and time domain that is relative to mains frequency, just about fine with opto isolator. SMPS measurements would get bit too involved at this point. That would be high atennuation symmetrical front end and differential amplifier?

I haven't really considered digital isolators....how ignorant of me. Had to search net and I think that I have seen those, sometimes packaged with a DC/DC converter . Something like this?
http://www.analog.com/en/products/interface-isolation/isolation/standard-digital-isolators/adum1200.html

or any reasonably good with DC/DC converter to power front end opamp?

I have been betting on wrong horse all the time?

Pekka

Offline PekkaNF

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Re: Scope external trigger "box" for TTL and Mains
« Reply #5 on: February 18, 2016, 02:55:01 PM »
Found some high speed optoisolators, but haven't used this type ever. It does not look that diffiicult to use, based on cursory read of the spec.

http://www.ti.com/product/6N137
www.vishay.com/docs/84732/6n137.pdf

Bit more reading on those digital isolators reveals that on the other side they seem to be easy to use. On the other hand, they must have some issues on their own like high switching frequency, probably need some more shielding than opto isolators.

Pekka

Offline PK

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Re: Scope external trigger "box" for TTL and Mains
« Reply #6 on: February 18, 2016, 06:38:39 PM »
Those high speed opto's look good. You can see that they have some signal shaping and output circuitry rather than just a photo transistor. It's not hard to do this bit yourself if you have access to the base of the transistor.

Re noise on digital isolators. I used one on our load cell calibration jig. It has a 24bit ADC on it and I experimented by turning the isolator off and on. Made no difference..

Offline PekkaNF

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Re: Scope external trigger "box" for TTL and Mains
« Reply #7 on: February 19, 2016, 03:58:21 AM »
Reasuring.

Re. noise. Have been reading some more and it looks like the noise of the digital isolators should not be issue (high frequency, easy to filter and low power levels).

however it looks like it needs some attention on those devices that have a DC/DC converter (too) in a "plastic" IC. I really did not know, just stumbled on this one AN-0971
http://www.analog.com/media/en/technical-documentation/application-notes/AN-0971.pdf
apparently not a show stopper or anything on general signal levels.

I remember having some layout issues with SMPSU circuits long time ago.

I read some limits also on magnetic/electric fields w/ digital isolators. Can't really relate it to yet on real life situations. I remember that HAL-sensors used on rotary encoders were big NO with big AC motors, a bit of a gaus and encoders were vomiting out whatever.

Now it looks like it is doable either way, each has it's forte and it looks like it all boils down what kind of front end I want there. One of the modern one that has digital isolators and DC/DC converter would offer low component count and size...old school is available on trough hole components and there is no need for me to learn to make PCB for itty bitty SMD:s :lol:

Pekka