Phototransistor optocoupler question

[Freddybaby]

Not too familiar with these devices. They give access to the internal transistor base and a few datasheets mention adjusting the on/off speed with this pin but not a lot of application information.

I need this thing to shut off in about 10uS but it's very slow (milliseconds)- the spec says 5uS on/off...

I did try Bob Pease' suggestion using a 2n3904's base tied to the emitter of the opto but I just can't make it work with this device...

I might be up against a wall in that I'm driving the input from a full wave rectifier (line driven) and about 0.1uF of filtering so the decay of the cap might be slowing me down.

My next attempt with be a comparator set to trip maybe a few millivolts below Vcc to catch the initial slope and hopefully trigger on that..

Any help with the opto's would be greatly appreciated

 

[karesz]

Hi Freddybaby,
A schema would be better to see...
k

 

[flatulent]

In general opto isolators with diode outputs are much faster than ones with BJT outputs.

 

[karesz]

Yea,
but he has a systematic problem/failure, than hes speed is ca by 1000(!) worser as datasheet predict...
K.

 

[Freddybaby]

Sorry, should have posted a schem but have been changing it around many times.. This is the basic
concept and what I have now.. It's still too slow...

 

[karesz]

Hi,
How much is your input current, and what about C2; a 100nF II to the 10MOhm?
K.

 

[Freddybaby]

I tried with and without C2, did not increase off time. I increased C2 and it slowed it down so I figured leave it out till I need it. Input current is about 7-10mA

I'm replacing the Schmitt with an LM319 and will set the threshold to a little below the max Vin and enough feedback that I can reduce the filter cap across the bridge (or I get an 8.3mS squarewave). See any problems with this approach ?

Again, I need fast HI when voltage is present and a fast LOW when it's removed. This is sensing voltage across a solenoid coil delivered by a zero crossing solid state relay.

 

[karesz]

I asked you over Input current, how much is it, are the LED resistores/impedances in range of data sheet?
The ca. 5usec Tsw is specified with in/out currents of 1mA! Check for fig. 8 pls...
For 117VAC their proposed 180K+ 22nF/200V combination or ohmic 285KOhm in part of LED!
I have had read somewhere, that (photo)transistors can be speeder switch if BE has a Schottky (Cathode at B) on(this is a knowed effect)..
Your switching time must be independent of schmitt trigger as load!, but more depend of the input part_why do you need the 100nF on the bridge?
I dont understand why you have it, if you are free of these "filter" your opto koppler wil, or can, follow the input half sinuses and that is what you need_I think!? If you need smaller/shorter output pulses as the halfwave(8.3ms) you must differenciat it befor connected to the schmitt (Highpass R-C filter)...
K.
Sorry I overflyed the input current of 7-10 mA...
I dont know how these component swithching time changes with higher Input current_but i think, that your factor of ca. 1000 is not from that, but more from the input filtering 100nF. I think you must verify it_is it (more as 1mA Led current) relevant by these copplers switching times or not?

 

[Freddybaby]

I tried input currents from about 1-10 with minor changes in speed.

I agree, the "delay" is from the filter cap but I needed this to keep the opto from switching at current zero. Trying to minimize component count

 

[karesz]

I think for zero current switching is better another types to select, maybe older MOCxxxx, I remember in some parallel topic is subject here. Title was some similar as how switch I my Triac...Is it nothing for you?
K.

Added after 19 minutes:

Moment pls;
you told yet delay, do you have a slow switch time or some switch time but with delay after your line pulse?

Added after 6 minutes:

If you apply a parallel load resistor on the bidge_you will speeder decrease the charge volumen in your 100nF. But it must be a pwoer resistor...

 

[Freddybaby]

Let me explain the actual application, perhaps I'm going about this the wrong way..

This is for a relay contact travel timing tester. I need a clean logic high (or low) the instant AC voltage is applied to the relay coil. Then a second sensor goes high/low when the contacts close.

I also test time to open contacts when the coil is de-energized. This is where I need a high/low the instant the AC voltage is removed from the coil.

My timing resolution is in the microsecond range so these 5-10 millisecond uncertainties are not going to work.

By the way, thank you so much Karesz for the great help..

 

[karesz]

Hi Freddybaby,
Tell me pls; is your relay a "true" AC type or DC, & is it i.e. a 5V/24V/... relay or higher voltage?
K.

 

[Freddybaby]

The relay coil is 115 VAC, 60Hz, May need 240V operation as well.. I wish it was DC, that would be simple..

 

[karesz]

OK,
I think your solution is if you will search for "Zero through going" switch-optocopplers...
I mean, synchron with the line pulse it will be (some delayed) "activated"...
Are the working contacts on AC voltages to sens too?
K.

 

[Freddybaby]

I will check into this type of opto. I still think it will "chatter" ever 8.3mS unless it latches but of course I'm not familiar with this device.

No the contacts will have a 10V DC bias and run into a Schmitt trigger, then to a digital count up timer.

 

[karesz]

Do you check for prellings too?

Added after 2 hours 47 minutes:

Hi Freddybaby,
Check pls these datasheets..
K.

 

[karesz]

Hi, I found on a parallel topic two zero crossing detectors for Triacs:
**broken link removed**
Tha one attache...
K.

 

[karesz]

Hi,
I found a new Toshiba Gate Driver for IGBT & MOSFETs, in a magazin as newts component, but I didnt get datasheet.....
TLP358x, has +/- 6A Output currents, up tu +125Cels,with 2mA input!, swiching times max.0.5 usec!
K.

 

[karesz]

Hallo,
If its actuell; I see today an articel of new components:
CPC1966, 3Aac,600Vp_Dual optically isolated power SCRs (www.clare.com...)
Clare, Inc., a wholly owned subsidiary of IXYS Corporation, has introduced the CPC1966, 3Arms AC Power Relay which is an AC solid state switch utilizing dual optically isolated power SCRs as an integrated alternative to discrete optocoupler and Triac circuits.

The device provides a blocking voltage capability of 600 Vp. In addition, zero-cross circuitry ensures AC load switching while minimizing the generation of transients.
**broken link removed**
K.