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Optoisolator spec question

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Mark_R

Member
Hi,
can't believe I'm struggling with this.

I'm trying to select an opto isolator to control 12vdc, the part under consideration is here --> https://www.electro-tech-online.com/custompdfs/2009/10/ps2801.pdf

The spec lists the Collector to Emitter Voltage as max 80V, Emitter to Collector Voltage as max 5V.

Am I interpreting correctly that;

1.- The 80v c to e voltage is max open circuit when the transistor is switched off and the collector is positive in relation to the emitter, the direction one would normally use it, (thus 12v would be fine) and

2.- The 5 volt e to c voltage is the maximum it can withstand reversed biased (emitter positive in relation to the collector) without letting out the factory smoke?

Thanks for the brain check.
 

ericgibbs

Well-Known Member
Most Helpful Member
Hi,
can't believe I'm struggling with this.

I'm trying to select an opto isolator to control 12vdc, the part under consideration is here --> https://www.electro-tech-online.com/custompdfs/2009/10/ps2801-1.pdf

The spec lists the Collector to Emitter Voltage as max 80V, Emitter to Collector Voltage as max 5V.

Am I interpreting correctly that;

1.- The 80v c to e voltage is max open circuit when the transistor is switched off and the collector is positive in relation to the emitter, the direction one would normally use it, (thus 12v would be fine) and
The 80V refers to the maximum permissible voltage to the collector/emitter of the photo detector, NOT the actual working voltage that will be much lower, say 12Vdc.

2.- The 5 volt e to c voltage is the maximum it can withstand reversed biased (emitter positive in relation to the collector) without letting out the factory smoke?
The Vr is the max reverse voltage.

Thanks for the brain check.

Hi
I would suggest you post a circuit on how you think the opto should be connnected into a circuit, we can take it from there.:)
 
Last edited:

Roff

Well-Known Member
Hi,
can't believe I'm struggling with this.

I'm trying to select an opto isolator to control 12vdc, the part under consideration is here --> https://www.electro-tech-online.com/custompdfs/2009/10/ps2801-2.pdf

The spec lists the Collector to Emitter Voltage as max 80V, Emitter to Collector Voltage as max 5V.

Am I interpreting correctly that;

1.- The 80v c to e voltage is max open circuit when the transistor is switched off and the collector is positive in relation to the emitter, the direction one would normally use it, (thus 12v would be fine) and

2.- The 5 volt e to c voltage is the maximum it can withstand reversed biased (emitter positive in relation to the collector) without letting out the factory smoke?

Thanks for the brain check.
You are correct.
 

Mark_R

Member
diagram

Diagram attached.

Trying to transmit an open collector output from an RF modem chip to a micro-controller 50' away. Want to step up from 3.3v to beat the distance and increase SNR. Also want to isolate the OC output on the (expensive) transmitter. Intend to have another opto on the other end to interface eith the 5V micro-controller input.

OC is a open collector output of the modem It conducts to ground when active, high impedance otherwise. The spec on the OC states 10ma Max.
Speed is not an issue, the OC just activates to indicate the modem is active.
Modem operates on 3.3v.
 

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ericgibbs

Well-Known Member
Most Helpful Member
Diagram attached.

Trying to transmit an open collector output from an RF modem chip to a micro-controller 50' away. Want to step up from 3.3v to beat the distance and increase SNR. Also want to isolate the OC output on the (expensive) transmitter. Intend to have another opto on the other end to interface eith the 5V micro-controller input.

OC is a open collector output of the modem It conducts to ground when active, high impedance otherwise. The spec on the OC states 10ma Max.
Speed is not an issue, the OC just activates to indicate the modem is active.
Modem operates on 3.3v.

Hi.
Am I correct in assuming that the 50ft distance is via a wire pair.?

The usual method is to have the load resistor in the opto RX collector.
 

Mark_R

Member
Hi.
Am I correct in assuming that the 50ft distance is via a wire pair.?

The usual method is to have the load resistor in the opto RX collector.

Revised sketch attached. (Like that?)

The connection is via 7 core shielded cable.

This is an equipment monitoring control system. The host controller is indoors connected to an RF transmitter. The transmitter does not have enough power to remote it from the (patch) antenna more than a foot or so, so the transmitter has been integrated into the antenna housing. The antenna housing must be outside under open sky.

The transmitter communicates with the host controller at 9600 baud, 3V TTL. Using a MAX232 on both ends to convert to RS232 to beat the distance, improve the SNR, and make the transmitter able to communicate with a PC for other implementations. Similarly, I would like to step up the OC signal for the trip down the cable. The transmitter runs on 3.3V, the micro-controller runs on 5V.

The wires are;
+12V
Gnd
RS232 TX
RS232 RX
RS232 CTS
RS232 CTS
Open collector signal

The OC indicates modem status, it goes to an input (via another isolator) of the micro-controller. In the scenario where it communicates to a PC, I would run it to the RS232 RI pin.

Not using twisted wire, didn't feel it was required using shielded at 9600 up to 50'.
 

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ericgibbs

Well-Known Member
Most Helpful Member
hi.
I think I follow your description OK.:)

As you have free wire pair I would suggest if you are using the emitter follower output of the sender opto, that you break the emitter to 560R connection and insert the open end of the pair between the emitter and 560R.

At the receiver opto connect the pair across the opto diode.

This will provide a path for the from the sender opto to the receiver emitter.

Do you follow or would a diagram be helpful.?

EDIT:
This circuit is one option
 

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crutschow

Well-Known Member
Most Helpful Member
The usual method is to have the load resistor in the opto RX collector.
I don't believe it makes much difference with an opto isolator whether the load is in the collector or emitter, since the transistor is floating.
 

ericgibbs

Well-Known Member
Most Helpful Member
I don't believe it makes much difference with an opto isolator whether the load is in the collector or emitter, since the transistor is floating.

hi Carl,
I know that it dosnt matter in most cases, I was a little concerned about how the 50ft cable was going to be connected to the remote via the 50ft.

Look at the option circuit I have posted.:)
 

Mark_R

Member
hi.
I think I follow your description OK.:)

As you have free wire pair I would suggest if you are using the emitter follower output of the sender opto, that you break the emitter to 560R connection and insert the open end of the pair between the emitter and 560R.

At the receiver opto connect the pair across the opto diode.

This will provide a path for the from the sender opto to the receiver emitter.

Do you follow or would a diagram be helpful.?

I think I follow, but it is important to maintain compatibility with standard RS232 to a PC if needed down the road. I would like to have 1 pin which is 0V or 12v which I could run into the RI (ring indicator) pin on a serial port of a PC.

I'm assuming I could use the pin to sink current to the cathode of the remote opto? See attached....
 

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Mark_R

Member
how about...

I think I like this way better.....

Output wouldn't be inverted.
 

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Mark_R

Member
Hi Eric,

Just noticed your drawing. Makes sense, but I'm not clear on the reason for the diodes if unscreened. Are they to shunt out noise?

I took electronics 20+ years ago then barely used it since. I'm a bit (OK a lot) rusty.

Thanks for all your help.
 

creakndale

New Member
I prefer to place the current limiting resistor (R1) between the Supply voltage (3.3V) and the Opto's anode. The Opto operates the same but this orientation provides a little more safety when testing or troubleshooting.

As it is now, if someone, while probing the circuit with a meter or scope, accidentally grounds the cathode of the Opto, it could be damaged or destroyed. If the resistor is placed as suggested the Opto is protected from accidental short circuits.

creakndale
 

Mark_R

Member
I prefer to place the current limiting resistor (R1) between the Supply voltage (3.3V) and the Opto's anode. The Opto operates the same but this orientation provides a little more safety when testing or troubleshooting.

As it is now, if someone, while probing the circuit with a meter or scope, accidentally grounds the cathode of the Opto, it could be damaged or destroyed. If the resistor is placed as suggested the Opto is protected from accidental short circuits.

creakndale

Good point.
 

crutschow

Well-Known Member
Most Helpful Member
hi Carl,
I know that it dosnt matter in most cases, I was a little concerned about how the 50ft cable was going to be connected to the remote via the 50ft.

Look at the option circuit I have posted.:)
Why do you use two opto isolators? One would be enough to provide isolation between the source and the receiver. And two isolators requires a separate isolated supply which a single isolator doesn't.

You would only need two isolators if you want to isolate both signal and receiver grounds from the external common for some reason and I don't see that as a requirement here.
 

Mark_R

Member
Why do you use two opto isolators? One would be enough to provide isolation between the source and the receiver. And two isolators requires a separate isolated supply which a single isolator doesn't.

You would only need two isolators if you want to isolate both signal and receiver grounds from the external common for some reason and I don't see that as a requirement here.

The host equipment runs on 12V which runs some field devices, Etc. The 12V is extended out to the remote transmitter. The remoter transmitter has a 12 to 3.3V converter to power the transmitter IC as that is what it requires.

The host equipment has a 12 to 5V converter to power the µC as that is what it requires. The host µC and transmitter both speak TTL level 9600b serial, and can both tolerate 3-5 volts on the UART so they *could* be connected directly.

I figured that I should step the serial up to +/-12v RS232 levels due to the distance and to offer some noise tolerance. As I said, I would like to be able to use the transmitter module connected to a PC serial port for a different project down the road.

In the PC connected configuration I would need the OC output to trigger the RS232 RI (ring indicator) pin of the PC.

The 3 different voltages, along with the scheme to make the unit PC compatible led to the 2 opto idea.

That and the fact that the customer would make the final connection of the transmitter module cable to the host controller. I'm trying to make it as smoke proof as possible should someone cross the connections.
 
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ericgibbs

Well-Known Member
Most Helpful Member
Why do you use two opto isolators? One would be enough to provide isolation between the source and the receiver. And two isolators requires a separate isolated supply which a single isolator doesn't.

Carl,
Its simply because the OP wants to use 2 opto isolators.:)
Looking at his opening post.
 

ericgibbs

Well-Known Member
Most Helpful Member
I think I follow, but it is important to maintain compatibility with standard RS232 to a PC if needed down the road. I would like to have 1 pin which is 0V or 12v which I could run into the RI (ring indicator) pin on a serial port of a PC.

I'm assuming I could use the pin to sink current to the cathode of the remote opto? See attached....

hi,
To ensure noise immunity on a long RS232 cable run, the RI PC's, RS232 pin should really swing +/-5V [ at least]
 

Mark_R

Member
hi,
To ensure noise immunity on a long RS232 cable run, the RI PC's, RS232 pin should really swing +/-5V [ at least]

So then what if I just get a MAX3237 and run the OC through that as well. Right now I'm using a MAX232, but it only has 2 drivers and 2 receivers, which are all being used by TX, RX, CTS and RTS. The MAX3237 has 5 drivers and 2 receivers.
If I understand correctly, the 3237 will take the 0-3.3V logic and transmit them as -7/+7 (inverted) signals

Thanks for all the input.
 

ericgibbs

Well-Known Member
Most Helpful Member
So then what if I just get a MAX3237 and run the OC through that as well. Right now I'm using a MAX232, but it only has 2 drivers and 2 receivers, which are all being used by TX, RX, CTS and RTS. The MAX3237 has 5 drivers and 2 receivers.
If I understand correctly, the 3237 will take the 0-3.3V logic and transmit them as -7/+7 (inverted) signals

Thanks for all the input.
hi,
I have used MAX232's on marine applications to connect 'dc & pulse type' signals over distances upto 50mtrs, works very well.

For you application it sounds ideal, use a pull up on the o/c output to +3.3v in order to drive the MAX input.:)
 
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