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Normally Closed Optoisolators

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1

$begingroup$

I understand that typical optoisolators can logically be considered as "normally open". However I have a situation where I'd like the circuit controlled by the opto to be normally closed, mainly for the failure state but also so that the opto's led doesn't have to be activated for 99% of the time.

How can this be done on the transistor side?

Failure states aside, should I be concerned with having the opto activated indefinitely (in a normally open circuit)?

relay opto-isolator normally-open

share|improve this question

asked 8 hours ago

SpammySpammy

1833 bronze badges

$endgroup$

• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  $begingroup$
  Use an SSR instead, you can get isolated SSR's that are NO or NC
  $endgroup$
  – Voltage Spike
  8 hours ago
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  $begingroup$
  Can you recommend a part number? The circuit that needs to be NC is 12vdc
  $endgroup$
  – Spammy
  7 hours ago
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  $begingroup$
  Try CPC1117N. These normally closed relays are often called Form-B.
  $endgroup$
  – Steve G
  6 hours ago
  

  
  
  
  

add a comment | 

1

$begingroup$

I understand that typical optoisolators can logically be considered as "normally open". However I have a situation where I'd like the circuit controlled by the opto to be normally closed, mainly for the failure state but also so that the opto's led doesn't have to be activated for 99% of the time.

How can this be done on the transistor side?

Failure states aside, should I be concerned with having the opto activated indefinitely (in a normally open circuit)?

relay opto-isolator normally-open

share|improve this question

asked 8 hours ago

SpammySpammy

1833 bronze badges

$endgroup$

• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  $begingroup$
  Use an SSR instead, you can get isolated SSR's that are NO or NC
  $endgroup$
  – Voltage Spike
  8 hours ago
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  $begingroup$
  Can you recommend a part number? The circuit that needs to be NC is 12vdc
  $endgroup$
  – Spammy
  7 hours ago
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  $begingroup$
  Try CPC1117N. These normally closed relays are often called Form-B.
  $endgroup$
  – Steve G
  6 hours ago
  

  
  
  
  

add a comment | 

$begingroup$

I understand that typical optoisolators can logically be considered as "normally open". However I have a situation where I'd like the circuit controlled by the opto to be normally closed, mainly for the failure state but also so that the opto's led doesn't have to be activated for 99% of the time.

How can this be done on the transistor side?

Failure states aside, should I be concerned with having the opto activated indefinitely (in a normally open circuit)?

relay opto-isolator normally-open

share|improve this question

asked 8 hours ago

SpammySpammy

1833 bronze badges

$endgroup$

share|improve this question

asked 8 hours ago

SpammySpammy

1833 bronze badges

share|improve this question

asked 8 hours ago

SpammySpammy

1833 bronze badges

asked 8 hours ago

SpammySpammy

1833 bronze badges

asked 8 hours ago

SpammySpammy

1833 bronze badges

2 Answers
2

active

oldest

votes

6

$begingroup$

Instead of switching the actual load with the opto, use the opto to drive a transistor which switches the actual load.

However, note that if the opto fails short then that will open the transistor.

The risk could be potentially reduced by having two optos wired series on the output and parallel on the input so that both must close (or fail) for the transistor open.

^{simulate this circuit – Schematic created using CircuitLab}

share|improve this answer

edited 7 hours ago

answered 8 hours ago

DKNguyenDKNguyen

5,18011 gold badge55 silver badges2424 bronze badges

$endgroup$

• 
  
  
  

  
  

  
  
  
  
  
  

  
  $begingroup$
  Any thoughts on the second part of my question? Should I even bother - is it considered normal usage to just keep an opto activated for extended periods of time? I've ordered some 2N3904s and 2N3906s to add to my parts box anyway.
  $endgroup$
  – Spammy
  6 hours ago
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  $begingroup$
  LEDs wear out, but then again, so do transistors while conducting so it's six of one or half a dozen of the other. The LED would also consumes mA though that might not matter depending on how much your load current is conducting when normally closed. I guess it matters how fail-safe your thing has to be.
  $endgroup$
  – DKNguyen
  6 hours ago
  
  

  
  
  
  

add a comment | 

0

$begingroup$

Yes, there are DIP opto-MOS relays with depletion-mode MOSFETs on the output.

Or you can use a photovoltaic gate-drive optocoupler with a discrete depletion-mode transistor. Just remember to connect it to drive the gate negative.

share|improve this answer

answered 4 hours ago

Spehro PefhanySpehro Pefhany

220k55 gold badges175175 silver badges457457 bronze badges

$endgroup$

add a comment | 

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6

$begingroup$

Instead of switching the actual load with the opto, use the opto to drive a transistor which switches the actual load.

However, note that if the opto fails short then that will open the transistor.

The risk could be potentially reduced by having two optos wired series on the output and parallel on the input so that both must close (or fail) for the transistor open.

^{simulate this circuit – Schematic created using CircuitLab}

share|improve this answer

edited 7 hours ago

answered 8 hours ago

DKNguyenDKNguyen

5,18011 gold badge55 silver badges2424 bronze badges

$endgroup$

• 
  
  
  

  
  

  
  
  
  
  
  

  
  $begingroup$
  Any thoughts on the second part of my question? Should I even bother - is it considered normal usage to just keep an opto activated for extended periods of time? I've ordered some 2N3904s and 2N3906s to add to my parts box anyway.
  $endgroup$
  – Spammy
  6 hours ago
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  $begingroup$
  LEDs wear out, but then again, so do transistors while conducting so it's six of one or half a dozen of the other. The LED would also consumes mA though that might not matter depending on how much your load current is conducting when normally closed. I guess it matters how fail-safe your thing has to be.
  $endgroup$
  – DKNguyen
  6 hours ago
  
  

  
  
  
  

add a comment | 

6

$begingroup$

Instead of switching the actual load with the opto, use the opto to drive a transistor which switches the actual load.

However, note that if the opto fails short then that will open the transistor.

The risk could be potentially reduced by having two optos wired series on the output and parallel on the input so that both must close (or fail) for the transistor open.

^{simulate this circuit – Schematic created using CircuitLab}

share|improve this answer

edited 7 hours ago

answered 8 hours ago

DKNguyenDKNguyen

5,18011 gold badge55 silver badges2424 bronze badges

$endgroup$

• 
  
  
  

  
  

  
  
  
  
  
  

  
  $begingroup$
  Any thoughts on the second part of my question? Should I even bother - is it considered normal usage to just keep an opto activated for extended periods of time? I've ordered some 2N3904s and 2N3906s to add to my parts box anyway.
  $endgroup$
  – Spammy
  6 hours ago
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  $begingroup$
  LEDs wear out, but then again, so do transistors while conducting so it's six of one or half a dozen of the other. The LED would also consumes mA though that might not matter depending on how much your load current is conducting when normally closed. I guess it matters how fail-safe your thing has to be.
  $endgroup$
  – DKNguyen
  6 hours ago
  
  

  
  
  
  

add a comment | 

$begingroup$

Instead of switching the actual load with the opto, use the opto to drive a transistor which switches the actual load.

However, note that if the opto fails short then that will open the transistor.

The risk could be potentially reduced by having two optos wired series on the output and parallel on the input so that both must close (or fail) for the transistor open.

^{simulate this circuit – Schematic created using CircuitLab}

share|improve this answer

edited 7 hours ago

answered 8 hours ago

DKNguyenDKNguyen

5,18011 gold badge55 silver badges2424 bronze badges

$endgroup$

share|improve this answer

edited 7 hours ago

answered 8 hours ago

DKNguyenDKNguyen

5,18011 gold badge55 silver badges2424 bronze badges

answered 8 hours ago

DKNguyenDKNguyen

5,18011 gold badge55 silver badges2424 bronze badges

answered 8 hours ago

DKNguyenDKNguyen

5,18011 gold badge55 silver badges2424 bronze badges

0

$begingroup$

Yes, there are DIP opto-MOS relays with depletion-mode MOSFETs on the output.

Or you can use a photovoltaic gate-drive optocoupler with a discrete depletion-mode transistor. Just remember to connect it to drive the gate negative.

share|improve this answer

answered 4 hours ago

Spehro PefhanySpehro Pefhany

220k55 gold badges175175 silver badges457457 bronze badges

$endgroup$

add a comment | 

0

$begingroup$

Yes, there are DIP opto-MOS relays with depletion-mode MOSFETs on the output.

Or you can use a photovoltaic gate-drive optocoupler with a discrete depletion-mode transistor. Just remember to connect it to drive the gate negative.

share|improve this answer

answered 4 hours ago

Spehro PefhanySpehro Pefhany

220k55 gold badges175175 silver badges457457 bronze badges

$endgroup$

add a comment | 

$begingroup$

Yes, there are DIP opto-MOS relays with depletion-mode MOSFETs on the output.

Or you can use a photovoltaic gate-drive optocoupler with a discrete depletion-mode transistor. Just remember to connect it to drive the gate negative.

share|improve this answer

answered 4 hours ago

Spehro PefhanySpehro Pefhany

220k55 gold badges175175 silver badges457457 bronze badges

$endgroup$

share|improve this answer

answered 4 hours ago

Spehro PefhanySpehro Pefhany

220k55 gold badges175175 silver badges457457 bronze badges

answered 4 hours ago

Spehro PefhanySpehro Pefhany

220k55 gold badges175175 silver badges457457 bronze badges

answered 4 hours ago

Spehro PefhanySpehro Pefhany

220k55 gold badges175175 silver badges457457 bronze badges