Help: lowest input current optocoupler

Thread Starter

Alex1965

Joined Feb 8, 2018
13
Hi all,
I am searching for the lowest input current optocoupler to turn on led.
I have tested 4N35, it has 0.3mA, but I want a low current possible.
I have 5Vdc input, speed is not a problem, it works in DC only.
focus is to turn on a 12Vdc relays on the transistor side.
Do you have some suggestions for the right component?
 

Alec_t

Joined Sep 17, 2013
10,896
I am searching for the lowest input current optocoupler to turn on led.
I'm not aware there is a lower current limit to 'turn on' a LED. Even a few uA should give some optical output, and hence a detectable current through the output transistor. I think you need to search through some datasheets and pick an optocoupler with a relatively high CTR (current transfer ratio).
 

philba

Joined Aug 17, 2017
960
Like others, I'm not exactly sure what you are looking for. Context is important here.

However, the LDA111 darlington optoisolator appears to have super high transfer ratio and a very low If (70 uA). Not sure I'm reading the datasheet correctly, though, as it talks a lot about Ifs in the more typical 1-20 mA range elsewhere.

edit: by the way, it looks to be blinding slow on turn off - > 300 uS.
 
Hi all,
I am searching for the lowest input current optocoupler to turn on led.
I have tested 4N35, it has 0.3mA, but I want a low current possible.
I have 5Vdc input, speed is not a problem, it works in DC only.
focus is to turn on a 12Vdc relays on the transistor side.
Do you have some suggestions for the right component?
I think that the number of responses that you have already received to your first post on the site evidences the willingness of folks here to help.

Maybe only for my own education, but also toward helping...

The If measure for an optoisolator is the forward current. Seems to me that alone, it is almost meaningless. What you want to know is the CTR at an If, and specifically a very low If.

When you say "I have tested 4N35, it has 0.3mA, but I want a low current possible." I am a bit puzzled. I don't see anything in the 4n35 data sheets to suggest that at .3mA your CTR is going to be more than .3mA - in fact, I think it would be much less - that is what I mean by the lowest If being meaningless unless you link it to a CTR, since the current switching capability on the transistor side is what you need to operate the 12V relay - right?

So, for example, look at the data sheet for a 6N139M https://www.mouser.com/ds/2/308/6N139M-1118397.pdf
I see on page 4 that the CTR at .5mA If (at 4.5V) is 400 minimum and 2000 typical. So, theoretically, you could switch a load on the transistor side of 200 mA at least and typically, 1000mA. Of course, there are limits on the current handling and power dissipation of the transistor side, so I would not be expecting to switch a 12v, 1A load with .5mA through the led at 4.5v.

But, hopefully, you get what I am saying.

So, anybody, correct me if I am wrong (and I certainly can be), but my thinking is that If (except at 0 to evaluate current leakage of the transistor) is meaningless without a correspondin CTR measure.
 

LesJones

Joined Jan 8, 2017
2,493
You could save power on the driver side of the coupler by avoiding it being permanently driven. The first way is to have 2 opto coulpers. On the receiving side you would have a flipflop which was set by one of them and reset by the other. You would then only use power for a few tens of microseconsds (Depending on the speed of the opto coupler.) each time you needed to switc the load on or off. The second way would be just to use one opto coupler and send one code for on and a different code for off. Saying EXACTLY what you are trying to achieve would be a great help to getting a suitable answer.

Les.
 

Thread Starter

Alex1965

Joined Feb 8, 2018
13
I want to turn on a 12Vdc relay finder type 40.61 (RL1 is the coil).

I need a low current optocoupler on led side, in order to turn on led (and RL1) with a smartphone 5Vdc.

Low current is necessary for SmartPhone battery duration.

I have tested attached circuits.
In the first with an old TLP571 on R1 I measure 0.5mA.
In the second with 4N35, on R1 I measure 0.3mA.

R1 is the maximum resistance possible in order to turn on the relay with the minimum current on led side.
 

Attachments

philba

Joined Aug 17, 2017
960
To save power on the LED side of the opto, use a flip-flop on the other side to create latching relay. First pulse is on, second is off. The pulses can be on the order of 100s of nanoSeconds so the power consumption would be tiny.
 

Thread Starter

Alex1965

Joined Feb 8, 2018
13
To save power on the LED side of the opto, use a flip-flop on the other side to create latching relay. First pulse is on, second is off. The pulses can be on the order of 100s of nanoSeconds so the power consumption would be tiny.

Great! You are a genius!
Could you send me a simple circuit example, using a 4N35 or TLP571?
 

philba

Joined Aug 17, 2017
960
It's not that hard to do. just use the pulled high output from the phototransistor in the opto as the clock to a flip flop and route the /Q output to the input. Then the Q output goes to your relay driver. I don't have time to lay out the circuit, maybe some other kind soul could help.
 

ebp

Joined Feb 8, 2018
2,332
Be careful using an optocoupler to provide a clock for an edge triggered device. It is certainly possible to do, but rise and fall times from ordinary couplers are long, which may cause problems. A schmitt trigger will clean up the signal. If two signals are available on the input side of the coupler, a low-cost dual coupler could be used to drive an RS latch. This has the advantage of always going to a known state. With a clocked FF it is easier to get into an unknown state (state at power-on is indeterminate so a power-on reset is required; POR circuits, depending on demands placed on them, like proper behavior with brief power drops, are surprisingly hard to do; though there are many POR ICs available - they tend to be stupidly expensive; low performance POR can be done with a resistor and a capacitor & maybe a diode) Another option if two signals can be used is to use a magnetically latching relay. They are a bit more expensive & limit choices. A two-coil type is generally easiest to use.

LED current and CTR both come into play. High CTR devices almost always use darlington transistors at the output and are slow as old socks. as a post-doc fellow I knew used to say. CTR is usually quite non-linear at the extremes of LED current, often falling substantially at low current. Testing one or two pieces of an optocoupler and relying on the results is a bad thing to do. Couplers age, CTR changes with temperature and there is considerable unit to unit variation. A 2 to 1 variation is not unusual. Data sheets usually will tell you what you need to know. Otherwise, for reliability you would need to test a great many parts from several different production batches to accumulate enough data for meaningful analysis.
 

Thread Starter

Alex1965

Joined Feb 8, 2018
13
I am lost!
I'll try to reformulate the question:
I need a circuit to turn on a 12 Vdc relay when I have 5Vdc and turn off relay when zero volt.
Two focus:
1) optoisalted circuit
2) lowest current possible on optoisolator led side

I cannot use a magnetically latching relay
 

LesJones

Joined Jan 8, 2017
2,493
Can you give some details of the interface that you are using on tha smart phone. (I know nothing about smartphones. I just assumed they were a tablet and a phone in the same case.) I thought that the only interfaces avilablable would be audio out, USB, Bluetooth and a memory card socket. Would it not be possible to use the Bluetooth interface to send two ASCII characters. One to switch the relay on and one to switch it off. What condition is generating the output signal and what is it controlling in the car ?

Les
 

LesJones

Joined Jan 8, 2017
2,493
So am I correct that you are just detecting if a USB cable is plugged into the phone or not ? Or is the USB cable left plugged into the phone and it is just to remind you that you have fogotten to switch the phone off ? (I assume the USB socket is only powered when the phone is switched on.)

Les.
 

Alec_t

Joined Sep 17, 2013
10,896
Personally, I'd probably forget about the optoisolator and just use a logic-level MOSFET to switch the relay. The gate could be connected to the USB +5V via a diode to block anything nasty from getting back to the USB. The gate could have a 470k or so to ground to ensure turn-off when the USB cable is disconnected.
 
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