using Optocouples as a Relay

[fm101]

hi,
Can we use optocoupler as a electrically controlled relay. I want to turn on another system(switching on a battery in that circuit) when a sensor sends signal to the optocoupler, will this work?

 

[dick_freebird]

Depends on your "load". Most optos are an infrared
emitter and a NPN bipolar phototransistor. You have
limited "base current" hence NPN output current, a
saturation voltage that's probably 100mV or so and
a nonlinear I-V output characteristic (NPN IC-Vce
curve).

A relay on the other hand is milliohms of contact
resistance and pretty linear across a wide current
range.

There are newer isolated switches and drivers
using inductive or capacitive isolation. They can
be on a par with optos and relays for isolation
although the opto guys like to point to some
special (and likely gamed, by them) specs that
give optos an edge in some applications.

Key is probably how much battery current you
need the switch to pass. If a lot, then maybe your
play is an isolated FET driver and a big ol' power
MOSFET.

Along those lines but lesser current is "PhotoMOS"
optically coupled MOSFET switches.

 

[KlausST]

Hi,

An optocoupler's function is almost the same as a relay.
* when input voltage (current) is below a certain limit it is OFF
* when input voltage (current) is above a certain limit it is ON
(Both threshold values are given in the datasheet, for relay as well as optocoupler)

When ON there is some output "ON resistance"
When OFF there is some output "OFF resistance"
(Both values are given in the datasheet, for relay as well as optocoupler)

Both are isolating devices with specified isolation voltage

Some differences:
* a relay can switch AC and DC, a standard optocoupler DC only
* a relay is more rugged, it will not be immediately killed on overload (overvoltage, overcurrent)...but it will be killed sooner or later
* a relay usually is specified for higher output current
* an optocoupler is much faster
* an optocoupler needs less power
* an optocoupler may be immediately killed on (output) overvoltage.

Like always with all electrical and electronic parts:
One needs to read the datasheets to operate them reliably.

Klaus

 

[c_mitra]

Can we use optocoupler as a electrically controlled relay. I want to turn on another system(switching on a battery in that circuit) when a sensor sends signal to the optocoupler, will this work?

Yes, you can.

If you want to turn on/off power to another system, perhaps you can use a relay better. Perhaps you can use them in series for best results.

You should NOT use an electromechanical relay in case you are planning to turn on/off the power very often (say every second or more often).

Your input voltage should be like a step, either on or off (no chattering of relay).

You did mention that you going to switch on/off a battery to the load; select the relay with suitable volt and current rating.

Use a combination of opto coupler and a relay for most reliable operation. If the input to the opto coupler is not logic level signal, you may need some signal conditioning.

 

[danadakk]

Power isolated relays -

Products - Solid State Relays - Page 1 - Wolf Automation

www.wolfautomation.com

Working With Optically Isolated Relays - Power Electronic Tips

www.powerelectronictips.com

Some are isolated, some not.....


Regards, Dana.

 

[fm101]

hi,
why does this circuit not work? its proteus simulation but i think it won't work in reality also.

 

[danadakk]

Buzzer part number/ datasheet is ?


Regards, Dana.

 

[schmitt trigger]

Many buzzer types require to be driven by an AC signal of a specific frequency range.

What type do you have?
Self driven or external drive?

 

[fm101]

its a normal buzzer, works with dc

 

[c_mitra]

The optcoupler on resistance is high. You need to use a reed relay after the optocoupler.

 

[KlausST]

Hi,

A photo can't replace datasheets.
Datasheets show technical data like voltage, current, resistance...

If you want to know about electrical specifications, then you need to read datasheets.
Everybody needs to read them. We professionals, too. Every day, many documents with many pages.

A device without datasheet has to be considered as an unreliable device in my eyes.

Klaus

 

[fm101]

hi
-hi klaus, cannot find the datasheet from google, it is labelled CLDZ(bought it from local market), from what is specified from sellers(googling) it says ony 5V but I tested and worked with 3V supply too, resistance between the terminal is in the range of 5MOhm(!)
- hi c_mitra, i think u r right in that it seems be due to buzzer resistance(the circuit works with LED with resistor), but on reed relay, actually i wanted to replace relay with optocoupler thats my question in first place

any other solution i can try?

 

[KlausST]

Hi,

Cannot find the datasheet:
Without datasheet, without technical specifications, it's impossible for you to correctly design a circuit.
My clear recommendation: If you want to be on the safe side: don't use thus part, but buy a part with specifications.
(In worst case you buy cheap parts that failed to pass the production test.)

And you need to understand that we - in the forum - have even less informations. Indeed every recommendation how to use this parts is pure guessing. You can't rely on this. Not better than trial and fail on your own.

The device's load surely is less than 5Mohms. With 5Mohms the current and the power is too low to produce any useful loudness.

Your circuit of post#6 generally looks correct. It could work (or not) depending on the device's parameters.
It seems like a simulation tool schematic. If so simply use the tool to determine voltages and currents.
That's what a simulation tool is made for.

Klaus

 

[danadakk]

Is buzzer magnetic, a coil, or piezo ? If former optocoupler may not be able
to supply current needed. If latter then does it have integrated driver, or just
a raw piezo element. Is it rated for 5V or 12 V or.......

guessing guessing guessing......no data sheet......


Regards, Dana.

 

[FvM]

According to internet advertising, it's an electromagnetic 3V buzzer, 30 mA current draw. You should use a switch transistor along with the optocoupler.

 

[d123]

Hi,

I was wondering about what Dana said, too. You could try driving an npn/nmos or a pnp/pmos with the 4N35 transistor to power the buzzer, just to see if the problem is lack of power from the optocoupler.

 

[c_mitra]

Just try a 10uF capacitor across the opto transistor (5 and 4 pins). It may work- try a simple simulation first. Also try reducing R2 (push the opto transistor in the saturation region).

 

[danadakk]

If it is coil buzzer you are missing the protection diode for the L transient
it will generate when being switched. off. In fact you may have already
fried the 4N35 output transistor.

Regards, Dana.

 

[FvM]

According to internet sources the buzzer has an internal oscillator and should be DC operated. The suggested circuit can work for DC as well.