Probably stupid questions about opto isolators/couplers...

[mvagusta]

I was wondering why people use opto-isolators/couplers to run extra leds from a mobo led header, such as a 4N28, instead of just using a much cheaper solution such as a bc548 & resistor?

Do we really need that much isolation? I haven't tried it but i'm fairly sure using a simple bc548 as a switch would work fine. I think both the 4N28 & bc548 have 100ma collector current continous ratings, so why spend the extra money on an opto-isolator such as a 4N28?

You could even get a bc639 with a massive 1a contious rating for less than half the cost of the 4N28! The cheaper bc548 & bc639 solutions are even slightly more compact in thier TO-92 cases! Cheaper & smaller...

Jaycar in aus sell 4N28 for $1.40, and bc548 for $0.26, bc639 for $.60, and resistors are like nothing.There must be an advantage i am missing here, it just doesn't seem correct?

Wait, is it possibly of the tiny risk of a transistor failing and shorting collector to base, sending the supply voltage into the mobo header? Is this risk not a possibility or have i answered my own question? So maybe if you want more current capability than the 4n28 provides, you use the 4N28 to trigger a large transistor?

 

[A.Anand Srinivasan]

here the main concern is the risk of a transistor failing... the reason behind that is that there could be sudden spike in the supply as well as the input due to the vehicle....

 

[mvagusta]

Re: Probably stupid questions about opto isolators/couplers.

Actually, if triggering say 10 x 30ma leds from the mobo header, and taking 5v from the pc's psu, i don't think there isn't any risk to the mobo when using a transistor such as the bc639 to supply the 300ma!

Even if the worst case scenario of the collector shorting to the base occured, the pc's 5v will be simply connected to the mobo's 5v pin, which would have no effect - especially considering the base is connected to the 5v pin via a large resistor! So the mobo will be safe and the pc would continue to run unaffected even in the worst case scenario...

Anyone have another reason as to why an opto-isolator is required?

I think the safe, more compact, more powerfull and cheaper option of using a small transistor is better.

 

[nxtech]

Re: Probably stupid questions about opto isolators/couplers.

Optocouplers/isolators are used for electrical isolation and sometimes for noise reduction (johnson noise).

E

 

[dfullmer]

Re: Probably stupid questions about opto isolators/couplers.

Hi mvagusta,

I use them for high voltage isolation very often. I must control 2kv circuits on a routine basis. opto's are very good for this. They have some draw backs when you are trying to use them in linear circuitry but this too can be done..


regards


dfullmer

 

[nandhu015]

Re: Probably stupid questions about opto isolators/couplers.

Hi

Opto isolators are very much useful when controlling high voltage with microcontroller.

for example dimming 230v light with microcontroller u cant do it with a bc547.

Far more than connecting neutral of 230v to the ground of microcontroller is :x

Nandhu

 

[davidgrm]

Re: Probably stupid questions about opto isolators/couplers.

Opto Isolators come in handy when the high voltage part of your circuit fails. You dont get hundreds of volts destroying your cpu connected to your pc connected to your printer connected to you router ... you get the picture

 

[mvagusta]

Re: Probably stupid questions about opto isolators/couplers.

Thanks for all the explanations of what an opto isolator is good for guys, but the question here is "why people use opto-isolators/couplers to run extra leds from a mobo led header, instead of just using a much cheaper solution such as a bc548 & resistor?"

Well based on my reasoning in my second post at the top of the page, i say there is no reason to use an opto isolator to drive extra leds from a low current ouput, such as a mobo header. As leds are a light, which do not provide or supply high voltages or spikes! there is no need to protect a mobo from leds, the mobo only helps help to power extra leds.

The cheaper option of using a transistor, seems perfectly safe, and can be more powerfull whilst still remaning cheaper.

 

[A.Anand Srinivasan]

take a look at the power dissipation of both..... 4n28 has about 250mW of total device dissipation at room temp and bc548 has 625mW.....