Differences between using transistor, MOSFET and opto-isolator as a switch

Hi guys,
I have been looking to other people's design and found out that transistor can work as a switch, MOSFET can be work as a switch, and opto-isolator can be work as a switch too.
What i want to know is, application wise what are the difference between this 3 components?

Re: Switches difference.

Hi,

All the answer is referring to SWITCH operation ONLY, transistor and Mosfet can operate other means too.

Transistor Base current will go to Emitter (NPN) when used as switch, in Mosfet Gate current will not flow into Drain. In Opto-isolator, you will drive the LED part which will drive the transistor base without any contact (via light) which will trigger Collector to Emitter current flow, so, in away opto-Isolator work like Mosfet, but gives option to have different ground between gate(base) and output(Collector/emitter).

Hope this is clear.

M.Pathma

Re: Switches difference.

Thanks for the explaination.
But what i want to know is, applicational wise. When to use a transistor as a switch? When to use a MOSFET as a switch? And when to use a opto isolator as a switch?

Re: Switches difference.

Hi,

Well, I will explain in my designs;

When to use a transistor as a switch?
if MCU need to switch a LED on-OFF, or activate Relay, or buzzer or ...., I use NPN transistor, because, I do not worry about ground difference and no issue on huge current flow and temperature rise.
Since NPN will invert my logic, if I needed non-investing logic I use PNP.
Transistor as switch for application with mA only, larger current will create heat.

When to use a MOSFET?
If I need to SINK very large current say, 1000mA or 5000mA, then I use Mosfet because low heat lost.

opto isolator as a switch?
When I need to isolate input/output ground to system ground.

Again this refer to my design logic, other might have other reason.

M.Pathma

Re: Switches difference.

mpathma said:

Hi,

Well, I will explain in my designs;

When to use a transistor as a switch?
if MCU need to switch a LED on-OFF, or activate Relay, or buzzer or ...., I use NPN transistor, because, I do not worry about ground difference and no issue on huge current flow and temperature rise.
Since NPN will invert my logic, if I needed non-investing logic I use PNP.
Transistor as switch for application with mA only, larger current will create heat.

When to use a MOSFET?
If I need to SINK very large current say, 1000mA or 5000mA, then I use Mosfet because low heat lost.

opto isolator as a switch?
When I need to isolate input/output ground to system ground.

Again this refer to my design logic, other might have other reason.

M.Pathma

Click to expand...

So, can I say that the difference of transistor switch and MOSFET switch is that for transistor switch, it is use in low current application while for MOSFET switch, it is used in high current application. If my current is 2A, should I use transistor switch or MOSFET switch?
For opto-isolator, when is it is the time to isolate io ground and system ground? Any example?

Re: Switches difference.

Hi;
You can use mosfets for high current ratings as M.Pathma said. Mosfets is used for power management in general.
ie you have a device which you want to control. You can put a mosfet in series with that device supply voltage. So that, your mcu can on and off that device at any specifed time.
Optocouplers are used especially for SMPS design, as i know. For feedback from secondary to primary side (since primary has no actual ground, it is not always connected to neutral, it may be phase (line) depending on how do you insert the AC plug into the wall). In that case any physical/electrical connection will damage your cct. So you need a pyhsical isolation. Then you can use "light" to pass electrical signals. That is the internal of optocoupler cct. A LED and a photo transistor.

Hope helps

Re: Switches difference.

Thanks for the explanation.
If my mcu which output a 5v signal to turn on other device for example the 74hc595, which switch should i use? The MOSFET switch, the transistor switch or the opto isolator switch? And why?

Re: Switches difference.

HI,

74HC595 is SIPO, what you mean "turn-on", if you wanted to connect your MCU I/O pint to 74HC595 , and extend the serial output of your MCU to parallel (8-bit), then connect them directly, no need for any switch, remember, MCU has good SINK type NPN connected to I/O port, and a weak PNP for PULL UP, the NPN is strong enough to drive LED directly, (SINKING).

Some good information on using 595, https://www.sparkfun.com/products/733 read the user comments.


M.Pathma

Re: Switches difference.

mpathma said:

HI,

74HC595 is SIPO, what you mean "turn-on", if you wanted to connect your MCU I/O pint to 74HC595 , and extend the serial output of your MCU to parallel (8-bit), then connect them directly, no need for any switch, remember, MCU has good SINK type NPN connected to I/O port, and a weak PNP for PULL UP, the NPN is strong enough to drive LED directly, (SINKING).

Some good information on using 595, https://www.sparkfun.com/products/733 read the user comments.


M.Pathma

Click to expand...

What I mean is to use one of my mcu io pin to send signal to MOSFET switch or opto switch and then to power on the (give 5V to pin VCC) 74HC595. I thought you guys just said MOSFET is for power management and controlling the power?