Reason for using NPN transistors for Relay switching

[muruga81]

Hai all,
can any one tell me the reason for switching 12v,24v relays using NPN transistors and not PNP transistors?

theoritically both transistor types can ben used for this purpose, but mostly NPN transitors are used, van any one tell me the correct reason.


murugan

[vidur]

hi

There is no rule. you can use pnp versions ....but

npn are much more popular than their pnp cousins..its the same story in almost all applications...pnp versions are now limited to select applications like, push-pull ckts etc.

Reasons why not to use pnp

Reason-1-> npn-they're much more faster for the same device size....esp. on IC circuits where speed is an issue...this is bcoz charges in p-type semi-c move slowly compared to n-types

Reason-2->Standardization of bias values etc...imagine the chaos if half of the world uses circuits on npn transistors & the rest use pnp transistors... It would be tuff to interface such dissimilar ICs.Many
people (including me) have an intrinsic dislike of negative numbers...imagine if someone says bias Vce= -5V....plus you'll have to deal with negative power supplies more often

:-)

vidur

[jack]

Sure you can use both NPN as well as PNP transistor for switching. But the output in the latter case will be inverted, because PNP transistor comes into conduction when you apply -ve supply to its base unlike NPN where you put +ve supply.

I agree with Vidur that pnp transistor have slower switching speed compared to npn, but this fact no useful if you are taking about relay switching, things are only apparent at high frequency!

[muruga81]

thanks for the reply,

but some of my friends said that when we use micrcontrollers to drive the relay transistor mostly NPN tr is preffered because in NPN volatage goes out of the device to NPN enitter, but when we use PNP to drive high volatage transistors some of the high volage spike volatge comes in to the IC pin(microcntrollers are TTL based ans very sensistive to over volage spikes) and cause a abnormal behavoiur or damage the pin.
so most prefer NPN over PNp transistors for high volage switching.....
is it true, can any answer me..


regards
murugan

That is bull ****

Voltage spikes occur when you turn the relay off, irrespective of whether NPN or PNP is used. Free wheeling diodes should be used if your ckt can be harmed by this.

Now to the real reason:
Microcontrollers usually work off a low voltage supply, typically 5V. Hence their output voltage swing is limited to 5V as well. To drive a PNP transistor switching a 12V relay, you need an output swing of >11.5V. Where as with a NPN >0.6V swing is sufficient.

[muruga81]

http://www.allaboutcircuits.com/vol_3/chpt_4/5.html

Please refer the site about NPN and PNP transistors switching, its given that only during amplification purposes bias volages are considered, but for switching NPN transistor only 5v requried for base, and for PNP 0v is required. can any one answer me.........

murugan

[Vivek]

To swith ON a PNP transistor you do need to provide 0v.
And thats what that website implies.

But to switch it "OFF" -- you need ?
think

draw a PNP transistor circuit with a load and guess the voltage on base ..
assume a 12 V motor!

[nchinoy]

Choice of NPN or PNP is done for two reasons....

First if you want a Normally Open Switch or a Normally closed switch....

Second and more design related is the question of sourcing and sinking currents...

The question of speed i would doubt.. in most applications where relays are to be controlled "switched on/off" the switching frequency is not that high... and also time delay between a NPN and a PNP might be in way less than some nano seconds ... doesnt really matter as much ... considering the delay induced by the relay coil and further which ever contact the relay is switching....


just my 2 cents worth....

Nimesh