What transistor to choose for Relay Switch

Thread Starter

yuvi1

Joined Feb 1, 2013
37
Hi all,

I see many circuits over the internet of a PIC controlled Relay,

what are the differences and considerations of choosing the right transistor for this kind of circuit ?


DC Source : 5vots
Relay : 5volts

Thanks !
 

ericgibbs

Joined Jan 29, 2010
9,104
hi y1,
For a 5V relay which has a coil resistance of 100R thru 500R, almost any low/medium power general purpose transistor is OK.
The 2N2222 or 2N3904 are common choices.

The other consideration is what are the relay contacts being used to switch.?
That will determine the contact type.
E
 

ericgibbs

Joined Jan 29, 2010
9,104
What do you mean "what are the relay contact being used" ?
You may be switching a high current, voltage, low level signal etc.
Relays contact types are available for many different applications, do you have a project in mind.?
 

Thread Starter

yuvi1

Joined Feb 1, 2013
37
If i want to switch a 220v load up to 200 watts, it depends on the transistor i'll choose ? i thought that the load that i want to switch
depends on the relay , am i right ?
 

ericgibbs

Joined Jan 29, 2010
9,104
You must get a relay that is contact and insulation specified to work with mains voltages.
For 200W, assume 2Amps, so get a relay with with at least 3A rated contacts

Its important that the 5V relay coil has a suppression diode across the coil in order to protect the transistor
 

Thread Starter

yuvi1

Joined Feb 1, 2013
37
i don't think it's the same table that i sent in the link but i understand what you wrote ,

A relay with max switching voltage of 220volts
and a max switching current of 2A will work with a 400watts load ?
 

bertus

Joined Apr 5, 2008
20,180
Hello,

In the given datasheet is given a maximum resistive load of 60 Watts.
The 400 Watt load will destroy the relais.

Bertus
 

Thread Starter

yuvi1

Joined Feb 1, 2013
37
Thanks for your answer, Bertus.


So the relay determines which load i can use ? and the transistor that drives the relay does not affect or determines the load ?
 

bertus

Joined Apr 5, 2008
20,180
Hello,

A larger relays will also need a bit larger transistor to drive it, as the mass of the switch contacts will be larger.
This is dependend on the coils resistance in the relays.
There must be a coil current be given in the relays datasheet.
Use a transistor that can switch at least 2 X the coil current for safety.
Also a back EMF diode must be used.

Bertus
 

Thread Starter

yuvi1

Joined Feb 1, 2013
37
so if i understand, coil current and switch current of the relay are not the same thing,

coil current is the current that's need to drive the relay
and switch current is the max current of the load that is switched by the relay .
am i right ?
 

MaxHeadRoom

Joined Jul 18, 2013
19,421
Hi all,

I see many circuits over the internet of a PIC controlled Relay,

what are the differences and considerations of choosing the right transistor for this kind of circuit ?


DC Source : 5vots
Relay : 5volts
Thanks !
You could use a device that was specifically designed to interface with a power device with a PIC etc, namely the 2n7000 'Fetlington'.
Cheap and has the advantage of not loading the μp.
Max.
 

Attachments

hp1729

Joined Nov 23, 2015
2,304
Hi all,

I see many circuits over the internet of a PIC controlled Relay,

what are the differences and considerations of choosing the right transistor for this kind of circuit ?


DC Source : 5vots
Relay : 5volts

Thanks !
You need to choose the relay before you choose the transistor.
 
Top