Driving a relay or a High Current Load using a Transistor
may sometimes be confusing like, what value of resistor to use to drive the
Transistor's Base.
Once I had a problem while toggling a relay using BC547. My
relay contacts have got welded due to lack of Holding Current. Due to that I made
3 relays useless. So, Then I read some basics and know how to calculate the
base resistance of the transistor required to Switch the load successfully.
So, heres how i got this to work.
1. Determine which transistor will be
suitable.
As shown in Figure Relay rated Voltage = 12Volts,
Relay
resistance = 200 ohms
Relay's
holding Current = 12v/200 ohms = 60mA
So, we can use BC547
for switching the relay which has Icmax = 100mA and Vcemax = 45V
Switch any load by driving the transistor in Saturation mode
will makes sure the load gets its rated current, Holding Current in case the
Load is a Relay.
2. How to drive BC547 or any Transistor into Saturation mode:
We want to find the Base Current(Ibase) required to drive
bc547 into saturation mode(Ic in this mode will be greater than relay's holding
current i.e >60mA)
we cannot calculate Ibase by using beta of bc547, because
beta value is varying with Ic and Vce.
So, we can see the Vce(sat) voltage on the datasheet of
BC547. There we can find the Ibase(max) of BC547. see the screnshoot of
datasheet below.click to see enlarged image.
From the above fig.
IC=100mA(max), therefore Ib = 5mA(max).
So BC547 can withstand upto 5mA Base Current
Set Ibase just 10 to 20% lower for safety.(80% of 5mA = 4mA, actually we are Overdriving the transistor just to put it in Saturation mode)
So Rbase(RB) = [5V(Logic High From uC)]/4mA
Therefore Rbase(RB) = 1.25kohms ~= 1.2kohms
That’s all ..we are ready to drive a relay. Similarly we can
find RB required for any Transistor.
nicely described. It helped me thank you. i will try it.
ReplyDeleteYou are welcome
DeleteBro, can share your e-mail address? , i have some questions
ReplyDeleteMuito bom
ReplyDeleteThank u sir u explained very well
ReplyDeleteGod bless you very nice
ReplyDelete