# Transistor Power Supply

#### Sibithomas

Joined Oct 27, 2011
6
Hi All,

I am trying to design a transistor relay driver circuit.My driver output requirement is 10mA. Here my power supply is a variable kind and it can be varied from 5V to 15V. I have to ensure that my relay driver is working in whole range of supply voltage, ie. from 5V to 15V. so I designed my driver ciruit to work even at minimum supply voltage of 5V. Now my question is what will happen if supply voltage changes from 5V to 15V?

As per my understanding when supply volatge changes Ic will also change; subsequntly beta too. Will these changes affect Ib?

In case if my base driving source is giving the maximum current even at 10V; what will happen to output current at 15V. Will my driver work properly at 15V supply?

Sibi Thomas

#### jimkeith

Joined Oct 26, 2011
540
What you need to do is to overdrive the transistor so that when driven it is always in saturation--this is usually done by making Ib = Ic / 10 or so.

Note that the beta remains essentially constant

Also, if the base drive is derived from the same voltage source as the relay driver, the ratio or Ic /Ib remains constant so that it tracks the voltage source.

Another issue here is the power dissipation in the relay with the voltage at 3 times normal. P = E² / R So it is a factor of 9 times higher. There are different ways of dealing with this, but it is beyond the scope of the question.

#### Sibithomas

Joined Oct 27, 2011
6
Hi jimkeith,

Still I am not clear. My question is; as we know beta = Ic / Ib...
here suppose if Ic increases, to keep beta constant ( as per my understanding beta is constant for the transistor) Ib should be adjusted (either in +ve direction or in - ve direction). At this moment if my base driver ( it is a different source ex. output of an IC / micro controller) is not sufficient to drive the base; what will be the output / what is going to change at the output?

Hope you got my point....

I am attaching my circuit tooo

Sibi

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#### crutschow

Joined Mar 14, 2008
34,070
The beta value from the data sheet is for small signal circuits, not switching circuits. You don't worry about keeping beta constant, you just want the base current to be sufficient to switch the transistor under worst-case conditions. As jimkeith noted, to insure that the transistor is well turned on (into saturation) you need to use a forced beta value of about 10 (no more than 20). Thus you calculate a base current that will drive the largest collector load at 15V (a base current of Icmax/10). That will insure that all loads will be switched properly.

If the current from your micro controller is insufficient to provide this base current then you will need to add another transistor to provide gain.

#### Sibithomas

Joined Oct 27, 2011
6
Thank you !!!!

I understand ...so..I need to design the cirucit for worst condition ie. base current required to drive the transistor @ 5V and with maximum collector load.

I am right?

Sibi

#### crutschow

Joined Mar 14, 2008
34,070
I understand ...so..I need to design the cirucit for worst condition ie. base current required to drive the transistor @ 5V and with maximum collector load.

I am right?
If 5V is the voltage supplying the base resistor current then yes.

You always want to design for worst-case conditions, if possible.

#### Sibithomas

Joined Oct 27, 2011
6
Thank you All!!!!!!!!
Great help!!

#### sheldons

Joined Oct 26, 2011
613
another thing is that yr relay and lamp are correct voltage wise regarding what supply voltage you use......there is a min/max regarding the pull in voltage /drop out voltage relay wise