# Help choosing a transistor

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#### volks

##### New Member
I have received a lot of great help on this site now I need help choosing a transistor for my project.

I am using a controller with a 5v output and some transistor to switch a relay. The manufacturer of the controller said to use a 220-440ohm resistor on the base of the transistor. The coil of the relay is powered by 9v and is 500ohm so Ic=9/500= 18mA. I am got sure if i am calculating hfe correctly. I came up with a value of around 8 using a 440ohm resistor on the base. Will a bc107 NPN transistor work?
https://www.st.com/stonline/books/pdf/docs/9293.pdf

Also, will a 10kohm resistor on the emitter prevent leakage when the transistor is off? Just making sure I am on the right track.

I know I need a protection diode on the coil. What do i need if the relay contact is seeing 24vdc, max 8A?

Thanks

#### Hero999

##### Banned
I think 2k2 will be fine.

As a general rule of thumb the base current should be 1/10th of the collector current.

If the relay uses 18mA, the base current needs to be 1.8mA.

R = (5 - 0.6)/0.0018 = 2444 so use a 2k4 or even a 2k2 resistor.

#### volks

##### New Member
Thanks, that explains a lot. I was not sure how to figure max IC current for the base. So based on my equations I have the following:

Vc = 5v
Ic = 18mA
hfe = (5 x Ic)/Max IC = (5x18)/1.8 = 50
Rb = (Vc x hfe)/(5 x Ic) = 50/.018 = 2777ohms

So i would use a 2k4 ohm resistor on the base.

Any suggestions on what transistor to use? Will the one mentioned before work for my application?

#### crutschow

##### Well-Known Member
Also, will a 10kohm resistor on the emitter prevent leakage when the transistor is off? Just making sure I am on the right track.

I know I need a protection diode on the coil. What do i need if the relay contact is seeing 24vdc, max 8A?
You would connect the 10kΩ resistor between the base and emitter if you were worried about high temperature leakage. But that's not needed if the controller source driving the transistor input resistor is pulled to ground for logic zero.

The relay contact may need protection if the contact load is inductive. What are the contacts controlling?

#### volks

##### New Member
Relay contacts are controlling a 24vdc linear track actuator. It is rated at 2A no load to 8A full load. I will be no where near full load.

So no leakage protection will be needed?

#### Hero999

##### Banned
Use a general purpose transistor, BC338, BC548, PN2222A or even the BC109 you linked to - it doesn't matter.

Add a diode (1N4001 will do) in reverse parallel with the relay coil to protect the transistor from the high voltage pulse produced when it's turned off.

#### volks

##### New Member
Hero999 and crutshow, thanks so much for your help.

If I chose to use the BC548 transistor will i need to use its min hfe (110) to calculate the resistor for the base or can I stick with the 2k4Ω as originally planned?

rb = (Vc x hfe)/(5 x Ic) = 110/.018 = 6111Ω

Sorry for all the questions but I am new to this.

#### Hero999

##### Banned
You can't use 110 for the hfe because it's specified with an Ic of just 2mA and a Vce of 5V which is too higher voltage loss.

Use a forced hfe of 10 as I explained previously giving you a resistor value of 2k4.

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#### crutschow

##### Well-Known Member
To clarify, the beta given in the data sheet is for AC operation at the give collector current and voltage. If you want to use the transistor as a switch you want to make sure it is completely turned on, with minimum saturation voltage. Thus you use a low value of beta to insure this (which is commonly selected to be a value of 10).

#### volks

##### New Member
Ok, thanks for everything. I will try using a BC548 transistor with 2k4Ω on the base. Also a 1N4001 as a protection diode.

#### Woodnit

##### New Member
You can't use 110 for the hfe because it's specified with an Ic of just 2mA and a Vce of 5V which is too higher voltage loss.

Use a forced hfe of 10 as I explained previously giving you a resistor value of 2k4.

Hero999: could you tell me if I'm understanding this correctly?
When you say to use a forced value of 10 for gain (which is hfe and beta, right?), the transistor you put in will be able to pass way more collector current based on its actual hfe. So your 'factor of safety' to make sure the transistor-as-switch is on is the actual hfe/forced value.

I'm doing a similar circuit, using a 555 output to a 2N3904 Base to operate a relay coil requiring about 90mA @ 5V.

But the ~470Ω base resistor

(5-.6)/.009 = 477Ω

didn't seem to allow enough collector current to flow so I switched to a 100Ω and it worked. I assume a higher gain transistor also would have worked, so how does the actual gain figure into the calculation?

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