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How to pick a transistor to switch a relay
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johnkenyon:
How to chose a relay driving transistor from your spares bin:

1) Characterise your load
What's the coil voltage?
What is coil rated current?

The OP has already stated: 12V 75mA

2) Characterise your selected part(s) - in this example I have a BC547, a 2N2222 and a big bag of BC337.
a) Get hold of the data sheet for the transistor, and look for the "Absolute Ratings" part
b) Look for the Collector Current (lc - that's 'eye'-'cee'). Is it sufficient to drive your relay? (BC547 - 100mA, 2N2222 - 800mA, BC337 - 800mA)
c) Look for the emitter-collector voltage (Vceo). Is it sufficient to withstand the voltage across the relay? (BC547 - 45V, 2N2222 - 30V , BC337 - 45V)

From this information:
All three transistors can handle the voltage (worst device 2N2222 @ 30v, +150% margin)

The BC547 can handle the current, but there isn't much margin. The 2N2222 and BC337 can both handle >x10 the expected current.
The relay may also draw a transient current when energising - that's when your safety margin becomes useful - more margin, the less likely that the transistor will fail prematurely.

The 2N2222 has a TO18 case, and the BC337 and BC547 have a TO92 case.

I would leave the BC547, and 2N2222 for another project, and use the BC337, with the following reasons

i) The BC547's low Ic sends it to the back of the queue
ii) The 2N2222's metal TO-18 case would be better used in case where a higher dissipation is required (i.e. linear operation rather than being driven into saturation)
iii) I have lots of BC337's because that's my jellybean part.

Note that this selection process should be carried out when you purchase your jellybean parts - there is a reason why I have a bag of BC337s.

So going back to the beginning, if I were the OP and only had a couple of BC147's in my spares box, I would order some BC337's (to the first price break) rather than 2N2222 solely on the higher Vceo rating (one for the project and the other 9/19/24 for the spares box)



moffy:
Just for accuracy, I think your data about the 2N2222a is a little out of date: Haven't seen a TO-18 metal case in a while, they're TO-92 like most others. The other specs seem a little off too: https://www.onsemi.com/pub/Collateral/P2N2222A-D.PDF
:)
mikerj:

--- Quote from: WattsThat on February 07, 2020, 05:47:24 am ---While some very good points have been made, I see different specs out there for BC547 based on who’s making it. What I see that causes concern on most datasheets is an absolute max collector current of 100ma (based on the Fairchild DS on Mouser). That’s not enough margin for me.

https://www.mouser.com/datasheet/2/149/BC547-190204.pdf

--- End quote ---

I strongly suspect the Fairchild datasheet from Farnell showing 500mA max is incorrect.  Parts from every other manufacturer I've found, including the later Fairchild datasheet in your link shows 100mA.
moffy:

--- Quote from: mikerj on February 07, 2020, 11:43:58 am ---
--- Quote from: WattsThat on February 07, 2020, 05:47:24 am ---While some very good points have been made, I see different specs out there for BC547 based on who’s making it. What I see that causes concern on most datasheets is an absolute max collector current of 100ma (based on the Fairchild DS on Mouser). That’s not enough margin for me.

https://www.mouser.com/datasheet/2/149/BC547-190204.pdf

--- End quote ---

I strongly suspect the Fairchild datasheet from Farnell showing 500mA max is incorrect.  Parts from every other manufacturer I've found, including the later Fairchild datasheet in your link shows 100mA.

--- End quote ---

Thanks for that. I thought the BC547 was 100ma until I looked at the Farnell data sheet, which said 500ma. The power rating on the Mouser sheet is 500mw, looks like they mistyped that into the current rating. Based on that I would stay away from the BC547 also.
poorchava:
I routinely use MUN2211 for such things. Same as cheap as any other transistor, but less stuff to solder. Normal BJTs require 2 additional resistors and MOSFETs require at least one or they are susceptible to spontaneous turn-on due to noise if control lines are long.
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