Whats is a BCX38B!

T

Tasp

Member
So I wish to connect a 5v relay to an 08M, manual 3 page 7 shows a BCX38B npn darlington transistor.

However do you think I can find someone that stocks it!

This is the relay;
https://www.rapidonline.com/productinfo.aspx?tier1=Electronic+Components&tier2=Relays+%26+Solenoids&tier3=PCB+Relays&tier4=Subminiature+relay+SPCO+1A+-+Tianbo&moduleno=60257&catref=60-0680#

I've searched the forums for another part number or reference to which one I should use but to no avail.

Perhaps this one is appropriate, could someone confirm;
http://www.rapidonline.com/sku/Electronic-Components/Discrete-Semiconductors/Transistors/Medium-power-NPN-transistors/66248/81-0080

Thanks very much.
 
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premelec

premelec

Senior Member
Resistance of relay coil?

URL wouldn't load for me - likely you can drive the relay with a high gain NPN transistor depending on the coil resistance. Do you actually have the relay in hand?
 
M

MPep

Senior Member
Hi Tasp,

The BCX38 is a high current device, however for a relay coil you most likely won't need anywhere that amount. The BC639 (from RapidOnLine) should be fine to use.

MPep.
 
W

westaust55

Moderator
The relay you gave a link to, presuming that you are using the 5Vdc version, with a coil resistance of 69 Ohms you will have a continuous current of around 70mA and a peak somewhat higher until the armature movement closes the airgap in the magnetic circuit.

As already mentioned, the BCX38B is a high power transistor. This has a high gain so Hfe (gain) = 10K at Ic =0.5A but at just a few mA load the gain also drops off and may be as little as 200.

The BC639 ihas an Ic rating of 800mA and a Hfe of 25. So capable of working but needs more base current although all fairly nominal in your example.

You could even consider a simple NPN signal transistor such as the BC548B with Ic = 200mA and typically Hfe of 200 for 5V operation.
With respect ot the A, B or C on the end of some transistors this typically relates to the gain (Hfe) grouping of the transistor. C has higher gain than B has higher gain than A.
 
R

rolfi@tisco.no

Member
Hi, Tasp, - I have seen a quite number of hobby circuits, specifying this or that transistor, as if there is only one type that will do the job. And more often than not, it is obvious that ANY small signal switch or amplifier NPN will do. Like in this case.
If your supply voltage is 4.5 to 5V, just use ANY small signal NPN transistor you might have lying around! (It is really hard to find a transistor unable to switch sub 100mA relay currents with 5V supply!)
While a darlington will be fine with 10kohm base resistance, 1kohm will be adequate for "normal" types!
 
B

BeanieBots

Moderator
rolfi@tisco.no said:
It is really hard to find a transistor unable to switch sub 100mA relay currents with 5V supply!
Click to expand...
Famous last words!
Ironically, I'd NOT use BCX38 for such a purpose as it is actually one of the most likely to fail at the task on account that it is a darlington and will have a much higher volt-drop than a regular transistor. To lose ~1.2v from a 5v supply is a significant percentage drop.
 
T

Tasp

Member
Er...ok...

Slightly confused but I eventually went for the BC639 as previously recommended, and I'll hope for the best, as long as the chip is ok the transistor can be changed for higher value.
 
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C

cactusface

Senior Member
Hi,
Whatever you use, don't forget the diode across the relay coil, otherwise you could fry the trany. I recently discovered FET's and just find them easier to use, very little power or voltage loss, very little drive current needed and no heat, well not in a circuit like this....
Hope it helps
Regards
Mel
 
R

rolfi@tisco.no

Member
Well, three things will kill a transistor: Current high enough to melt the bonding wires between the chip itself and the pins, chip temperatures high enough to destroy the doping profile, and hot spots at the base-collector junction caused by excessive dissipation in combination with high emitter-collector voltage.
Which one of these killed your BCX38s? Or did I miss out on sth, - or did someone forget the diode across the relay coil?
And I agree: Best to avoid darlingtons in low voltage circuits because of their inherent high emitter-collector saturation voltage, which is at least .8V, even at small currents.
 
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