# BJT or MOSFET for 4017 Relay Driver Circuit retaining High-Level Logic Output

#### e44-72

##### Member
Hello there,

I'll start by saying I'm by no means an expert in electronic design, so please bear with me. Also my schematic and the datasheets I'm using will all be attached to this post.

I'm designing a circuit whereby the output of a 4017B IC is driving a relay via a BC337-25 BJT Transistor (or maybe a MOSFET), but also want to ensure the output voltage remains high enough to drive other CMOS Logic Inputs. The circuit uses a 12V DC supply and the relay uses 140mA, my calculations for determining the base current are below (please correct me if they're wrong):

After the protection diode D1, the supply voltage for the circuit is about 12 - 0.7 = 11.3V.
Using the Minimum hfe value for BC337-25 from the datasheet of 160, with a collector current of 140mA I get a base current of 0.14/160 = 0.000875A.
To ensure saturation I've multiplied by a factor of 3 to get 0.000875 x 3 = 0.002625A = 2.63mA.
To determine the base base resistor I've used the VBE(on) value of 1.2V so the voltage across the resistor is 11.3 - 1.2 = 10.1V
I then get a resistor value of 10.1 / 0.002625 = 3847.62 ohms, so I've chosen a 3.9k resistor.
(Just realized I didn't take into account the voltage drop across the 1N4148 diodes so resistor value will probably be a little less)

What I want to determine is the maximum output current a 4017B can source and maintain a high enough voltage level to drive other inputs. I've looked at the Texas Instruments datasheet for the 4017B, on the second page in the Static Electrical Characteristics table. I'm reading the Min output high current with a 10V supply at 25 degrees C is 1.3mA or typically 2.6mA, with an output voltage of 9.5V, so I'm guessing it would still maintain a higher enough output voltage if I'm reading it correctly? I'd like to know how much the output voltage of the 4017B drops as output current increases and I'm guessing Fig. 7 probably tells you that but I'm not exactly sure how to read it, so if anyone could help me with that it'd be great.

So would a BC337-25 be appropriate for this or maybe I should pick a transistor with a higher gain like a BC337-40, or would it even be best to use an N-Channel mosfet such as an IRF510 perhaps?

My Schematic and datasheets are all attached below, many thanks for reading and helping me out.

BC337-25 Datasheet: https://www.onsemi.com/pub/Collateral/BC337-D.PDF
Schematic attached to post:

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#### crutschow

##### Well-Known Member
Using the Minimum hfe value for BC337-25 from the datasheet of 160, with a collector current of 140mA I get a base current of 0.14/160 = 0.000875A.
To ensure saturation I've multiplied by a factor of 3 to get 0.000875 x 3 = 0.002625A = 2.63mA.
You don't use the data sheet hfe value for calculating required base current in a BJT used as a switch.
That value is for operation in the linear region.
If you look at the saturation voltage specification in the data sheet (below) you will see that they typically use a forced hfe value of 10 (base current is 10% of the collector current) for minimum saturation voltage, but you could probably use a value of 5% if necessary.
That means the base current should be 7 to 14mA when switching 140mA.

Since the 4017B can't provide that much current, you might be better using an N-MOSFET such as the 2N7000 to drive the relay coil, which requires only a gate voltage (and essentially no current) to turn on. #### e44-72

##### Member
Thanks very much for that, I'll remember that in future. I have some BS170 N-channel mosfets which look pretty similar to 2N7000 except there rated at 500mA instead of 200mA so they'd probably be ok. I was concerned though that using an N-channel mosfet in a TO-92 package like BS170 with an RDS(on) value of 5 ohms might get too hot so was thinking maybe something in a TO-220 might be more appropriate.

BS170 datasheet: https://www.onsemi.com/pub/Collateral/BS170-D.PDF

Thanks for the help.

#### crutschow

##### Well-Known Member
I was concerned though that using an N-channel mosfet in a TO-92 package like BS170 with an RDS(on) value of 5 ohms might get too hot
Watt's Law tells us no.
Power is I²R so for 140mA, the MOSFET power dissipation is 0.14A² * 5Ω = 98mW, which is well below the 350mW dissipation capability of the TO-92 case at room temperature, so there should be no problem using that MOSFET.

• gophert

#### e44-72

##### Member
Ok that makes sense, so that should work just fine. I'll prototype it on breadboard first just to be sure it works. Thanks very much for your help. 