# Door Strike transitor/resistor selection

#### ShopRat59

Joined Nov 27, 2013
56
Hi, I’m working on an Arduino project and have computed all the type/values for the needed discreet components, but am struggling with one. I’ve attached a Spice diagram of this piece of the circuit:

I need to drive a door strike that has the following characteristics:

Operating Voltage = 12

Current Draw = 750 mA

RL (calculated) = 16 ohm

I plan to use a transistor and while a 2N2222 is rated at 1 amp, my understanding is that you should not drive a transistor beyond 2/3 of its max current. Therefore, I selected a BD135 which has a max IC of 1.5 amps. From its datasheet:

Vbe = 1 V

Vce = 2 V

hFE = 250

Using the calculator at http://www.petervis.com/GCSE_Design...ator/transistor_base_resistor_calculator.html

I calculate an Rb of 533Ω, the closest I have on hand is a 510Ω or 680Ω.

Sorry for the long background, now my questions:

1) Is the BD135 a reasonable choice for the transistor? If not, what is a better choice?

2) Is my Rb calculation correct?

3) Assuming it is correct, do I use the 510Ω or 680Ω resistor and why?

4) In transistor circuits I often see a second resistor connecting the emitter to the base (R2 in the Spice diagram). Is this needed and if so why and what value?

5) Since the door strike is an inductive load I believe I need a flyback diode – do I have D1 (in the spice diagram) in the correct bias/location?

6) If I used a higher rated transistor, like a TIP-120 (5 amps) would it run cooler?

7) Since the strike is 12V would it be smarter to isolate it from the Arduino with a optocoupler or relay?

Tom

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#### AlbertHall

Joined Jun 4, 2014
11,610
The circuit won't work as drawn. You need to connect the emitter direct to 0V and move the diode and door strike to between the collector and +12V.
1. BD135 should be OK, but a MOSFET would be better and would run cooler.
2. To get good base drive for the transistor the base resistor would ideally be ten times the collector resistor i.e. 160Ω. That would ensure the transistor was turned on hard and with the minimum collector/emitter voltage and hence lowest heat. However, I doubt that the arduino would supply the current too well. A MOSFET requires no gate current so would fix that problem.
3. See 2.
4. The second resistor is for any leakage current to flow through and ensure the transistor switches off completely. Not needed if you use a MOSFET.
5. The diode polarity is correct. When moved to the collector/drain, in the diagram just move it to the left, don't rotate it and it will still be correct.
6. MOSFET recommended.
7. If you want to use an opto coupler that will work too and again is easier with MOSFET. Connect the phototransistor between +12V and gate. You will now need R2, perhaps 10k, but no Rb is needed.

#### ShopRat59

Joined Nov 27, 2013
56
1. BD135 should be OK, but a MOSFET would be better and would run cooler.
Albert,

Thanks for the quick response, never used a MOSFET before so I had to do some research. Based on my requirement it seems that an IRF520 is appropriate? Also, I read that the gate has to be pulled to 0 to shut off so I added a 10K pull-down resistor.

I have redone my Spice circuit with a MOSFET, would you mind confirming I have it correct now?

Thanks,

Tom

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#### AlbertHall

Joined Jun 4, 2014
11,610
It might work with the IRF520, but the gate threshold is rather high so the arduino might not turn it on fully (depends on which end of the range the one you get happens to be). The one below is better. The circuit is OK, but the arduino pins will get very close to zero so you don't need R1. It would be a good idea to include a low value resistor, say 100Ω, between the arduino pin and the MOSFET gate.
https://www.rapidonline.com/stp36nf06l-mosfet-logic-n-60v-30a-47-0552

#### ShopRat59

Joined Nov 27, 2013
56
Albert,

Again, thanks for your quick response. I have not been able to find a good (and cheap) source for the MOSFET you recommend, but did find the RFP30N06LE, which has very similar characteristics, will get them Thursday and be able to finish my project thanks to you.

I do have one last question. Could you opine on why most Arduino web pages discussing driving high loads use a power transistor, when it is now obvious to me that a logic MOSFET is clearly a better solution?

Thanks,

Tom

#### AlbertHall

Joined Jun 4, 2014
11,610
That MOSFET looks fine to me.
On the Arduino web question, it may be because MOSFETs are relatively new and folk are not very familiar with them and what is available.