Double check base resistance values and TVS diode choice, please.

  1. Yo yo yo. What up homies? I am once again humbled to be in need of your gracious assistance. I did all the work this time. I would just like some fresh minds to double check. I am driving two relays with this transistor. This is the relay. It's coil is rated at 11.6mA.

    Now here comes the tricky part. I chose a 7.5KOhm resistor. I looked at the datasheet for the transistor and saw that the minimum Beta (Current Gain) was 100. I used 50mA as my target Ic and got .5mA as my Ib. Now, 5V go into my PIC, 4.3 come out. On top of that, 0.7V is lost from the base to the emmiter, right? So I now have 3.6V/.0005A = 7200Ohm. And I chose 7500Ohm, because the part was the best fit. Is that all correct?

    I have two other transistors that power some other relays. I did all the math the same way, except that they are Darlington's so I used a 1.4Vbe. the only thing I'm pretty wary on is whether I used the right value for Beta. I used 750. Can someone confirm or deny this? Datasheet.

    Ok. Now we come to the good ol' TVS diodes. This is what I chose. The clamping voltage is 64.5V. The transistors have a max voltage of 100V. Gravy, yeah?

    Thanks for the soon-to-come help.
  2. jcsd
  3. berkeman

    Staff: Mentor

    Could you post the schematic for us to see it all together? Apologies if it's in your links already, but I didn't see it so far. Thanks. (BTW, it sounds like you are mostly doing things correctly)
  4. Thanks for the reply. Here's the whole schematic. The transistors are right-ish and up-ish. On the actual circuit board layout, D12 is connected so that the connections from the relays touch the diode first and then go on to the transistor. I'm not sure if I can use one diode for 2 relays like I did. Any input there would also be helpful.

  5. I know there are people out there who know this. Please just double check my calculations. Thanks.
  6. berkeman

    Staff: Mentor

    Is there a reason you are using TVS components instead of just diodes? What voltage TVSs are you using.

    Also, I'm not understanding the Start and Run Darlington drive connections. It looks like the collectors are going to something associated with -12V?

    LOL the Doohickies.
  7. Thanks for the reply. I am using TVS diodes because I read some articles and discussions saying that a simple diode will cause long opening times of the relay contacts which in turn will cause arcing and shorten the lifespan of the relay. In this article, it says that the best combination is a diode in conjunction with a Zener. The other stuff I read led me to the bidirectional TVS didoes and they seem to be the best for the job. I have a bunch of saved links on the subject if you want them. The TVS's have a clamping voltage of 64.5V.

    The +12<| symbol means that that connection hooks up to the +12V supply source. I used the symbol for neatness's sake. The -o o- things are connections points. The relays that hook up there aren't on the circuit board. I guess the plus and minus 12V symbols on the far right should be changed to plain old + and -.
  8. berkeman

    Staff: Mentor

    Sounds okay. What is the breakdown of the Darlingtons? Hopefully more than 65V + 12V.
  9. Yea the Darlington's have a 100V breakdown.
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