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

Click For Summary

Discussion Overview

The discussion revolves around verifying calculations related to resistor values and the selection of TVS diodes in a circuit driving relays with transistors. Participants explore the appropriateness of component choices and configurations, including the implications of using TVS diodes versus standard diodes.

Discussion Character

  • Technical explanation
  • Debate/contested
  • Mathematical reasoning
  • Experimental/applied

Main Points Raised

  • One participant details their calculations for selecting a 7.5KOhm resistor based on transistor beta values and relay specifications, questioning the correctness of their approach.
  • Another participant asks for confirmation on the use of a Darlington transistor's beta value, suggesting uncertainty about the chosen value of 750.
  • Concerns are raised about whether a single TVS diode can adequately protect two relays, with a request for input on this configuration.
  • Discussion includes the rationale for using TVS diodes over standard diodes, citing potential issues with relay lifespan due to arcing.
  • Clarifications are made regarding circuit symbols used in the schematic, including the meaning of the +12V connections.
  • Participants express uncertainty about the breakdown voltage of the Darlington transistors in relation to the circuit's voltage levels.

Areas of Agreement / Disagreement

Participants express varying levels of certainty regarding calculations and component choices, with no clear consensus reached on the correctness of the resistor values or the adequacy of the TVS diode configuration.

Contextual Notes

Some calculations depend on specific assumptions about transistor beta values and the configuration of the circuit, which remain unresolved. The appropriateness of using one TVS diode for multiple relays is also under discussion without a definitive conclusion.

¡MR.AWESOME!
Messages
35
Reaction score
0
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 http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name=FMMT493CT-ND" 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? http://www.fairchildsemi.com/ds/BD/BD675A.pdf" .

Ok. Now we come to the good ol' TVS diodes. http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name=SMAJ40CALFCT-ND" 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.
 
Last edited by a moderator:
Engineering news on Phys.org
¡MR.AWESOME! said:
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 http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name=FMMT493CT-ND" 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? http://www.fairchildsemi.com/ds/BD/BD675A.pdf" .

Ok. Now we come to the good ol' TVS diodes. http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name=SMAJ40CALFCT-ND" 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.

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)
 
Last edited by a moderator:
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.


th_PushbuttonCircuit4.jpg
 
I know there are people out there who know this. Please just double check my calculations. Thanks.
 
¡MR.AWESOME! said:
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.


th_PushbuttonCircuit4.jpg

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.
 
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 http://relays.tycoelectronics.com/appnotes/app_pdfs/13c3311.pdf" 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 -.
 
Last edited by a moderator:
¡MR.AWESOME! said:
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 http://relays.tycoelectronics.com/appnotes/app_pdfs/13c3311.pdf" 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 breakdown 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 -.

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