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Do I rneed a voltage regulator?

Discussion in 'General Electronics Discussion' started by silverfox, Mar 31, 2012.

  1. silverfox

    silverfox

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    Mar 31, 2012
    I am not very experienced at circuit design but I am designing a circuit that I hope will become part of a low-cost commercial product of my own design. Consequently, I am trying to keep component count to a bare minimum. My question is, if my gadget has its own 3V CR2032 coin battery which powers a microcontroller, EEPROM, RTC, and some optoisolators, is there any need for a voltage regulator? Can I get away just leaving it out?
     
  2. davenn

    davenn Moderator

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    What are the voltage and current requirements of the circuit? That is, what voltage range will they work over. If say, it's 3 V plus or minus a volt then there shouldn't really be any problem
    Have you worked out how long the circuit is going to last on a CR2032 ?


    Dave
     
  3. silverfox

    silverfox

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    Mar 31, 2012
    How to best switch 5V with a 3V circuit?

    A circuit for a commercial gadget I am designing only requires 3V but one of the functions of the circuit is to occasionally supply +5V to a jack into which an external device is plugged. When the user wants to use that device, they plug an external battery pack into a jack on my gadget and the application program in my microcontroller turns the device on and off as required by switching the 5V supply. I am thinking that one way to do this is with a solid state relay that can switch the 5V on and off but can be operated entirely with 3 volts. But I wonder if this is the best solution. I am trying to make this circuit as inexpensive as possible since the end product needs to be very low cost and hopefully manufactured in high volume. Is there a better way than the relay?
     
  4. silverfox

    silverfox

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    Mar 31, 2012
    I am not really sure about the range of voltage and current requirements. I guess I need to check that information on the datasheets of the individual components. And I don't know for sure how long the CR2032 will last but I am aware of a similar circuit for which the CR2032 is quite adequate. It seems like it would be tough calculation to make since there is a microcontroller involved and I think that the current draw depends on what it is doing, which is not entirely predictable.
     
  5. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Jan 21, 2010
    I see several ways:

    1) power the circuit from 5V, not 3V

    2) employ a boost regulator to get 5V from your 3V

    3) use another relay that will operate from 3V

    4) use something other than a relay (SSR, optocoupler, triac, mosfet, etc -- possibly in combination)
     
  6. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Also look at the datasheet for the battery. It's not a particularly difficult problem once you have all the information.
     
  7. silverfox

    silverfox

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    Mar 31, 2012
    1) I am planning on using a CR2032 because size and weight are an issue here for the most common usage. If the user wants to attach a device that is powered from my device then he can attach a battery. But that is not the primary application and I want the unit to be as small and light as possible for the primary usage.

    2) I don't think a boost regulator is the best idea because the external device could drain the little coin battery too fast. I want them to use their own battery if they want to power their devices off my gadget.

    3) Another relay? Other than what?

    4) So what are the advantages of any of those (optocoupler, triac, mosfet, etc ) over using a solid state relay as I suggested? That was my question. Are any of them cheaper, use less power, smaller? And are there 3V versions of all of them? I was thinking about an optocoupler but just wasn't sure if there would be any problems. I think that they have current limitations but probably a SSR does too. I am not sure which is more tolerant. Something to research.
     
  8. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    If you don't need the output to be isolated, you can use your circuit to switch a mosfet. That will require almost no additional power from your battery. The SSR will require much more because it is essentially operated by you powering a LED.

    I initially misinterpreted you as saying that you provide an external 5V to operate a relay. Since you've started a new thread for this I did not realise this was about the thing operated from a small cell.
     
  9. silverfox

    silverfox

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    Mar 31, 2012
    Thanks, that was the answer I was looking for. Now I will have to research MOSFETs. That is what I like about doing things like electronic projects on my own but with help from the kind people in forums. I learn a lot more this way.
     
  10. silverfox

    silverfox

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    Mar 31, 2012
    I am having a little trouble finding the correct MOSFET device to use. I found this site that is useful: http://brunningsoftware.co.uk/FET.htm. But they are using 5V to trigger the MOSFET it isn't clear (to me) if the same device will also work with 3V. I did a Digikey search but I didn't see any filters that specified the minimum gate voltage to trigger the MOSFET. Any ideas?
     
  11. BobK

    BobK

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    Jan 5, 2010
    Look for a logic-level MOSFET. These have a lower gate voltage requirement.

    Bob
     
  12. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    What you're looking for is threshold voltage. Digikey list it as "Vgs(th) (Max) @ Id". Some value around 1V would probably be good.

    Then you need to look at the datasheet to determine the saturation current you're going to see at your minimum supply voltage (less however much your gate drive differs from that). to see what load the device can switch. Also relevant is Rds(on) for this gate voltage. Both of these can be used to calculate the max load depending on the amount of heatsinking your device has (which you then de-rate for safety)

    edit: OK I did this and picked the first N channel mosfet I found. it's a NDS331NTR-ND, and it's datasheet tells me that it has a RdsOn of 0.21 ohms at Vgs of 2.1V (it also gas a BVdss of only 20V) It's saturation current will be something over 3A (it's only rated for 1.3A continuous)

    At 1A, the power dissipation would be 0.21W, and the junction to ambient thermal resistance is 250 degC/W, so the junction would rise about 52 degrees over ambient. That would be acceptable, but I'd label the device as being able to switch 12V at 500mA just to be sure.

    You could go searching for other devices (that may be more easily available or have different specs) if you have a need for them. I don't recommend this part, as I said it was the first one on the list.
     
    Last edited: Apr 2, 2012
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