Connect with us

Overvoltage protection

Discussion in 'Electronic Design' started by Mark Ebert, May 21, 2006.

Scroll to continue with content
  1. Mark Ebert

    Mark Ebert Guest

    I'm looking for a method of protecting a circuit from overvoltage
    (perhaps by turning off a FET in the presence of higher voltages).
    The input voltage is normally +5v, but if plugged into the wrong
    system it approaches 30 volts which destroys the downstream
    circuitry. I'm trying to find a way to isolate the overvoltage
    without shorting the input (so a simple fuse or TVS is out), but
    have been unsuccessful so far. My original plan was to use a
    comparator to switch a MOSFET off when a pre-determined threshold
    is exceeded, but I'm unable to come up with a proper driver for
    the FET gate.
    Any suggestions would be greatly appreciated!

    Thanks,
    Mark
     
  2. A problem with turning OFF devices is it assumes they still work.
    Series fuse, 5 watt 6.2 volt Zener to ground. Fuse is a 'Dang'
    indicator Use a self healing poly fuse if you don't want to replace the
    fuse every time.

    I don't get how a fuse shorts the input.

    GG
     
  3. Fred Bloggs

    Fred Bloggs Guest

    There are things called LDO's. If you can't find one suitable then make
    one. A PMOS type set to regulate at say 5V +15% will be fully driven to
    RDS,ON with 5V input/output, but clamp at 5.75V output. There will be
    thermal protection considerations if the loading is heavy. Or maybe you
    want to just shut the thing down if 30V is applied, just extend the LDO
    idea except now it is the input and not output that is sensed.
     
  4. John  Larkin

    John Larkin Guest

    Please define "a circuit". Is the input a logic signal to the circuit,
    or does it power it? How much voltage does the circuit actually need
    to function?

    John
     
  5. John Fields

    John Fields Guest

    ---
    View in Courier:


    Vin>--+------+--------+---S D----->OUT
    | | | G
    [10K] [10K] [10K] |
    | | | [1K]
    +------|--|+\ | |
    | | | >--+-----+
    [10K] +--| /U1A LM393
    | |
    | [LM385-2.5]
    | |
    GND>--+------+-+------------------->GND
    |
    +--------+
    | |
    +--|+\ |
    | | >--+
    +--|-/ U1B
     
  6. Mark Ebert

    Mark Ebert Guest

    Actually, the "circuit" that is driven by the 5 volt supply is
    just a series of backlight lamps for a control panel. The normal
    voltage range is 0-5 volts (dimming), but if the panel is plugged
    into the wrong product (we have field problems with this), it
    burns out the bulbs. We have used fuses to protect this in the
    past, but because of the location of the PCB its a pain to replace
    so I want to improve on it to provide some sort of overvoltage
    sense, then open the output in the presence of this. The current
    draw is 2.5 amps max. and the inout voltage can reach 32 volts,
    otherwise I would use a commercial overvoltage device, such as a
    On Semi NCP346 driving a FET.
     
  7. Ken Smith

    Ken Smith Guest

    Polyfuses are near useless in overvoltage situations. When the voltage
    goes too high and the crowbar fires, a polyfuse will usually be
    destroyed, unless something else protects it..

    Check out the "Max. current" ratings of Polyfuses.
     
  8. John  Larkin

    John Larkin Guest

    Sounds like you could use a polyfuse followed by a 5-volt power zener
    or transzorb.

    Or just a 5-volt LDO regulator, although the 2.5 amps makes it tricky.
    If you could split the load into sections, 3 maybe, you could use
    three small ldo's. Below 5 volts, output would track input pretty
    well, but above 5v in they'd regulate and eventually thermal limit.

    John
     
  9. Ken Smith

    Ken Smith Guest


    How about this:

    TIP-36
    e c
    --+-------+------ ----------- To bulbs
    ! ! \ /
    / ! -----
    \ ! !
    / 0.5W \ !
    ! 2.2K / 1W \
    zener/-/ \ 1K / 50R
    4.3V ^ ! \ 2W
    ! ! 22K !
    +-----------/\/\-+
    ! ! !
    ! ! !
    ! ! !/ c
    ! --+------! 2N2222
    ! !/ !\ (2 EA)
    +---! !
    ! !\ GND
    / !
    \ GND
    / 1K
    !
    GND
     
  10. Maybe a LMV431, BJT, MOSFET + 5R would do the trick.


    Best regards,
    Spehro Pefhany
     
  11. John Fields

    John Fields Guest


    ---
    This is easier:

    PCH
    Vin>--+------+--------+---S D----->OUT
    | | | G
    | [10K] [10K] |
    | | | |
    [10k]<---|--|+\ | |
    | | | >--+-----+
    | +--| /U1A LM393
    | |
    | [LM385-2.5]
    | |
    GND>--+------+-+------------------->GND
    |
    +--------+
    | |
    +--|+\ |
    | | >--+
    +--|-/ U1B


    You don't need the gate resistor, and just set the pot for whatever
    turn-off threshold you want.

    Here's the LTSPICE circuit file:

    Version 4
    SHEET 1 880 680
    WIRE -256 272 -256 -16
    WIRE -256 432 -256 352
    WIRE -256 464 -256 432
    WIRE -144 272 -144 160
    WIRE -144 432 -256 432
    WIRE -144 432 -144 352
    WIRE -16 -16 -256 -16
    WIRE -16 16 -16 -16
    WIRE -16 192 -16 96
    WIRE -16 272 -16 192
    WIRE -16 432 -144 432
    WIRE -16 432 -16 352
    WIRE 144 160 -144 160
    WIRE 144 192 -16 192
    WIRE 176 -16 -16 -16
    WIRE 176 144 176 -16
    WIRE 176 432 -16 432
    WIRE 176 432 176 208
    WIRE 256 -16 176 -16
    WIRE 256 16 256 -16
    WIRE 256 112 256 96
    WIRE 256 176 208 176
    WIRE 256 176 256 112
    WIRE 416 112 256 112
    WIRE 464 -16 256 -16
    WIRE 464 96 464 -16
    WIRE 464 272 464 192
    WIRE 464 432 176 432
    WIRE 464 432 464 352
    FLAG -256 464 0
    SYMBOL Comparators\\LT1017 176 176 R0
    SYMATTR InstName U1
    SYMBOL res -32 0 R0
    SYMATTR InstName R1
    SYMATTR Value 10k
    SYMBOL res -32 256 R0
    SYMATTR InstName R2
    SYMATTR Value 8200
    SYMBOL voltage -144 256 R0
    WINDOW 123 0 0 Left 0
    WINDOW 39 0 0 Left 0
    SYMATTR InstName V1
    SYMATTR Value 2.5
    SYMBOL res 240 0 R0
    SYMATTR InstName R3
    SYMATTR Value 10k
    SYMBOL voltage -256 256 R0
    WINDOW 3 24 104 Invisible 0
    WINDOW 123 0 0 Left 0
    WINDOW 39 0 0 Left 0
    SYMATTR InstName V2
    SYMATTR Value PULSE(0 30 0 1)
    SYMBOL pmos 416 192 M180
    SYMATTR InstName M1
    SYMATTR Value Si9407AEY
    SYMBOL res 448 256 R0
    SYMATTR InstName R4
    SYMATTR Value 2
    TEXT -290 506 Left 0 !.tran 2


    I simulated with an LT1017 because I don't have a model for an
    LM393, but I can't imagine why the LM339 wouldn't work.
     
  12. AJ

    AJ Guest


    ...........This might help....
    http://www.maxim-ic.com/appnotes.cfm/appnote_number/760

    Regards


    AJ
     
Ask a Question
Want to reply to this thread or ask your own question?
You'll need to choose a username for the site, which only take a couple of moments (here). After that, you can post your question and our members will help you out.
Electronics Point Logo
Continue to site
Quote of the day

-