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7805 drops to 4,6 volts? — Parallax Forums

7805 drops to 4,6 volts?

ArchiverArchiver Posts: 46,084
edited 2004-06-09 14:06 in General Discussion
hai,

build a 7805 power supply. with normal specs . (input voltage 16 volts, 4
diodes to to make alternate voltage "flat" condensator 4700 microfarads, and
by the 7805 two condensators of 0.1 microfarad) measure with no load is 5.04
volts, but if i make a resistor load of 100 ohms the voltage drop to 4.67
volts. is this normal behaviour??

thanks pieter-jan

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Comments

  • ArchiverArchiver Posts: 46,084
    edited 2004-06-06 21:28
    <snip>
    This is perfectly normal. Try it with Ohm's law. V= IxR

    Regards,
    John Baker


    With all due respect, I don't think so. Ohms law does not directly apply
    here. As long as the vreg max current or max power, or dropout voltage are all
    within range for normal operation, the output should remain at (very close to) 5
    volts.

    5v / 100 ohms yields 50 mA.

    You mentioned capacitors. Do you have a cap 0.1uF on the input and the output
    of the regulator, very close to the regulator?

    Ken
    ======================


    Pieter-Jan van Diepen <diepenpj@h...> wrote:
    hai,

    build a 7805 power supply. with normal specs . (input voltage 16 volts, 4
    diodes to to make alternate voltage "flat" condensator 4700 microfarads, and
    by the 7805 two condensators of 0.1 microfarad) measure with no load is 5.04
    volts, but if i make a resistor load of 100 ohms the voltage drop to 4.67
    volts. is this normal behaviour??

    thanks pieter-jan


    [noparse][[/noparse]Non-text portions of this message have been removed]
  • ArchiverArchiver Posts: 46,084
    edited 2004-06-06 21:47
    1. You didn't say if the 16 volts was AC or DC.
    Since you used 4 diodes to rectify it, I'll assume
    it's AC. So, you have 'full-wave' rectified DC --
    which is DC with ripple on it -- the ripple filtered
    by the 4700 uF capacitor. This is good.

    2. The input and output caps on a 7805 should be much
    larger than 0.1 uF -- more like 22 uF.

    Putting 5 volts into 100 ohms is 50 mA. A 7805
    TO220 package should happily put out 1.5 amps at
    +5 volts, though it will get warm doing it.

    The amount of heat depends on the watts being
    dissipated in the 7805 (Voltage Drop * Current)

    4. Different 7805 packages (transistor type, and
    SMT-mount) have different wattage.
    I should think ALL of them would happily put out
    50 mA without losing regulation.

    So NO, what you are seeing is NOT normal, and you
    probably need larger input and output capacitors
    for the 7805.

    --- In basicstamps@yahoogroups.com, "Pieter-Jan van Diepen"
    <diepenpj@h...> wrote:
    > hai,
    >
    > build a 7805 power supply. with normal specs . (input voltage 16
    volts, 4
    > diodes to to make alternate voltage "flat" condensator 4700
    microfarads, and
    > by the 7805 two condensators of 0.1 microfarad) measure with no
    load is 5.04
    > volts, but if i make a resistor load of 100 ohms the voltage drop
    to 4.67
    > volts. is this normal behaviour??
    >
    > thanks pieter-jan
    >
    > _________________________________________________________________
    > Hotmail en Messenger on the move
    > http://www.msn.nl/communicatie/smsdiensten/hotmailsmsv2/
  • ArchiverArchiver Posts: 46,084
    edited 2004-06-06 23:36
    What is the max current of the 16 volts going in? part of the problem
    could be the 11 volt drop the 7805 has to produce in heat in addition to
    the load. In this case only 50 ma. Is it getting hot to the touch? It
    should be ok. The 7805 should handle up to about 35 volts going in with
    heat sink.
    are you using a 7805 in a TO-220 case (big one) or the 78L05 TO-92 case
    (small transistor style) the small one won't produce much power out.
    Without digging out the formula, sounds like the 4,700 uF capacitor should
    be plenty. It should be placed on the input side of the 7805.

    BTW, you "speak" better English than some of the people I know over there.
    [noparse]:)[/noparse] Probably just a coincidence but got a good friend Marsel Pieter in germany.

    Daryl



    At 01:57 PM 6/6/2004, you wrote:
    >hai,
    >
    >build a 7805 power supply. with normal specs . (input voltage 16 volts, 4
    >diodes to to make alternate voltage "flat" condensator 4700 microfarads, and
    >by the 7805 two condensators of 0.1 microfarad) measure with no load is 5.04
    >volts, but if i make a resistor load of 100 ohms the voltage drop to 4.67
    >volts. is this normal behaviour??
    >
    >thanks pieter-jan
    >
    >_________________________________________________________________
    >Hotmail en Messenger on the move
    >http://www.msn.nl/communicatie/smsdiensten/hotmailsmsv2/
    >
    >
    >
    >To UNSUBSCRIBE, just send mail to:
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    >Body of the message will be ignored.
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    >
    >
    >
    >
  • ArchiverArchiver Posts: 46,084
    edited 2004-06-06 23:43
    This is perfectly normal. Try it with Ohm's law. V= IxR

    Regards,
    John Baker
    http://www.geocities.com/johnsrobotics/

    Pieter-Jan van Diepen <diepenpj@h...> wrote:
    hai,

    build a 7805 power supply. with normal specs . (input voltage 16 volts, 4
    diodes to to make alternate voltage "flat" condensator 4700 microfarads, and
    by the 7805 two condensators of 0.1 microfarad) measure with no load is 5.04
    volts, but if i make a resistor load of 100 ohms the voltage drop to 4.67
    volts. is this normal behaviour??

    thanks pieter-jan

    _________________________________________________________________
    Hotmail en Messenger on the move
    http://www.msn.nl/communicatie/smsdiensten/hotmailsmsv2/



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    [noparse][[/noparse]Non-text portions of this message have been removed]
  • ArchiverArchiver Posts: 46,084
    edited 2004-06-07 02:03
    Having worked with the 7805 regulators for many years, bear in mind that
    many of the older types require a certain minimum load on them in order
    to regulate properly. The load is typically on the order of a few
    milliamperes, but once in place, it should be stable across the whole
    range of input voltage/output load (assuming it's not getting too hot).

    David Erbas-White

    smartdim@a... wrote:

    ><snip>
    >This is perfectly normal. Try it with Ohm's law. V= IxR
    >
    >Regards,
    >John Baker
    >
    >
    >With all due respect, I don't think so. Ohms law does not directly apply
    >here. As long as the vreg max current or max power, or dropout voltage are all
    >within range for normal operation, the output should remain at (very close to) 5
    >volts.
    >
    >5v / 100 ohms yields 50 mA.
    >
    >You mentioned capacitors. Do you have a cap 0.1uF on the input and the output
    >of the regulator, very close to the regulator?
    >
    >Ken
    >======================
    >
    >
    >Pieter-Jan van Diepen <diepenpj@h...> wrote:
    >hai,
    >
    >build a 7805 power supply. with normal specs . (input voltage 16 volts, 4
    >diodes to to make alternate voltage "flat" condensator 4700 microfarads, and
    >by the 7805 two condensators of 0.1 microfarad) measure with no load is 5.04
    >volts, but if i make a resistor load of 100 ohms the voltage drop to 4.67
    >volts. is this normal behaviour??
    >
    >thanks pieter-jan
    >
    >
    >[noparse][[/noparse]Non-text portions of this message have been removed]
    >
    >
    >
    >To UNSUBSCRIBE, just send mail to:
    > basicstamps-unsubscribe@yahoogroups.com
    >from the same email address that you subscribed. Text in the Subject and Body of the message will be ignored.
    >
    >Yahoo! Groups Links
    >
    >
    >
    >
    >
    >
    >
    >
  • ArchiverArchiver Posts: 46,084
    edited 2004-06-07 02:09
    Well like I said in my earlier reply. It depends what the max current of
    the 16V going in is. The 7805 regulator should not drop any voltage if all
    is kept within limits. A good example would be a computer power supply. The
    DC resistance of 3.3V of some motherboards is less than 1 ohm but yet the
    power supply still keeps a steady 3.3V because theres like 80amps going in.


    BTW: Pieter, whats the price of wood and building materials over there now.
    It is ridiculous here like 4X the cost


    At 05:43 PM 6/6/2004, you wrote:
    >This is perfectly normal. Try it with Ohm's law. V= IxR
    >
    >Regards,
    >John Baker
    >http://www.geocities.com/johnsrobotics/
    >
    >Pieter-Jan van Diepen <diepenpj@h...> wrote:
    >hai,
    >
    >build a 7805 power supply. with normal specs . (input voltage 16 volts, 4
    >diodes to to make alternate voltage "flat" condensator 4700 microfarads, and
    >by the 7805 two condensators of 0.1 microfarad) measure with no load is 5.04
    >volts, but if i make a resistor load of 100 ohms the voltage drop to 4.67
    >volts. is this normal behaviour??
    >
    >thanks pieter-jan
    >
    >_________________________________________________________________
    >Hotmail en Messenger on the move
    >http://www.msn.nl/communicatie/smsdiensten/hotmailsmsv2/
    >
    >
    >
    >To UNSUBSCRIBE, just send mail to:
    >basicstamps-unsubscribe@yahoogroups.com
    >from the same email address that you subscribed. Text in the Subject and
    >Body of the message will be ignored.
    >
    >Yahoo! Groups Links
    >
    >
    >
    >
    >
    >
    >
    >Do you Yahoo!?
    >Friends. Fun. Try the all-new Yahoo! Messenger
    >
    >[noparse][[/noparse]Non-text portions of this message have been removed]
    >
    >
    >
    >To UNSUBSCRIBE, just send mail to:
    > basicstamps-unsubscribe@yahoogroups.com
    >from the same email address that you subscribed. Text in the Subject and
    >Body of the message will be ignored.
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    >
    >
    >
  • ArchiverArchiver Posts: 46,084
    edited 2004-06-07 02:54
    My 2 cents.
    Have you connected a oscilloscope to the regulator output?
    The regulator may oscillating.
    You should see a ac signal with a frequency of 8.33 msec.
  • ArchiverArchiver Posts: 46,084
    edited 2004-06-09 13:01
    This could be normal.

    If you take a look at 78xx spec. you can see a Vout Tolerance parameter with load. For the most of 78xx this parameter is +-5%.

    This tolerance depends on Manufacturer, Type of device, Input voltage and Load conditions.

    Examples, look for Output_voltage parameter:
    http://www.st.com/stonline/books/pdf/docs/2143.pdf
    http://focus.ti.com/lit/ds/symlink/ua7805.pdf

    Luis.

    Mensaje original
    De: Pieter-Jan van Diepen [noparse]/noparse]mailto:[url=http://forums.parallaxinc.com/group/basicstamps/post?postID=wtH66jneUq_l623jGbQUJ_IBwTdXQe4bsPD8bdfm7hjYip6X7Qn7pFz71U7QktwSjbD_EeV78PiYl7ov]diepenpj@h...[/url
    Enviado el: domingo, 06 de junio de 2004 20:57
    Para: basicstamps@yahoogroups.com
    Asunto: [noparse][[/noparse]basicstamps] 7805 drops to 4,6 volts?


    hai,

    build a 7805 power supply. with normal specs . (input voltage 16 volts, 4
    diodes to to make alternate voltage "flat" condensator 4700 microfarads, and
    by the 7805 two condensators of 0.1 microfarad) measure with no load is 5.04
    volts, but if i make a resistor load of 100 ohms the voltage drop to 4.67
    volts. is this normal behaviour??

    thanks pieter-jan

    _________________________________________________________________
    Hotmail en Messenger on the move
    http://www.msn.nl/communicatie/smsdiensten/hotmailsmsv2/



    To UNSUBSCRIBE, just send mail to:
    basicstamps-unsubscribe@yahoogroups.com
    from the same email address that you subscribed. Text in the Subject and Body of the message will be ignored.

    Yahoo! Groups Links
  • ArchiverArchiver Posts: 46,084
    edited 2004-06-09 14:00
    A point is missing.
    There is a big difference between the Initial Tolerance and the
    Voltage Regulation.
    The initial Tolerance yes! it is +/- 1%,5% or 10% or whatever.
    However, the Voltage Regulation depends in the Open Loop gain of the
    internal Amplifier, and the thevenin equivalent output impedance.
    It is not uncommon Open Loop gains of 60 and up DBs.
    Not even the Internal Voltage Reference has any effect in the Voltage
    Regulation not counting Tempco (Thermal coefficient) effects.
  • ArchiverArchiver Posts: 46,084
    edited 2004-06-09 14:06
    Um, not to be difficult, but 5% of 5 volts is
    0.25 volts. So, 4.75 volts would be the
    lowest acceptable with that tolerance.

    Just a thought.

    --- In basicstamps@yahoogroups.com, "Luis" <lullamas@y...> wrote:
    > This could be normal.
    >
    > If you take a look at 78xx spec. you can see a Vout Tolerance
    parameter with load. For the most of 78xx this parameter is +-5%.
    >
    > This tolerance depends on Manufacturer, Type of device, Input
    voltage and Load conditions.
    >
    > Examples, look for Output_voltage parameter:
    > http://www.st.com/stonline/books/pdf/docs/2143.pdf
    > http://focus.ti.com/lit/ds/symlink/ua7805.pdf
    >
    > Luis.
    >
    >
    >
    Mensaje original
    > De: Pieter-Jan van Diepen [noparse][[/noparse]mailto:diepenpj@h...]
    > Enviado el: domingo, 06 de junio de 2004 20:57
    > Para: basicstamps@yahoogroups.com
    > Asunto: [noparse][[/noparse]basicstamps] 7805 drops to 4,6 volts?
    >
    >
    > hai,
    >
    > build a 7805 power supply. with normal specs . (input voltage 16
    volts, 4
    > diodes to to make alternate voltage "flat" condensator 4700
    microfarads, and
    > by the 7805 two condensators of 0.1 microfarad) measure with no
    load is 5.04
    > volts, but if i make a resistor load of 100 ohms the voltage drop
    to 4.67
    > volts. is this normal behaviour??
    >
    > thanks pieter-jan
    >
    > _________________________________________________________________
    > Hotmail en Messenger on the move
    > http://www.msn.nl/communicatie/smsdiensten/hotmailsmsv2/
    >
    >
    >
    > To UNSUBSCRIBE, just send mail to:
    > basicstamps-unsubscribe@yahoogroups.com
    > from the same email address that you subscribed. Text in the
    Subject and Body of the message will be ignored.
    >
    > Yahoo! Groups Links
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