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MC7808BTG drop out voltage?

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  • MC7808BTG drop out voltage?

    This seems to be what i need and says it's LDO which i assume means low drop out, but i can't find the spec on the drop out. I want 8 volt output (actually prefer 8.5v but Enzo suggested a diode will get me there) to keep a rechargable that charges to 11.6v at a steady 8.5 preferably. But from what i'm told i need maybe 100mv DO in order for it to work down to 8.1v. Will this do that? Also, how does using a diode to keep it at 8.5v instead of 8 affect the DO ? Will it still work till 8.1v when the output using a diode 8.5? And finally, in laymans terms how would i wire the diode and what diode(s) would get me 8.5v?

  • #2
    Where does it say it's LDO ? I don't think that one is.
    Datasheet shows dropout voltage as 2V.
    "Everything is better with a tube. I have a customer with an all-tube pacemaker. His heartbeat is steady, reassuring and dependable, not like a modern heartbeat. And if it goes wrong he can fix it himself. You can't do that with SMD." - Mick Bailey

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    • #3
      Figures, oh well. I can't recall where i first noticed it but i was searching for LDO ones and somewhere it said that was LDO but didn't show a spec and i looked elsewhere for the spec but couldn't find it. Back to the drawing board. I don't think there are many that aren't either $15 or 99 cents plus $10 shipping.

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      • #4
        Everything will have the minimum shipping. If you have a local electronics component supplier that is the only way around it. Or a local repair shop might sell you one.
        LM2940T-8 is LDO. Dropout is 0.5V above output voltage. So if you want to maintain 8.5V output (with diode to ground pin), you need to maintain at least 9V at the input.
        There are LDO's with even lower dropout rating, like 200mV. So with that you would need to maintain at least 200mV more at the input than the output.
        "Everything is better with a tube. I have a customer with an all-tube pacemaker. His heartbeat is steady, reassuring and dependable, not like a modern heartbeat. And if it goes wrong he can fix it himself. You can't do that with SMD." - Mick Bailey

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        • #5
          I know about shipping but what i was trying to do is find one with the right specs then try and find it on ebay where there are usually free ship options. I think that one was $5/free ship which is why i was interested in it.
          200MV would be fine because it would drop out at 8.2volts and the battery charges to 11.6 and takes hours to get down to the 8v range so i just want it to hold between 8 or 8.5 for most of it's charge. Even that one at 9 i suppose i'd be good with if i can't find one at 200mv DO.

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          • #6
            Originally posted by g1 View Post
            Everything will have the minimum shipping. If you have a local electronics component supplier that is the only way around it. Or a local repair shop might sell you one.
            LM2940T-8 is LDO. Dropout is 0.5V above output voltage. So if you want to maintain 8.5V output (with diode to ground pin), you need to maintain at least 9V at the input.
            There are LDO's with even lower dropout rating, like 200mV. So with that you would need to maintain at least 200mV more at the input than the output.
            You sure it's 500mv? Because i found a spec sheet on the National LM2940T-8.0 and it says 1v. I'd like to know for sure because i found one on ebay for $5.40 shipped.

            Edit: now i find another that says .5v. Geez. How do u know which is right. Quite sure the first spec i saw was LM2940T-8.0

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            • #7
              It looks like the drop out voltage varies depending on load and temperature. So for the LM2940t, both figures are correct.
              On top of that, it seems LDO's have quiescient current that may be problematic for battery life.
              Maybe others here can comment on that. Datasheet attached.
              (also, if any of those ebay 'free shipping' items are shipping from China, they may be fakes)
              Attached Files
              "Everything is better with a tube. I have a customer with an all-tube pacemaker. His heartbeat is steady, reassuring and dependable, not like a modern heartbeat. And if it goes wrong he can fix it himself. You can't do that with SMD." - Mick Bailey

              Comment


              • #8
                Originally posted by g1 View Post
                On top of that, it seems LDO's have quiescient current that may be problematic for battery life.
                Maybe others here can comment on that. Datasheet attached.
                Anyone? I'd probably go with t`his one even at 1v because this would keep the battery at 8v then 9 once dropout occurs and thats fine. I just don't want to spend the first 3 hours playing at 11.6 on down to 9 which takes about 4 or 5 hours. But now i have the above to worry about so anyone know if there's an issue like G1 suggested may be the case?

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                • #9
                  The quiescent current is between 12mA and 15mA for a current draw of 500mA for the LM2940T. The dropout voltage is perhaps 0.3v or less with 500mA draw. Both figures reduce with lower temperature, lower output current and low input/output differential. The charts give a better picture than the data. 3-terminal devices have poorer specs than more modern 8-pin SMD designs, some of which have 50mV dropout and just a few microamps quiescent draw for use in phones and automotive applications. I would think that this device would give pretty good results with low-current effects. What are we looking at - about 40mA per pedal?

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                  • #10
                    Originally posted by Mick Bailey View Post
                    The quiescent current is between 12mA and 15mA for a current draw of 500mA for the LM2940T. The dropout voltage is perhaps 0.3v or less with 500mA draw. Both figures reduce with lower temperature, lower output current and low input/output differential. The charts give a better picture than the data. 3-terminal devices have poorer specs than more modern 8-pin SMD designs, some of which have 50mV dropout and just a few microamps quiescent draw for use in phones and automotive applications. I would think that this device would give pretty good results with low-current effects. What are we looking at - about 40mA per pedal?
                    A lot less actually. Running 3 boosts off one battery, no digital stuff. One is 16mA, one is 4, and the tube screamer i think is under 10. So we're talking a total of around 30mA.

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                    • #11
                      For 100mA load you'd get a dropout voltage voltage of around 0.11v. That's as low as the spec sheet shows, and you may get better than that in practice. Also, once the regulator ceases to regulate due to the supply voltage being is less than the regulated voltage+dropout voltage, the output remains linear to the input (apart from a small internal loss within the regulator). Seems to me to be a good choice of device for this application.

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                      • #12
                        Thanks Mick, perfect. Heres the one i will get. So if it doesn't work out i lose very little. Under $6 shipped. Actually i'll grab a couple t b safe at only $1.49 more https://www.ebay.com/itm/20236133278...4AAOSw60NbPmfG
                        Last edited by daz; 11-24-2021, 03:14 PM.

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                        • #13
                          Don't forget that unlike 78xx regulators you really do need the 22uf or higher capacitor across the output (close to the pins), otherwise the output voltage can be way off what it's intended to be.

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                          • #14
                            Originally posted by Mick Bailey View Post
                            Don't forget that unlike 78xx regulators you really do need the 22uf or higher capacitor across the output (close to the pins), otherwise the output voltage can be way off what it's intended to be.
                            Yes, i just looked at the data sheet and see a 22uf with + to out and - to ground. Also, where would i put a diode, what type, and what direction to up the output to 8.5v as Enzo suggested?

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                            • #15
                              Ok, went back and found Enzo's replies and i now see a schematic he posted where the diode goes on the ground leg with anode to the regulator. I don't get why he said it wasn't in series, as to me thats in series with ground. But that aside, what about the type. Any 4000 series ? And what about voltage....if i wanted to make it bring the voltage to 9v can i do that by adding another diode?

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