Hello,
I need 4v, I am dropping it from 14v with adjustable smps (buck converter), I also need linear regulator after the smps because of voltage ripple.
The ideal solution would be to use adjustable linear regulator ajdusted to 4v and supply 2 volts higher to it from smps.
But I have L7805CV 5v regulators at home.
I am wondering if I can use L7805CV and supply 4v or a bit more to it and get 4v out without the risk of damage (3.7v - 4.3v is in the tolerance).
Also I have been told once by a member of this forum to add reverse biased diode across the linear regulator if using it after smps to avoid damaging the linear regulator... I did a research, but couldn`t find where exactly to put it, my guess is across input and output pins on linear reg?
Thank you for help
I need 4v, I am dropping it from 14v with adjustable smps (buck converter), I also need linear regulator after the smps because of voltage ripple.
The ideal solution would be to use adjustable linear regulator ajdusted to 4v and supply 2 volts higher to it from smps.
But I have L7805CV 5v regulators at home.
I am wondering if I can use L7805CV and supply 4v or a bit more to it and get 4v out without the risk of damage (3.7v - 4.3v is in the tolerance).
Also I have been told once by a member of this forum to add reverse biased diode across the linear regulator if using it after smps to avoid damaging the linear regulator... I did a research, but couldn`t find where exactly to put it, my guess is across input and output pins on linear reg?
Thank you for help
Linear regulators have an internal voltage reference to set the output voltage, so that will be fixed.
The input voltage for this device must be 2V more than the fixed output voltage for the regulator
to function. You can't trim the output voltage by dropping the input voltage with a linear regulator,
since it will just stop regulating instead.
A protection diode can be connected to this positive regulator, with its anode to the output terminal,
and its cathode to the input terminal. For a negative voltage regulator, reverse the connection.
The input voltage for this device must be 2V more than the fixed output voltage for the regulator
to function. You can't trim the output voltage by dropping the input voltage with a linear regulator,
since it will just stop regulating instead.
A protection diode can be connected to this positive regulator, with its anode to the output terminal,
and its cathode to the input terminal. For a negative voltage regulator, reverse the connection.
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Two diodes in series will drop ~1V. I' am using a USB charger and a couple of diodes to power my 3V doorbell this way.
If you have not already bought the L7805CV, consider an LM317 instead. It can be adjusted with external resistors down to 2.5V. Easy to use just like the 7805.
Thank you all for anwsers.
In that case I will use appropriate regulator.
I think that I will go for L200CV, I could use LM317, but if I must buy, I will go with 2A to have some space.
Anyways if someone knows of a better alternative to L200CV please let me know, I need as least Quiescent drain Current as possible.
In that case I will use appropriate regulator.
I think that I will go for L200CV, I could use LM317, but if I must buy, I will go with 2A to have some space.
Anyways if someone knows of a better alternative to L200CV please let me know, I need as least Quiescent drain Current as possible.
If the input to output voltage difference is lower than the drop out voltage of the 7805 ( roughly 2V depending on current draw), then it's not regulating, and you won't get any help on the ripple or noise. You'd be better off doing it correctly using an adjustable like an LM337.
And yes, the diode goes between the input and output, with the cathode on the input so it's reverse biased by the input to output voltage difference. A SMPS can drop to zero volts very quickly at turn off. The linear regulator has a good sized capacitor which can hold the output voltage up after turn off. This would reverse bias the regulator and can blow the pass transistor. The diode limits the difference to one diode drop.
Bill
And yes, the diode goes between the input and output, with the cathode on the input so it's reverse biased by the input to output voltage difference. A SMPS can drop to zero volts very quickly at turn off. The linear regulator has a good sized capacitor which can hold the output voltage up after turn off. This would reverse bias the regulator and can blow the pass transistor. The diode limits the difference to one diode drop.
Bill
How about the dropout voltage? Both the LM317 and the LM200CV have a dropout voltage that's actually a bit too tight to go from 6 V to 4 V. Or can that 6 V also be 7 V?
Thank you for the explanation. Yes input voltage could be 7v, but I will rather go for LDO regulator, something like PQ30RV21 . I need as least wasted current as possible. It will be in the car running all the time to keep android tablet in standby.
After some more research my head is getting bigger and bigger and starting to hurt.
Are LDO regulators not as good for filtering porpuses than ordinary ones? I see that they have less ripple rejection.
That said would it be better to drop higher voltage with linear reg in terms of noise filtering and ripple rejection?
If so I would use smps for when car is off to minimize current draw and use a relay to switch over to linear reg when I turn the key to ACC.
Can I use smps before the line reg to get same resoults or is it better not to use smps before the linear reg at all?
Is there anything I can do to make sure that I if build this circuit my android tablet wont shut down when relay switches from smps to linear reg? Should I add a capacitor on the output to make sure it won`t turn off?
I guess that I sould add diodes on the outputs of both regulators to avoid damaging one or another regulator?
Are LDO regulators not as good for filtering porpuses than ordinary ones? I see that they have less ripple rejection.
That said would it be better to drop higher voltage with linear reg in terms of noise filtering and ripple rejection?
If so I would use smps for when car is off to minimize current draw and use a relay to switch over to linear reg when I turn the key to ACC.
Can I use smps before the line reg to get same resoults or is it better not to use smps before the linear reg at all?
Is there anything I can do to make sure that I if build this circuit my android tablet wont shut down when relay switches from smps to linear reg? Should I add a capacitor on the output to make sure it won`t turn off?
I guess that I sould add diodes on the outputs of both regulators to avoid damaging one or another regulator?