Can you "backfeed???" all/some/none voltage regulators? - Page 1
Forum section: electronics / beginners

author: cemelec
author_rank: Contributor
timestamp: July 17, 2022, 05:26:00 pm
content: Example - I have numerous working projects over the years, analogue, digital fed from unregulated or just higher voltage sources to a voltage regulator which output typically +/-12v, 5v, 3.3vBut say I have a 5V>3.3v onboard regulator (e.g for USB to logic), can I feed 3.3v directly to the output to power the circuit? e.g from a 18650 cell? (approx 3.3v)I've read loads of voltage regulator data sheets but I can't find any info on this. A blocking diode is a partial but not always an ideal solution.ThanksCharles

author: Siwastaja
author_rank: Super Contributor
timestamp: July 17, 2022, 05:41:35 pm
content: Most linear regulators fall into either of the two categories:* They have integrated diode from Vout to Vin, which makes your 3V3 (minus diode drop) appear on theinput, which is pretty bad if someone shorts the input, etc.;* They just blow up if you do this due to lack of the integrated diode. (i.e., 78xx, hence the example circuit where Vout->Vin diode is externally added)You can of course then just add a series diode from 5V to Vin, so that 3V3 appearing on Vin is blocked.

author: Peabody
author_rank: Super Contributor
timestamp: July 17, 2022, 06:51:54 pm
content: I think it depends on what the regulator's input is connected to when power is applied directly to the output. If the input is disconnected from its normal supply, then I don't think it matters much - other than the small amount of current that will flow through the ground pin. But if it's still connected to a supply that has gone to ground, then you could have a problem. For example, any regulator that uses an internal P-channel mosfet as the pass transistor will also have a body diode that will pass current from the output to the input. So if the input is grounded, lots of current will flow.But after seeing various Arduino schematics, for example, it looks like modern regulators handle this ok. There appears to be no problem powering a Nano at the 5V pin after the regulator so long as Vin is disconnected, or maybe even if it's still connnected.In the end, you may have to set up a test to see if current flows back through the regulator, either to the input or to ground.The question I have is how you would work the 18650 into the 3.3V line as a backup, if that's what you have in mind. It's not obvious how you would do that. It might be easier to work it into the 5V line so the power source with the higher voltage would always be selected. So aside from the backflow question, I'd like to see your schematic. :-)

author: David Hess
author_rank: Super Contributor
timestamp: July 17, 2022, 09:12:56 pm
content: It depends on how the regulator is implemented, but most will fail. Some are designed to safely block a higher output voltage up to some limit.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: July 17, 2022, 09:26:14 pm
content: Synchronous switching regulators can, if enabled to do so.Tim

author: Gyro
author_rank: Super Contributor
timestamp: July 17, 2022, 10:01:40 pm
content: It's probably worth mentioning that, whatever linear fixed regulator you might use, you would be using it outside its datasheet specs (if it isn't in the datasheet, it isn't spec'd!). This definitely isn't advisable if you're planning to back drive it with a high current source, like the 18650 cell you mentioned.If you do want to use a linear regulator then a 3 terminal adjustable one would be better, something like an LM317 or LDO alternative. These have well documented protection methods in the datasheet to protect the adjust pin and pass transistor using diodes. It won't stop the other source discharging to ground through the adjustment resistor, but at least it will do so via a minimum of several hundred ohms.

author: bdunham7
author_rank: Super Contributor
timestamp: July 17, 2022, 10:19:48 pm
content: Quote from: cemelec on July 17, 2022, 05:26:00 pmIn many cases you can put the blocking diode before the regulator so that the output voltage is still correct, although the dropout voltage will increase by 0.6V or so. For a typical linear/LDO you will still be leaking current through the regulator divider in many configurations so this may not be a workable solution for very low power systems with small batteries.

author: EEVblog
author_rank: Unknown
timestamp: July 17, 2022, 10:40:52 pm
content: Quote from: Siwastaja on July 17, 2022, 05:41:35 pmThis.If you knew you wanted to do this and were designing the circuit then you'd design in the diode on the output before the sense feedback resistors.If it's an existing circuit then it's a bit of crap shoot what's going to happen.

author: David Hess
author_rank: Super Contributor
timestamp: July 17, 2022, 10:49:18 pm
content: TheLT1129is an example of a regulator intended to accept an output voltage higher than the input voltage without failure.Also the input may be connected to ground or a reverse voltage without reverse current flow from output to input. This makes the LT1129/LT1129-3.3/LT1129-5 ideal for backup power situations where the output is held high and the input is at ground or reversed. Under these conditions, only 16μA will flow from the output pin to ground.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: July 17, 2022, 11:38:49 pm
content: There are also automotive regulators that tolerate not just (externally-)powered outputs, but wide input voltages including negative values. Infineon and onsemi I think have lines of these.Tim