7805 in series - Page 1
Forum section: electronics / beginners

author: metrologist
author_rank: Super Contributor
timestamp: July 02, 2017, 04:11:17 pm
content: What would happen if I wired two 7805 regulators in series from 48V? I mean with the ground of one connected to the input of the other. Would they output 5V on one and 10V on the other, something unstable, or just some smoke?

author: rdl
author_rank: Super Contributor
timestamp: July 02, 2017, 04:55:53 pm
content: Since 48 volts exceeds the maximum input voltage specification they may just be destroyed, but if the first one actually works at 48 volts in, the second will just drop the voltage slightly because it needs an input-output difference in order to regulate.

author: mariush
author_rank: Super Contributor
timestamp: July 02, 2017, 05:03:33 pm
content: Yeah, most regulators in the 7800 series have an absolute maximum voltage of 35-40v ... my guess is the transistors inside the IC would be damaged.Look for linear regulators with HV at the end of their name, those typically can handle up to 60v input voltage, and in theory then you can have one regulator take it down from, 48v to around 24v and then a second one (which could be a non-hv one i guess) take it down from 24v to 5vBut remember, the linear regulators work by dissipating the voltage difference times current as heat .. so unless you use nice heatsinks, with high voltage differentials you won't be able to have high output currents.A better option would be to use a buck regulator to take 48v down to ~7v and then use a linear regulator.

author: JacobPilsen
author_rank: Regular Contributor
timestamp: July 02, 2017, 06:14:07 pm
content: Quote from: metrologist on July 02, 2017, 04:11:17 pmNo, it will not work.There will be 5V on one output, and about 48V (unstabilised) on the other output.What about ground of one connected to theoutputof the other ???

author: mikerj
author_rank: Super Contributor
timestamp: July 02, 2017, 09:09:23 pm
content: Ignoring the voltage issue (i.e. selecting high voltage regulators) then in principle this should work.By connecting the ground pin of the first 7805 to the output of the second, you have created a tracking pre-regulator, so the first one will output 10v and the second one will output 5v. However you will need to deal with the quiescent current of the first regulator, i.e. the current coming out of the ground pin. If this has no path to go (i.e. no load on the output of second reg) then you will lose regulation. The fix could be as simple as a fixed resistor on the output of the second 7805.

author: rdl
author_rank: Super Contributor
timestamp: July 03, 2017, 04:37:27 am
content: Quote from: metrologist on July 02, 2017, 04:11:17 pmI completely missed that part. It's not exactly what I think of as "in series", but it should work assuming, as mikerj said, you allow for the current out of the ground pin and the input voltages are reasonable.

author: digsys
author_rank: Unknown
timestamp: July 03, 2017, 05:37:01 am
content: As well as - depending on what current you are after - the 1st regulator has to pass the full current of the 2nd regulator, plus it's own.I'm guessing you want to do this cheap or simple - work out the total current you will need, then use a say 5-20W resistor to drop at least ie 30V 48 > 18VThen have an 18V 5W+? Zener to clamp the voltage when no / low currents re used. Then have both regulators in parallel. Using a resistor to dump most the heat is amuch nicer method. Option 2 is to put the Zener in series and have it drop out say 30V, but it'd need to be a pretty big zener (or zener + pass transistor).There is also the LM317HVT adjustable regulator, which will easily work to 60V. As others have said, lots of better options.

author: ebastler
author_rank: Super Contributor
timestamp: July 03, 2017, 06:25:53 am
content: Quote from: metrologist on July 02, 2017, 04:11:17 pmI assume this is a typo? If you connect the second regulator's ground to the input of the first one (i.e. to 48V), where would you connect the second regulator's voltage input?!

author: David Hess
author_rank: Super Contributor
timestamp: July 03, 2017, 04:22:14 pm
content: If you are dealing with a higher input voltage than a single regulator can handle, then it may be acceptable to place two regulators in series with the first one configured to drop half of the voltage distributing the power dissipation evenly across the regulators; note that this is easier to do with 317 type regulators than 7805 type regulators because the adjustment pin current of a 317 is lower than the common pin current of a 7805. This may even be preferable than a single high voltage regulator because it distributes the power dissipation across two parts and more area.But it may be less expensive to use a single power transistor in place of the first regulator and if this is done correctly with both parts thermally coupled, then the thermal and safe-operating-area protection of the integrated regulator can be extended to the power transistor.You have to be very careful about start up, shut down, and output short circuit conditions to make sure that the maximum voltage of one regulator is never exceeded. It is also probably a good idea to externally limit the current to as low a value as possible and to implement foldback current limiting. By the time do you all of that, a regulator based on the 723 starts to look appealing.

author: Zero999
author_rank: Super Contributor
timestamp: July 03, 2017, 08:23:42 pm
content: Another transistor can be added to drop the voltage before the regulator.Here's an example with the LM317Unfortunately this increases the minimum load current required to maintain the voltage regulation. Some of the current bypasses the regulator, via D1, to the load.It will work with the LM7805 too. Another transistor can be added to reduce the current bypassing the regulator.