Changing 5V Voltage Regulators - Page 1
Forum section: electronics / projects

author: Dipski
author_rank: Contributor
timestamp: August 28, 2017, 07:58:09 pm
content: Hi folks, I've got some old power supplies that use LM3405T (5V 1A) voltage regulators - I'd like to replace them and higher (amp) rated regulators seem to be cheaper, and would on paper be more robust if say the power draw spiked over the normal 1A draw. The replacements I'm looking at are TS7805CZ which are 5V 2.2A, the only difference I can spot is an accuracy drop from 2% to 4% in changing to the TS7805CZ which should be fine - I just thought I'd check if there's anything else I haven't thought of to take into consideration in changing the regulators.

author: Zero999
author_rank: Super Contributor
timestamp: August 28, 2017, 08:05:09 pm
content: Quote from: Dipski on August 28, 2017, 07:58:09 pmThe only issue could be that the regulator will no longer be able to protect any upstream components such as the rectifier and transformer.

author: Dipski
author_rank: Contributor
timestamp: August 28, 2017, 08:33:53 pm
content: Excellent point, it would take away the potential for the regulator to act as a fuse over 1A (if it opens on failure) so sticking with the original would be safer - thanks!

author: edavid
author_rank: Super Contributor
timestamp: August 28, 2017, 09:23:31 pm
content: Most vendors use the same IC and sell it as both LM340-5 and 7805. So, there is no reason to make this swap.Also, these regulators have robust short circuit protection and thermal shutdown. They will not fail just because you try to draw a little extra current.

author: rdl
author_rank: Super Contributor
timestamp: August 28, 2017, 10:28:49 pm
content: Quote from: Dipski on August 28, 2017, 07:58:09 pmAppears the TS7805CZ is rated 1 amp, same as all the other 7805 regulators. There is a "Peak Output Current" spec of 2.2 amps, but I did not see an explanation of what that exactly was. Maybe the point where the internal current limiting kicks in? I doubt any other 7805s are much different, at least the TO-220 sized ones.http://www.taiwansemi.com/en/products/details/TS7805CZ

author: Zero999
author_rank: Super Contributor
timestamp: August 28, 2017, 10:48:13 pm
content: Yes, I believe the current limit typically kicks in at 2.2A on the LM78xx regulators but it could be as low as 1.5A or as high as 3A. Note that I'm recalling these figures from memory because I've seen them somewhere but can't find a reference at the moment, so could be wrong.

author: David Hess
author_rank: Super Contributor
timestamp: August 29, 2017, 12:33:07 am
content: I have run into this problem before. All of the 340 and 78xx regulators are 1.5 amps and there are no higher current fixed voltage versions. My solution was to use an LM350 (3 amps) or LM338 (5 amps) adjustable regulator with an external resistor voltage divider.

author: edavid
author_rank: Super Contributor
timestamp: August 29, 2017, 01:46:55 am
content: Quote from: David Hess on August 29, 2017, 12:33:07 amLM323 +5V 3A78H05 +5V 5A (TO3 only)LAS1405 +5V 3ALAS1605 +5V 2ALAS1905 +5V 5ALAS3905 +5V 8A

author: mariush
author_rank: Super Contributor
timestamp: August 29, 2017, 02:11:02 am
content: Look into LM1086 (1.5a) or LM1085 (3A) - there are fixed 5v versions available.But note the maximum input voltage is lower compared to other regulators - i think for LM1085 the max voltage is 25v

author: David Hess
author_rank: Super Contributor
timestamp: August 29, 2017, 02:31:17 am
content: Quote from: edavid on August 29, 2017, 01:46:55 amI have used the LM323 before but the higher current fixed voltage parts were never available in TO-220 packages unlike the adjustable TO-220 parts.Quote from: mariush on August 29, 2017, 02:11:02 amThese are low dropout improved performance alternatives to the less expensive 3 amp LM350 and 5 amp LM338 and originally made byLinear Technologyand second sourced by National/TI and others. They are great parts but cost more.One thing I always wanted was a higher current LM337 (1.5 amp adjustable negative) but nobody made that either.

author: edavid
author_rank: Super Contributor
timestamp: August 29, 2017, 02:57:59 am
content: Quote from: David Hess on August 29, 2017, 02:31:17 amLM323Tiswas certainly available. Not any more thoughQuoteLT1033 (3A)

author: David Hess
author_rank: Super Contributor
timestamp: August 29, 2017, 07:46:04 pm
content: Quote from: edavid on August 29, 2017, 02:57:59 amIs it? ON and STM discontinued it. Who is still making them?QuoteThat is a good catch. I searched the Linear Technology web site yesterday and missed that.My solution for higher power in a TO-220 package has been to add one external transistor, diode, and two resistors but that is not a solution if one just wants a higher current TO-220.

author: westfw
author_rank: Super Contributor
timestamp: August 30, 2017, 03:58:45 am
content: Note that these 3 terminal linear regulators tend to be limited more by power dissipation issues than max current. Sticking a 3A regulator in the same place you had a 1A regulator probably won't help much unless you give it more heatsink as well.

author: David Hess
author_rank: Super Contributor
timestamp: August 30, 2017, 02:36:26 pm
content: Quote from: westfw on August 30, 2017, 03:58:45 amThe larger die yields a lower junction to case thermal resistance but you are correct; the cooling requirements go up with higher currents which is why TO-220 packages are not used above 5 amps except with low input to output voltages.This also made TO-3 packaged regulators useful even at lower currents; their lower case to heatsink thermal resistance made cooling easier.