μA7805UC - Page 1
Forum section: electronics / projects

author: mojoe
author_rank: Frequent Contributor
timestamp: October 08, 2025, 04:53:51 am
content: I don't find much information on this part, but it cross references to a MC78M05CT. Are there any specific reasons why the μA7805UC would be used in place of the MC78M05CT? I ask because I'm repairing a power supply in a piece of equipment, and it uses both parts. The MC78M05CT (3 of them) are used along with some op amps and zeners to create +15, -15 and +5. The μA7805UC is used for a separate +5 for the digital circuits.Strangely, the μA7805UC is the only one that isn't blown. All of the digital logic is working, and I get a display on the front panel.In any case, I just placed a Mouser order, including some new electrolytic caps. They're from the late 1980's, so it wouldn't hurt to replace them.

author: amyk
author_rank: Super Contributor
timestamp: October 08, 2025, 05:21:47 am
content: It's just the usual 7805 high-dropout linear regulator, they probably used what was available at the time.

author: mojoe
author_rank: Frequent Contributor
timestamp: October 08, 2025, 05:39:36 am
content: The schematic actually specifies the two different parts, so there must have been a reason.

author: inse
author_rank: Super Contributor
timestamp: October 08, 2025, 07:09:04 am
content: xx7805xx is the 1.5A version while xx78M05xx can only handle 500mAThey are different packages, too

author: gamalot
author_rank: Super Contributor
timestamp: October 08, 2025, 07:28:00 am
content: Quote from: inse on October 08, 2025, 07:09:04 amActually, some of the 78Mxx regulators come in TO-220 packages too, just like the regular 78xx ones.

author: PGPG
author_rank: Frequent Contributor
timestamp: October 08, 2025, 11:08:02 am
content: Quote from: mojoe on October 08, 2025, 04:53:51 amIn 80s we were using 7805 to get VPP voltage needed in EPROM programming. VPP should be switched between 5V, 12V, 21V, 25V.We had 30V at input and get needed voltage by switching Zeners between 7805 GND pin and our GND.When for example you had 21V at output and wanted to finish VPP pulse (get it down to 5V) by shorting 7805 GND pin to GND than some types of 7805 behaved as we wanted but some get dropped their internal feedback and instead of 5V at output we get 28V (30V minus minimum drop). 7805 was not broken, but just jump out of feedback.We had a list of 7805s to be used in this circuit and a list that can't be used.

author: mojoe
author_rank: Frequent Contributor
timestamp: October 08, 2025, 05:04:52 pm
content: The current capacity explains it. Thanks. I coudln't find a datasheet for the μA7805UC. That's probably also why it was the only one to survive. The other three regulators are blown.

author: gamalot
author_rank: Super Contributor
timestamp: October 08, 2025, 05:11:37 pm
content: Quote from: mojoe on October 08, 2025, 05:04:52 pmEven decades ago, 78xx regulators from different manufacturers were everywhere, and honestly, they were pretty much all the same.

author: David Hess
author_rank: Super Contributor
timestamp: October 09, 2025, 01:03:24 am
content: Fairchild's 7805 and 78M05 are the same as everybody else's 7805 and 78M05.The Fairchild 1978 Linear Data Book is a good place to track down these old Fairchild parts:https://www.rsp-italy.it/Electronics/Databooks/Fairchild/_contents/Fairchild%20Linear%20Data%20Book%201987.pdfIt is possible that the design is relying on the lower 0.5 amp current limit of the 78M05, so I would not substitute a 7805, especially since the 78M05 is readily available.

author: wraper
author_rank: Unknown
timestamp: October 09, 2025, 01:50:13 am
content: Quote from: David Hess on October 09, 2025, 01:03:24 amThat's nonsense. There is no 0.5A current limit, it's how much current it's guaranteed to provide. Current limit is not precise and should not be used for anything. It's there for IC self protection only. TI LM78M05:

author: mojoe
author_rank: Frequent Contributor
timestamp: October 09, 2025, 03:47:12 am
content: Thankfully the 1.5A regulator works fine. I did order exact replacements for the 0.5A regulators from Mouser.