How does a resistor between 5v and 12v on a power supply work? - Page 1
Forum section: electronics / beginners

author: MGaddict
author_rank: Contributor
timestamp: February 27, 2025, 09:24:18 pm
content: Okay, I am a hobbyist. I know what all these components are and what they do, but I don't often understand how they work together and this one is seriously messing with me.I am attempting to get an Intel 486 workstation working. (Yes, it's an OEM Intel workstation. Specifically LP486/33E) The rubber feet had turned to goo and was all over the inside of the case and inside the power supply.I decided to just check for shorts between 5v and ground, and 12v and ground, all good. But, there appears to be a short between 5v and 12v. The resistance is only 81ohms.I opened the supply and there was black goo everywhere. So I started removing components and cleaning this crud off with goo gone because that's the only thing I could find to remove it. Every single part, after being cleaned off, tests good. Every capacitor tests within tolerance. Every resistor, literally everything. This thing looks practically new and I was extremely careful to make sure I didn't bridge anything putting it back together. But, what appears to be a short between 5v and 12v is still present.I then started following traces and a large resistor, 82ohm based on the rings, appears to be located directly between 5v and 12v. It appears IT WAS LITERALLY DESIGNED THIS WAY. Removing that resistor clears the issue. And I noticed, under the resistor is written, "OCP +5V" which I'm assuming means, over current protection for the 5v rail. What I'm not understanding is, how does this not allow 12v to freely flow on the 5v rail? I mean, a resistor is bi-directional, correct? If this was a diode, I would get it. I'm pretty sure the power supply is fine and I should trust it, I'm just asking because this just doesn't make sense to me.P.S.Also, there is a dead short on the motherboard between 5v and ground I have to deal with before I put these 2 things together, but I'll save that for a future post if I can't figure out what's going on there. This whole project was really just meant as a learning experiment for myself.The 2 pictures below are the top and bottom of the low side of the power supply. 2 caps are still not reinstalled and the offending resistor isn't installed. You can see the empty holes on the right hand side of the bottom of the PCB. My red probe is pointing to 5v and the black is pointing to 12v (At least that's the pinout based on the documentation I could find. This power supply is very specific to this motherboard.)

author: Doctorandus_P
author_rank: Super Contributor
timestamp: February 27, 2025, 11:58:23 pm
content: I can think of two reasons for this design.First is that it was common (easier?) in those days to design power supplies that had a minimum load current. With this resistor, the 12V rails has a minimum current of (12-5)/81 = 86mA. And instead of shunting that current to GND, it does something useful at the 5V power rails, which is probably assumed to always have a load.Second, OCP may indeed be over current protection. In this case, the 12V rails comes up first, and then the voltage on the 5V rails is measured. If the voltage is too low, the power supply never powers up the 5V rails and goes into a fault mode.

author: Konkedout
author_rank: Frequent Contributor
timestamp: February 28, 2025, 04:52:29 am
content: Your power supply looks old and simple. It seems likely that the 12V is connected in series with the 5V. This could either be done with windings connected in series at the transformer (The 5V diode is just connected to a tap on the +12 winding) or the +12 winding is really a separate +7V winding which is stacked in series with the +5VDC rail.I have designed multiple output power supplies which work this way; it helps the "cross regulation.".Anyway that resistor is probably also there to establish a minimum load on the +12 output without loading the +5 output. This is particularly reasonable to do if the +12windingis connected to the +5DCas I mentioned. It is probably the way it is supposed to work and would not be a problem.

author: MGaddict
author_rank: Contributor
timestamp: February 28, 2025, 02:20:16 pm
content: Old? Come on, it was just built in 1989.The +7v thing makes a ton of sense and would actually make the rest of this layout a lot more logical. Looking at it with that in mind, I think I can actually follow what's going on. Thank you so much.