Replacing transformer with SMPS, question about removing components - Page 1
Forum section: electronics / beginners

author: Mp3
author_rank: Frequent Contributor
timestamp: June 22, 2019, 08:01:27 am
content: Hello, I previously replaced two noisy linear PSU's in some old 1U rack gear with Mean Well switching PSU's. Both times it worked great and i am very happy with the results. I'm now looking to do this sort of power supply replacement in another piece of equipment I have.The problem i have is because on the swaps I already did, the power supply board was separate from the main board, thus it was easy to wire the new SMPS to the power supply board's outputs.I think I should be removing the power supply regulators as well as capacitors, would that be correct? I've attached a clip of the schematic and I have highlighted the regulators, power supply capacitors and where I think I should be wiring the SMPS' outputs to. I have also attached a youtube screenshot showing the power supply area of the device i'm working with.Thank you in advance for any assistance on this.

author: MosherIV
author_rank: Super Contributor
timestamp: June 22, 2019, 09:02:10 am
content: Hi. I think I saw your post.No, I do not think you have it right.In you previous post, it looks like you mounted mains based smps.What you are now proposing, it looks like you will need buck switching regulators (totally different type to mains based smps).You need to leave the big 1000uF caps in place to create an un-regulated supply for the buck regulator to work from.You can then get rid of the components after the regulator.

author: madires
author_rank: Super Contributor
timestamp: June 22, 2019, 09:58:03 am
content: Noisy linear PSUs? Bad caps? SMPSUs are noisy (caused by the way they work) and need additional filtering to lower the noise. The 78xx are 1A regulators, and based on the filter capacitors' values the total power consumption won't be more than 20W. The +/-15V rails might be for OpAmps which benefit from a low noise linear PSU. Is there a problem with the device's power supply or why do you want to replace it with a SMPSU?

author: Ian.M
author_rank: Super Contributor
timestamp: June 22, 2019, 11:20:34 am
content: Crazy-Crazy!You have a >25 year old piece of equipment that if recapped with high quality capacitors for a relatively modest cost will probably be good for another 25 years, and if not should still be repairable.You want to put in modern SMPSUs that will probably only last five years, unless you buy really expensive western branded ones with extended warranty, then you may get ten or fifteen years out of them. This will cost a *LOT* of money and if the equipment uses low level analog signals the circuits may not have enough PSRR to handle the resulting HF ripple on the supply rails and still stay in spec. Also, when they do die, there's a significant risk*of them severely over-voltaging the supply rails blowing the s--t out of all the ICs on the board and rendering it B.E.R.*Greater IMHO than the risk of a well designed and constructed linear regulator run within its ratings failing shorted and doing the same, because there are more possible points of failure.

author: cyborg16
author_rank: Contributor
timestamp: June 22, 2019, 09:49:22 pm
content: I agree with Ian. Replace the caps and you're very likely good to go. About the noise; have you measured it? Or is it just guessing? Linear PS are a lot less noisy than SMPS's