Drop a 12v PSU to 11v or less? - Page 1
Forum section: electronics / beginners

author: CZroe
author_rank: Contributor
timestamp: January 15, 2018, 05:24:30 am
content: I have a device that is missing the original power adapter: a NEC TurboGrafx-CD dock. The original specs: 5.5x2.5mm, center negative, 11v, 1.53A. It’s ‘80s tech so the adapter was linear. The dock is internally regulated to 5v but I have reason to believe that even the specified 11v is pushing things, temperature-wise, and that anything down to 9v would be appropriate.The commonly-recommended Radio Shack replacement is no longer available, but it was apparently 9v. The Japanese version of this dock (“IFU30 Interface Unit”) uses the same internal hardware as the US device but specifies 9v, 1.45A with the same polarity and plug dimensions. They are interchangeable between regions, which tells me that a replacement within the 9v, 1.45A+ and 11v range should be acceptable. In fact, it sounds an awful lot like the exact same adapter with the 10-20% greater input voltage creating a proportionally higher output voltage (US 110-120v compared to JPN 100v).I went to a thrift store and test-fit various loose plugs until I found one that fits: a modern Hyundai-branded 12v, 5A switching PSU brick that looks like it’s for an LCD monitor or something. I reversed the polarity and tested it and it works great, but these old CD docks are known for failing caps and we all know that electrolytic caps aren’t going to like the extra heat over the long-term.IIRC, internal regulation is handled by two 7805s. Based on the Japanese spec, it seems that the US version is already running them hotter than their Japanese counterpart by feeding a higher voltage. I’d like to drop that voltage closer to the Japanese spec for increased reliability... anywhere in between 9v and 11v. I know I could just splice the end onto a 9v 1.45A+ plug, but I can actually use the extra amps here for some other projects and I want to be able to move the PSU between them.I’ve never done anything like this but I assume it can be done with a couple diodes and a resistor, so here come the questions:How do I know what specs to order?Could I just salvage a couple of them from the rectifier of an old device that takes 12v AC?Will this 5A diode from Mouser be ideal?https://www.mouser.com/productdetail/vishay-semiconductors/egp51g-e3-c?qs=sGAEpiMZZMtbRapU8LlZDxCc6FzR8ItptId7%252bqIFh04e8bI1655czg%3D%3DHow do I determine the value of the resistor I should pair with it (assuming that’s what I’m supposed to do)?Thanks in advance.

author: phil from seattle
author_rank: Super Contributor
timestamp: January 15, 2018, 05:36:21 am
content: Yes, a couple of diodes will drop the voltage - it's fine if you are feeding the 7805 regulators. But, you should be able to find a 9V DC wall wart, they are fairly common. The thing about wall warts is that the rated voltage occurs at the specified current. So that 5A one will probably run unloaded at around 16V and >13V at 1.5A.If you can't find the right size/polarity plug, you can replace it with the correct one. Cut and solder. Use a little shrink wrap on each wire and a 3rd piece to keep them together (put the shrink wrap pieces onbeforeyou solder - don't ask me how I know that...).

author: kalel
author_rank: Frequent Contributor
timestamp: January 15, 2018, 05:39:58 am
content: Quote from: phil from seattle on January 15, 2018, 05:36:21 amI know it very well too. I wish there is an effective external way to apply heatshrink (or something similar) after such a mistake that just happens easily.

author: CZroe
author_rank: Contributor
timestamp: January 15, 2018, 06:09:02 am
content: Quote from: blueskull on January 15, 2018, 05:28:26 amThe original voltage regulators are actually still in the dock, so is there still a reason to use an external 7809 instead of a couple diodes? Thanks.Quote from: phil from seattle on January 15, 2018, 05:36:21 amLOL! I’ve made that mistake a a couple times myself.I checked beforehand and the replacement appears to be a rock-solid 12v due to being a switch-mode PSU. I just used liquid electrical tape and a single piece of heat shrink for my first splice but I plan to do it nicer once I have selected the proper components to include.Thanks!

author: lordvader88
author_rank: Frequent Contributor
timestamp: January 16, 2018, 05:52:20 pm
content: LM317 is a very common reliable and useful chip, and cheap too (i think?) Very easy to setup too, just 2 resistors and 2 caps.LM3173-Terminal Adjustable Regulatorhttp://www.ti.com/lit/ds/symlink/lm317.pdfQuote