Powersupply 12VDC 2-3A DC to 9VAC <1A and 5VDC <1.5A - Page 1
Forum section: electronics / beginners

author: fasman
author_rank: Newbie
timestamp: February 14, 2014, 04:57:23 pm
content: Hi everyone , I am a retro game collector , and recently I picked up a Commodore Vic 20, I know just enough about electronics to blow stuff up.The problem is the Vic 20 didnt come with a power supply, andoriginal power suppliesare ... ... well known not to be too stable, and weigh a tun to making importing not only risky but expensive.So I thought if it might not be possible to DIY one for myself, but theres soo much conflicting information on the internet about it I have no idea where to start.What the computer needs two power inputs to work , 9v AC at slightly less than 750ma, and 5v at slightly less than 1.5a.The 9v AC is what I find the most tricky to pull off, the system uses the 60hz as a internal timer of sorts , and hence needs AC current.Originally what I wanted to do was get a9VAC power supply and rectify a part of it off to a 7805 for 5V, not knowing if this will work, and only being able to find a 1.5a 9VAC PSU meant that I had to abandon that idea...Ive been told that I can simply connect two different wall warts to get the desired effect, but was also told this is not ideal as they dont share a common ground and can be dangerous if the system is turned off and on a lot.TL:DR Version:So my question is this, is there any easy, diagrams, or examples I can follow to build a power supply that can take a regulated 12VDC source and spit it into two different voltages one being 9VAC the other 5V DC?

author: mariush
author_rank: Super Contributor
timestamp: February 14, 2014, 05:17:40 pm
content: Buy a 18v AC center tap (ideally with two separate windings) not just 3 wires on the secondary side. Use a winding for 9v AC , rectify the second and then use a dc-dc converter board to produce 5v.For example, a 18v 30VA+ may be enough, for example something like this:http://uk.farnell.com/pro-power/ctfc50-9/transformer-50va-2x-9v/dp/1780856it has 2 x 115v on input so it works on 115v (parallel windings) or 230v mains (series), and two separate secondary windings.One winding will be 9v 25VA (about 2.7A) so more than enough for 750mA 9v ACOn the other winding, you can use a bridge rectifier to convert it to dc => 9v x 1.414 = 12.7v - 2x voltage drop on rectifier diodes = ~ 11v peaks. Add a capacitor (or several in parallel, something like 2 x 3300uF 16v would be fine) to smooth out the dc voltage and keep it to around 9-11v, depending on the load. The current will be about 0.62 @ 2.77A = 1.72A.You can now use a dc-dc converter like those lm2576 or lm2596 boards on ebay like these:http://www.ebay.com/itm/LM2576HV-LM2576-DC-DC-Step-Down-Adjustable-Power-Supply-5V-60V-to-1-25V-30V-/371005942182?pt=LH_DefaultDomain_0&hash=item5661ad69a6http://www.ebay.com/itm/5-pcs-DC-DC-LM2596-Power-Supply-Step-Down-Adjustable-Converter-Module1-3V-35V-/130984321712?pt=LH_DefaultDomain_0&hash=item1e7f4622b0to obtain 5v @ 2-3 A from that 9-11v. These dc-dc converters are about 70-80% efficient so to produce 5v @ 1.5A (7.5w) they'll use about 10w which is around 9-11v @ 1A and you hae about 1.7A available so it would all work out fine.sotransformer -> secondary winding - > straight to 9v actransformer -> secondary winding -> bridge rectifier -> capacitors to smooth dc - > dc-dc converter to 5voptionally you can add a mains fuse (50va/230v = 0.21A so use a 350-400mA slow blow/time delay fuse)

author: fasman
author_rank: Newbie
timestamp: February 14, 2014, 06:15:04 pm
content: Hmm so recreatethe original power supplyexcept instead of using a Regulator use a modern DC to DC convertor?I had a quick look at RS (probably the only place I can get transformers locally) and the cheapest one they have isthis one.That is about $24 (US) , but they dont stock the DC to DC convertors so I will have to import one.Is there any other way to do it maybe that will allow me just to buy all the components from RS or utilize a PSU Ive allready got?PS Thanks for the super fast response!!

author: sync
author_rank: Frequent Contributor
timestamp: February 14, 2014, 06:49:40 pm
content: Are you sure that it is the right power supply for your VIC-20? Mine had only a simple AC transformer (two wires).

author: mariush
author_rank: Super Contributor
timestamp: February 14, 2014, 06:50:54 pm
content: The original power supply uses just two diodes for rectification, which results in lower efficiency. A bridge rectifier would be a bit better.The boards on eBay basically follow the typical application design in the datasheet of that IC. You can buy all parts separately, but you would have to solder them on PCB and the total of those parts will be more expensive than those eBay boards.As an alternative to such dc-dc converter, you could go with a linear regulator like LM1085 :http://za.rs-online.com/web/p/low-dropout-voltage-regulators/5339404/which basically would only need a small output capacitor to give you 5v at up to 3A BUT since it produces this 5v from up to 11v, it will dissipate the difference as heat so you need a heatsink capable of dissipating those 5-8 watts of heat. Maybe something like this :http://za.rs-online.com/web/p/heatsinks/2342362/(or something with similar specs but and it would still stay at around 70-90c while running (less if you put a small fan blowing some air over it).Or a bit more expensive but it allows you to choose a smaller transformer and not bother with large heatsinks and heat, you could just go for a ready made power adapter for the 5v, for example this one will give you 5v @ 2A :http://za.rs-online.com/web/p/plug-in-power-supply/7653311/Just cut the micro usb jack and you have 5v DC. And then you can get a smaller 9v AC 10-20 VA transformer which should be enough if you say it only need 9v AC - 750mA.And then you can go for a smaller transformer which can have

author: fasman
author_rank: Newbie
timestamp: February 14, 2014, 07:34:15 pm
content: Quote from: sync on February 14, 2014, 06:49:40 pmThere was two revisions of the Vic 20 the first did the voltage conversion internally, the second used it externally and used a power brick similar to the C64, I actually like the first version a whole lot moreI actually gambled on getting the first one, but sadly the person I bought it from didnt know the difference and couldnt post pictures, I took the chanceQuote from: mariush on February 14, 2014, 06:50:54 pmThat makes a lot of sense , thank you, I might just go for a linear regulator then, and use a bigger enclosure with a fan, was going to anyway, considering the parts list I think its a much more elegant solution than two wall power supplies, and will cost about 1 1/2 times the price, which I think is still not bad, far more affordable than importing one from overseas.PS I also foundhttp://za.rs-online.com/web/p/non-isolated-dc-dc-converters/7158544/while it does look like it needs a bit more components to get working it looks like its very similar to the ones you posted on ebay?Thanks again for all the help, I will be placing a order soon and posting the result