Commodore 64 power supply - Page 1
Forum section: electronics / beginners

author: huggy
author_rank: Newbie
timestamp: September 19, 2014, 05:54:18 pm
content: My Commodore 64 psu is retired i think so i'm looking at building a new one myself. The outputs required are a 9 volt AC at 9VA (1a nominal) and a 5 volt DC with a minimum current capacity of 1.5 amps but i prefer to increase the max current on the dc line to double or 2 amps at least.With me being a newbie at electronics i'd like to build a linear dc output as opposed to a switching module because i'll learn more rather than just wire up the ac mains to a switching module. For the DC i'd like to use two 7805's tied together at the output pins with a 1 watt resistor for load balancing, so it should give me either 2 amps or 3 dependng on the 7805 max current. I know there's two versions of this package, one has 1 amp and the other 1.5. Also a smoothing capacitor of 4700uf and of course four 4001 diodes to rectify.I need a transformer with two secondary's that are isolated from each other, one will go to the DC circuit and the other straight out to the Commodore as AC 9v. I'm not up on wiring a transformer but am i right in thinking that dual output secondary windings are isolated from each other? I know it's elementary stuff but like i said i've not wired one before.The transformer will need to be at least 30VA because of the DC circuit requiring either 2 amps or 3 depending on the 7805's capacity. Also will each secondary winding of lets say a 30va transformer put out 3.3amps each or is that the total of both windings?

author: Simon
author_rank: Unknown
timestamp: September 19, 2014, 06:12:45 pm
content: Quote from: huggy on September 19, 2014, 05:54:18 pmDon't over complicate your life, external power adapters are often already well speced, 2A fine.QuoteYou need a 7805CVfor 1.5A:http://www.sparkylabs.co.uk/shop/product_info.php?products_id=13&osCsid=9mjv46jj6una7c10mt3f27h343You can use a single part bridge rectifier, use a couple of smaller capacitors in parallelQuoteIf you have 2 wires each then yes they are isolatedQuoteThe VA will generally be rated for the whole thing, You will need a good bit more than the theoretical because the power draw through the transformer is not linear. Once your capacitors are charged up only parts of the waveform that have a higher voltage than the capacitor+voltage drop in the bridge will flow current into the capacitor in large short spikes that can overheat a tightly speced transformer.

author: Simon
author_rank: Unknown
timestamp: September 19, 2014, 06:14:13 pm
content: I'm not sure that you connect 2 regulators with a balance resistor, you can use one regulator and a BJT in parallel with a balancing resistor.

author: Neverther
author_rank: Regular Contributor
timestamp: September 20, 2014, 08:12:24 am
content: If youre lazy you could just grab barreljack and usb connector and make feed cable.9V AC adapter from old modem and some generic USB supply and I was good to go.Took under an hour from the point of primaries shorting on the old one to having working C64 again....Almost messed up becouse there was two types of din plug for the power. Pins in circular array and in U shaped array, cant remember which one it is but I almost used the wrong one.

author: huggy
author_rank: Newbie
timestamp: September 20, 2014, 12:50:17 pm
content: Quote from: Neverther on September 20, 2014, 08:12:24 amThanks man, yeah i did know about using two wall wart type adapters and joining them at the din plug but i'm going to make one in a metal enclosure so i can bolt the two 7805's to the metal case as a heat sinkThe two output pins being tied in series via a 1 watt 1 ohm resistor so i can have a bit more juice on the dc line for any expansions or whatever, and less heat for each regulator.

author: Simon
author_rank: Unknown
timestamp: September 20, 2014, 01:21:27 pm
content: You won't be using regulators in series to increase current. I take it you mean parallel with a resistor each joining them to help balance the load.

author: David Hess
author_rank: Super Contributor
timestamp: September 27, 2014, 01:10:24 pm
content: Quote from: Simon on September 19, 2014, 06:14:13 pmTwo regulators in parallel with ballast resistors on the outputs can work but it depends on accurate output voltages to keep the ballast resistor values low or load regulation will suffer. LT1086 regulators can be used this way successfully but run of the mill 7805s are not really accurate enough.It would be easier to use a single LM323 or LT1085 higher current regulator or a PNP bipolar pass transistor like Simon suggests.Multiple regulators in parallel or adding the pass transistor have the advantage of distributing the power dissipation over multiple devices so cooling is easier.

author: Simon
author_rank: Unknown
timestamp: September 27, 2014, 01:16:07 pm
content: if you use 0.1 ohm I suppose you will only loose 0.1V at full load. if you use a parallel BJT you can have all the current you want with a part that will be about the same price and same size.

author: David Hess
author_rank: Super Contributor
timestamp: September 27, 2014, 02:20:28 pm
content: Quote from: Simon on September 27, 2014, 01:16:07 pmThe LT1083 (7.5 amps) datasheet recommends 0.015 ohm resistors for ballasts when operating in parallel so an appropriate value for the LT1086 (1.5 amps) would probably be about 0.075 ohms. Regulation for a 7805 is about 2.5 time worse so they would require almost 0.2 ohms for a similar level of current sharing.

author: Sigmoid
author_rank: Frequent Contributor
timestamp: September 29, 2014, 02:29:58 pm
content: Speaking of C64 power supplies...If I have a transformer with two secondary windings, is it okay to use them as separate circuits entirely? (One for the 9VAC, and one for the DC circuit?)I think that's how the original C64 adapter was built, but I have never seen this practice in any books, guides or specsheets, so I'm thinking it might be some kind of antipattern.

author: Simon
author_rank: Unknown
timestamp: September 29, 2014, 02:40:27 pm
content: you will have 2 saeperately isolated supplies, whether or not you commonise them is up to you. Many transformers that produce different voltages are "tapped" off but many have totally separate secondary coils.

author: AmmoJammo
author_rank: Frequent Contributor
timestamp: September 29, 2014, 09:39:14 pm
content: The problem with linear regulators, is they get hot! Here's some pics of two of my C64 PSUs side by side...Somehow, the deformed case one still works, but its really quite worrying!My replacement runs a 9volt, 2amp transformer (although 1amp is enough) and a 5volt, 2amp switchmode power supply PCB, that came from a digital camera charger.

author: Timmay
author_rank: Contributor
timestamp: October 01, 2014, 07:59:50 am
content: Throw that junk away and order a Tan brick. Then follow this tutorial if it applies to your system, it did to mine. Everything was janky back then, especially power.http://www.lemon64.com/forum/viewtopic.php?t=48451&highlight=

author: huggy
author_rank: Newbie
timestamp: October 09, 2014, 07:47:17 pm
content: Quote from: Sigmoid on September 29, 2014, 02:29:58 pmYes that's how the original design is and how i made mine, but with two regulators bolted to my metal enclosure for heat sinking.