Replacing a 7805 linear regulator with a DC-DC step-down module

[shoestring]

Just sharing some info that might help someone here.

 

I'm a little particular when it comes to powering my precious 8 bit machines with old power supplies. The Atari power supply pictured below was one of the first types distributed with Atari machines in Australia during the early days. There is also an identical looking one that goes with the c64. The only good thing about these is that they're rebuildable. Besides there being no mains fuse anywhere they utilise a very simple circuit and a 7805 linear regulator which runs very hot ( even with a heatsink ) and is very inefficient.

 

 

 

Here is the schematic which almost matches this circuit for the original power supply I have.

 

 

I want to replace this with the more efficient switching regulator so I chose this DC-DC buck converter. I begin by doing some prep work and cut two wires to equal lengths.

 

 

All soldered up to the input and outputs.

 

 

I remove the cover of the power supply and the heatsink.

 

 

Two things have to go. The voltage divider ( two resistors ) and the voltage regulator. I also remove the large cap temporarily to test it before putting it back in. At the time of writing I realise that I don't need the smaller cap on the output so I'll remove this later once I get my fuses.

 

 

I power up the supply to locate 12v with my meter. This will be the left most pin ( input ) of where the 7805 used to be.

 

 

Diagram of the device from the front.

 

 

I solder the red wire to where the input used to be and the black wire ( ground ) to the ground pad of PCB then power it up. The module has a blue LED that lights up when it's powered on.

 

 

 

I connect my DMM to the output of the module to check the voltage which I'll need to turn down.

 

 

There is a little trimpot on the module. You'll need to turn it anti-clockwise quite a bit before the voltage starts to change. I adjust the voltage to just over 5v because I know it's going to drop under load slightly.

 

 

I now connect the other ground wire to the ground pad and the red wire ( positive ) to output of where the 7805 used to be.

 

 

With the Atari 65XE powered up the voltage only dropped by a hair.

 

 

Final adjustment to 5v

 

 

Optional step on the scope. Checking for a nice clean DC signal

 

 

Just making sure she pasts the hardware test.

 

 

I've been running this for a few hours now and the voltage and temperature is really stable. Even without a heatsink this barely gets warm. Not bad for a few bucks

 

 

[Rybags]

Nice one. I have that same type of power supply. I've left it going for months at a time though haven't used it much lately.

[+mytek]

Here's another option if you just want to replace the regulator. It's a small DC switch-mode power regulator designed as a drop-in fit for the 7805. Puts out a healthy 1.5 amps assuming that your transformer can supply that. I use these on my 1088XLD project, and they work great, putting out exactly 5 VDC. However at $6 that's probably pricier then the Chinese DC Switcher boards being sold on eBay. But since these are produced by Texas Instruments I suspect the reliability would be much better (how much is your Atari 8-Bit worth to you?).

Mouser P/N: 595-TPSM84205EAB

[_The Doctor__]

finally a clean output from one of these things! Normally there is noise and ripple, how does it looks close up. does it have an open failure mode or would a crow bar protection be in order?

Edited February 2, 2019 by _The Doctor__

[manterola]

That 65XE is calling for a couple of TMS4464 and an EMMU. Sorry I can't help it

[xrbrevin]

ive done a similar voltage regulator conversion to my 400, a couple of 1050s and a sinclair spectrum - all happy!

[shoestring]

It uses the LM2596 rated at 3a with a heat sink, adjustable version. See data sheet below for details and specs.

 

http://www.ti.com/lit/ds/symlink/lm2596.pdf

 

Yes there are plenty of cheap Chinese fakes that cost a couple of bucks. This was around $7.

 

Ive been using one in my C64 powersupply for awhile. Ive had no issues yet.

Edited February 2, 2019 by shoestring

[shoestring]

finally a clean output from one of these things! Normally there is noise and ripple, how does it looks close up. does it have an open failure mode or would a crow bar protection be in order?

It has thermal protection and current limit protection. With that in mind it shuts itself down.

 

These are national semiconductor branded. I believe TI bought them out and probably still have a huge stockpile of these.

 

I’ll take some close ups of it when I get home but you can probably find some pretty good ones online

[shoestring]

That 65XE is calling for a couple of TMS4464 and an EMMU. Sorry I can't help it

I was getting to it when I was in the process of rebuilding the board. But I was convinced that it deserves an Ultimate from Lotharek.

[+mytek]

Just to make sure I was perfectly clear in what I proposed as an alternate solution back in this POST... that is a single self contained drop-in for the 7805. Unlike the LM2596 No other parts required.

Installation: de-solder and remove the original 7805 --- solder in the TPSM84205EAB --- and for just a tad over $6 you're done .

[shoestring]

In a stock machine that would be perfect, also a great alternative to the stock 7805 inside the C64 . Id still like to get a heatsink on there somehow if I could. I have nil experience with these so how does that perform when powering a fully loaded machine + upgrades ?

Edited February 2, 2019 by shoestring

[+mytek]

In a stock machine that would be perfect, also a great alternative to the stock 7805 inside the C64 . Id still like to get a heatsink on there somehow if I could. I have nil experience with these so how does that perform when powering a fully loaded machine + upgrades ?

 

No heat sink required even when drawing the full 1.5 amps. It's so efficient that it hardly even gets warm to the touch. The same would hold true for the LM2596, but then you would need to build the support circuit with the inductor, diode, ect.

 

Edit: 1.5 amps is more than capable of powering the 8-Bit plus upgrades. A stock 8-Bit normally draws 0.6 amps (0.7 amps max).

[shoestring]

I'll have to order a couple of those and give it a try. Thanks for the heads up.

[_The Doctor__]

1.5 isn't really, if fully decked and you run a couple of sio2sd's etc.. they already struggle at 1.5 and a decked out (with old memory upgrade etc) machines

[+mytek]

Reality check is that the transformers in most all of the Atari 5V power warts probably couldn't safely provide more than that amount of current on a continuous basis. But last time I checked, on a XEGS I had stuffed with a U1MB, a TK-II Stereo board, a UAV, and a Sophia, I was only seeing a tad over 1 amp on the 5V side. If you got so many upgrades that you need more then 1.5 amps to power it, then you shouldn't be using a stock PSU to begin with.

 

Of course the benefit of installing the DC switch-mode regulator is that the transformer will see less load, and also run cooler as a result. So even those smaller wall warts normally rated for 1 amp, will likely now be capable of more.

 

And for the sake of argument, I think we can assume this discussion is aimed more at a stock or moderately upgraded machine, not some over stuffed monster (not to say those don't exist).

[phoenixdownita]

I use this (3A 5V):

https://www.ebay.com/itm/5V-3A-Switching-Regulator-TO-220-size-LM323T-Replacement/252433083539

 

I wrote to the guy years ago when he didn't have a 3A part and he created one "for me", at the time I was trying to replace both 7805 in my MegaDrive1 with a single one .... it works great (I ended up using 2 anyway as at the time I was attempting to replace my MD1 CXA1145 with a CXA2075 and getting mixed results, turned out it was something else entirely and the MD1 was working just fine with only one of these).

One note, if you want to literally drop-it-in you'd need to file/sand the back side where the header is soldered and apply some tape to avoid touching metal parts (sometime it is really hard to get rid of the existing heatsinks )

 

EDIT: the guy also has his own storefront https://www.ezsbc.com

at the time he was extremely gracious with me and because he was beta-testing these 3A parts I got such a deal (if memory serves I bought 2 and got 4 or something like that).

I bought all my other regulator replacements from him as well and ALL of my consoles have had the 7805 yanked out and replaced.

If I could go back likely I'd replace all of my 7805 with these 3A parts so that I would know no flash-cart would put too much stress on them as it is now some have the 1A part, and a couple the 3A part (where required obviously)

[BillC]

I use this (3A 5V):

https://www.ebay.com/itm/5V-3A-Switching-Regulator-TO-220-size-LM323T-Replacement/252433083539

 

I wrote to the guy years ago when he didn't have a 3A part and he created one "for me", at the time I was trying to replace both 7805 in my MegaDrive1 with a single one .... it works great (I ended up using 2 anyway as at the time I was attempting to replace my MD1 CXA1145 with a CXA2075 and getting mixed results, turned out it was something else entirely and the MD1 was working just fine with only one of these).

One note, if you want to literally drop-it-in you'd need to sand the back side where the header is soldered and apply some tape to avoid touching metal parts (sometime it is really hard to get rid of the existing heatsinks )

 

EDIT: the guy also has his own storefront https://www.ezsbc.com/

at the time he was extremely gracious with me and because he was beta-testing these 3A parts I got such a deal (if memory serves I bought 2 and got 4 or something like that).

I bought all my other regulator replacements from him as well and ALL of my consoles have had the 7805 yanked out and replaced.

If I could go back likely I'd replace all of my 7805 with these 3A parts so that I would know no flash-cart would put too much stress on them as it is now some have the 1A part, and a couple the 3A part (where required obviously)

Your storefront link doesn't work, it needs the "www." removed.

The corrected link: https://ezsbc.com/

[shoestring]

1.5 isn't really, if fully decked and you run a couple of sio2sd's etc.. they already struggle at 1.5 and a decked out (with old memory upgrade etc) machines

 

I have the same module in my c64 PSU. It runs a little hotter than the PSU powering the stock 65XE when I have Ultimate-II+ cartridge and a USB device connected to the back of the c64. The 600XL has an Antonia upgrade and the PS barely gets hot.

 

I'm going to run some tests and see how much they draw. But according to the datasheet it should be able to deliver 3a without problems as long as there is a heatsink, without a heatsink it's guaranteed to provide 2a. I'm assuming the thermal protection will kick in if its delivering more than 2a without adequate dissipation of the heat.

[Level42]

.....and now you can pray this will run 35 years just like the original power supplies have....

 

 

I'll never change them to cheap Chinese switching stuff....78XX's may not be the most efficient, they are VERY reliable and IF they die, they die peacefully by lowering their output voltage to something safe. Switching PSU's can die in much worse ways.....killing your machine.

Edited February 3, 2019 by Level42

[+mytek]

I thought it would be good to mention that the LM7805 used in the original Atari PSU was only rated for 1.5 amps, and that assumes it had a big enough heat sink to dissipate the heat. The real limit is it's body temperature rising to the internal thermal shutdown setting. On those smaller PSUs that usually came with the XEGS, I doubt that it's heat sink was able to safely dissipate the heat generated at anything much above 1 amp, even though the LM7805 rating said it could go beyond that.

[Gunstar]

I love my OEM universal Atari PSU's. I have the same 3+ amp ones on all my peripherals and computers. I add upgrades to my heart's content and never worry about not enough amps for my upgrades on any of my peripherals or computers. They have lasted 35-40 years with only an occasion fuse to be replaced over the years. They may not be efficient, but I don't care. They are reliable beyond belief and I never have to worry about them taking out my electronics if their fuse blows. The simplest circuits of all the PSU's. Keep it simple stupid.

 

I still refrain from modern 5V alternatives, even ones that have the amperage I want. I doubt they will last 35-40 years myself.

Edited February 3, 2019 by Gunstar

[+kheller2]

Hmm. Tracpower/recon or the tpsm. Hmmm. Decisions.

[shoestring]

.....and now you can pray this will run 35 years just like the original power supplies have....

 

 

I'll never change them to cheap Chinese switching stuff....78XX's may not be the most efficient, they are VERY reliable and IF they die, they die peacefully by lowering their output voltage to something safe. Switching PSU's can die in much worse ways.....killing your machine.

You’ve only described one of its failure modes. They can fail quite spectacularly as well, when they short v_input to v_output. Bye bye Atari...

[Mathy]

Hello guys

 

I just recently bought a bunch of 7805 and 7812 replacements from Tracopower for my drives and my 1200XL and some none Atari stuff.

 

Sincerely

 

Mathy

[Level42]

You’ve only described one of its failure modes. They can fail quite spectacularly as well, when they short v_input to v_output. Bye bye Atari...

 

 

In my 35 years of electronics I haven't found one 78XX go bad like that. You might be confusing this with other Atari power supplies which do NOT use 78XX's but other voltage regulators instead. These control a power transistor and THOSE DO go bad and can short input to output.....like the famous Ingot does....

Edited February 4, 2019 by Level42