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

[+remowilliams]

Tracos still going strong : )

 

http://atariage.com/forums/topic/279606-1200xl-traco-7805-replacements/

[shoestring]

 

 

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....

 

https://electronics.stackexchange.com/questions/179233/making-lm78xx-fail-safe-against-short-open-reverese-bias-flyback-etc

[phoenixdownita]

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

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

It wasn't the "www." but an escaped "/ " (slash+space) at the end of the URL, link now fixed + yours works as well for all the browsers that automatically prefix "www." to URLs with less than 3 parts in the hostname (all modern ones ), at any rate thanks for posting a working link!!!

 

EDIT: @shoestring, why does your Tektronix says "Australian Defence Force"? ... what's your deal man?

[shoestring]

Hey Phoenix.. this might explain it.

 

http://www.ferret.com.au/c/tekmark-australia/tekmark-supply-the-australian-defence-force-with-tektronix-digital-phosphor-oscilloscopes-n1830793

 

I bought it second hand from an auction. Mine is not the C model though but still damn good for what I paid. Only powered up ~20 times when I got it so it was virtually unused. My guess is that their budget is large enough to updating this stuff to the latest tech.

Edited February 4, 2019 by shoestring

[phoenixdownita]

Hey Phoenix.. this might explain it.

 

http://www.ferret.com.au/c/tekmark-australia/tekmark-supply-the-australian-defence-force-with-tektronix-digital-phosphor-oscilloscopes-n1830793

 

I bought it second hand from an auction. Mine is not the C model though but still damn good for what I paid. Only powered up ~20 times when I got it so it was virtually unused. My guess is that their budget is large enough to updating this stuff to the latest tech.

... I was conjuring up an image of you wearing a SCUBA kit (combatant diver style) and being deployed everywhere to defend the sovranity of AUS against the baddies of the world and maybe the occasional black-op in NZ (just by proximity) .... lol .... I should stop drinking rum now.

[Level42]

 

https://electronics.stackexchange.com/questions/179233/making-lm78xx-fail-safe-against-short-open-reverese-bias-flyback-etc

 

That is a lot of questions, not proof of it happening....

[shoestring]

... I was conjuring up an image of you wearing a SCUBA kit (combatant diver style) and being deployed everywhere to defend the sovranity of AUS against the baddies of the world and maybe the occasional black-op in NZ (just by proximity) .... lol .... I should stop drinking rum now.

I literally laughed out loud at my desk at work.

 

Hope you aren't too hung over in the morning.

[1050]

Every time I lost ram due to over voltage it was a 7805

shorted straight thru. Lightning surge will do it, so

will years of no heat sink while trapped inside a slug

of cracked epoxy as is done in the ingot style. When the

epoxy wasn't gaped open by shrinkage cracks there was

at least some heat sinking going on. Thank God it doesn't

happen very happen, but it does happen.

 

Not happy with the 7805. It's killed at least three

of mine.

[_The Doctor__]

Why would anybody use an ingot, we've known about the issue within a few months of them coming out. Almost every users group and bbs as well as internet venue has had a warning about them. You can see just about any forum still alive has a warning about them and a hate the ingot club that tells folks almost instantly if they see hear or smell one around!

 

That being said... anything is possible. I've got nothing but hate for the ingot, and mad respect and love for the 7805. The hum bars and other issues of the ingot shows the design is the issue, the 7805 not so much.

Edited February 5, 2019 by _The Doctor__

[shoestring]

I still think a crowbar / voltage clamp is in order and so I made some small changes. Kicks in once I wind the pot past 5.7v. Gonna have to play around with the resistor and see if I can get it to blow the fuse at a slightly lower voltage. But I'd say success at this point...

 

Edited February 9, 2019 by shoestring

[_The Doctor__]

I like it when it doesn't go past 5.2 to 5.29 v (depending on the meter) on the actual output under load to the computer. Never lost a component with those limits and nothing gets starved.

what does output look like up until it kicks in?

 

It really does limit damage to a machine when you do this, really nice to see it employed. I always add a scrap piece of metal as a heat sink on the regulators. Nothing exciting or large just a helper to keep is slightly more happy.

Edited February 9, 2019 by _The Doctor__

[Forrest]

My suggestion is to use a new 5V power supply for the Raspberry Pi, and just make an adapter cable from the micro-usb cconnector to the 7 pin DIN plug. These power supplies are made by many companies, including https://www.shopqvs.com/store/c/495-Raspberry-Micro-USB.aspx and these are CE certified and labeled. Microcenter sells these power supplies

[shoestring]

I like it when it doesn't go past 5.2 to 5.29 v (depending on the meter) on the actual output under load to the computer. Never lost a component with those limits and nothing gets starved.

what does output look like up until it kicks in?

 

It really does limit damage to a machine when you do this, really nice to see it employed. I always add a scrap piece of metal as a heat sink on the regulators. Nothing exciting or large just a helper to keep is slightly more happy.

 

I haven't put it on the scope yet but I suspect the DMM might be a bit off. So far I've only tested the circuit on the bench supply which can provide 5a and is happily blowing 3a fuses. I'm not sure about the transformer in the Atari supply pictured in the first post. It's rated at 1.5a but that might be based on 7805 itself. Will have to try a bunch of fuses in steps and activate the clamp to see which one is going to be suitable. The datasheet claims it can provide 3a with heatsink but I am sceptical. A fully decked out machine will draw over 1.5a so I might need something around there or slightly higher.

 

I'm struggling to find heat sinks that will fit that small place. I don't fancy cutting any down to size..

[+mytek]

A fully decked out 10088XEL/XLD will still have reserves, not taking it all the way up to 1.5 amps. Reason being is that a lot of CMOS gets used for the glue logic, and SRAM eliminates several support chips, as well as being more efficient then DRAM. I also suspect that the U1MB is a lower power draw then the chips it replaced.

 

A lot of the stock 74LSxx chips on the stock Atari could also get replaced with 74HCTxx equivalents, thus reducing power requirements. One exception would be the 74LS08 which should be exchanged for a 74F08 instead.

[shoestring]

I think a 1.5a fuse should do it then, I just don't have any on hand right now besides these 3a ones. I don't have a lot of upgrades. Just the Antonia board in the 600XL which is fantastic and a couple of other bits and pieces.

 

The stock breadbin c64 draws a little more than the Atari with the same 5v circuit and even more with the Ultimate-II+ cartridge ( that thing sucks a lot of juice ) , so I'm definitely going to get myself a bunch of heat sinks for these.

[davince]

On 2/3/2019 at 1:34 PM, Level42 said:

...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.

The problem with C64 PSU and lm7805 is that over time they output higher voltage, eg. 5.4V which can ruin your system.