New power supply project for TI-99/4a

[retro_doog]

Hey all, since FinalGROM99 beat me to the market with a fine multi-cart solution, I've been thinking up other useful retro projects to do...

 

I've always HATED the large heavy bricks that come with the TI-99, even the "wall mount one" complete with screw hole so you can anchor it in place using the screw that holds your duplex outlet junction box cover on!

 

Not completely unrelated, I also cannot stand the large, heavy power brick that comes with an Apple IIc, although it is "prettier" than the TI brick. Also, these bricks take up a lot of space in my collectibles box/trunk.

 

So.... I've devised a way to allow much more compact "switcher" wall adapters work with these machines. In fact, my goal is to be able to use the same wall supply with both my TI-99 and my Apple IIc.

 

Of course, the TI-99 uses an AC/AC wall or floor brick and has a sizable internal power supply board to both rectify the low voltage AC and to regulate the +5/-5/+12 supplies required by the main logic board. On the earlier revision units, the individual regulators are linear and are terribly inefficient. For example, To create the 5V rail from the 12V rectified/regulated source, which itself is derived from a slightly higher rectified filtered voltage, you basically "burn off" the 7V in a power transistor that acts as a variable resistor. So more power is wasted/converted to heat than is actually delivered to the logic board! Now, I believe the later QI supply switched from linear to switching supplies(unavoidable pun), but I'm not 100% sure as I haven't seen the schematic for the QI supply board. However, I suspect it too is far less efficient than modern supplies are capable of.

 

Anyway the goal of this project is to make a replacement power board that can fit into either Black/Silver or Beige units(sans power LED - or you can drill a hole in your case) and allow these units to run with a variety of compact switcher AC/DC supplies. I've found 18W 15V units as small as 2.5x1.5x1.1"! A goal is to allow a fairly wide range of input voltages depending on whether my 12V regulator is strictly a buck(requires higher input voltage than output) or what is called a "buck-boost" which can actually take an input voltage that is either lower or higher than the output voltage. The 5V supply will definitely be a buck and the -5V may just be a switched capacitor inverting supply as it has much lower current draw from measurements on my console.

 

With that I did some current draw measurements on my Non QI supply and logic board and got this:

+12V : 250mA

+5V : 1.0A

-5V : 100mA

 

So, despite the wall brick being rated for 22Watts, only about 8.5W is being delivered to the logic board. Surprisingly, I saw very little variation between idle and while playing Parsec. It appears that keypresses more than anything caused load fluctuations, but the load variations I saw were in the 5%-ish range. Now I suppose the unit would need more power if you stack a bunch of sidecars onto the console. I'm not as familiar with those as I never had more than the cassette interface myself. Does, say the disk drive get power from the console or does it have it's own supply?

 

Regardless, I'm going to make this specced for the power requirements of just a bare console, with a reasonable amount of overhead (maybe 20-25%) to allow for some of the more modern things people are making that attach to the consoles (FA18, that CF floppy emulator thing, that RaspberryPi other thingy, etc...). The rear power connector will be replaced with a thin panel(metal or just made from PCB material) with a standard 5.5mm barrel jack that most generic wall supplies can be gotten with(as long as it's not one of those oddball ones that puts power on the outer barrel!). Since the TI console alone only uses 8.5W, I'd be interested to see if I can run it off of the 6V 1.8A radio shack supply I have that basically looks like a box shaped power plug and never blocks other power sockets when in use. I bought like 50-100 of these when someone was liquidating after Radio Shack went more or less out of business, so I'd like to find a use for these...

 

Anyway, this is a long introductory post, but I was wondering if there would be an interest in a product like this in the community? I'm going to make this thing for myself anyway, but If I can make one I can make more if there is a market. Thanks for reading!

[+arcadeshopper]

The speech synth is the only ti sidecar that needs power from the console all the other original ones had their own supply's due to the speech doesn't pass the power though too the external connector.

There are some newer devices such as the SAMS and 32k sidecars that use console power as an option.. they both draw less than 500ma

Sent from my LM-G820 using Tapatalk

[retro_doog]

3 minutes ago, arcadeshopper said:

The speech synth is the only ti sidecar that needs power from the console all the other original ones had their own supply's due to the speech doesn't pass the power though too the external connector.

There are some newer devices such as the SAMS and 32k sidecars that use console power as an option.. they both draw less than 500ma

Sent from my LM-G820 using Tapatalk
 

Oh! Great reminder - I should repeat my measurements with the Speech Synth attached, although I suspect it probably only add a couple hundred mA of load to the 5V and a negligible amount to the other rails. I'd be surprised if things like the modern 32K sidecar use much more than 100mA assuming it's using fairly a modern SRAM component. Thanks for the input!

[+arcadeshopper]

No problem, if you come up with something feasible and it works well I'll be happy to carry it in my store if you like

Sent from my LM-G820 using Tapatalk

[retro_doog]

8 minutes ago, arcadeshopper said:

No problem, if you come up with something feasible and it works well I'll be happy to carry it in my store if you like

Sent from my LM-G820 using Tapatalk
 

Thanks for the offer! I'll definitely reach out when I get closer. I'm putting this design on the fast track as I hope to reuse the regulation circuits on a number of similar projects.

[Omega-TI]

Yes, some people are now hanging the speech, the TIPI/32K and Raspberry Pi ZERO off of the TI as well.  I also have the @Tursi designed keyboard enhancement drawing power as well, so I'm of the school that it's always a good idea to factor for some extra overhead, especially because you never know what the next cool gadget might be.

 

The MeanWell power brick is one avenue people have come out with as well.  I always want to "push the envelope", so if a new power supply comes out, I'd like to see a "smart power supply", one that could be shut down with a command from the TI via the TIPI or Raspberry Pi.  How exactly one would do this, I'm not completely sure, I figure it would require at least some logic on the board itself and maybe some a couple of wires to send the impulses to the board.  Overkill?  I dunno, but I'd use it if there was software support for it.

[retro_doog]

Quote

I'm of the school that it's always a good idea to factor for some extra overhead,

Yeah, there will definitely be reasonable overhead. I'll probably design the regulators to handle a significantly higher load so the end requirement will lie in whatever adapter the end user chooses for their system.

 

Quote

The MeanWell power brick

Oops! You lost me at "Brick"

 

Quote

one that could be shut down with a command from the TI via the TIPI or Raspberry Pi

I usually refer to that as "soft power"(being a vintage Mac guy). I really don't see a need for that here as the TI-99 has no power management capabilities. Also anything that would involve extra tie ins to the logic board or software/firmware hooks is an anti-goal. It takes less energy to slide that switch that's a few inches away from the keyboard than to type a command and hit carriage return  As far as automated startup shutdown remotely from a networked R-Pi or similar, that's probably a very small niche. I could add a test header with the regulator enable signals in case someone else wanted to gain access to those. Anything "Smart" or "Soft" power related would have to live somewhere else anyway. Or... ugh now I'm digging myself a rathole  , I could put a small ATTiny "Power Management Unit" on the supply board and provide a UART header to whatever wanted to hack it. And I cold make it a BOM stuff option or DIP Switch configurable. I'm thinking of making the power LED an RGB and having a dip switch to allow for 7 colors to be chosen. Well, there U go, I think I just defined the final feature set for this thing. Thanks for the input!  

[WhataKowinkydink]

Awesome project idea

[dgrissom]

I would be interested in an alternative power supply especially if there was less heat  inside the console.  I've thought of looking at a Meanwell.

 

I absolutely hate the Black and Chrome slider on/off switch. Almost any other power on/off solution would be preferable.   A rocker is a good choice.

 

It would need to power a my console, speech synthesizer and my my TIPI/32k with PI Zero W in a speech synth case (Although, I prefer to power my PI from an external adapter.)

 

Seems like a good idea!  

 

[retro_doog]

4 minutes ago, WhataKowinkydink said:

Awesome project idea

Thanks! Encouragement is fuel!

[retro_doog]

11 minutes ago, dgrissom said:

I would be interested in an alternative power supply especially if there was less heat  inside the console.  I've thought of looking at a Meanwell.

 

I absolutely hate the Black and Chrome slider on/off switch. Almost any other power on/off solution would be preferable.   A rocker is a good choice.

 

It would need to power a my console, speech synthesizer and my my TIPI/32k with PI Zero W in a speech synth case (Although, I prefer to power my PI from an external adapter.)

 

Seems like a good idea!  

 

The new supply will absolutely produce less heat! Like I demonstrated before, more power is being burned than delivered to the logic board with the existing pre-QI supply at least. I'll be using regulators with 80-90% efficiency at load.

 

Can't do much about the switch as I'm going for a drop in replacement. However the stock supply uses a dual-pole and I'll only need a single pole, so there could be less friction?

 

I'm sure I can design a supply that is capable of powering any combination of modern devices attached short of a backlit TFT display! I found a candidate switcher for the 5V rail that is good for 2 Amps, so I'll probably use that. The main requirement will be on the size of wall adapter the user chooses.

 

I thought about the dream scenario of putting an AC/DC inside the box and having a common IEC power cable attach to the unit, but then I'm getting into the realm of dangerous voltages and possible fires/injuries that could expose me to liability. Something non-obtrusive that attaches at the wall was my compromise.

 

Again, thanks for all the great input!

Edited July 3, 2020 by retro_doog
Typos

[dgrissom]

1 hour ago, retro_doog said:

 

I thought about the dream scenario of putting an AC/DC inside the box and having a common IEC power cable attach to the unit, but then I'm getting into the realm of dangerous voltages and possible fires/injuries that could expose me to liability. Something non-obtrusive that attaches at the wall was my compromise.

 

 

I agree.  Low voltages in the console is much safer for both humans and machines.  A modern laptop style power adapter would be great!

[Omega-TI]

6 minutes ago, dgrissom said:

I agree.  Low voltages in the console is much safer for both humans and machines.  A modern laptop style power adapter would be great!

The Mean Well is a modern laptop style power adaptor that would supply all the correct voltages.  If you do not plan to make a "soft power" supply, a switch and connector leads could be done on a PCB without many components, the internal board would generate no heat at all to speak of.

 

Mean Well GP25A13A-R1B

[retro_doog]

16 minutes ago, dgrissom said:

I agree.  Low voltages in the console is much safer for both humans and machines.  A modern laptop style power adapter would be great!

Even a laptop supply is bulkier than I'd like. - and at 50-90W is way overkill. I'm thinking commonly available 12-18V wall adapters with the standard 5.5mm barrel connector. Some of these can be very compact at sub 20W ratings. If i can efficiently handle 5V input, a lot of very compact USB type chargers would even be sufficent, although most of those are only in the 5-10W range and may be underpowered. Still the barrel jack will allow for a lot of options as some users can get by with a lot less input power and some will want more...

Edited July 4, 2020 by retro_doog

[retro_doog]

15 minutes ago, Omega-TI said:

The Mean Well is a modern laptop style power adaptor that would supply all the correct voltages.  If you do not plan to make a "soft power" supply, a switch and connector leads could be done on a PCB without many components, the internal board would generate no heat at all to speak of.

 

Mean Well GP25A13A-R1B

Still too "Bricky" for me. Also, while that potentially replaces the entire internal power board, you'd still need a PCB inside the box to work with the factory switch and to provide the LED for B&SS systems. The meanwell is also still a floor brick, although smaller, and requires "assembly" in the form of a second IEC power cord. All make me go "Yuk!"

 

I'm thinking more like this:

https://www.digikey.com/product-detail/en/tri-mag-llc/L6R18-180/364-1273-ND/7682636

 

As far as soft power, it's far better to bring out a communication interface that connects to something "smart" than a raw switch. To me the smart power would still use the console switch to connect/disconnect the input power. The soft power would then enable the regulators to actually power the console. Soft power would require a small "trickle" regulator to power a simple MCU(I'd use an ATTiny AVR part). An added benefit of having a communication interface like UART is that you can do lot more power management type stuff like poll any indicator signals from the regulators (Overcurrent/Under voltage) or even set the power LED color remotely. In addition the ATTiny has a small bit of EEPROM so you can store settings in it that persist when wall power is removed. I actually made a project with one of these that gives you an RGB power LED for the Macintosh Color Classic and overrides the volume and brightness buttons to set the LED mode while still retaining the main volume and brightness functions of those buttons.

 

Also, since I'm planning multiple platform power supply projects based on these switcher designs, I like the idea of the universal single(but flexible) input voltage over a standard barrel connector. My solution may not end up being for everyone, but I think I'm designing for the "common case" of Console + a few modern low power add-ons...

 

Thanks for the continued input

 

[retro_doog]

Also, I have a bunch of these which might be able to provide enough power for just a console + a small addon:

 

 

Wouldn't THAT be something?

 

[wierd_w]

I am still of the opinion that a picoatx with a breakout and a small circuit to prove -5v from one of the redundant +5v rails would fit in the original Qi board's area just fine, and could be powered from a suitable, bog-standard positive tip 12v barrel wall wart supply.

 

The one that *REALLY* needs a proper replacement is the supply inside the PEB though.  That thing as a 10lb transformer coil in it!

[retro_doog]

9 hours ago, wierd_w said:

I am still of the opinion that a picoatx with a breakout and a small circuit to prove -5v from one of the redundant +5v rails would fit in the original Qi board's area just fine, and could be powered from a suitable, bog-standard positive tip 12v barrel wall wart supply.

 

The one that *REALLY* needs a proper replacement is the supply inside the PEB though.  That thing as a 10lb transformer coil in it!

I'm sure that and many other things could be made to work, but they're all piecemeal hack-together solutions: PicoATX+Neg5VCircuit+Power Switch+LED+How to mount it all... 

 

I like to make drop in boards that have the same form factor as the original so there's nothing glued, hacked, or flopping around inside the case. ?

 

As far as the expansion box, I suspect it has the AC/DC supply inside and just takes a power cord? There are lots of ways to lighten that up, I'm sure, but again I'm trying to stay away from anything that involves line voltage. UL exists for a reason, ya know! Also, and I could be wrong, but I suspect the console only "users" outnumber PEB by something close to 10:1.

[retro_doog]

Hmmm, I'm rethinking the use of lower than 12V for the input supply. Even with an exactly 12V supply I'd need what is called a Buck-Boost or Flyback style regulator which are larger, more complex, and more costly than a Buck regulator. I don't want to fix the input supply requirement to be 12V only and even then the wall supply would be unregulated and not really suitable for the 12V rail as-is...

 

So, I'm considering making the input supply range 15V-18V. I need a bit more time to research the flyback options. I should also locate a 12V supply and see what the unregulated voltage actually measures. Sometimes these 12V supplies are actually 13-14V with a min at 12V under full load(because usually unregulated internally). If that is the case, I could use an ultra low dropout linear regulator for the 12V rail since it's only drawing 250mA.

 

I've already identified a couple of low cost/complexity and very efficient (90%+) 5V switchers at 2-3A ratings, so the rail with the highest power requirements is already sorted out.

 

On to the -5V. 100mA is borderline for switched-Cap inverters. I may need to do a inverting boost reg instead. From looking at the schematic, it appears that only theCPU,  DRAM, and GROM ICs are actually using -5V. I doubt any modern addon hardware will add extra load requirements to this rail. I should probably see what the draw is with the Speech Synth attached, though...

[+Lee Stewart]

1 hour ago, retro_doog said:

I've already identified a couple of low cost/complexity and very efficient (90%+) 5V switchers at 2-3A ratings, so the rail with the highest power requirements is already sorted out.

 

On to the -5V. 100mA is borderline for switched-Cap inverters. I may need to do a inverting boost reg instead. From looking at the schematic, it appears that only theCPU,  DRAM, and GROM ICs are actually using -5V. I doubt any modern addon hardware will add extra load requirements to this rail. I should probably see what the draw is with the Speech Synth attached, though...

 

I believe both +5V and -5V appear at the cartridge port.

 

...lee

[retro_doog]

1 minute ago, Lee Stewart said:

 

I believe both +5V and -5V appear at the cartridge port.

 

...lee

Sure, because... GROMS   My 100mA -5V measurement included with a cartridge playing, so I suspect the load requirements will not be much higher. With that, I found a 200mA capable Inverting Switched Cap regulator IC, so I think we're going to have good margin on these supplies ? Again, the end user will simply need to choose a power brick with enough wattage for their particular setup...

[Omega-TI]

... amperage right.

[retro_doog]

7 hours ago, Omega-TI said:

... amperage right.

It's actually wattage  Depending on the voltage chosen for the power adapter, the amperage requirements will be different. If the power input requirements of the computer are, say 18Watts, the end user could use a 12V/1.5A supply or a 15V/1.2A supply or an 18V/1.0A supply equally well. I still plan to allow for some range of input voltages, I'm just not sure if I'm going to handle the case of <=12V yet. I may design for both so I can test on the first prototype as there should be plenty of board space.

[Kchula-Rrit]

When I saw the part about "burning off 7V" I just had to comment that, after a year of service, one of the regulators on my console died of 'heat stroke' after giving out a piercing shriek.  An autopsy confirmed that one of the voltages had gone.  That panel next to the cartridge go hot enough to hurt my fingers.  No computer should die that way.

 

Anyway, I went down to the surplus store and found a +12,+/-5V brick to replace the TI one, and just wired it directly to the motherboard.  It's worked great ever since.  I'd like to find a +12V to a three-output converter so I can run mine off a car battery during power outages.

 

As far as the power switch goes, I got a four-pole toggle switch to switch the brick's output.  I'm not a fan of the current trend of having everything controlled by the computer, just too complicated and adds expense.  Also, when the computer goes "haywire" or locks up it's just so satisfying to throw a switch, rather than just holding a d*mn button for several seconds and hoping it's not too late.

 

Also, why don't computers have reset-buttons any more?  They think programs never lock up?

 

K-R.

 

[wierd_w]

hard reset on systems that expect graceful shutdown is not kind to them.

 

It's all part of the "make computing fit for the masses, instead of educating the masses about what is proper when computing."

 

You know, like all the "mittens mode" bullcrap found in windows and MacOS.