What are the power specs for the 7800 JS Ports?

[flickertail]

I see that the Easy 78 Input/Output board from Edladdin takes advantage of the 5V power coming from the 7800's joystick ports to power "low amp" add-ons... aka LEDs.

 

What are the JS port's power specs on the 7800? I know that it's 5V, but what are the amps? Are they the same as the 2600?

 

Thanks,

 

Aaron

[edladdin]

Following.  I dont know the answer to this either.  I'd love to update the spec so people know what can really be done with that feature.

 

FWIW, I have built one custom controller where we replaced stock fire buttons with four illuminated buttons.  The 7800 powered those with no problems.

[flickertail]

Hey Edladdin, I'm starting to think a lot could be done with it.

 

I managed to use the power pins from the 7800 ports last night to boot an overclocked Raspberry Pi Zero W in back-power mode.

 

It had bluetooth on, wifi on, and usb on. The monitor wasn't connected, but it booted just like it would if it were connected to a power supply.

 

And it was it stable. I even connected to the Pi's bash prompt it using SSH from my desktop.

 

All I did to shut off the Pi was to turn off the 7800.

 

I wondered if shutting it down so abruptly would corrupt the operating system, so I booted the Pi right after using just a wall plug. It fired back up just fine - uncorrupted.

 

So apparently there is enough current to do that. The 0W runs at 150 mA at idle and not overclocked. A 1.2 Amp power supply is recommended.

 

I'm guessing the ports have close to 1 Amp worth of current.

[+-^CrossBow^-]

I doubt it is an amp. The factory 7805 is only rated to output 750mA to begin with. And a stock 7800 running a pokey game and nothing else is already drawing 700mA. That is why it is advised to replace the stock VRs with something beefier since the system is already running near the output capacity to begin with.

 

[+Mitch]

10 minutes ago, -^CrossBow^- said:

I doubt it is an amp. The factory 7805 is only rated to output 750mA to begin with. And a stock 7800 running a pokey game and nothing else is already drawing 700mA. That is why it is advised to replace the stock VRs with something beefier since the system is already running near the output capacity to begin with.

 

Hmm, for some reason I thought they were rated for 1 amp. It has been a while since I looked at the specs though.

 

Mitch

[+Stephen Moss]

3 hours ago, Mitch said:

Hmm, for some reason I thought they were rated for 1 amp. It has been a while since I looked at the specs though.

Me too, from memory the small 78L05 in the TO92 package are 100mA, the TO220 packege as used in the 2600/7800 are 1A and those in the TO3 package are 3A.

But it may vary slightly from manufacture to manufacture, and in use is dependant on the amount of heatsinking attached to the device and the input voltage so as to not exceed the maximum power rating.

Personally I try to use National Semicondutor (Now part of Texas Instruments) as they initially developed them and in my experience National regualtors were always bang on spec unlike some manufacture where the output voltage could be 0.5V off.  

 

But to answer the original question of how much current you can get from the Joystick power, the first things is that the controller ports on Atari are designed to have pull down inputs, and so are not really intended to supply voltage. That said you can work out how much current you can take from the ports.

The first limiting factor is the current specification of the power unit, you should never exceed that, and no, getting more current is not as simple as just getting a power unit that can supply more current because on unregulated power units as typically used from 2600/7800 unused current = higher input voltage = more power dissipation in the regulator.

The second limiting factor is the amount of current you can take from the voltage regulator itself, so there is no point getting a higher current rated power unit if you cannot take that current from the regulator.

 

So if we assume you are using the standard power unit and voltage regulator to work out how much current you can take from the Joystick ports, desolder the output leg of the voltage regulator so that you can use a DVM to measure the current output.

Measure the current consumption from the regulator with a few different games inserted (use ones that have an attract mode if possible) but no controllers connected.

Subtract that current from the specified maximum current of either the power unit or the regualtor (whichever is the lowest) and what is left should be what you can safely take from the joystick ports without damaging anything.  

Edited April 22, 2020 by Stephen Moss
Added answer to the original question

[flickertail]

Stephan and everyone, thanks for the info and recommendations. I will experiment as suggested. I haven't tinkered with it for a several days as I finally had to stop procrastinating on my end-of-the-semester paper deadline.