+12V current draw

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Does anyone know how much current can be safely drawn from the +12V supply at the keyboard connector? The Apple-1 manual is clear about the availability of 1.5A on the +5V supply and, clearly, this is the preferred source for powering external devices. But I do need 12V. I could, of course, use a boost converter to step-up the 5V to 12V but that does seem overkill when 12V is available in any event.

My understanding is that the LM340-12 is rated at 1A. However, T2, which is itself only rated at 1A, is required to supply the -12V and -5V lines as well. Accordingly, the +12V line seems to be rated at 500mA. As far as I can tell, the +12V is only used for the RAM chips. Each MK4096 draws about 1.5mA on standby. So if all 16 sockets are populated, that would be a total of 24mA - leaving 476mA available - more than enough for my purposes. However, when active, the RAM chips can draw as much as 30mA. If all of them were active, this would be a total of 480mA, leaving only 20mA which is far from sufficient for my purposes. However, I would have thought (but this is pure guesswork on my part) that only one chip would be active at any one time, so that the current draw from the RAM would likely be 15 x 1.5mA + 1 x 30mA = 52.5mA, again leaving more than enough for my purposes.

I suspect I need a minimum of 170mA from the 12V line. Ideally I would prefer to comfortably have over 200mA available. Do you think I will be able to do this with the +12V supply?

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speedyG's picture
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Joined: Nov 16 2011
Posts: 2413
Re: +12V current draw

Hello lazarusr,
your calculations are quite well done by the availiable current....
in fact you should always keep your conclusions restricted to what engineers call:
the "worst case scenario"
in fact there will be some current availiable with 250 milliamperes....

BUT:
That´s not the point you should worry about !

The real trap is burried in the fact that the +12 Volt regulator is normally operated without heatsink !
That´s your real problem !

Bear in mind that depending to the transformer you might use, at the electrolytic capacitor you might have
up to 18 Volt to 20 Volt and that the regulator must "convert/destroy" by "consumption" quite a bunch
of Watt resulting from difference between inputvoltage and outputvoltage multiplied with the amount of current !
So you better measure with the real amount of input voltage you are dealing with !

By math : Input Volts - Output Volts = dropvoltage x Ampere or i.e. :
18 Volt - 12 Volt = 6 Volt and
with normal usage of 0,4 Ampere ( = 400 milliampere )the powerconsumption equals to :
6 Volt X 0,4 A = 2,4 Watt
BUT with additional load of 0,2 Ampere ( = 200 milliampere ) the same math :
6 Volt x 0,6 A = 3,6 Watt !

In such case if you don´t supply your regulator with additional heatsink you will be starting
to grill the regulator slowly but for sure !

In such case it´s recommended to use at least a sink like this one:
http://www.jameco.com/webapp/wcs/stores/servlet/Product_10001_10001_2029608_-1

just to understand the data: 16 degrees Celsius/W means that by each single Watt applied to that sink it heats up
by 16 degrees ! So at "consumption of 3,6 Watt that heatsink will heat up 57,6 degrees above room temperature.
That´s already far above what you would like to touch !

So you will be looking for a heatsink that fit´s into that availiable area and that has value better
( less degrees celsius / Watt ) than the one i linked to - that´s just the absolute Minimum !

And don´t forget: the value of powerdisaption is only valid if thermal paste has been applied to ensure transition
of heat with optimal condition !

Unless you don´t think about that side of the problem, you surely will soon have to replace soon some parts of your supply area !

sincerely
speedyG

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Re: +12V current draw

That is extremely helpful. I think my problem is always that I look at electronics like coding software: if you string the right components together it should work, without considering real-life issues such as, noise, power loss/inefficiency and, of course, heat dissipation.

I am keen to have a solution that does not involve any physical changes to the Apple-1 itself. I think I can draw most of what I need from the +5V supply. I can get the +12V down to a maximum of 75mA. (In practice, it is likely to be less than this). I am aware that many people use an Apple II keyboard (with suitable rewiring) on the Apple-1 keyboard connector. My understanding (see here) is that this can draw up 50mA from the +12V supply. So I am only looking to draw an extra 25mA. Does this seem achievable without extra cooling or heatsinks?

Your comments have also made me think about the demands on the +5V supply. Here, I am looking to draw about 225mA, but I would like to be able to go up to 675mA. Do you think that the heatsink on the LM323 would be able to cope with this? (I appreciate that airflow and ambient temperature are relevant considerations here.)

speedyG's picture
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Posts: 2413
Re: +12V current draw

Hello lazarusr,

i´m sorry - but i´m a bit confused......
the reference you spot to, is the connector of the apple II and at that connector only minus 12 Volt is availiable.

At the other hand it´s true that at the Apple-1 connector of the keyboard there is also plus 12 Volt availiable
which you can´t access at the Apple II keyboard socket !

Compare both sockets at:
http://myownapple1.blogspot.de/2012/06/keyboard.html
or at the manual of the PS2 Apple-1 adaptor.

But if you mention in fact the minus 12 Volt pin at the Apple-1 or Apple II keyboard socket
there is another point to pay attention to:
At the Apple-1 the output of the minus 12 Volt also supplies the input of the minus 5 Volt !
So usage at the minus12 result from adding up current use at the minus12 Volt and minus 5 Volt together !

Maybe it´s usefull to explain what you really intend to do, so i can really spot out
details according to your real needs.....

Up to this point my explenations made previously above in first posting are only related to the plus 12 Volt at the keyboard socket of the Apple-1, while here in this posting i explain details related to the minus 12 Volt usage.

Please recognize the difference i explained here.

sincerely
SpeedyG

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In early days I had a lot of money but no time - now I have no money but a lot of time....
the second part includes less friends but a lot more joy on life....

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Re: +12V current draw

speedyG wrote:

the reference you spot to, is the connector of the apple II and at that connector only minus 12 Volt is availiable.

Please completely ignore the reference to the Apple II keyboard. I had forgotten that this was -12V and not +12V. It is not relevant.

I think I know what I need to do now. I have to make a prototype of the device I have in mind and (i) measure how much power it is actually drawn from the various power lines; (ii) measure the temperature rises I am getting on the voltage regulators to see whether this kind of power draw is sustainable. If both current draw and temperature are OK, then that is good news. If not, I will have to power the device with an external power supply.

speedyG's picture
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Posts: 2413
Re: +12V current draw

Hello lazarusr,

i really didn´t want to blame anybody - i just was confused and seeking
for "clear view to the beach"....
and the text was other than the reference....
Just back to your conclusions in last posting...
bear in mind that such a cooling sink is "clipped" to the IC -
so it´s removable and after removing the sink, the paste can be wiped off too....

So it´s only 2 or 3 bucks to operate at the safe side and apply the sink
while measuring the current and then - if it´s possible without trouble,
you just may later remove the cooling sink - if it´s not required....
that surely will be the best option ... at least the safest one...

i operate at same option by myself by using safe options, but keeping aim that they also
are completly removable.... repair of damaged chips in most cases gets more expensive...

just my 5 cents....
sincerely
SpeedyG

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In early days I had a lot of money but no time - now I have no money but a lot of time....
the second part includes less friends but a lot more joy on life....

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Re: +12V current draw

Thanks.

I think I am going to follow your advice in another thread of learning by mistakes. I think I can get away with drawing only 25mA from the +12V supply, which I doubt would be a problem. Even if it is, my Mimeo-1 does not have a genuine LM340-12. It is actually using an LM7812 (which can be replaced for only 99p/US$1.44/€1.32).

Everything else will be from the +5V supply and I'll make sure I have a big fan running across the heatsink.