add on board power

Modern, Memotech inspired, hardware projects
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g8zpd
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Joined: 28 Oct 2015 23:10

add on board power

Post by g8zpd » 01 Nov 2015 20:06

I'm playing with adding an expansion board with eprom, dart and ram to my mtx (I know this has already done by people on here but its fun!). What sort of current can I safely draw from the existing 5v line. Also what can I expect from the standard PSU i.e. when do I add another 7805 and when do I need to add an extra PSU.

Thanks for any help,
Paul.

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1024MAK
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Location: Looking forward to summer, in Somerset, UK

Re: add on board power

Post by 1024MAK » 01 Nov 2015 23:30

If you use CMOS EPROM (27Cxxx) and SRAM chips, high speed CMOS logic (74HCTxxx or 74HCxxx logic chips) or CPLDs you should be fine using the main board +5V. The +12V and the unregulated -V for the RS232 is already accounted for in the power supply design :D

Mark

Martin A
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Re: add on board power

Post by Martin A » 04 Nov 2015 00:25

The standard MTX power circuitry has enough spare capacity to power 2 internal expansions. Typically a RAM board and RS232, as would have been found on any number of systems back in the day.

I've just checked the photo's on Andy's site, there would have been 22 chips on a fully populated RAM board and discounting the higher voltage line drivers 10 more chips on the RS232. And being mid 80's parts, none of those are CMOS chips.

So as long as you stick to CMOS and keep your chip count under 30 you should have no issues.

To put some numbers together, A quick look at the data sheet shows a typical 64k x1 ram chip of mid 80's vintage would draw 45ma, 16 of those on a ram card is 720ma. The DART would draw another 100ma, the 15 logic chips would have been drawing something as well. I wouldn't start to worry unless you're trying to draw more than an amp.

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1024MAK
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Re: add on board power

Post by 1024MAK » 04 Nov 2015 11:55

Yes Martin. But... That was when the machines were new. Having done some work on the power supply section and having had a good look at the transformer, in my judgment two expansion boards in a MTX would have been pushing the transformer into overloaded territory. It's stated current rating is 1A AC (see here). That is the AC current rating. Of course, it is possible that the wording on the transformer/PSU is misleading and it actually has a higher current rating. I would have liked to know the VA rating. But then again it is a similar size and weight to what I would expect for that rating of transformer.

The calculated maximum DC current for all the power supply rails is tricky to work out because the transformer is multi-tapped and three DC supplies are derived from it. So assuming most of the current is for the 5V DC supply, so the transformer is working in centre tapped mode with full wave rectification only, the maximum DC current is 1 x the AC current. But the 12V DC supply works in half wave mode using some of one winding that also supplies the 5V DC line plus another section of winding that is only used for the 12V DC supply. maximum DC current in this case for this mode is 0.28 x the AC current.

So taking the best case, that's a total current of 1A for all DC supplies. Now the negative supplies can be ignored because they take so little current. But the current on the 5V and the 12V supplies with just one expansion board (32k bytes memory) is significant.

Now, over 30 years later, the transformer will not have changed, but the electrolytic capacitors may have lost capacity, and will be less able to smooth the "rectified DC' and keep the voltage on the regulator inputs within range. So given that CMOS parts are either the only type available new, or are cheaper than or the same / similar price as NMOS or TTL/74LSxxx parts, it is sensible to use CMOS parts.

CMOS parts use much less current compared to NMOS or TTL/74LSxxx parts, so will put less stress on the power supply system components.

Mark

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