Power consumption and the voltage regulator

Discussions about Sinclair ZX80 and ZX81 Hardware
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patters
Posts: 122
Joined: Thu Mar 11, 2021 12:34 pm

Power consumption and the voltage regulator

Post by patters »

I have just used my multimeter to measure current draw but putting it in series with the supply current. I touched the tip of the power jack against its rightful contact on the socket (from the side), and used the multimeter to bridge the ground pin to ground on the board by jamming the other probe under the edge of the metal lid of the modulator. Unfortunately I don't have a thermometer in the house, only an in-ear one for children, so my temperatures are vague.

My results:

Code: Select all

    Test                    PSU            Draw       Heat sink temp
16K comp* ZX81          Sinclair UK700     290mA    Quite hot
16K comp* ZX81          9V regulated       290mA    Only a bit warm
16K comp* with WESPI    9V regulated       490mA    Uncomfortable to grasp
*all tests on a composite modded system (modulator removed) and with a 32K SRAM wired simply for 16K.

1) Does anyone have a power draw figure for an unmodded 1K ZX81? My guess is that removing the modulator and consolidating the RAM might have lowered the current draw a bit.

2) Should I convert to a switching 1A DC-DC regulator for a machine containing a WESPI? Presumably the Sinclair design anticipated a fair amount of heat inside the case, however that did bring notorious reliability problems back at the time didn't it?
My guess is that while it could run hot with the WESPI on a regulated 9V supply, if someone used an original Sinclair PSU it could be hazardous. I want to sell one of the two machines I'm restoring so I need it to be safe. I guess I've convinced myself...

3) How much current headroom do you need to leave (for the DC-DC regulator) to power a ZX Printer to avoid having to give it its own PSU? I have one that I got restored by Mutant Caterpillar so it would be fun to try it without risking a restored Zeddy. I think I'll probably buy a 2A PSU for the ZX81 that I keep for this reason. Will I need a 2A rated regulator?

4) Where's a good place to buy one? All the electronics suppliers seem to have shipping or handling charges that just make small orders non-viable. Retroleum has the 2A Traco, but not the 1A version and it's quite pricey coming in at nearly £15 shipped. Do Chinese fakes of these components exist?
Last edited by patters on Fri Apr 02, 2021 12:23 am, edited 2 times in total.
zx-heinz
Posts: 41
Joined: Thu Apr 27, 2017 8:56 pm

Re: Power consumption and the voltage regulator

Post by zx-heinz »

Hello patters,
we've detected the same problem in https://forum.tlienhard.com/phpBB3/view ... 046#p49046. The internal regulator will be overheated. A switching regulator is strongly recommended to overcome this problem. I usually use cheap Chinese step down regulators as shown in the picture.They are good up to 3A. Of course, the power supply should be strong enough. The ZX printer takes its current mainly from the 12V line, the consumption on the 5V line is less than 20mA.
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patters
Posts: 122
Joined: Thu Mar 11, 2021 12:34 pm

Re: Power consumption and the voltage regulator

Post by patters »

Great thanks for the info! I have to say the WESPI is an astoundingly good project. Mostly in software, and such an elegantly simple circuit design. Really fantastic. I just can't get over the WiFi config being done on the actual ZX81 also :)

I received my replacement keyboards and just tested again with the motherboard back in the case and the heat sink got almost hot enough to burn my fingers so I'll order some regulators.

So if I understand you correctly, the extra load for the ZX Printer will all be on the main PSU so as long as I have at least 1.2A like the later Sinclair revision then that should suffice.
Last edited by patters on Fri Apr 02, 2021 1:16 am, edited 1 time in total.
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1024MAK
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Re: Power consumption and the voltage regulator

Post by 1024MAK »

A unmodified ZX81 draws around 330mA from the nominal 9V DC input.

Yes, using a DC-DC switching converter in place of the 7805 voltage regulator is wise. I posted how I did it in this topic.

A ZX Printer uses about 210mA. But Sinclair supplied a UK1200 1.2A PSU for use with the ZX81 and ZX Printer. The ZX Printer actually draws most of it’s power from the nominal 9V rail rather than the +5V rail. The 7805 is rated at 1A, so a DC-DC converter rated at 1A should be fine.

There is no easy answer to buying small amounts of electronic components unless you live not too far from a electronics shop or trade counter. RS Components I think now has the most ‘branches’. But they are not always the cheapest supplier. I normally order online and then collect when buying from RS.

The other way, is to wait until you have enough projects and therefore you need a number of items. Or to top up your order with extra parts that you are likely to use, or buy suitable tools. Then it’s not too difficult to get over the threshold so you don’t pay the small order charge.

There is a topic over on StarDot forums about good electronic component suppliers ;-)

Mark
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patters
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Re: Power consumption and the voltage regulator

Post by patters »

Thanks Mark. I got lucky - I bought the last two Traco TSR 1-2450 regulators from retrorevivalshop.co.uk for £15 shipped. Once I have that bad composite mod sorted both machines will be golden.
zx-heinz
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Re: Power consumption and the voltage regulator

Post by zx-heinz »

Hi patters,
look for Step Down regulators at e Ba* as an alternative.
patters
Posts: 122
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Re: Power consumption and the voltage regulator

Post by patters »

I considered that but weighed it up as being a high probability of receiving counterfeit or faulty goods. Having been burned by buying non-working composite mods containing substandard NE555 ICs (as identified by 1024MAK) I decided that buying from a genuine retro shop that will be buying larger quantities of them was the safest option, even if it costs slightly more. It's still a lot cheaper than a small order form RS, Farnell, Conrad, Mouser etc.

EDIT - I have found another stockist of the Traco regulator at a lower price:
https://store.activeconsult.co.uk/shop/ ... eplacement
on further scrutiny, the printing on it looks fake though, no space between TSR and the 1. Probably a counterfeit.
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