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Old 23rd Jul 2020, 6:56 pm   #1
indiosse
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Default Looking for voltage reduction off 12vdc supply

Hi

I have an old computer from the 80's and it doesn't like my new modern supply giving it 12vdc. Actually like almost all the one round my garage, its giving out 12.22 volts, but i need to be more or less dead on 12.00 volts

The old supply uses an AC input and it looks like it uses 2812 regulator at least that's where it seems to end up when i follow the traces to keep the voltage steady at about 12.02.

Can someone advise me on a simple way (if possible) to get the 12.22v down to 12.00 volts so the thing will turn on first time, rather than once in 50 times.

I'm a bit rubbish with circuit diagram so anything in contemporary English would be great. Oh and i still need to able to let through 500mA after converting it.

Much obliged

Rich
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Old 23rd Jul 2020, 7:33 pm   #2
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Default Re: Looking for voltage reduction off 12vdc supply

3 1N400xs in series?

You could always replace the 7812 with another one that is closer to spec. I'm surprised that an extra 0.22V matters that much though.
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Old 23rd Jul 2020, 7:41 pm   #3
indiosse
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Default Re: Looking for voltage reduction off 12vdc supply

hi i can try the 1n400xs, the only diodes i have lying around are 1n4148's i presume that is a different spec ? is there any calculation i can use on these diodes i can read up on ? cheers
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Old 23rd Jul 2020, 7:46 pm   #4
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Default Re: Looking for voltage reduction off 12vdc supply

I was thinking of a power shottkey in series or possibly a load of parallel germanium diodes in series. Actually, maybe a turned-on-hard Ge power transistor might have the right drop.
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Old 23rd Jul 2020, 7:52 pm   #5
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Default Re: Looking for voltage reduction off 12vdc supply

Sorry, I meant generic silicon rectifiers which have a voltage drop of about 0.7V. The 1N400x series is a range of such diodes, rated at 1A where the last number indicates the voltage rating (so 1N4004 is 400V, 1N4007 is 1kV etc.) Any of the series will be fine for this application.

There is also the 1N540x series which is rated at 3A.

You can buy lots of 1N4007s for pennies from eBay sellers. Send me a PM if you want a few FOC.
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Old 23rd Jul 2020, 8:09 pm   #6
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Default Re: Looking for voltage reduction off 12vdc supply

Rich, do report back when you solve the issue, as there is (as always!) a chance it's not as simple as it appears.

Thanks
Dave
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Old 23rd Jul 2020, 8:21 pm   #7
indiosse
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Default Re: Looking for voltage reduction off 12vdc supply

will do, i have ordered some 50v 1n4001 10 pack, lets see when they arrive, it is interesting the computer doesnt boot off 12.2v i actually proved the point by trying it on another exact computer and it did the same thing, by running the 12v linedirectly off a working supply it fixed it rightaway proving that it was indeed the .2v that was crashing it
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Old 23rd Jul 2020, 8:44 pm   #8
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Default Re: Looking for voltage reduction off 12vdc supply

Only 0.22v needs dropping. Three 1N400x diodes in series would be 2.1v; even one diode would be too much.

Last edited by Steve_Bell; 23rd Jul 2020 at 8:50 pm.
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Old 23rd Jul 2020, 8:50 pm   #9
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Default Re: Looking for voltage reduction off 12vdc supply

that was a waste of a few quid then, how do i drop only 0.22 v ? is there another diode that will do a little bit only, need to still get through 500ma
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Old 23rd Jul 2020, 9:18 pm   #10
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Default Re: Looking for voltage reduction off 12vdc supply

Sorry about that, I misread your post. There are Schottky diodes that drop that voltage, but I'm still puzzled that such a small overvoltage is causing problems.

You could try a single 1N4001 and see if it helps.
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Old 23rd Jul 2020, 9:29 pm   #11
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Default Re: Looking for voltage reduction off 12vdc supply

Quote:
Originally Posted by llama View Post
I was thinking of a power shottkey in series or possibly a load of parallel germanium diodes in series. Actually, maybe a turned-on-hard Ge power transistor might have the right drop.
Graham
hi do you have any model numbers in mind on this stuff cheers?
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Old 23rd Jul 2020, 9:53 pm   #12
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Default Re: Looking for voltage reduction off 12vdc supply

Quote:
Originally Posted by The Philpott View Post
Rich, do report back when you solve the issue, as there is (as always!) a chance it's not as simple as it appears.

Thanks
Dave
Hi.

I agree. It does seem strange the computer is very fussy over such a small increase in supply voltage. I was wondering if Rich is measuring the output voltage from his power supply with it connected or unconnected to the computer?
A possible scenario is the power supply could actually be loaded down by the computer due to poor regulation. A voltage check with it actually connected should prove this. A 'scope check to see if there's any significant ripple may be worthwhile. It's feasible there could be dried out electrolytics in the power supply as the computer seems to work after many attempts.

EDIT: If a decent bench power supply is available and with an accurate digital voltmeter connected to the output, you could prove whether the computer will operate successfully on a nominal 12.00V and on 12.22V. Be careful when increasing the output voltage though!

Regards,
Symon

Last edited by Philips210; 23rd Jul 2020 at 10:11 pm. Reason: Additional info
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Old 23rd Jul 2020, 11:00 pm   #13
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Default Re: Looking for voltage reduction off 12vdc supply

Sounds fishy to me, doesn't an 80's computer just take the 12V input and regulate it to 5V anyway? What computer is it?
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Old 23rd Jul 2020, 11:41 pm   #14
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Arrow Re: Looking for voltage reduction off 12vdc supply

The added 'edit' in post #12 needs to be done before any serious headway can be made with diagnosing and then fixing the reported fault. Without that being done - and assuming that a 'scope is not available - almost any other diagnostic route will simply be guesswork.

Al.
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Old 24th Jul 2020, 7:08 am   #15
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Default Re: Looking for voltage reduction off 12vdc supply

Quote:
Originally Posted by McMurdo View Post
sounds fishy to me, doesn't an 80's computer just take the 12V input and regulate it to 5V anyway? What computer is it?
I agree. This sounds more like a PSU that can't supply the initial inrush current of the PC booting and so it just crashes out.

If the 'new modern supply' (post #1) is an SMPS rather than an old linear type then it may well be glitching with any sudden initial current demand, particularly if there are any large value electrolytics in the incoming feed to the PC..
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Old 24th Jul 2020, 1:37 pm   #16
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Default Re: Looking for voltage reduction off 12vdc supply

hi all i was indeed surprised by the findings myself, but the measurements are taken under load i will know more when i drop the voltage down to exactly 12v on the modern supply

the computer is a dragon 32, it has a -5v feed, a 12v feed and a 5v feed to the motherboard coming from an external supply which feeds 28v ac and 8va (yes really) to the regulators. i did think of using the internal regulator which converts the ac input to 12vac but it looks like it needs a minimum of 14vac to function and i was a bitch twitchy about hooking up 12.2v dc to it, can you even hook up dc to an ac 2812 regulator ?

you may all be right about the fact its modern psu as oppose to an old one which may be the reason for the no-start but as i've just ordered some cheap Schottky Barrier Rectifier Diodes we ill see what happens first before returning to the drawing board.

Last edited by indiosse; 24th Jul 2020 at 1:43 pm.
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Old 24th Jul 2020, 6:56 pm   #17
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Default Re: Looking for voltage reduction off 12vdc supply

Hi.

OT but relating to this thread, can someone explain the significance of the little symbol to the left of the thread title in each post? If I hover the mouse over the symbol in say posts #13 and #15 (the same symbol) it shows as "Default". In Al's post #14, there's a black arrow in a green circle. Placing the mouse over this says "Arrow". Just wondered what this is for. Also noticed something similar in other threads.

Regards,
Symon
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Old 24th Jul 2020, 7:29 pm   #18
Philips210
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Default Re: Looking for voltage reduction off 12vdc supply

Hi.

OK, I've just realised they're post icons, I must be Mr Thicko for not noticing the option to apply one when replying in a thread, I've never had cause to so hence my question.
Most of the icons are self explanatory but did wonder what the arrow icon is for.

Regards,
Symon
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