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12v DC supply


AndrewP
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9 hours ago, melmerby said:

 

You must buy rubbish PSUs :(

Many modern wall supplies are switched mode and are much better regulated. I've just tried some lying around.

A 12v 1.25A one was 12.14v, a 12v 2A one was 12.3v and a 5.1V 3A one was 5.28v all off load.

(In fact they not very modern, all are 10+ years old!)

Just stuff hanging around from old kit like mobile phones, lights etc.  Probably all well over 10 years old too.

 

3 hours ago, kevinlms said:

Most "12 Volt" power supplies supplied with equipment are not regulated, but when connected to the specified load, give roughly the correct voltage. Any unregulated power supply will invariably be high on no load.

That sounds like a plausible explanation.  I was expecting some variation but not the readings I got.

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On 15/09/2019 at 23:33, melmerby said:

 

You must buy rubbish PSUs :(

Many modern wall supplies are switched mode and are much better regulated. I've just tried some lying around.

A 12v 1.25A one was 12.14v, a 12v 2A one was 12.3v and a 5.1V 3A one was 5.28v all off load.

(In fact they not very modern, all are 10+ years old!)

 

Interesting, were any of them sold for Model Railways?    If they are for computers, printers, phone chargers etc then I would expect the voltage to be somewhere close to that stated on the label.    My collection of model railway power units shows even more variation than Teaky's, 21/ 24 volts off load, and my car battery chargers deliver around 14 volts on the 12 volt setting, measured by a variety of electronic and good old moving coil voltmeters.

 

On 15/09/2019 at 12:03, Junctionmad said:

One needs to be careful bandying about terms here 

 

The ( Average )  DC voltage output of full bridge rectifier with capacitor smoothing is a value determined by the nature and type of load current , and both the peak Vac and hence peak Vdc is a meaningless figure unless accompanied by the known load.  People are mixing up DC peak values, VacPeak and Vac RMS here 

 

Junctionmad  is absolutely correct.  We had a lot of trouble in the early 1980s with flickering dashboard lights on some cars, it was really annoying rapid changes in intensity.   Initially it was on Magnetti Marelli alternators, we couldn't find anything wrong, voltage was fine, bang in the middle of the 12 volt range of 13.8 to 14.2 volts in the specification as the charging voltage so the customer had to put up with it.   Later Lucas started selling alternators of similar design under their brand and my MG Maestro had one and lo and behold flickering dash lights, headlights you name it.  My voltmeter showed no faults, too well damped, so after a short while I binned it and fitted the old style Lucas alternator from an Ambassador and it was transformed.  It must have been very high peak voltage, obviously, (or not) cars don't run at a fixed number of cycles  as the number of cycles/ pulses is derived from an alternator with an operating speed range of from memory 2000 to 20 000 rpm, geared around 3:1 so its charges from tick over around 650/ 850 rpm.    We used to test them by switching every electrical thing on and seeing if it charged at tick over. If the voltage was 13 and rising it was good, falling was a fail.  Likewise exceeding 14.2 at high revs off load was a fail.  

 

To reiterate, a fully charged 12 volt car battery is around 14 volts.  [EDIT ] A fully charged 1.5 volt Non rechargeable battery is somewhere round 1.8 volts, a similar rechargable battery somewhere round 1.47 volts [END]  and, your computer 12 volt circuit will be 12 volts.  

Edited by DavidCBroad
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1 hour ago, colin penfold said:

 

So if you want one get it before 31.10.19. :jester:

Why?

 

Volts will be still Volts, Current Amps, Power Watts, Frequency Hertz and for that matter, Resistance will still be measured in Ohms.

 

At least with electricity, everyone worldwide uses the same terminology.

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All this is very interesting but back to the OP.

 

On 14/09/2019 at 21:42, AndrewP said:

I have a standard Hornby R695 controllor but have just been given a motorised turntable that seems to need a DC output but the controllor only has 16vAC output. Just wondering if anyone can advise how to wire up the turntable?

 

thanks

Andy

 

Hi Andy

 

As John said in the second post, buy another cheap train set controller. It will give some control over how fast you spin the turntable and direction you wish to go. It will also be easier to wire up than adding switches, rectifiers etc to your existing controller if you are unsure about electrics.

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6 hours ago, DavidCBroad said:

 

Interesting, were any of them sold for Model Railways?    If they are for computers, printers, phone chargers etc then I would expect the voltage to be somewhere close to that stated on the label.    My collection of model railway power units shows even more variation than Teaky's, 21/ 24 volts off load, and my car battery chargers deliver around 14 volts on the 12 volt setting, measured by a variety of electronic and good old moving coil voltmeters.

 

To reiterate, a fully charged 12 volt car battery is around 14 volts. A fully charged 1.5 volt rechargeable battery is somewhere round 1.8 volts and, your computer 12 volt circuit will be 12 volts.  

No. They were for a variety of things, I have also bought new power supplies and they are also switched mode.

Why wouldn't they be? SMPS are cheaper to make as there isn't a large transformer which is expensive and heavy. They are also more efficient.

If you have a rechargeable battery that is 1.8 volts, it's knackered.

 

Strange about the car alternators as they are three phase and when rectified should be a very smooth output, especially at the frequency they operate at.

 

Edited by melmerby
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2 hours ago, melmerby said:

No. They were for a variety of things, I have also bought new power supplies and they are also switched mode.

Why wouldn't they be? SMPS are cheaper to make as there isn't a large transformer which is expensive and heavy. They are also more efficient.

If you have a rechargeable battery that is 1.8 volts, it's knackered.

 

Strange about the car alternators as they are three phase and when rectified should be a very smooth output, especially at the frequency they operate at.

 

 

Q-Factor, source impedance, load, and maximum power transfer are well known concepts in PSU behaviour and design.

(Interpretation, and application, of such concepts are much well less understood!!!)

 

"3-Phase Rectification", verses "2-Phase Rectification", is inherently "smoother".

(3-Phase rectification uses a 6 Diode-Bridge whereas 2 Phase rectification uses the much more well known 4 Diode-Bridge.

 

 

Look them up.

(The maths is even more interesting!)

 

 

Kev.

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While ordinary AA/AAA’s are rated at and seem to give 1.5v when fresh, all the rechargeable versions I have only give just 1.25v fully charged. Is there another type/size of 1.5v rechargeable around these days (C/D size?).

 

Izzy

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34 minutes ago, Izzy said:

While ordinary AA/AAA’s are rated at and seem to give 1.5v when fresh, all the rechargeable versions I have only give just 1.25v fully charged. Is there another type/size of 1.5v rechargeable around these days (C/D size?).

 

Izzy

In use NiMh & NiCd batteries are usually about 1.2v-1.3v  and whilst charging go up to 1.5v-1.6v anything higher is due to increased internal resistance caused by deterioration.

Most sizes of batteries are available as rechargeables and always have been, C & D are quite common but beware of C cells masquerading as D cells by being inside a D size sleeve

Compare these two D cells:

https://www.rapidonline.com/Catalogue/Search?Query=rechargeable batteries&Attributes={"Battery Size":["D"]}

 

Which is the better value?

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SHMD mentions '2 phase supply', I assume he means single phase. In reply to melmerby, AAA, AA, C, andD cells are all nominal 1.5 volt. It is the current/capacity which varies. I used to teach electronics to GCSE level, and my first lesson was always a simple one about volts and amps. It always amazed me how little kids knew about the basics. Ask the average 15 year old the voltage of our mains supply and the answers vary from 1 to 1000.

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2 minutes ago, cliff park said:

SHMD mentions '2 phase supply', I assume he means single phase. In reply to melmerby, AAA, AA, C, andD cells are all nominal 1.5 volt. It is the current/capacity which varies. I used to teach electronics to GCSE level, and my first lesson was always a simple one about volts and amps. It always amazed me how little kids knew about the basics. Ask the average 15 year old the voltage of our mains supply and the answers vary from 1 to 1000.

Rechargeable batteries are not a nominal 1.5v and never have been.

Where did you get that from?

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10 hours ago, Izzy said:

While ordinary AA/AAA’s are rated at and seem to give 1.5v when fresh, all the rechargeable versions I have only give just 1.25v fully charged. Is there another type/size of 1.5v rechargeable around these days (C/D size?).

 

Those rechargeables are totally compatible with any properly-designed gear intended to work with dry cells.

 

Although dry cells are nominally rated at 1.5V and give around that when new, they are not considered totally flat until they have discharged down to around 1V - so starting at 1.2 - 1.3V and down to 1V flat from a NiCd or NimH cell is absolutely fine, or should be.

 

The problem arises when equipment makers skimp on specifications and do not design things to work over the full voltage range, so they pack up or show batteries being flat long before they really are.

 

That's common problem with things using 9V batteries and 7805 type voltage regulators - the battery consists of six 1.5V cells and can provide power down to 6V, but the regulator does not work properly below 7V...

 

Typical per-cell discharge voltage curve for dry cells - anything that cuts out at 1.2V is wasting a good part of the cell capacity:

 

AA-100mA.png

 

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25 minutes ago, RobjUK said:

 

Those rechargeables are totally compatible with any properly-designed gear intended to work with dry cells

 

Physically, yes. There are two main reasons why gear that is not "properly designed" will often come with a recommendation not to use rechargeables.

 

1.The lower voltage

2. More important, the lower internal resistance. Rechargeables will allow much higher power to be delivered when something goes wrong. 

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18 minutes ago, Clive Mortimore said:

I still think if Andy can buy a train set controller he is on a winner.

IMHO The rest of the discussion is just peripheral rubbish as far as the OP is concerned.

I would do as you suggest, a second basic controller just for the TT.

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25 minutes ago, dhjgreen said:

I'm not sure what the OP will make of this topic if he ever comes back.  He posted on Sat at 21:41 and was last on RMWeb an hour later.

 

I've long since learnt not to ask any simple electrical questions on here and this thread is the perfect example of the reason why. :rolleyes:

 

David

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People want a simple answers.

Electronics is not simple.

Language is not so precise and neither is it's interpretation.

There are (usually) many more ways of achieving the required goal.

There is lots of experience and knowledge here.

 

A 4-diode Bridge rectifier can be used across 2 phases or between 1 phase and the star point (usually termed neutral and usually connected to earth at the sub-stations secondary - in the UK ((if it's not a delta secondary that is!)) ). I used the example of a 4-diode bridge, across 2 phases, as a lead on how the 6-diode bridge works.

 

Happy to inform (where I can) and sorry to inform if you don't want to learn why something should be done a particular way or not.

 

 

:)

 

 

Kev.

 

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Most answers on all forums are not of mercantile quality - and that is the price of an open forum. (Moderated or not.)

Some answers are dangerous - to varying degrees - and some are priceless (and that is why we ask), some are just wrong.

 

The best thing to do is either build a consensus of opinion or you can just blindly follow the first answer!

I know which I prefer.

 

 

Kev.

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4 hours ago, Mallard60022 said:

So many answers on here are a load of tosh. I would have ignored it but it is such tat I just had to say this. Enough to put someone off asking any questions here for life.

Clive is sensible though...………………….

 

The question posed is simple , the fact is the answer is not , so can 16vac output be used to power a nominally 12v dc turntable , the correct and only answer is “ it depends “ ( on a number of factors and you need far more info to answer the question correctly ) 

 

 

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