chunky 12v transformer

[virgilns]

Hi - I'd like to have 12v available in the workshop but at a high amperage (tobe able to power car parts for some testing work without the need for constant battery being topped up)

i need to runs fans, motors and other items which will draw quite a load.

I think ui read somewhere that the simpel 'stick welders' step 240 volt down to 12 but if i remember rightly they are all A/C (DC being used for alumimium welding and more expensive)

WOuld it be easy to rectify a high current 12v ac souorce to DC?

Also if the welder core is not any use is there anything easily available to use to step the voltage down with out shellign out huge bucks for a commercial 12v trans?

Thanks,

Virgil..

 

[bernardgreen]

Electrical equipment in cars is designed to work from a nominal 12 volt supply from a generator / battery combination. This is smooth DC supply.

The DC created by rectifying the 12 volt AC from a transformer is not smoothed, It turns on and off 100 times a second. Nothing like the supply in a car.

So best low cost option is to use a car battery and a battery charger. A simply charger will do if you charge before use.

 

[Jackrae]

"quite a load" Now that's an interesting specification.

As previous writer, get yourself an old (or better still a new) car type battery and a battery conditioning charger which can be left permanently connected.

 

[Robin0577]

Bernard and Jack are quite right in saying that the battery is going to be the cheapest simplest option.

If you do want to go for a bench supply, you're going to need something smoothed and regulated like this... http://cpc.farnell.com/jsp/search/productdetail.jsp?sku=IN06012

 

[virgilns]

cheers guys.

Have used several batteries of various vintage/state, but just wanted a permanent high current 12v fixture really.

If I keep my current battery charger continuously connected in trickle mode can I cause it any damage if I then place a large load on the battery? Or could I just connect this up leave it to 'maintain' and then just 'use it' when needed with out discinencting it?

I guess the answer, if not, is just a DP switch in the circuit to the battery from the charger...

 

[virgilns]

If you do want to go for a bench supply, you're going to need something smoothed and regulated like this... http://cpc.farnell.com/jsp/search/productdetail.jsp?sku=IN06012[/QUOTE]

Thats exactly the kind of thing I was looking for...didnlt realise they were that cheap...

Still will try the battery idea for a while longer though and see how it pans out...

 

[JohnW2]

If you do want to go for a bench supply, you're going to need something smoothed and regulated like this... http://cpc.farnell.com/jsp/search/productdetail.jsp?sku=IN06012[/QUOTE]
Thats exactly the kind of thing I was looking for...didnlt realise they were that cheap...

One thing to beware of with that sort of thing is that they (certainly the cheaper ones) are usually 'switch-mode' power supplies and, as such, may have a minimum current load below which they won't work.

Kind Regards, John

 

[bernardgreen]

usually 'switch-mode' power supplies and, as such,

will have a lot of high frequency ripple on the output. Not a problem for motors but radios and other audio equipment can be affected by the electrical noise this ripple produces.

 

[JohnW2]

usually 'switch-mode' power supplies and, as such,

will have a lot of high frequency ripple on the output. Not a problem for motors but radios and other audio equipment can be affected by the electrical noise this ripple produces.

...just as 'conventional' ones, particularly high current ones, will have some 50Hz ripple - but I get the impression that the OPs interest is primarily in 'electromechanical' loads, which won't care about the ripple.

Also, I doubt that switch-mode PSUs such as the one linked to are as bad as you may fear, since one of the common uses of such things is for powering high-power '12V' electronic equipment, such as amateur radio transceivers, which would not like too much ripple.

Kind Regards, John

 

[virgilns]

Partly electromechanical, but also ecu, injectors, sensors etc...basicaly most of the stuff in the engine bay, but also starters and window winders, fans etc...

 

[ericmark]

All the commercial test benches I have used had a battery in them most with a regulated battery charger as well. Many years ago I worked as an auto electrician and I wanted an output of around 300 amp and your not going to get that as a cheap power supply.

 

[JohnW2]

Partly electromechanical, but also ecu, injectors, sensors etc...basicaly most of the stuff in the engine bay, but also starters and window winders, fans etc...

I actually suspect that anything electrical or electronic in a car will be very tolerant of a fair degre of 'dirtiness' of the power supply. Although a healthy battery will get rid of most of the 'dirt', cars also need to be able to work when the battery is on its last legs, when the cleaness of the power is likely to deteriorate. However, I do agree with all those others who have said that a battery-based system (with a charger) probably makes most sense (and is likely to be the cheapest/simplest).

Kind Regards, John

 

[SimonH2]

I actually suspect that anything electrical or electronic in a car will be very tolerant of a fair degre of 'dirtiness' of the power supply.

You suspect correctly - at least for competently designed devices.

The spec for underbonnet stuff is "interesting" to say the least. Nominal voltage is normally around 14.0 V with engine running, down to as low as 12V when engine not running. Then when cranking, stuff needs to stay working down to 8V or perhaps less. Then allow for someone connecting up the battery the wrong way round (so -12 to -14V on your +12V input), and jump starting from a 24V vehicle - so 24 to 28V on your 12V input.

And that's only the benign stuff ! Load dump surges are a biggie. When you turn off a large load, the alternator takes a finite time to reduce its output - so the excess generated power causes a spike (much of which is absorbed by the battery). And then there's something like 20kV sparks, electrical noise from the alternator, electrical noise from stuff like motors, and so the list goes on.

And a new one, not yet common. Some are suggesting a move to 42V electrics (3x the current 14V) to reduce currents. This would (in practice) mean having two systems as there will be a lot of 12V equipment to be run for many years. What happens if the ground connection fails ? You have 12V equipment with connections to 42V equipment with potentially -42V on it's inputs.


But back to the OPs question. I'd also suggest a battery. Combine it with a float charger (it must be regulated if you don't want to have to manually switch it on and off to avoid drying out the battery). Unless you intend doing long tests, a relatively small AGM battery would give a similar discharge current to a larger wet battery - just leave it connected to a 13.5 to 13.8V float supply.

If you choose correctly, you can leave the supply connected. Some will trip if overloaded, but others will current limit and simply return to normal once the load is reduced. For example, this one from RS is designed to float a LA battery, and simply current limits if you draw too much from it (either with a large load, or with a heavily discharged battery). I've got one in a project at work (some LED lighting with battery backup).
However the 13.5V will be slightly lower than you'd normally find in a running vehicle.

If you want the full 14V, then you'll want a higher output supply - and it'll need to be able to power the load of the battery voltage will drop.


And as a thought, how about a standard alternator (plus battery) driven by an AC motor (and belt drive to up the speed). With a 2 pole motor (3000rpm), a step up drive of around 3:1 or higher should suffice to emulate a vehicle with the engine running.

 

[mfarrow]

The alternator idea is a good one, but I would leave it as 1:1, as at 1000rpm the alternator doesn't really output a great deal, if anything. You can also leave it closely coupled. If you want to change the ratio, how about a cheap Clark bench drill, modified with an alternator in the chuck.

 

[JohnW2]

The alternator idea is a good one, but I would leave it as 1:1, as at 1000rpm the alternator doesn't really output a great deal, if anything. You can also leave it closely coupled. If you want to change the ratio, how about a cheap Clark bench drill, modified with an alternator in the chuck.

These disciussions are all very well but, since you'd need a battery as well, why not just use a simple conventional charger? AFAICS, these alternator ideas would only really make sense if one wanted very protracted periods of high current from the battery - which I doubt the OP does.

Kind Regards, John