12v DC Motor

Decided to use a windscreen motor/gearbox to power my cam grinder (not the grinding head) as with only a little tweaking with sprockets can get the speed I want (about 20rpm). How to power it is the question. It takes in the order of 1.9 amps when it's running off my battery charger, anyone any ideas on using an ATX power pack from an old computer? seem to recall that they do a 12v connection and the wattage I think is adequate. Can any electronics guy out there give some guidance as to feasibility?

Cheers

GW

Hi Graham,
The 12 volt output usually requires that the main 5 volt output has a reasonable load on it before it is able to deliver a reasonable current. To enable the outputs you need to short two pins together on the connector that normally goes to the motherboard. (I cannot remember which pins but I have a note of it somewhere if you need it.) This is not a very efficient way to get 12 volts as there would probably be more power dissipated in the load you would have to put on the 5 volt rail. An alternative way would be to use an old 12 volt 50 watt lighting transformer (A real transformer not an electronic one.) with a bridge rectifier. You would probably not need a smoothing capacitor. Most electronic transformers specify a minimum load which is why a suggested a real transformer.

Les.

.

What about an old power supply for a CB radio or similar. - Try ebay.!

There is a pot inside them for altering the output voltage. (usually set to 13.8v)

Nick

Graham,

I and others have used the converter from a Campervan or similar, which connects to the mains when on a camp site. Mains input down to 12 volts and can deliver up to 10 amps. Usually available on the well known auction site

GW, I use a transformer from a Xerox photo copier, it's about 6" square(the bare tranny), it has a bridge rectifier, and a 2500uf electrolytic capacitor. The transformer was an autotransformer, so to isolate the secondary I wound on an extra winding to give 18 volts.

If you feel like a bit of work, the transformer from an old black and white TV(valve type) about 3" + sq. can be rewound after taking off the secondary.

Ian S C

Something like THIS from your local car boot will do?

Neil

Thanks for the replies. Thought to use an ATX PSU as there is an old computer tower in the loft gathering dust but had an inkling it wouldn't be straightforward, thanks for the info Les. Was also trying to keep the spend down which is why I didn't go for the caravan inverter. Think there may be a few transformers and bridge rectifiers about of U/K spec, brother was into old radios etc, may play with those then. Battery chargers are a bit ungainly Neil, was looking to have something more compact.

Cheers

GW

Funny to see my newest battery charger being described as 'Vintage......Classic....". Just because it doesn't have a chip in it . I was using my second newest this morning so I guess that would be described as " Veteran...." My oldest must be " Prehistoric......." though there will be some kids who think everything before Facegoogle qualifies as prehistory.

FWIW I would advise as simple a psu as possible - transformer + bridge and maybe a cap - say 10,000uF, oh and a fuse is nice too. I have found fancy power supplies don't like motors because motors tend to draw a heavy current at startup and throw a clever psu into overcurrent shutdown. As they say - KISS.

Try ebay look for switch mode power supply , at least 5 amps you may get one local but more likely from china,if you have issues with start up current a series resistor to limit the supply output to 80-90% of rated output followed by some large value capacitors ( in parallel) will help to buffer this.

The old tranny/ rectifier unit from a second hand battery charger will be the cheapest , i would go for 6amps minimum but that's just me ! Maybe you could incorporate this into the machine base so it is hidden ?

I just took a look at my laptop supply and it can supply 19v up to 3.6 amps and measures 110x45x35 mm so that may be another avenue also. A regulator circuit or maybe an old regulator out of a car alternator to sort out getting the 19v down to the 13.5 v the motor is designed to operate from then i would use a pulse width modulated speed controller as this will give you a more precise control on motor speed.

One thing i have never tried is running two switch mode supplies in parallel to give a higher current rating - might be worth blowing up some old phone chargers to see what happens !

Ian

Or, buy a 12 volt gel cell battery, the type they use for fire alarms, UPS etc. They are dirt cheap and come in 12 amp/hour and bigger. Then just hook it up to your existing trickle charger.

This dodge works well too for old battery drills etc whose original batteries have died.

My power supply is about 18" wide x 8" deep x 8" high, and screwed to the wall behind the milling machine, there are leads off to the front of the workshop for a compressor, to pump up my bike tires. and for the electrolytic bath (rust removal). There is another plug for a 18v battery drill, And an overhead hoist above the lathe for transporting chucks (8"/200 mm)

Ian S C

Tried Jason's suggestion and got some info from the web searching under bench top supplies from ATX. Seemed to work, until I connected the motor. My meter was showing 11.3 volts, motor turned a couple of times and stopped, checked on battery and motor still OK. Connected the meter to PSU again and it showed 11.2 volts but began to immediately drop down to less than 1, no idea why so left that. Now switched to Laptop power unit as I've got one left over from a defunct laptop, 18.5 volts DC at 3.5amps. Now my extremely limited knowledge (no idea what switch mode or pulse width modulated speed controller are) using Ohms Law gives me 3.25 ohms resistance with a 6.5v (18.5 - 12) drop with 2amp current, and to get that I need to use 3 x 9 ohm resistors in parallel using the formula from Maplins. As the motor is about 25watts and the max wattage per single resistor on Maplin's web site is 10w, and before I buy them, would 3 x 9 ohm 10watt resistors work ????

All your help and suggestions appreciated

GW

Obtain an old micro wave transformer remove the high voltage. Wind on a new secondary, not a long job for 12 or 13 volts, secure with air drying varnish. Be careful when passing the copper wire through the core, I stick masking tape around any sharp edges. Connect the new winding ends to a bridge rectifier and to an electrolytic capacitor ( I use 1000 micro farad per amp ) and you have useful power supply for very little cost . It should mounted in ventilated enclosure and EARTHED.

Graham, since you already have a battery charger, why don't you just buy a cheap 12v car or leisure battery and use that as your power source & just top it up when necessary with the battery charger?

Regards Martin.

Hi Martin.

That's a possibility but anything with some capacity would be larger than I would like and I wouldn't want it to flatten when grinding a cam. As Neil earlier suggested I could use a battery charger, mine powered the motor OK but they're usually bulky The laptop charger looks like it could be incorporated in the base, it's quite small, and wouldn't take up a lot of room, just unsure of the values of resistors needed (or indeed if any such are available) to use the laptop power unit I have with the motor I have.

Cheers

GW

In your original post you said the current was 1.9A, is that under load?

You also said you had some flexibility in drive ratios, if so I would run the motor straight off the laptops PSU. The 18 Volts is going to make it run a bit faster but with a higher reduction ratio the current will be less. Another minor advantage is that the laptop PSU will be regulated so the motor will run at a fairly constant (and repeatable) speed.

I presume you are running the windscreen motor in the direction it was designed for, the armature can usually only take thrust in one direction.

Ian P