OT: AC to DC transformer source needed

Recently I purchased a medical exam light (like what a dentist uses) from my local used tool retail store - don't know how the lights got there but they had several different types of medical lighting lamps there. THis light uses a 20W 12v DC bulb. I need to get a replacement transformer. I need 1.7+ amps at 12v DC. Once upon a time I was able to find an on-line company that had a great selection of AC to DC transformers. I have surfed and surfed and I can't seem to find the on-line store I once had discovered, nor anyone else w/transformers greater than 1.5A at 12v DC. I am not interested in paying the money for a regulated power supply. I am antsy to get this light functional - I figure if its good enough for a dentist looking into a mouth cavity, it should be just as good when looking into the cavity of a mechanical device. Off in google groups land (no newsgroup access thru work) awaiting your responses.

Reply to
Aribert
Loading thread data ...

Check

formatting link
If this links does not work, try and search on Power Supplies. Several listed at 12 volts Item SWITCHING SUPPLY CAT# PS-1242 IS 12 VOLTS 3.5 AMPS.

Reply to
Howard R Garner

If it is an ordinary incandescent filament bulb it doesn't really care a jot whether it's running on DC or AC, so all you need is a plain and ordinary

12v transformer

Andrew Mawson

Reply to
Andrew Mawson

Marvin at surplustraders I bet.

formatting link

Reply to
Jim Stewart

As Andrew says, any transformer with 12 volt output will do. Then, provided it's only a normal bulb, run it on ac If you want a power supply, try one for a CB set. These are generally reasonably well regulated and cheap.

John

Reply to
John Manders

Go down to the local home improvement store (Home Depot or Lowes?) and go look at the under cabinet low voltage lighting. They should have replaecment transformers good for 60w for under $20.

-- Joe

-- Joseph M. Krzeszewski Mechanical Engineering and stuff snipped-for-privacy@wpi.edu Jack of All Trades, Master of None... Yet

Reply to
jski

THis light uses a 20W 12v DC bulb. I need to get areplacement transformer(clip) ^^^^^^^^^^ I wonder why they run it on DC. Andrew is absolutely right. A light bulb doesn't know or care whether it is on AC or DC, so long as the RMS voltage is correct. Is it possible that they are running it on AC, but use 12 volt automotive bulbs because they are available?

If you DO actually need 12 volts DC, your search may work better if you don't call it a "transformer." Transformers take AC input and create AC output. You would need a DC power supply, if, indeed, you need DC to run the lamp.

Reply to
Leo Lichtman

It will surely also work with AC if it is really a bulb!

You could search for an old PC power-supply, you may find one for free on the scrap!

Reply to
Reto

Aribert,

This and a rectifier-

formatting link
Jim

Aribert wrote:

Reply to
James E. Baldock

If this is an ordinary filament type (regular or halogen) light bulb, there's no such thing as a DC bulb - AC or DC will work equally well.

Many halogen reading/desk/... lamps use 12 volt bulbs and transformers.

20W is a very common size and many have the transformers as wall warts. Perhaps you can find a broken lamp from which you can get the transformer.

Ted

Reply to
Ted Edwards

The only difference is a diode or bridge rectifier... Thats all you need to change your ac transformer to dc... One diode would do it... Most of the time it cuts your voltage in half... So if you use a 24 volt transformer and put one diode in one of the output lines you should get around 12 volts dc....

Reply to
Kevin Beitz

The local thrift store has a tub of wall-warts with some well up into that range, at $1 or less. Maybe there's one near you.

Bob

Reply to
Bob Powell

I find lots of this sort of stuff here:

formatting link

Reply to
Charles

Try Signal Transformer

formatting link
There's a 24 volter in your current range look at the A41-25-12 a 12 volt 2 amp unit
formatting link
sales reps and pricing

Reply to
Bradford Chaucer

Probably a safety thing, what with all the metal items around and water and working inside someones mouth and such.

I havent read the whole thread, but I think an old computer power supply would likely power it up and also provide for excellent isolation.

Reply to
PrecisionMachinisT

[ ... ]

One consideration is that computer power supplies are usually switching power supplies (Since the first IBM PCs at least), and often require a minimum load on the 5V output before they will put out a full current 12V output. Some of the more recent ones, designed to handle a lot of disk drives, may be more forgiving about this, but older supplies for XT machines often had resistor load on the 5V output when only one floppy (and no hard drive) was plugged in -- or when one instead of two hard drives were installed.

And -- I wouldn't consider a PC power supply a good thing to run in the shop, unless you have an extra housing around it to protect it from metal chips, as they usually have several slots for cooling air to flow through, and at *least*, have the fan with a grille too coarse to keep out the chips.

Good Luck, DoN.

Reply to
DoN. Nichols

Get a light dimmer turn it down untill it reads 12 volts then hook up the light .

Reply to
TLKALLAM8

Hmm ... I don't think so -- unless you have a Variac (or other brand of autotransformer) as a dimmer. And even with a Variac, you have the problem that there is this big knob which people will be tempted to turn. I've seen a shelf of electronic equipment on a 240V Variac set to produce 120V, and a kid walking up to the big steering-wheel looking knob and spinning it to the right, frying the only piece of equipment which was turned on at the time -- a Magnecorder.

And one thing you probably should determine *before* you put this into service is how available replacement lamps for this are, and how much they cost. Otherwise, it may suddenly cease to be worth having, once the lamp burns out.

Most dimmers are triacs which switch on at a variable point along the AC curve. Some meters read peak voltage and apply a correction factor to give you an approximation of the RMS voltage. Others (very few) read true RMS. The two can vary wildly with a waveform like what is coming out of a lamp dimmer set low. The true RMS is what matters to a lamp. I've got a pretty good collection of test equipment, but don't have a voltmeter which reads true RMS.

There are a lot better ways to do this -- including picking up an old broken desk lamp which uses 12V bulbs, and extract the transformer from the remains of the base. (As others have said, there is no need to have DC for normal incandescent lamps.

Good Luck, DoN.

Reply to
DoN. Nichols

Hmm ... I don't think so -- unless you have a Variac (or other brand of autotransformer) as a dimmer. And even with a Variac, you have the problem that there is this big knob which people will be tempted to turn ^^^^^^^^^^^^^^ Aside from the true RMS problem, I don't see any inherent difficulty with using a dimmer, except this, which is the same one you would have with a Variac: The knob will be running about 10% up from zero, so small changes in knob position will produce large relative changes in voltage. A small amount of overvoltage to a light bulb causes a drastic reduction in bulb life, and you may not be aware that you are causing it.

Reply to
Leo Lichtman

Thanks all. I just assumed that since the label on the light listed the output voltage as: 12v dc 1.7A, that I needed a DC output. The bulb is a bi-pin capsule bulb that costs $17 from the light maker - I bought one just so that I could try to read the fine print on the bulb so that in the future I could source on thru McMaster or Grainger.

One of the first places that I looked for a transformer was in a woodworking supply catalog - specifically for the low voltage lights found in some cabinetry. In my online surfing, I found numerous 12v AC transformers that were in ther right amp range and also cheap - just did not realize I could use one of them.

Reply to
Aribert

PolyTech Forum website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.