Powering a 12v Halogen Clamp lamp from a computer PSU-OK or not??

[Old Sparky]

The switchmode 12v 36 watt (3Amp) PSU supplied with a Habitat 'Graziella' Clamp lamp has failed and replacement PSUs are, it seems, nolonger available. My initial thoughts are to use one of the commonly available 12 volt 3amp psus designed to power/charge Laptops, etc. Is this (a) feasible - I would have to change the connector on the new PSU, as the lamp uses a 2-pin plug/socket rather than the 'standard' DC power type? (b0 is such a PSU likely to be safe and reliable powering a Halogen lamp?

[Eddystonepete]

Hi Sparky, any 12v DC supply should be OK to power a lamp load, but you may run into problems at start up as the PSU sees a cold lamp as nearly a short cct. Another idea is to use an"electronic transformer" , the type used to power 12 halogen GU10 lamps. Would probably be cheaper too. Bear in mind that these devices dont give DC but a very peaky AC. I once tried to use one into a bridge rec and capacitor. Gave very strange results!. If youve got an old PC power supply you could use the 12v DC output from that. With only a lamp load it would probably not need the fan so would be silent. Would need the PSU start pin on the big white plug shorted to neg to allow the PSU to start. That would be the cheap Scots way!

Peter

[Nick]

The frequency of oscillation is typically in the order of 10's (sometimes 100s) of kHz - a good overview of the general approach can be found in this ST Application Note - http://www.st.com/web/en/resource/techni...003902.pdf

Rectifying them is generally not a good plan - they rely on the RMS value being correct only...

[Eddystonepete]

Yes Nick I found out the hard way !. The DC started rising at switch on and kept rising, I grabbed for the switch about 80v . Thinking about it its only the RMS that matters for lamps. So much for my cheap power supply.

Peter

[Skywave]

My approach would be to build a basic 12v. 3-amp. linear PSU using a suitable mains transformer. If a regulator cct. is needed, a simple design based on a 2N3055 should be sufficient. A generously-rated circuit and thermal design should give a PSU which will last for years without failure. What's your salvage stock like for the necessary bits?

Al.

[Mark Hennessy]

I don't see the need to convert to DC. Assuming you don't already have one in stock, a simple 50VA mains transformer will cost between £10 and £15 from one of the big suppliers (RS, Farnell, CPC, etc) - perhaps less from places like eBay - and should do the job reliably, and with no RFI or other problems. I'd mount in a die-cast enclosure hidden away under your desk, making sure it has a firm connection to PE and a 1A slow-blow fuse in the primary. Job done

[Nick]

The proper (and cheapest) solution is to go to your local electrical factors (TLC, A1, whatever), even, dare I say it, B&Q, and get a halogen 12V PSU - they're designed for exactly this job, e.g. http://www.tlc-direct.co.uk/Products/LTYT50.html

[Mark Hennessy]

Wow - that's staggeringly cheap! And it appears to include PFC - how do they do it for that price? Not as EMI-friendly as a 50Hz transformer, and I'd bet that it won't last as long as a 50Hz transformer, but it's hard to argue with the price...

All my halogen desk lights are old enough to have 50Hz transformers mounted in their bases. It does help with stability - indeed, I've seen newer ones that have a weight in their base because the SMPS is so light

I've said it before, but I still find it staggering that at these sorts of power levels, it's cheaper to design, test, seek approvals, and then manufacturer a complex switched-mode PSU than it is to buy some copper and iron. The BOM costs for most of these supplies is about a dollar in the quantities they're produced in. With the rising price of commodities, you can see how things have gone this way - even if it goes against our instincts...

[Old Sparky]

That transformer from TLC, or a similar one stocked by CPC (LA02938) @ £5.94, inc VAT seems ideal. The original SMPSU is mounted in a 13A plugtop case similar to a 'wall-wart', but one of the in-line types such as these will be OK.

[Old Sparky]

I obtained a Dais DA-J60 'electronic transformer' from a local electrical supplier. This was designed to be wired in series with the mains supply and the lamp, so I removed the PCB from it's plastic case, connected it's primary to the L & N pins of the 'wall-wart' case which housed the original, replaced the flying leads of the o/p with the 2-pin socket from the original. The assembly was fitted into the 'wall-wart' case (Fortunately this one was just clipped together!), connected to the clamp lamp, which now works perfectly. The Dais PSU cost les than £8, and the job above took all of ten minutes. The olny downside is the I now have a faulty PSU PCB which will be scrapped, because it's just not cost-effective to find and fix the fault! (BTW the Dais DA-J60 is rated 20-60W/VA so should power a 35-36 Watt Halogen Bulb for years to come)