Good sources for old transformers?

[sweesiong78]

Where/which type of appliances should I like out for in order to salvage a 24-30v AC transformer, if I am looking to build a linear bench power supply?

[mazurov]

Look into old linear bench power supplies. You may be able to fix it, too; in my experience, transformers usually survive.

Older (heavy) wall warts also contain low-frequency transformers. A 24V AC wall wart can be had in the Home Depot in the sprinkler section.

If you are close to Boulder, CO, check JB Saunders.

[sweesiong78]

thanks mazurov, how about amps for computer woofers or audio speakers? I know most of the audio stuff is 70V so it would require another transformer for 120V/70V?

[HooRide]

There are plenty of places in the Silicon Valley but I assume you're not located here since you're asking.

[Teledog]

Ham swaps,
I used to sell boat anchors for pennies on the pound. (haggle with the sellers, they don't want to haul the weight back home!) ;-)
Old discarded 1960-1970's tube televisions are a good score..just have to figure out the taps..
If you want 12VDC or less, an old computer power supply is fine.
Google ATX PSU

[Kjelt]

Look inside old "disco" halogen lamps, the ones which project images that spin etc. (Snowflakes, diamonds etc.) their halogen bulbs usually operate @ 24V  200+ Watts. Usually not the best transformers (made in china tinplate quality) but still made to last several thousands of hours.
Also take a look at your local iron scrapyard, they take the transformers out of all the equipment they get for the copper, some of those companies are smart and leave the transformers intact for sale. I often see some very nice heavy toroidal transformers there.

[mzzj]

Overhead projectors have quite often a 200-500 watt  24v transformer. Some of the better units have really nice toroidal transformers!

[sweesiong78]

I m thinking more along the lines of 5A 30v supply , so it would it be necessary to have a multitap transformer? want to keep it a linear PSU

[tautech]

I m thinking more along the lines of 5A 30v supply , so it would it be necessary to have a multitap transformer? want to keep it a linear PSU

I built mine to exactly those desired specs using 2 old pre-SMPS 12 VAC halogen transformers in series.
Switched the secondaries for series or parallel operation for lower heatsink requirements at low voltages.
But check the switching carefully for parallel operation so that the secondaries are "in phase".
Voltmeter should indicate 0 V if you have it correct.
Use a DPDT switch for that option if needed.
I used a LM338 for 1.2-32 V 5 Amp output.

Edit
Ask your local electricians if they are doing any refits for old 12 VAC halogen transformers.

[paul18fr]

I've the same approach 
 from what I know, cooker hoods have a 12V transformer ; personally I'm foccusing on household electrical appliances ...
other interesting (but rare) device: an inverter for PC's

[N2IXK]

24VAC transformers are commonly used for HVAC control systems, to provide power to thermostats, gas valves, etc.

[sweesiong78]

I built mine to exactly those desired specs using 2 old pre-SMPS 12 VAC halogen transformers in series.
Switched the secondaries for series or parallel operation for lower heatsink requirements at low voltages.
But check the switching carefully for parallel operation so that the secondaries are "in phase".
Voltmeter should indicate 0 V if you have it correct.
Use a DPDT switch for that option if needed.
I used a LM338 for 1.2-32 V 5 Amp output.

These halogen transformers would not be the 'electronic' type right?
http://www.ebay.com/itm/OSRAM-halogen-electronic-transformer-halogen-35W-105W-12V-lamp-220-240V-/171477213848?pt=US_Light_Bulbs&hash=item27ecd6a298

Could I just cut open these electronic transformers and use the coils inside?

I was looking at the HVAC transformer too like these:
http://www.homedepot.com/p/Packard-20VA-120-20-240-Volt-24-Volt-Secondary-2-ft-Mount-Transformer-PF42420/203565056?MERCH=RV-_-RV_gm_pip_rr-4-_-NA-_-203565056-_-N#specifications

but it doesnt look like it has multiple taps, then again its only 20VA, not sure what the typical VA ratings are for standard 5A 30V PSU's?

[Seekonk]

I get old home computer UPS inverters from the town dump recycling center.  The ones with 12V battery have 24V center tapped at about 10-20A depending on wattage.  I've used them for battery chargers and buck boost transformers.  I have a small 400W ONAN gas generator that won't run a vacuum or miter saw.  I used one of these as a buck transformer to drop the voltage down to 90V  and I can run both now.

[SeanB]

If it says electronic transformer, and you can lift it in one hand it is not suitable. You want something heavy, with steel laminations and copper windings, where shipping it requires a box and not plastic bag wrapping.

Reusing inverter transformers requires care, as most of these are not designed for long term use, only being rated thermally for running 5 minutes every once in a while. They might be usable with care provided you derate them considerably.

[tautech]

I built mine to exactly those desired specs using 2 old pre-SMPS 12 VAC halogen transformers in series.
Switched the secondaries for series or parallel operation for lower heatsink requirements at low voltages.
But check the switching carefully for parallel operation so that the secondaries are "in phase".
Voltmeter should indicate 0 V if you have it correct.
Use a DPDT switch for that option if needed.
I used a LM338 for 1.2-32 V 5 Amp output.

Could I just cut open these electronic transformers and use the coils inside?

No, they must be a standard transformer not SMPS as the small transformers inside them are designed for use at ~50 KHz not mains frequencies(50 or 60 Hz)

but it doesnt look like it has multiple taps, then again its only 20VA, not sure what the typical VA ratings are for standard 5A 30V PSU's?

V x A = W therefore 150 Watts output for 30 V 5 A.
For a good conservative design ensure transformers are capable of 200 W.

The 20 VA transformer you linked will provide less that 1 amp @ 30 V.
Look for say 40 or 50 VA and use a LM317 for ~ 30 V 1.5 A that should suit most bench use.

Get the datasheets for 317 & 338 and have a study.

[Paul Moir]

Pallet jack chargers often have a big just over 24v.  They're often built into the battery packs so generally they're tossed when the battery is replaced.  Any forklift company should be tossing them out regularly.  (Some are SMPS but they're easy to identify because they're light.)  And they're not much use to anyone as a charger because they're 24v.  Typically 15-20A though, so a little bit big.

[sweesiong78]

Get the datasheets for 317 & 338 and have a study.

Yup I actually plan to use this design (fig. 22) in the ON semi datasheet for 317 (seems like a 25V 1.5A) design

http://www.onsemi.com/pub_link/Collateral/LM317-D.PDF

Only thing is this design could be dumping out 20W of waste heat if I run it at say, 5V and 1A, I plan to run a PC fan inside it though and perhaps attach the 317's to the case.

[tautech]

Get the datasheets for 317 & 338 and have a study.

Yup I actually plan to use this design (fig. 22) in the ON semi datasheet for 317 (seems like a 25V 1.5A) design

http://www.onsemi.com/pub_link/Collateral/LM317-D.PDF

Only thing is this design could be dumping out 20W of waste heat if I run it at say, 5V and 1A, I plan to run a PC fan inside it though and perhaps attach the 317's to the case.

I chose not to use a fan, just 2 big external heatsinks instead. 1 for the bridge and the other for the LM338 reg. As it is something I rarely move, size & weight are not an issue.

Maybe you could use an extruded finned case to enclose the PSU.
That will serve as a good heatsink, 20 W is no problem if your case is the heatsink.
Select one large enough to accommodate the switches, meters, knobs and output terminals.
A vented case might also be a good idea to keep the E-caps temp low.
Resist mounting them close to any heat source.
 

[Kjelt]

Only thing is this design could be dumping out 20W of waste heat if I run it at say, 5V and 1A

Yup in some power supply designs that is why they "dim" the mains side of the transformer with a triac. So the ouput voltage is lower and the dissipation is lower.
That means an extra control loop. If you have more taps on the secondary side you could also do it with lets say multiple relais, but you need good hysteresis or your PS will keep on switching in some Vout position.