Need help to choose hard-wired 12v transformer

[peteski]

Ah, that's better.    I found some specs for it ( https://www.edwards-signals.com/files/c-88.pdf ). Looks like it is a 24VAC CT (Center-Tap) transformer.  Does it have 3 terminals on the secondary side?  Let me know (or post a photo).

If yes, then we would have to use another type of rectifier (using 2 diodes) with the center-tap as a common.  You should be able to use unfiltered DC for you strip lights (so no filter cap needed).

[Lemosteam]

Yes, there is one terminal for 12v or another for 24v output with a common in the center.

[peteski]

[ Guests cannot view attachments ]

John,

Diagram A is a standard center-tap transformer.  The secondary winding will output 24V but if but if you use the center tap you can get 12V from either side of the windings.

I suspect that the transformer you have is a center-type style but the terminals are marked differently (like diagram B).  Measure the AC voltage between 12VAC and 24VAC. That voltage should read around 12VAC (just like the voltage measured between the COMMON and 12VAC).  Then with the transformer unplugged measure the resistance between the 3 output terminals.  There should be very little resistance between all 3 terminals.  If there is enough resistance to measure then the resistance between COMMON and 12VAC should equal the resistance between 12VAC and 24VAC. Then the resistance between COMMON and 24VAC should be double of the previous readings.

If the above all checks out then you can add 2 diodes as shown in diagram C to get 12VDC you will need for the LED lights.  We'll figure out the diode specs once we determine that this circuit will work.

[Lemosteam]

@peteski , you were right about the stud locations, in your figure B. So figure C will just flip one diode to the center with the common at the top right?

Do you think I will need any resistors due to the higher voltage output below?

Voltage on 24v stud reads 29
Voltage on 12v stud reads 14

Resistance between coommon and 12v studs is:
[ Guests cannot view attachments ]

Resistance between common and 24v studs is:
[ Guests cannot view attachments ]

Resistance between 12v and 24v studs is:
[ Guests cannot view attachments ]

I was not sure where to set the dial when measuring the resistance, hopefully I did it right in the pictures. Not sure why they have so many choices or what they are for...

Whatever we do circuit wise, it will be on the low voltage side right? 

[peteski]

Yes, all we are dealing with is the low-voltage side.  But there are some things here that bother me.  Can you take a clear photo of where the leads on the low side exit the transformer?  The power rating is also confusing.

If it is wired the way I mentioned earlier (center-tap) and it is a 100W transforemr then 100W / 24V = 4.1A  So the transformer can only supply 4.1A. Hooking up the 12V side only uses half of the secondary winding, but the available amperage will still only be 4.1A. That is not enough to safely power up your LED lights.

Your resistance readings also seem off to me. You should be using the lowest resistance range (which you are). But the resistance of 21 or 27 ohm seems way too high.  I would have expected something around 5 ohms across the 24V terminals and 2 ohms across the 12V terminals.  Over 20 ohms seems way too high.  If you short the meter leads together does the reading go down to zero ohms?

Can you measure the voltage between the 12V and 24V studs? Is is also 14VAC?  BTW, 14VAC is not unexpected.

[Lemosteam]

Yeas ago I used this to power six under cabinet halogen puck lights. I think the they used 10 or 15 watt bulbs.

I had the settings for the ohm measurement on the 200 setting. Should I have had it on the 20 setting?

[peteski]

Your multimeter has a 20 ohm range?!  On majority of multimeters the lowest range is 200 ohms.  Yes, use the lowest range you have (make sure the transformer is not powered). Also make sure that the reading goes to zero when the leads are shorted.

Then check the AC voltage between the 12 and 24 terminals while the transformer is plugged in.

[Lemosteam]

@peteski , Nope it is 20k ohm setting... 

These are the best pics I can take inside.



Not sure why the values are so different this time...

Resistance test of multimeter did not go to zero upon short:


Resistance at posts:




Voltage at posts:

[Lemosteam]

Hi @peteski , did the values above make more sense?

[peteski]

Well, yes and no.  This thing is a bit of an enigma.  The voltage readings are what I would expect from a center-tap transformer. But the winding resistance seem way off. For the type of amperage this transformer can supply I would expect the resistance to be less than one ohm. What I also find confusing is the power rating.

I'm now thinking that it is a dual-secondary (not center-tap) winding transformer.  The 12V winding probably uses thicker wire to be capable to supply 100W worth of amperage.  My current way of thinking is to just use the 12V side of the transformer and attach it to a standard bridge rectifier (I would get a bridge rated for 15 or 20 Amps to be on the safe side).  So just connect the COMMON ans 12V leads from the transformer to the AC leads of the bridge rectifier.  The positive and negative unfiltered DC can then be used for the LED lights.

Here are some inexpensive bridges:
http://www.goldmine-elec-products.com/products.asp?dept=1133
This one looks well suited for this project:
http://www.goldmine-elec-products.com/prodinfo.asp?number=G20440

If it gets hot during operation you can easily mount it to a heat-sink.

[Lemosteam]

One more question, do you think I should add some sort of resistor to keep it 12v or below?

[peteski]

One more question, do you think I should add some sort of resistor to keep it 12v or below?

I don't think so. The bridge rectifier will drop around 1.4V across the diodes. PLus the transformer voltage will drop slightly under load. Even if the LEDs get a bit over 12V, they should be able to handle that.

Unless there is some critical device needing tightly-regulated 12V supply, most 12V devices are designed to handle little over-voltage. Just like in your car: The battery itself puts out 12V, but then the car is running the alternator raises the in-car voltage to about 13.8V.

Once you get the circuit built, you can take some voltage and current readings (and also monitor the transformer and rectifier temperature for a while).

[Lemosteam]

As always, thanks @peteski , parts ordered.  I'll let you know how they go.  Probably a Christmas break project.  I also ordered a 95W resistor, just in case folks don't like how bright they are...

[peteski]

A 95W resistor? 

If the lights are too bright then simply remove parts of the LED string inside the fixture. That should work even better (and no heat will be generated).  You could actually split the light strings into multiple segments and wire them in through switches (that way you can have several brightness levels available).

[Lemosteam]

[ Guests cannot view attachments ] @peteski as always, your sage advice pulls me through, and at the same time has saved me booku dollars.  After the bridge rectifier was installed, the DC voltage output was just under 12 at 11.83. So [ Guests cannot view attachments ] I dared to plug in the lights after finding a male 5mm plug laying around...