How to go about testing a transformer?

[Illum]

I have several AC-AC transformers stored in a crate, some have multiple secondaries but I found one that has only a single secondary. input is two wires: black and white, no red...so its 120VAC, output is two yellow wires, which I presumedly as AC

Since these transformers by itself has no regulation, taking readings for OCV will vary alot than the spec. whats a way to "load" them down for a test? with DC, I built a small load using a series of high current diodes mounted on heatsinks, with contact pins per 5 in a string, assuming a fan is on nearby blowing at the heatsink I can test DC using 3V, 6V, 9V, and 12V loads...but using diodes to test AC sounds a bit scary, so should I use a rectifier bridge and a series of resistors? I have access to 17 ohm 150W power resistors, so heat won't be an issue

I figured since using line voltage it would be difficult to run LEDs but certain LED drivers have a better time utilizing stepped down AC, preferably in the 12VAC region

 

[mattheww50]

You usually have to substantially overload the secondary on a transformer to get significant voltage drop. The induced voltage is independent of load as long as the core doesn't saturate (and bad things happen when the core saturates), so the only loss in output voltage is from the resistance of the winding.

By design that is usually small compared to the intended load resistance unless you are talking about small signal transformers. For small signal transformers you want maximum power transfer, and then the resistance of the winding needs to equal the resistance of the load. This is impractical for power transformers since the transformer would need to dissipate 1 watt for every 1 VA of power delivered. That makes the transformer a heating element, rather than a transformer!

The far more interesting question is what the open circuit voltage on the yellow leads is.
Transformers are built for all sorts of applications.

You can estimate the power handling capacity on the transformer from the weight as long it doesn't use an exotic magnetic material for the core (like a Variac® tape wound core). Figure about 30 pounds to the Kilowatt.

 

[Dave_H]

I have several AC-AC transformers stored in a crate, some have multiple secondaries but I found one that has only a single secondary. input is two wires: black and white, no red...so its 120VAC, output is two yellow wires, which I presumedly as AC

Since these transformers by itself has no regulation, taking readings for OCV will vary alot than the spec. whats a way to "load" them down for a test? with DC, I built a small load using a series of high current diodes mounted on heatsinks, with contact pins per 5 in a string, assuming a fan is on nearby blowing at the heatsink I can test DC using 3V, 6V, 9V, and 12V loads...but using diodes to test AC sounds a bit scary, so should I use a rectifier bridge and a series of resistors? I have access to 17 ohm 150W power resistors, so heat won't be an issue

I figured since using line voltage it would be difficult to run LEDs but certain LED drivers have a better time utilizing stepped down AC, preferably in the 12VAC region

First, how big are these and what sort of power do you expect
to get out of them?

I'd start by measuring the AC voltage across the secondary
with no load. If it is within the range you expect, attach a load
that (if something goes wrong) is cheap and expendable;
a resistor or lamp (forget the strings of diodes, not a good
test load).

First, check the primary and secondary to make sure neither
is shorted. What exactly constitutes a short varies, especially
for a high-current secondary. I can try a few transformers
I have around here to give a range of what's "normal". For
a step-down transformer, expect the primary to have higher
resistance, due to more turns than the primary and usually
smaller gauge wire (lower current).

The open-circuit voltage will be "substantially" higher than the
voltage at rated load current (I disagree with the above comment).

Also remember that if you full-wave rectify and filter, the dc
voltage will go up about 40%. A transformer with 12vac RMS at
rated load will create about 17vdc. Take this into account with
your load (if you attach a 12v lamp, it will be overdriven).

Dave

 

[Illum]

First, how big are these and what sort of power do you expect
to get out of them?

theres how the little cube looks beside my E1e. My cameras charging, so the webcam will do

I plan to load it at 1A max as that is the highest my bridge rectifier will go is rated 1.5A.

I'd start by measuring the AC voltage across the secondary with no load. If it is within the range you expect, attach a load that (if something goes wrong) is cheap and expendable; a resistor or lamp (forget the strings of diodes, not a good test load).

lamp filaments resistance will very according to heat, would a 120V 4W night light bulb be something good to try out with?

First, check the primary and secondary to make sure neither is shorted. What exactly constitutes a short varies, especially for a high-current secondary. I can try a few transformers I have around here to give a range of what's "normal". For a step-down transformer, expect the primary to have higher resistance, due to more turns than the primary and usually smaller gauge wire (lower current).

when you speak of resistance, do you mean the resistance in the independent windings? or am I checking for resistance by tying a leg from the primary and connect the leg from the secondary? Do I look for it in Ω or mΩ?

The open-circuit voltage will be "substantially" higher than the voltage at rated load current (I disagree with the above comment).

Also remember that if you full-wave rectify and filter, the dc voltage will go up about 40%. A transformer with 12vac RMS at rated load will create about 17vdc. Take this into account with your load (if you attach a 12v lamp, it will be overdriven).

My mistake, I was thinking of unregulated AC-DC power supplies.
So once powered if the secondary measures ~9VAC I should expect to see 13VDC on my bridge without load? got it, :thanks:

I plan to test and power it on sometime tomorrow after work, I wasn't able to get to it this weekend

 

[Dave_H]

I would tend to rate the rectifier bridge with more margin,
for a 1 A continuous load, say use a 3A-6A bridge. They should
not be expensive or hard to find.

Winding resistance is through either the primary or secondary,
but you could also check for primary-secondary shorts, which
could be quite catastrophic. The winding resistances for a
transformer this size will be in the ohms, typically use the
200 ohms range (or lower if your meter has one, like 20 ohms).

If you put a capacitor on the output of the bridge, you should
get about 13vdc as noted, and of course if would vary with load.
With no capacitor you'd get full-wave rectified AC so would
read different on the meter (and would create flicker). Also,
give the capacitor a good margin, well above the no-load
voltage (say 20v for what you describe).

For load, why not try a small automotive bulb like the low
filament of a #1157 or something similar (high would be too
much load). The 115v 4W bulb resistance will be too high and
not load very much.

Last, the comment I was disagreeing with was the other
guy's, sorry I was vague.

Dave

Update: I measured resistances on a good-quality (Hammond)
25v 0.5A transformer, which is wired as a centre-tapped FWR
(equivalent to a 12.5v 1A transformer). Primary resistance is
about 41 ohms. Secondary resistance is about 3.5 ohms
across both windings. Your transformer secondary is probably
around half that value.

 

[Illum]

managed to get to it today
Primary had about 57 ohms and secondary has about 0.57 ohms, no shorts were found between the two coils. A brief jolt of 120VAC gave me 12.3VAC

 

[Dave_H]

managed to get to it today
Primary had about 57 ohms and secondary has about 0.57 ohms, no shorts were found between the two coils. A brief jolt of 120VAC gave me 12.3VAC

Great...looks like you're moving ahead on this one. BTW was the
12.3vac with or without loading?

By the look of the transformer you should be good for 2A (maybe
3A) load current. You can hook up two or three of those 17 -ohm
loads, check the voltage drop, should not be more than 10-15%.
Also run it for a while and check that the transfomer iron doesn't
get uncomfortably hot. I like the lamps as loads, as they are visual...

Dave

 

[Illum]

12.3V without load, I'm guessing it'll be just below 12V under load:thumbsup:

I found a 1.5A bridge rated at 100PIV kicking around in my junk box, will it work on the secondary or do I need a 400PIV one? Being in the position of a power supply that will not be used to charge a battery, I see no possibility for the diodes to be reverse biased. PIV is supposed to vary with temperature so...