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bad transformer?

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james211

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I'm working on a project at home that uses a 25.2V Center tap 2.0A transformer. Prior to plugging everything in I used my multimeter to check the connections to make sure there were no shorts. I immediately found a short when I touched the meter (in continuity mode) to the two prongs on the household plug. I traced it back to what I think might be the transformer. The transformer has two black wires on the primary side and three (2 yellow, 1 black) on the secondary side. When I put the meter on the two black wires in continuity mode the buzzer goes off. I assume that is not normal, correct?
 
It's perfect;y normal. For DC, the transformer looks like a long wire and has a low resistance.

For AC, it will have a Reactance.

This low resistance also causes the large surge currents when a transformer is connected to the AC line.

Fuse it properly and you'll be fine. It's about 50 W, so if your mains are 120 use a fuse ~ 1.5 * 50 W or 75 W on the primary side or about 1.5 * the max load expected. P=VI and therefore I (fuse) ~= P/V or I <= 50 W/120 V. 120 can be 120, 230 or 240 etc.
The Primary voltage,

The 1.5x rule of thumb usually works for mostly resistive loads.
 
Yes, per KISS that is normal. I have a similar transformer sitting here. The primary side DC resistance is about 6 Ohms which when measured with my Fluke 87 in continuity mode (beep on) the meter will beep. All normal.

Ron
 
Ok, then explain this to me, and maybe this is just my bad luck. From what I understand, the two primaries connect directly to the power supply cord correct? One black primary to black supply, other black primary to white supply, correct? Or no?
 
From what I understand, the two primaries connect directly to the power supply cord correct? One black primary to black supply, other black primary to white supply, correct?

More information needed here.

What is your mains supply voltage? 120 volts as per USA or 240 volts as per most other places ?

The transformer, it has two primaries? ie a total of four connections for the primary?
A link to a datasheet or a good quality photograph would be helpfull at this point.

JimB
 
Ok, then explain this to me, and maybe this is just my bad luck. From what I understand, the two primaries connect directly to the power supply cord correct? One black primary to black supply, other black primary to white supply, correct? Or no?

That would be correct. I don't now your location but in the US the US mains household wiring for 120 VAC is Black (hot) and White (neutral) with green, when used, being ground. Transformers with leads extending typically use black (normally labeled) on the primary side.

Ron

Morning Jim who beat me by a min. :)
 
Morning Jim who beat me by a min.
As they say:
"great minds think alike"
But they also say:
"fools seldom differ"

I am not too sure which category I come in to.:D

JimB
 
As they say:
"great minds think alike"
But they also say:
"fools seldom differ"

I am not too sure which category I come in to.:D

JimB

I would say the Great Minds. :)

Back to transformers below are a few images of the transformer I mentioned earlier. Note the primary in this case is Black and the 24 VAC CT (Center Tapped) happen to be green with a Yellow CT.

Transformer Primary
Tran1.png


Transformer Secondary
Tran2.png


Schematic Representation
Stancor1.gif


Pins 1&2 are the Primary (Black) and pins 3&5 are the Secondary (Green) with pin 4 (Yellow) being the CT of the secondary.

That about sums it up. :)

Ron
 
Nice pics Ron.

A possible misundersatnding on my part here.
The OP said "the two primaries connect directly to...", which I interpreted as the being two primary windings.
What he probably meant was "the two wires which connect to the primary winding".

Appologies if I have confused the issue.

It would still be usefull if the OP could give more info about his transformer (if it is not very similar to the one shown by Ron).

JimB
 
Nice pics Ron.

A possible misundersatnding on my part here.
The OP said "the two primaries connect directly to...", which I interpreted as the being two primary windings.
What he probably meant was "the two wires which connect to the primary winding".

Appologies if I have confused the issue.

It would still be usefull if the OP could give more info about his transformer (if it is not very similar to the one shown by Ron).

JimB

You aren't alone Jim. My initial take was a dual primary, then as I thought about it I assumed (hopefully correctly) two primary wires. :)

Ron
 
Read this: **broken link removed**

And please don't try anything before doing thorough research first. Something could go haywire and you end up in hospital. (worst case scenario)
 
You can very approximately tell the capacity of a transformer by its dc resistance, low dc resistance is necessary to get good regulation, see the esp page linked to above.
 
It's perfect;y normal. For DC, the transformer looks like a long wire and has a low resistance.

For AC, it will have a Reactance.

This low resistance also causes the large surge currents when a transformer is connected to the AC line.
I don't think so.
The inductance of the transformer slows its current rise when it is powered. It is huge filter capacitors following a rectifier that causes a large surge current when they first charge.
 
I don't think so.
The inductance of the transformer slows its current rise when it is powered. It is huge filter capacitors following a rectifier that causes a large surge current when they first charge.
Certainly the capacitors can cause a large surge. But, independent of that, the first few input cycle's surge current can be very high, depending upon when in the waveform period the primary power is applied. See this article.
 
Certainly the capacitors can cause a large surge. But, independent of that, the first few input cycle's surge current can be very high, depending upon when in the waveform period the primary power is applied. See this article.
Thank-you, Zapper. I have only seen a transformer that is rectified and drives a huge high inrush current capacitor.
 
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