What is wrong with this freaking rectifier

[Noah]

I am powering these geckos off a home built power supply using the full wave bridge rectifier method. I have checked my connections again and again, but all I can get this thing to do is blow fuses and reset the breakers on my house. I am thinking there is a short somewhere in the rectifier, but it just does not make since that a rectifier could short out. I am applying 18 VAC at the two blank terminals of the 400 PIV 50 Amp rectifier. Right before the breaker or fuse blows the geckos display their fault light so they seem to be working. What else could be wrong.

[Noah]

pic 2

[Al_The_Man]

First, connect the transformer with no bridge rectifier connected, and power up, if ok connect the bridge with no output connected on the +- terminals and try again, then connect the caps only and see if that is ok.
Also make sure you do not have any part of the transformer secondary at ground potential. If you have a shorted rect. bridge it will blow the fuse/breaker with no output connected (+-).
Al

[hddjohns]

Can you post a schematic showing how the transformer primary is wired from the house AC outlet and how you wired the secondary to the bridge rectifier.

To help isolate the problem your may want to disconnect the secondary from the bridge rectifier and see if the house breaker still blows. If it does, then you may have a possible short some where in the AC side.

Check to see if the filter caps are reversed (+- swapped) this could cause a series secondary fuse to blow.

Do make sure that really have the transformer wired as per its spec sheet and no primary - secondary shorts or mis-wires.

Thats my $ worth,

Harley

[Noah]

Well you see thats kinda the interesting part. It won't blow with nothing connected to it. I already tried that, however, when I tried to measure the output on my scope which has 10 Megaohm connectors it blew its fuse as soon as I connected the leads.

[Noah]

Ok, I really don't understand all that about primary and secondary windings. Here is my transformer. I guess I don't see how I could have connnected it wrong since it is a separate unit. I just connected the two leads from an EIC power cord to the rectifier.

[hddjohns]

Can't use a variac as a power supply! period! no AC isolation!

The BIG problem using a variac is that one side of the 115VAC power circuit is connected to earth ground at the AC wall plug (that's one leg of the AC plug and the ground pin). And, if you connect the negitive output of the bridge rectifier to earth ground, then you got a ground loop short or ground fault.

And you don't want to get electrocuted!

You really need an isolarion transformer 115VAC input and 15 - 20 VAC output. A much safer method of building a power supply.

To figure the power output voltage, use this formula, secondary AC voltage * 1.4 = DC outout voltage after rectification. Example
15 * 1.4 = 21 VDC (if you know the secondary voltage) or
if you want a certain DC output then, just invert the voltage formula 18 VDC / 1.4 = 12.8 VAC

Harley

[Mariss Freimanis]

DO NOT USE A VARIAC!!!! You will kill yourself. I'd hate to see that happen to a fellow Junior Mint lover.

Mariss

[Noah]

I just found the culprit. It was the bridge rectifier. the new one is working great. I just checked it on the scope.

But you say the Variac transformer is a no-no. Is this really a big problem? I guess what I am trying to ask is, wouldn't this short circuit anyway if you touched the negative terminal to earth ground on a normal AC isolation transformer?

Are you guys really serious!?! I mean, is there any other way. I spent $120 on that transformer not to mention the plate I already designed.

[Noah]

Actually, after reading what you guys wrote again, I think I understand, but why would that negative terminal ever even get close to earth ground? I mean, once completed, the only power in will be from the transformer and the only power out will be DC for the servos, right?

What you are saying is that if I brushed up against an active lead while touching anything grounded I would get shocked. Is that right?

[davesaudio]

a variac is an autotransformer
there is NO isolation between the primary and secondary

[Ferenczyg]

In other words, the secondary of a autotransformer is only a fraction of the turns of the primary. That is the reason you can close a loop to ground if you tie the (-) of your bridge to the ground of your variac. Maybe with a picture..