power regulator smokin rectifier!

Discussion in 'General Electronics Chat' started by pastinsain, Sep 9, 2012.

Aug 25, 2012
160
2
Just bought this ebay power regulator and connected 17vac to the ac input everthing ok then when I connected the ground from the supply
the rectifier started smokin. I used a variac for power supply.
When I disconnected the ground, works good.

Why would connecting the ground cook the rectifier ???

2. R!f@@ AAC Fanatic!

Apr 2, 2009
9,616
1,082
Don't do tht dummy !..

By the way....where and how are u connecting the ground ?

Aug 25, 2012
160
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First of all don't call me a dummy , Its amazing how brave and bold people like you get on these forums. YOU WOULDNT SAY THAT TO MY FACE
"Senior Member EVIL"

Like I said in the starting post The ground from the variac was connected to the ground on the circuit

Last edited: Sep 9, 2012
4. Dodgydave AAC Fanatic!

Jun 22, 2012
6,398
1,014
When you connect the ground, your shorting out one of the diodes in the bridge.Use an isolation transformer for the supply.

PackratKing likes this.

Aug 25, 2012
160
2
Thank you however, what do you connect the ground to? Is not a ground
a ground? Its true that an isolation transformer will eliminate the problem
but what do you connect this ground to?

6. BMorse AAC Fanatic!

Sep 26, 2009
2,675
237
What do you mean ground from the supply? If it is an AC source, both lines should be connected to the AC inputs (1 and 2), you should not connect the AC ground to the DC ground!

Aug 25, 2012
160
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The ground on the variac. like all 120vac supplies you and a hot, neutral and "ground" wire.

Unless you use a isolation transformer .

Apr 5, 2008
16,954
2,957
Hello,

As you might know, an isolation transformer is a MUST for safety.
If you do not use an isolation transformer, you are violating the ToS of this site and the thread will be closed.

Bertus

9. BMorse AAC Fanatic!

Sep 26, 2009
2,675
237

AC ground wire does not connect to the DC ground, the "earth" ground can be connected to the power supply chassis (metal frame) in either case for safety.

For circuitry in situations where significant Earth ground currents can flow (like on electric-powered Subway trains), you will want to isolate all logic grounds from Earth/chassis grounds, and maybe only connect the two occasionally in the devices with a charge bleeder resistor like 1Meg in parallel with a small high-frequency cap like 1000pF.

PackratKing likes this.

Aug 25, 2012
160
2
Im am using an isolation transformer .

THE QUESTION IS, WHAT IS THE GROUND ON THE CIRCUIT USED FOR?

It obviously cannot be connected to the common on the variac.

For example the circuit ground is not an earth ground. why?

Aug 25, 2012
160
2
Thank you Bmoris, that was the answer i was looking for.
""
For circuitry in situations where significant Earth ground currents can flow (like on electric-powered Subway trains), you will want to isolate all logic grounds from Earth/chassis grounds, and maybe only connect the two occasionally in the devices with a charge bleeder resistor like 1Meg in parallel with a small high-frequency cap like 1000pF
""

Sometimes it takes a little longer to get the answer

Aug 25, 2012
160
2
Then Is it ok to connect the oscilloscope probe ground to an isolated ground?

Like the one in this curcuit. because the oscope has a 1meg resistor to protect it?

If I connected the Oscilloscoe probe ground to earth ground will
that damage the oscope?

13. ErnieM AAC Fanatic!

Apr 24, 2011
7,906
1,789
I suspect you are still powering this circuit with a variac. Those devices are NOT isolated so the AC line neutral is one of your bridge rectifier inputs.

The ground on a typical scope is also the AC line ground, so if you connect those things together you'll smoke another rectifier.

The solution is to go out and buy a proper power transformer to get full isolation from the AC line.

(We really should make this thread a sticky to show WHY we insist on no directly connected to the AC line circuitry.)

Aug 25, 2012
160
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yes I noticed when I connected the oscope ground, to the ground on the circuit it loaded and pulled amps so much that my variac started to hum!

So I connected an isolation transformer to the variac then circuit so I can do this .

1) isolation transformer > variac > circuit
2) oscilloscope non isolated earth ground "the way Tektronix recommends"

I need a fuller understanding of the types of "grounds " isolated vs earth
dc grounds vs ac grounds . and the relationship to oscilloscopes ground.
I have to remimber the oscope ground is a direct ac earth type.

thanks for the info

15. ErnieM AAC Fanatic!

Apr 24, 2011
7,906
1,789
The (earth) ground (third pin on the plug) and neutral (LARGER of the 2 blades) (when all wired correctly) are connected, usually back at the breaker panel. Neutral is thus close to earth ground in potential, only different by the drop from current is the wire.

Last edited: Sep 10, 2012
16. JMac3108 Active Member

Aug 16, 2010
349
66
Pastisain,

Let me try to answer what I think you're asking. Feel free to ask for clarification if I don't get at what you're really asking.

The ground on your variac is a safety ground. It eventually connects to the AC neutral at some point in your house wiring. This is why connecting it to you circuit "ground" causes a problem. As someone else pointed out, you are shorting out a diode in your bridge.

The "ground" in your circuit is a "common" point to reference voltages to. You can not connect it to the AC safety ground for the reasons mentioned above.

In many designs there is further isolation. For example, the rectified DC voltage fron the bridge might feed an isolated flyback regulator. A flyback is a switching power converter with a transformer for isolation. In this case the "ground" on the output side of the flyback converter circuit is completely isolated from AC mains and it is OK to connect the AC safety ground it.

As you can see, the concept of ground can be confusing. Hope this helps.

Aug 25, 2012
160
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thank you and very well explained.

18. crutschow Expert

Mar 14, 2008
16,163
4,315
Just to make sure it's clear, you cannot connect the bridge output common (ground) to the common of the input to the bridge, which is what you do when you connect the output of an AC powered bridge to the AC ground. That connects two of the diodes directly across the power which makes them act as a fuse, as you observed.

19. R!f@@ AAC Fanatic!

Apr 2, 2009
9,616
1,082
Did I hurt ur feelings ? I guess u never realized tht word is not offensive but rather a more bit of a compliment tht u were bold enough to ask a bit of a stupid but rather ignored question.

Still if u felt bad, I am sorry, it was a compliment.

I believe u got the solution. The earth loop thing that most of us get confused over.

Do not get angry and do not be harsh because you will never regret being kind $image=http://forum.allaboutcircuits.com/mimetex.cgi?\bullet&hash=b6817de3f76ee35987c5e21a6b44c061$