7812 voltage regulator, need help?

[gary350]

What is the correct way to hook up a 7812 voltage regulator.

First 2 drawing are online instructions. 1 drawing shows .33 uf cap another shows .47 uf cap. Tried them both 50 vdc rating.

Third drawing is how I have it connected.

I wired it as shown, turn on the DC power supply and the meter says I have 12 VDC on pin 3. It works.

With NO load, after 45 to 60 seconds they over heat and explode.

First 7812 blew the new 50 volt electrolitic capacitor and killed the 7812.

Second 7812 was testing good 12 VDC when it over heated and exploded into brown smoke.

Third 7812, too hot to tough after 30 seconds. I turned it off before it blew.

Caps all test good. I changed the caps the 7812 show 12 vdc each time but the darn thing over heats with NO LOAD.

Meter reads 12 volts from ground to pin 3 and 3 volts from pin 3 to +15. It seems to work, I dont know why they over heat with no load?

**broken link removed**

 

[tcmtech]

More cheap junk parts from China?

 

[Rich D.]

Sounds like a tricky one. It appears you know what you are doing...at least as much as I know what I am doing. When I get really odd mysteries like this one it usually comes down to 1) a lot of cussing and 2) in the end, some obvious assumption I was making turned out to be wrong. I usually think the world and the laws of physics are out to get me but in the end it is always something I never expected or considered. Given that...

Are you sure the 15 DC supply is really DC??
Are the regulators really marked 7812 or did the supplier/package just say they were.
Is your meter working / good batteries / set to the right range?
Capacitor polarity correct...are you absolutely sure about the polarity markings?

You get the idea. My advice is to start looking at the things that seem to obvious to check, because that seems to solve all the real head-scratchers.

-RD

 

[KeepItSimpleStupid]

Are the capacitors close to the device. Are they installed correctly.

Would they happen to be Tantalum caps. Typically tantalum caps don't like high ripple.

What's the ripple on the 15V supply? If you don;t have a scope, try to measure the AC voltage on the output of the 15V supply.

 

[rumpfy]

These ICs can sometimes oscillate. It sounds to me like this is the problem. This is due to poor layout as, for example, if you have the thing on a breadboard layout. I note you use capacitors at the in and out points but are these electrolytic types or are they ceramic types. Ideally you need some good high frequency types as bypass capacitors at the in and out points.
My guess is you do not have an oscilloscope. With a CRO you could easily see if the circuit is oscillating.

 

[audioguru]

The REAL SP7812 is made by Sipex Corp (I never heard of them) who are in California. Maybe they import cheap Chinese junk or maybe you got a fake one.
Try a Name Brand (Texas Instruments) one.

I agree that the 0.33uF input capacitor and the 0.1uF output capacitor should be ceramic as close to the pins including the ground pin as is possible.

 

[chemelec]

blew the new 50 volt electrolitic capacitor

Are you Sure you had the Correct Polarity?
And are you sure it is a DC Supply?
(Not AC)

If So, it must have been a Bad Cap before you started.

 

[gary350]

I have been getting crazy voltage readings with my meter of the power supply. AC from the transformer is 10 vac. Voltage after bridge rectivier and 3300 uf 50v electrolitic cap is 15 vdc but I also get a meter reading on AC that makes no sence. I get an AC and DC power supply reading with all 3 of my meters. A few minutes ago I am retesting things wishing I had a scope when the 3300 uf 50 volt cap went up in smoke killing the bridge rectifier. I replaced that cap with a 470 uf 200 vdc electrolitic cap and bridge rectifier, now I get only DC voltage readings. I am getting no more crazy AC meter readings on the DC power supply.

The 7812 voltage regulator meter reading is 12vdc but it is getting pretty hot. It warmed up to 120 degrees in 30 seconds. There is no load on the 7812 regulator. It must be pulling a lot of current to warm up that quick. I need to check no load current.

Does the 7812 voltage regulator need a current limiting resistor?

I have a lot of 7805 voltage regulators, I am going experement with them and save my 7812 until I figure this out.

 

[crutschow]

Sounds to me like the device is oscillating.
You need the decoupling caps to go directly from the input and output pins to the ground pins.

 

[gary350]

I got it fixed. It works perfect. Ground to voltage regulator + 12 vdc. Ground to power supply + 15 vdc.

As it turned out the third 7812 was bad too. The mystery is, was the very first 7812 bad or did the bad power supply cap kill it? OR did a bad 7812 damage the power supply cap? OR did the bad power supply electrolitic capacitor kill the all 3 of the 7812 voltage regulators?

I used a 3 prong TV plug to build a tester for all my 7800 series voltage regulators. I plugged all my 7805, 7809 and 7812 into the tester 1 by 1 they all test good except for the one that over heated on the project on the third test. Who says TV parts don't come in handy.