How to Drain transformer?

[skmdmasud]

Hi
In my circuit I have a big 220V-AC to 9VDC transformer which when turned off takes time to loose all the stored energy works somethings like a big capacitor. How do i make it go instantly off when disconnected? Any simple idea..

 

[ronsimpson]

"instantly" what does that mean to you. seconds, mS, uS, nS, pS? "0 seconds"?

 

[audioguru]

Why do people from a foreign country call an AC/DC power supply a "transformer"? A transformer does not have a DC output.

Why doesn't he know that it has a filter capacitor inside that "takes time to loose (spelling error for lose) all the stored energy" as it discharges.

A circuit could be added that quickly detects that the low voltage AC from the transformer has stopped.

 

[skmdmasud]

audioguru
Why do people from a foreign country call an AC/DC power supply a "transformer"? A transformer does not have a DC output.

lol dont know cuz i am not a pro.... to be more accurate its a AC/DC power supply.
Actually I do know that it has diodes and capacitor inside the power supply to convert the ac to dc.

I have also used a 6V voltage regulator (LM7806) to get a final 6V dc. Can i do something simple to the voltage regulator so that when the input of the regulator goes below 6V lets say 5.5V it will cut the output.

ronsimpson
"instantly" what does that mean to you. seconds, mS, uS, nS, pS? "0 seconds"?

lets say 500ms is good enough.

 

[audioguru]

When the power to an AC/DC power supply is cut, the low voltage AC from the transformer stops immediately. Detect its loss.

 

[skmdmasud]

When the power to an AC/DC power supply is cut, the low voltage AC from the transformer stops immediately. Detect its loss.

doesn't the low voltage(secondary coil) stores some charge? If its because of the filter capacitor inside the adopter then i will just remove it since i have a big capacitor after my voltage regulator.

 

[mvs sarma]

for 9V secondary adopters, place a 10K resistor across the electrolytic capacitor just after the rectifer9 full wave, or bridge. then on, after you remove the adopter from mains, the capacitor discharges thro' this resistor and the Led would be OFF much earlier .

 

[skmdmasud]

for 9V secondary adopters, place a 10K resistor across the electrolytic capacitor just after the rectifer9 full wave, or bridge. then on, after you remove the adopter from mains, the capacitor discharges thro' this resistor and the Led would be OFF much earlier .

Hello mvs Sarma. if i connect a resistor across the capacitor (2 legs of the capacitor) wont the resistor heat up or burn out?? my adopter supplies 350ma so wont the resistor eat up lots of ma?

 

[mvs sarma]

if the sec of the transformer is say 9V,
the rectified DC after the capacitor filter would be 1.414 times the (V, under No load.
once we load it would come down.
say 12V DC,
i suggested 10K
the current would be 1.2mA =12V/10000
now wattage would be I^2 R = (1.44/1000000)*10000 =.0144watts

works out to less than few mWatts
So, don't fear
connect a 1/4 watt 10K across the capacitor.

 

[skmdmasud]

if the sec of the transformer is say 9V,
the rectified DC after the capacitor filter would be 1.414 times the (V, under No load.
once we load it would come down.
say 12V DC,
i suggested 10K
the current would be 1.2mA =12V/10000
now wattage would be I^2 R = (1.44/1000000)*10000 =.0144watts

works out to less than few mWatts
So, don't fear
connect a 1/4 watt 10K across the capacitor.

Great nice maths... i will try it out. Thanks.

 

[audioguru]

doesn't the low voltage(secondary coil) stores some charge? If its because of the filter capacitor inside the adopter then i will just remove it since i have a big capacitor after my voltage regulator.

If you remove the filter capacitor that smooths the rectifier pulses then it feeds the regulator then the regulator WILL NOT WORK!
The regulator must have an input voltage that is at least 2V higher than its output AT ALL TIMES. The capacitor at the output of the regulator does not smooth anything, instead it stops the regulator from oscillating.

Great nice maths...

It is called OHM'S LAW. Its very simple rules are the basics of electronics and you should learn them.

 

[gary350]

Why do people from a foreign country call an AC/DC power supply a "transformer"? A transformer does not have a DC output.

Why doesn't he know that it has a filter capacitor inside that "takes time to loose (spelling error for lose) all the stored energy" as it discharges.

A circuit could be added that quickly detects that the low voltage AC from the transformer has stopped.

Why does everyone call a transformer power supply, ADAPTORS?

Go to Radio Shack or anywhere an ask for a 120 VAC to 9VDC power supply. They look at me like I'm stupid then tell me, We don't sell power supplies. Yea right.

30 years ago it was power supply, now the same thing is abaptor. Why does the name change to mean the same thing?

My Aunt was a nurse for 40 years. She liked to tell the story about how every 5 years the names of things changed because someone higher up decided it sounds better. In 1942 if you were sick at your stomach you threw up, then a few years later it was called vomit, then puke, then whale, then barf, then up chuck, then spew, then earl, then ralph, then you lost your lunch, then something else. LOL. STUPID!!!

If you put a relay and resistor across the output of your, (Transformer, Adaptor, Power supply) it will discharge the capacitor when it is turned off. The resistor ohms determines how fast it discharges.

 

[vlad777]

**broken link removed**

Hello.
I like to think up schematics, but I may not always be right though.
Gurus please comment.

Rb should be low enough for max pnp current.
Rg should be low enough for min switch off time but high enough for max pnp current.
Gate to source capacitance will keep mosfet conducting while ac is present,
but when ac is not present it will discharge and switch off rest of the circuit.

 

[duffy]

Go to Radio Shack or anywhere an ask for a 120 VAC to 9VDC power supply. They look at me like I'm stupid then tell me, We don't sell power supplies.

This is so annoying! Ever try to find a wall-wart on the RS site? Search is useless no matter what combination of "wall-mount", "power supply", "transformer", etc. you try. Is there a heading for it? Maybe under "batteries and power" - ? Nope. You will find a couple under "Cell Phones" somewhere... a couple more under "Home and Office" somewhere... you have to search through pages and pages. You know they have them, they just don't make it so you can find them. They assume all anybody wants is a replacement for a model 600-02B or whatever - not like anybody actually BUILDS anything, right?

And half the time they don't even tell you the output voltage and current! It's this same mentality that assumes anything remotely technical is unimportant because only dorks are interested. At RADIO SHACK.

 

[audioguru]

RadioShack doesn't have a clue about electronic parts and doesn't even know who made them.
The detailed spec's at RadioShack for an electronic part is its weight.
They sell cell phones and kids toys. Their clerks are .... clerks.

 

[davenn]

Why does everyone call a transformer power supply, ADAPTORS?
Go to Radio Shack or anywhere an ask for a 120 VAC to 9VDC power supply. They look at me like I'm stupid then tell me, We don't sell power supplies. Yea right. 30 years ago it was power supply, now the same thing is adapter. Why does the name change to mean the same thing?

In Australia and New Zealand and maybe a few other places, those that plug directly into the wall outlet are called "Plugpacks"
That name will also often be used in electronics suppliers catalogs.

D

 

[audioguru]

An AC/DC adapter converts the 115VAC or 230VAC at the wall socket to a low level DC like a battery. Then a device that sometimes uses a battery adapts to using the adapter.

In Australia and New Zealand they talk "funny" because they are "down under" and can call them anything they want, MATE!
I have never called a guy a MATE because my wife is my MATE. (I have never been a sailor).

 

[davenn]

I have never called a guy a MATE because my wife is my MATE. (I have never been a sailor).

thats your loss, MATE hahaha

 

[skmdmasud]

adding a 10k across the cap did not help. actually its much simpler to take an output from the rectifier bridge and use it to cut off the output as described by vlad777. I already have a relay to change over from adopter to battery so i will just control it from the rectifier bridge.