Bad voltage regulator?

Guest
I think I have a bad 7805 (TO-220) voltage regulator, but maybe I'm
just not using it properly. If I apply 5V to the input, I get a surge
of current (warming up my battery and cutting its voltage from the
normal 6V to about 1.5).
That's with the output unconnected.
With no power applied, I find there's a 4K resistance between ground
and output (is that normal??)
 
<strnbrg59@gmail.com> wrote in message
news:28fa4783-cda8-4273-9091-474cf277231c@e10g2000prf.googlegroups.com...
I think I have a bad 7805 (TO-220) voltage regulator, but maybe I'm
just not using it properly. If I apply 5V to the input, I get a surge
of current (warming up my battery and cutting its voltage from the
normal 6V to about 1.5).
That's with the output unconnected.
With no power applied, I find there's a 4K resistance between ground
and output (is that normal??)
Click to expand...
How hot does the 7805 get?
 
"ian field" <dai.ode@ntlworld.com> wrote in
news:40TIj.34443$jH5.30586@newsfe3-win.ntli.net:

With no power applied, I find there's a 4K resistance between ground
and output (is that normal??)
Click to expand...
Probably. I get 4.33 K on a known good 7805.
 
On Apr 2, 2:37 pm, "ian field" <dai....@ntlworld.com> wrote:
How hot does the 7805 get?
Click to expand...
Hot enough to give off an odor of burning plastic, and way too hot to
touch.
 
strnbrg59@gmail.com wrote:
I think I have a bad 7805 (TO-220) voltage regulator, but maybe I'm
just not using it properly. If I apply 5V to the input, I get a surge
of current (warming up my battery and cutting its voltage from the
normal 6V to about 1.5).
That's with the output unconnected.
With no power applied, I find there's a 4K resistance between ground
and output (is that normal??)
Those type of regulators back feed if the out side of the regulator
Click to expand...
is higher in voltage than the input side in which cause, your post
regulator will be acting like a short and your battery will be trying
to over come the 5 volts etc..

Normally this problem shows up in cases where you have a charged
capacitor on the output side and then there is no power to the input
in which case, causes the regulator to short the output to the input
and thus drain the cap via the reg.
This can some times damage the regulator and placing a diode on
the input and output normally fixes it.
In your case, you would need one in series.



http://webpages.charter.net/jamie_5"
 
strnbrg59@gmail.com wrote:
On Apr 2, 2:37 pm, "ian field" <dai....@ntlworld.com> wrote:
How hot does the 7805 get?

Hot enough to give off an odor of burning plastic, and way too hot to
touch.
Click to expand...

Look at the output with a scope to see if its oscillating.


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strnbrg59@gmail.com wrote:
I think I have a bad 7805 (TO-220) voltage regulator, but maybe I'm
just not using it properly. If I apply 5V to the input, I get a surge
of current (warming up my battery and cutting its voltage from the
normal 6V to about 1.5).
That's with the output unconnected.
With no power applied, I find there's a 4K resistance between ground
and output (is that normal??)
Click to expand...
You need to provide the input of the 7805 with *more*
than 5 volts. The regulator needs some "headroom".
Give it 8 volts or more on the input. Also, you may have
cooked it, so get a new one and give it at least 8 volts
input. You are using it to charge a battery, so you need
to protect it against current going the wrong way with a
diode.

Ed
 
ehsjr wrote:
strnbrg59@gmail.com wrote:

I think I have a bad 7805 (TO-220) voltage regulator, but maybe I'm
just not using it properly. If I apply 5V to the input, I get a surge
of current (warming up my battery and cutting its voltage from the
normal 6V to about 1.5).
That's with the output unconnected.
With no power applied, I find there's a 4K resistance between ground
and output (is that normal??)


You need to provide the input of the 7805 with *more*
than 5 volts. The regulator needs some "headroom".
Give it 8 volts or more on the input. Also, you may have
cooked it, so get a new one and give it at least 8 volts
input. You are using it to charge a battery, so you need
to protect it against current going the wrong way with a
diode.

Ed
Click to expand...
Make that "If you are using it to charge a battery, you
need to protect it against current going the wrong way
with a diode."

Ed
 
Don't forget to connect a capacitor parallel to the battery, I have
had a problem of power line oscillation before because lack of it.

On Apr 3, 1:41 am, strnbr...@gmail.com wrote:
I think I have a bad 7805 (TO-220) voltage regulator, but maybe I'm
just not using it properly.  If I apply 5V to the input, I get a surge
of current (warming up my battery and cutting its voltage from the
normal 6V to about 1.5).
That's with the output unconnected.
With no power applied, I find there's a 4K resistance between ground
and output (is that normal??)
Click to expand...
 
strnbrg59@gmail.com wrote:

I think I have a bad 7805 (TO-220) voltage regulator, but maybe I'm
just not using it properly. If I apply 5V to the input,
Click to expand...
WHY are you applying 5V to the INPUT of a 7805 ? It won't work properly !
It's a linear regulator . That means the input needs to be (several volts)
HIGHER than the rated output voltage.


I get a surge of current (warming up my battery and cutting its voltage
from the
normal 6V to about 1.5).
Click to expand...
A 7805 WILL NOT charge a 6V battery !!!!! You need around 6.8V to charge
a 6V (lead acid) battery.


That's with the output unconnected.
Click to expand...
Uh ?


With no power applied, I find there's a 4K resistance between ground
and output (is that normal??)
Click to expand...
Measuring the 'resistance' of ICs is pointless.

Learn about how to use these things before trying next time.

Graham
 
strnbrg59@gmail.com wrote:

"ian field" <dai....@ntlworld.com> wrote:
How hot does the 7805 get?

Hot enough to give off an odor of burning plastic, and way too hot to
touch.
Click to expand...
You've probably broken it.

Graham
 
Eeyore wrote:
strnbrg59@gmail.com wrote:


I think I have a bad 7805 (TO-220) voltage regulator, but maybe I'm
just not using it properly. If I apply 5V to the input,


WHY are you applying 5V to the INPUT of a 7805 ? It won't work properly !
It's a linear regulator . That means the input needs to be (several volts)
HIGHER than the rated output voltage.



I get a surge of current (warming up my battery and cutting its voltage
from the
normal 6V to about 1.5).


A 7805 WILL NOT charge a 6V battery !!!!! You need around 6.8V to charge
a 6V (lead acid) battery.



That's with the output unconnected.


Uh ?



With no power applied, I find there's a 4K resistance between ground
and output (is that normal??)


Measuring the 'resistance' of ICs is pointless.

Learn about how to use these things before trying next time.

Graham


You arrogant prick!. Why do you think he came here in the
Click to expand...
first place? to ask why it was happening! "To learn", you prick!

But then again, should I expect any better from you?

Your manners leave a lot to be desired, you British honker!


http://webpages.charter.net/jamie_5"
 
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