Dramatic voltage drop at voltage regulator output

[jim380]

yet on previous page you said it got really hot.....

yes. But when I was testing it today in the lab, it didn't seem to get hot, at all. Weird...

 

[Baluncore]

It is only weird because you are overlooking something very simple.
You must use a methodical approach to identify that problem.

Check all voltages relative to one reference point.
Check all joints are soldered and electrically continuous.
Shake the circuit while watching the output voltage to find the broken wire.

When output voltage falls under load...
1.7 volt / 12 ohm = 142 mA.
12V – 1.7V = 10.3V across the linear regulator.
10.3V * 0.142A = 1.46 watt power dissipated in the regulator so it should not get hot.
But why is it limiting the current to 142 mA ?

 

[jim380]

It is only weird because you are overlooking something very simple.
You must use a methodical approach to identify that problem.

Check all voltages relative to one reference point.
Check all joints are soldered and electrically continuous.
Shake the circuit while watching the output voltage to find the broken wire.

When output voltage falls under load...
1.7 volt / 12 ohm = 142 mA.
12V – 1.7V = 10.3V across the linear regulator.
10.3V * 0.142A = 1.46 watt power dissipated in the regulator so it should not get hot.
But why is it limiting the current to 142 mA ?

True. I will try to isolate the problem, and report back. Thanks for your help so far.

 

[jim380]

If the battery voltage does not also fall drastically then either the regulator circuit is incorrect or the regulator device is faulty.

okay. I think I found the problem. When the regulator is loaded with a 12 ohms resistor, the battery voltage drops from 12V to 3.4V . I've tried a couple new batteries, still the same problem.

 

[jim hardy]

Baluncore and jim380 :: ++++++

Jim - speaking of methodical
i like to measure all voltages "with respect to" some common point in the circuit. "With Respect To" just means hook you meter's negative lead to one spot, leave it there and write on schematic all your other voltages.

I'd have tried, in your circuit, to get negative meter lead on negative side of battery and leave it there... on battery itself not just nearby.
that-a-way you'll find the real sneaky troubles like a wire that's broke inside the insulation but looks fine from outside, or a corroded battery terminal like you find so often in automotive troubleshooting.

Methodical-ness is a good habit to develop.

 

[Baluncore]

okay. I think I found the problem. When the regulator is loaded with a 12 ohms resistor, the battery voltage drops from 12V to 3.4V . I've tried a couple new batteries, still the same problem.

Well done.
What type of 12 volt battery are you using, make, model, chemical technology and capacity?

 

[jim380]

Well done.
What type of 12 volt battery are you using, make, model, chemical technology and capacity?

http://www.digikey.com/product-sear...t=0&page=1&quantity=0&ptm=0&fid=0&pageSize=25

I'm using this one.

 

[Baluncore]

Data shows discharge Rate = 480µA
It is doing very well to produce 150 mA.

 

[jim380]

Data shows discharge Rate = 480µA
It is doing very well to produce 150 mA.

Yes. I figured. I also tried powering the 6v regulator using a power generator in my lab. Everything works fine when loaded with the same resistor. I'm literally running out of ideas.

 

[Baluncore]

If you have a lab supply that works, how come you are running out of ideas?
You need to get either an 8V to 12V power supply rated at 1 amp output that runs from the AC mains,
or a 12V gel cell with sufficient capacity to produce 1 amp.