Testing a voltage regulator in-circuit

Since you clearly don't understand how the circuit functions, how can you say that ?

What's a 'Huntron' btw ?

Graham

The diodes will have about a 0.7 volt drop if the anode is more positive than the cathode. Otherwise the drop will be whatever the existing voltage is since the diode will be reverse biased and not conducting. You need to pay attention to the polarity of the measurments you make. Same goes for transistors. If the transistors are bipolar NPN types, they will conduct when the base is more positive than the emitter by about 0.7 volts. If they are PNP types, the base must be more negative than the emitter for the transistor to conduct. Look more closely at the polarity of the measurments to figure out what is happening.

-Bill

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Can you post a schematic of the device?

JF

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If I can't figure it out I will try and trace out a schematic. There is about forty parts on a little 2" square board. You think with something this small I would be able to check each component and find a problem but nothing so far.

Well using my DVM I look for things like diode drops and opens on the resistors. If I suspect anything I remove one end and test. The Huntron I think is a great tool. You have used something similar I am sure. Just a V/I tester or Signature Analysis. Where you inject a low current/voltage sine wave into a component and view the results on screen. I have found many problems with it. But, it works best when using it to compare one signature to another taken from a known working board, of which I don't have in this case. By the way, has anyone ever seen a pocket size Huntron style tester. One that is about the size of a standard DVM? I would like to have one that I can take a long with me. Russ

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So what does it mean when testing a diode in circuit with power applied, 6VDC and you test 6VDC across the diode in either direction. With the meter set to read DVM. Does that show a problem?

As was posted before, you must pay attention to the polarity of that voltage you're measuring.

It could be a zener diode you're looking at, or simply, the voltage on the cathode side of a diode that is (+), which means no forward flow through the diode..

P.S. The cathode end is that of the band.

You may want to do a dead circuit test first.

"

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It depends on what you mean my measuring the voltage "across" the diode.

If you mean that the voltage reads the same polarity of 6V whether the
meter is hooked up like this: (View in Courier)


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Here's a general description which may help: Emergency lites such as you describe contain three basic functional circuits: power supply, charge controller and "light controller".

The power supply is the transformer, diode(s) and an electrolytic capacitor. It provides the input voltage to the charge controller, and 1 input to the light controller.

The charge controller is usually a simple float charge circuit. For a 6 volt light, it should provide something between 6.5 to 6.9 volts. That is likely what the LM340T5 is used for - with a couple of resistors added it can be set to provide the charge voltage.

The light controller senses the voltage from the power supply. If it does not detect any voltage, it turns the light on. It may have a second function - to turn off the lights if the battery voltage drops too low. Some emergency lights have that, some don't.

Some things you can do: 1) Test the battery and bulbs by connecting the bulbs to the battery. If you tested it with the DMM but without the bulbs glowing while testing, you cannot rely on the DMM reading. The battery can look good when using the DMM, but fail when the bulbs are conected. The *most common* failure of these emergency lights is the battery, and simply checking the battery with a voltmeter is useless. They _must_ be tested under load.

2) Post the part numbers for the transistors. 3) Look for a relay - often these lights use one in the light control circuit. 4) With the unit connected to the mains, measure the voltage across the battery It should rise above 6 volts. If it does not, the charge control circuit is not working. Let it run for a couple of hours if you don't see it rise right away and the check again. 5) Pull the circuit board and try to determine what completes the path from the battery to the bulbs.

The hardest thing is to draw a schematic based on the board, but that could ultimately be necessary. If you do it, check it over and over and over and over again. If you draw it wrong, it will impede your troubleshooting efforts. If you draw it right, you can then analyse what the circuit does, right down to what each component does - a good learning experience.

Ed

I am not sure the examples appear correct on my browser. But I think I understand what you mean. Just to clarify what I meant. If I measure the voltage drop directly across the diode, putting my meter black lead on the cathode of the diode in-circuit with the power on and putting my red lead on the anode I get 6VDC on the DVM. If I reverse the red and black leads I still get 6VDC. If I put the black lead on ground and the red lead on the cathode I get 6VDC. If I put the black lead on ground and put the red lead on the anode I get

0VDC. This can't mean a meter problem?

If I take a jumper and short the diode the light turns on! Somehow I should be learning something in all of this but it is not sinking in.

It won't matter which way you have your probes on it. the meter will just read the same value + or - etc...

YOu need to perform some in circuit non powered test. if something looks strange, pull it to test it.

Use a diode test for the basic primitive procedures on transistors and diodes.

"

Ed,

Everything you say matches what I am learning about this thing. Only that there is no relay on it. Everything is OK, except the little circuit board controller. I checked the battery, bulbs, transformer and they all seem OK,, I mentioned above that I shorted a diode that is connected to the base of a transistor that connects the positive battery terminal to the light bulb lead. This completes the light bulb path and it lights up. So something is not getting through to the fire the base. This is connected to the battery or bench PS. I will sketch the circuit out, but boy this is a lot of work for such a small circuit with maybe 30 components on it. I hope I learn something that will save time. I did find a resistor that was supposed to be 11 ohms but measured 44 ohms, and a cap that measured bad on my Sencore LC102 but after changing them it still didn't work.

The parts are

120AC to 12AC transformer. Q1 D44h8 Q2 2n4403 LM340t5 A bunch of diodes, a few resistors, five caps, a zener diode and that is it. Thanks so much Russ

Sounds intruiging but should be totally unnecessary to fix a simple circuit.

Do you have the schematic and can you post it somewhere ? There will be key places in the circuit that should form good 'test points'.

Graham

It means the diode is DEAD !

Graham

Not -6V ?

Graham

** ROTFL !!!!

DC voltages always read the same " in either direction " with a DVM.

Just the sign changes .....

...... Phil

"Uriah"

** Is the " - " segment in the display of your DVM not functioning ??

Or are you stubbornly choosing to ignore it?

BTW:

How are you sure the device is meant to run on 6 volts ??

Could it not be a 12 volt unit?

..... Phil

Which isn't what he said. If the sign didn't change it would be a dead diode.

Graham