Practical Test For Job Prospect

Thread Starter

Lasty

Joined Feb 12, 2019
2
I have a performance/practical test next week for a new job. I have already taken a written portion and passed. I have some pictures i'll attach. The first one will be component identification and the second a circuit with a 120v source AC into a fuse then to a step down transformer 24v output. Then a rectifier diode DC output to a resistor above the light bulb, not sure what type/ohm the resistor is, and finally the light bulb. All the TP labels are places you can get readings with a multi-meter. The labeled switches are different ways to fault the circuit. I know some steps for troubleshooting but want to get more comfortable with ways the circuit could be faulted. One example is the light bulb flashing which I believe is a fault in the rectifier. Wondering how other parts of the circuit could be faulted and how I would know. I was planning on checking continuity through all the components (ohms) and if that checks out start checking voltage drops. Not 100% sure of the exact scientific terminology for each component and could use a little help there. The job is a pretty big step for me any help is greatly appreciated.
 

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Uilnaydar

Joined Jan 30, 2008
118
I'm not sure exactly how they are administering this. The first thing I'd do is to create a schematic if none is provided. Seems pretty easy to do even with no view of the back of the fixture. I see things like variable resistors, different resistor values, and multiple transformer taps. They can throw all types of scenarios with you on this thing, just be glad it isn't me doing this... I'd have FUN with you! If you aren't allowed to draw up a schematic, just try and keep one in your head. Some stuff I'd throw at you are:
  • Why is the light dimmer/brighter?
  • Why is the light flickering more than usual?
  • Set the light to be "this" brightness (some random brightness level)
  • ... and my personal favorite. Dude, the light ain't for workin', why?
Good luck and pray the dude doing it isn't as warped and sadistic as I am.
 

pmd34

Joined Feb 22, 2014
503
Hi Lasty, It looks like the resistor, maybe an old variable resistor, with a slider on it? So you can dim the Light perhaps?
I suppose if half of the bride rectifier was not working you would notice a drop in light level but I doubt you would perceive flashing as such @ 60Hz.
I suppose possible things they can do:
Disconnect the light, so you have to check there is 24V going to it.
Short the light with a resistor, so the light dips in brightness - compare the DC voltage across the light and the voltage across the "unknown" resistor to see how much current is being used.
Short the diode bridge so you read an AC + DC or just AC voltage rather than DC.
Remove the diode bridge - no DC voltage but AC on the outlet of the transformer side - it looks like there is more than one low voltage winding on the transformer.. why?
Overload the transformer on the other windings, so 24V side drops in voltage.
Blow the inlet fuse (no ac voltage on the inlet side of the transformer).
"Blow the transformer" Ac on the inlet but nothing on the outlet

cant really think of much else!
 

Tonyr1084

Joined Sep 24, 2015
4,128
A flashing light typically indicates an overheated connection or component. When cool it makes contact. When it makes contact it heats up. When it heats up it breaks contact and the light goes out. Usually a diode is either good or bad. I've never seen a diode flash before, but I suppose anything is possible. Other switch configurations I can imagine is to either short the system out or to open the circuit. A switch can be used to short a component providing too much voltage or current to a device. The TYPE of light is important too. You say this is a 24 volt system (after transformer and rectifier). It COULD be an incandescent light bulb OR it could be an LED (Light Emitting Diode).

I would start by testing the voltages at every point and noting them. I would also test for voltages across resistors, as they drop some voltage. If you know the system is operating on 24 volts and you know the drop across the resistor you should be able to calculate the current. But you'll also need to know the resistance, which means YOU MUST TURN POWER OFF BEFORE TESTING FOR RESISTANCE. I made that mistake ONCE. Blew the hell out of an analog meter. If you blow up a meter - I wouldn't hire you.

Since there's a diode in the system I would also check for polarity. Switches can turn one around with the flick of a switch, so watch for that too, though I think that might not be so obvious if you're lighting an incandescent bulb. But if you're lighting an LED then yes, absolutely, the polarity WILL be an issue.

When checking the fuse - MAKE SURE THE POWER IS OFF BEFORE YOU PULL IT OUT! Pull it out before you check it for continuity. A switch can be used to simulate a blown fuse, so that's a point for checking for voltages.

Above all, make sure your meter is set properly before you perform a test. Likely the settings you will most need are AC and DC voltages, and resistance. Failure to do so can result in a blown meter. And a blown interview/test.
 

Thread Starter

Lasty

Joined Feb 12, 2019
2
I'm not sure exactly how they are administering this. The first thing I'd do is to create a schematic if none is provided. Seems pretty easy to do even with no view of the back of the fixture. I see things like variable resistors, different resistor values, and multiple transformer taps. They can throw all types of scenarios with you on this thing, just be glad it isn't me doing this... I'd have FUN with you! If you aren't allowed to draw up a schematic, just try and keep one in your head. Some stuff I'd throw at you are:
  • Why is the light dimmer/brighter?
  • Why is the light flickering more than usual?
  • Set the light to be "this" brightness (some random brightness level)
  • ... and my personal favorite. Dude, the light ain't for workin', why?
Good luck and pray the dude doing it isn't as warped and sadistic as I am.
Hi Lasty, It looks like the resistor, maybe an old variable resistor, with a slider on it? So you can dim the Light perhaps?
I suppose if half of the bride rectifier was not working you would notice a drop in light level but I doubt you would perceive flashing as such @ 60Hz.
I suppose possible things they can do:
Disconnect the light, so you have to check there is 24V going to it.
Short the light with a resistor, so the light dips in brightness - compare the DC voltage across the light and the voltage across the "unknown" resistor to see how much current is being used.
Short the diode bridge so you read an AC + DC or just AC voltage rather than DC.
Remove the diode bridge - no DC voltage but AC on the outlet of the transformer side - it looks like there is more than one low voltage winding on the transformer.. why?
Overload the transformer on the other windings, so 24V side drops in voltage.
Blow the inlet fuse (no ac voltage on the inlet side of the transformer).
"Blow the transformer" Ac on the inlet but nothing on the outlet

cant really think of much else!
A flashing light typically indicates an overheated connection or component. When cool it makes contact. When it makes contact it heats up. When it heats up it breaks contact and the light goes out. Usually a diode is either good or bad. I've never seen a diode flash before, but I suppose anything is possible. Other switch configurations I can imagine is to either short the system out or to open the circuit. A switch can be used to short a component providing too much voltage or current to a device. The TYPE of light is important too. You say this is a 24 volt system (after transformer and rectifier). It COULD be an incandescent light bulb OR it could be an LED (Light Emitting Diode).

I would start by testing the voltages at every point and noting them. I would also test for voltages across resistors, as they drop some voltage. If you know the system is operating on 24 volts and you know the drop across the resistor you should be able to calculate the current. But you'll also need to know the resistance, which means YOU MUST TURN POWER OFF BEFORE TESTING FOR RESISTANCE. I made that mistake ONCE. Blew the hell out of an analog meter. If you blow up a meter - I wouldn't hire you.

Since there's a diode in the system I would also check for polarity. Switches can turn one around with the flick of a switch, so watch for that too, though I think that might not be so obvious if you're lighting an incandescent bulb. But if you're lighting an LED then yes, absolutely, the polarity WILL be an issue.

When checking the fuse - MAKE SURE THE POWER IS OFF BEFORE YOU PULL IT OUT! Pull it out before you check it for continuity. A switch can be used to simulate a blown fuse, so that's a point for checking for voltages.

Above all, make sure your meter is set properly before you perform a test. Likely the settings you will most need are AC and DC voltages, and resistance. Failure to do so can result in a blown meter. And a blown interview/test.

You guys are awesome! gonna take all the input and make sure I know how to test for these possibilities. The example I mentioned about the bulb flickering happened to a guy I know who recently tested. I thought he said something about the AC or DC after the rectifier but he didnt pass so who knows lol. You guys confirmed my plans for initial testing, continuity with source off then voltage drops across each load. Gonna have to research how to check polarity on the diode and the examples pmd34 gave with the transformer. I think there is a schematic if not I could draw one up but it is timed. Heres hoping the tester isnt sadistic and that I dont blow a meter lol. Thanks again guys
 

MisterBill2

Joined Jan 23, 2018
5,162
The certain approach to determining the source of the failure is to first understand how the system works, and then see how it is working. By understanding what each element does it becomes clear which element is not right. Of course, the understanding is where the challenge lies. But that is the approach that has been very useful for me for a whole lot of years.
 

MrChips

Joined Oct 2, 2009
20,183
If I were the examiner, I would watch to see how you approach the problem.

For example, before you start putting test instruments to the test panel, did you pull the AC plug from the power outlet?
 

Uilnaydar

Joined Jan 30, 2008
118
If I were the examiner, I would watch to see how you approach the problem.

For example, before you start putting test instruments to the test panel, did you pull the AC plug from the power outlet?
Exactly, that's how I'd use it. I'd make an impossible to solve fault or one that had the answer "it depends". What I'd want to see out of a potential employee is if they did the work up front. If the kid asked me "Is there a schematic?" that's a point for him but I'd say "Nope". If the kid started to draw up a schematic I'd say "You only have 30 minutes, is that enough time to draw up a schematics?" answers like "A good schematic will save me time in the long run" or "The schematic will help if there are multiple faults" would earn him a point but I'd shrug my shoulders and say "Your choice whether its right or wrong".

When I hire techs, I don't want the robots out of the 2 year programs that can regurgitate ohms law and the NEC book. I want folks that can critically think. I want the folks that can point out that it MAY NOT be the piece of equipment that failed but the idiot (most likely me) that spec'd the incorrect piece of equipment in the first place.

Just be glad I'm not doing the test :)
 
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