Night Light Circuit -- Help Requested

peterboxer

Joined Mar 24, 2020
18
Hi,
Can someone help me understand how this circuit works? It is for a night light which plugs into a wall outlet and is supposed to come on in the dark, and it’s not working.

I believe I have traced it accurately and have checked 3 times, but it just seems wrong! I can attach pictures of the actual circuit board (both sides) if it would help.

I would expect that after the bridge rectifier I should have 120x1.414 = 170V DC. This then goes straight to the lamp, which seems crazy high, plus then what is the purpose of the rest of the circuit? And I am measuring only 105V DC at the lamp. The lamp itself is burnt out and I suspect that it is a low voltage (and of course low wattage) lamp that got zapped by high voltage.

Here are a few questions:
• What is the purpose of R1 and C1?
• R4 is burnt (open) and the markings are also burnt. I assume the circuit failure is because R4 has burnt out. What would be the appropriate value here, assuming that the output to the lamp should probably be 12 volts DC max, and most likely lower, maybe 3 to 6 volts. There are no markings visible on the lamp to help determine its rating.
• How does this circuit control (step down) the voltage to the lamp, the way it is drawn? Or, if I have drawn it incorrectly what is the more likely way it should be drawn?

Thanks, I would really appreciate any insights.

MrChips

Joined Oct 2, 2009
22,096
You have made some wrong assumptions.
How are you measuring voltage?
This runs off AC Line voltage and needs to be handled with due caution.

C1 and R1 is to reduce the current and voltage to the lamp. The lamp should be seeing full-wave rectified low voltage. Check that C1 and R1 are not both blown.

The way you have drawn it, (assuming that it is correct), at night, the CdS photo-resistor has high resistance and the SCR is in the off state. In daylight, the photo-resistor has low resistance which turns on the SCR. This shunts current away from the lamp.

Ylli

Joined Nov 13, 2015
986
That's one weird configuration.

Tonyr1084

Joined Sep 24, 2015
5,238
Can someone help me understand? It is for a night light which plugs into a wall outlet and is supposed to come on in the dark, and it’s not working.

And I am measuring only 105V DC at the lamp. The lamp itself is burnt out and I suspect that it is a low voltage (and of course low wattage) lamp that got zapped by high voltage.
I'm known to ask dumb questions; but did you try a working lamp?

The 170V thing is in error. Yes, your math is correct, but that's with a filter cap on the output. Without it the RMS value should stay the same; minus the forward voltage loss from passing through two diodes. But in theory that should be approximately 1.4V lost. Not enough to account for the 105 volts you're reading. But something in the circuit is taking away some power somehow. And I'm not sure you've drawn your circuit correctly. Photos would be advantageous for sure.

At this point all we can do is guess as to what is going on. Being the eyes and hands on scene - it's up to you to do the diagnosing. And that may start with a correct diagram.

Dodgydave

Joined Jun 22, 2012
9,666
Usually the thyristor puts the lamp on by putting the lamp on the Anode side,, would say you drew it wrong.

Post pictures of both side of the PCB.

MisterBill2

Joined Jan 23, 2018
7,306
The circuit is wrong in several places. First, the light is not in series with the SCR, and so it will be powered all the time. Next, with C1 and R1 in series with everything there could not be enough current to even lighta neon bulb. In addition, R3 and C2 would be part of the trigger circuit, not in series with the load current.
Aside from that, R4 burning up is not the cause of the failure, it is the result of the failure causing too much current to flow through it.
My guess is that R4 was actually wired across the light, or possibly in series with it.

peterboxer

Joined Mar 24, 2020
18
Thanks everyone for such quick responses! I am attaching pictures, hopefully they are clear enough, otherwise I can take some more. I have removed R4 and the bulb, but you can see markings on the board showing where they go (R4 and L1).

I have also checked R1 and C1, they are not short, they are measuring 325K and 880nF. I have not tried with a new bulb since I expect it would just blow.

I will also see if I can test the SCR.

Thanks again!

Ylli

Joined Nov 13, 2015
986
What type of 'lamp' is this driving? A pix of the lamp might be helpful too.

Put your ohmmeter across the photo resistor, now cover it with your finger. Does the resistance go higher or lower when the cell is darkened?

peterboxer

Joined Mar 24, 2020
18
Here are some more pics. I had checked the photoresistor and if remember right the resistance went up when covered, but I will recheck and post my readings.

BTW, just to be clear, the night light did work for many years and only just quit.

peterboxer

Joined Mar 24, 2020
18
Thanks, I will recheck. Does the rest of the circuit appear correct as drawn? For example, is C2 shown correctly?

And by LDR is that the light detecting resistor?

MisterBill2

Joined Jan 23, 2018
7,306
You have definitely drawn it wrong, the lamp goes to the middle leg of the scr which is the Anode and the ldr goes to the Gate..
Actually, it might be correct, because that big capacitor and 330K resistor are definitely in series with everything. So now I am guessing that it was a capacitive reactance voltage dropping arrangement. Certainly the bulb was a filament type bulb. At least that is what the structure in the picture implies. It appears that when light was present the SCR would be triggered, and that would shunt the current past the bulb through the SCR and that resistor. Then when it became dark and the resistance increased the SCR would not be triggered and the light would illuminate.
This product is not a circuit that I would ever recommend at any time under any condition. It would consume more power when it was off than when it was on.

peterboxer

Joined Mar 24, 2020
18
Yes, my thoughts exactly, why even bother to shunt it (from a power saving viewpoint).

However, to act as a shunt wouldn't the resistance have to be lower than that of the bulb filament? There is at least 220K in the "shunt", whereas the filament is probably around 25 ohms or so (assuming 12 Volts and 6 watts).

MisterBill2

Joined Jan 23, 2018
7,306
I am suspecting, from seeing the circuit, that the shunt resistor was R4, unknown, and that the 220 K and cap are not in the correct place on the drawing. And the bulb was not a 12 volt bulb, based on the length of the filament.
So my suggestion is to salvage the photo cell and possiby the reflector and get another night light. There are quite a few circuit designs available.

peterboxer

Joined Mar 24, 2020
18
Thanks. Yes, it's not worth repairing, more just trying to figure out how it works.

Thanks for you help.

LesJones

Joined Jan 8, 2017
2,726
I agree that the schematic does not look quite right. I think it may work by the SCR shorting out the lamp when the LDR is low resistance. (In daylight) The reactance of C1 at 60 Hz is about 3K so it would limit the current to about 40 mA, I think C1 may have failed short circuit to burn R4. I think the lamp has probably been blown if C1 has failed short circuit.
Edit. I actually wrote this post over one hour ago but forgot to click on the post button. I have just noticed MisterBill2's comment about it consuming more power when it was off than when it was on. I agree that it would take slightly more current when the light was off but it would consume almost no power as the current would be almost 90 degrees out of phase with the voltage.

Les.

Last edited:

Dodgydave

Joined Jun 22, 2012
9,666
A better photo of the PCB side getting all the tracks in would be better, the drawing is wrong as the Cathode is on the Negative terminal and you have drawn it in series with a capacitor.

peterboxer

Joined Mar 24, 2020
18
Hope these are clearer, I might take another from a different angle too.

peterboxer

Joined Mar 24, 2020
18
Here's 2 more froma a different angle.

I rechecked the LDR (CdS), it shows 12-14K ohms when light and 250-350K when dark.

I will retrace the circuit later based on all the suggestions here and see if I ccan find where I am tracing wrong.

LesJones

Joined Jan 8, 2017
2,726
I have taken the picture of the etch side of the board that was nearest to a perpendicular view (So the plug pins obscured the smallest amount of the etch.) and mirrored it and rotated it to best match one of the component side pictures. This makes it easier to trace out the schematic. This is the modified picture.

This is my interpretation of the schematic.

Les.

MisterBill2

Joined Jan 23, 2018
7,306
OK, now this circuit looks reasonable. When there islight, the photosensor has a lower resistance and triggers the SCR to shunt current around the light filament so that it does not illuminate. But when it is darker the 220K resistor and the cap pull the trigger input low so that the SCR will not trigger, allowing the light to come on. Given that R4 is what has burned up, it makes sense to check the SCR to see if it has become shorted. But it is more likely that the 880nF cap has developed some leakage, which an ohm meter check could verify, OR, that some higher frequency power had been present, which caused excess current to flow, which then fried R4, leading to the light to fail.
Using a series capacitor to limit current by it's capacitive reactance is only valid if the power is a constant frequency sine wave. Running the device from anything other than a perfect sine wave would certainly cause lots of damage due to over voltage. In this case it could be any device that used a switching mode power control. That could even include a motion sensor light control.