Oil Burner Photocell Repair
A common problem with oil fired heating is failure of the flame detector photocell. The cell can get sooted up or fail electrically giving problems like not starting or random shut downs. A Cadmium Sulphide (CdS) cell is often used to check for light from the burning flame by changing the cell resistance. In the dark, these cells have a very high resistance which then drops to a much lower value in the presence of light.
As you can imagine, it is a hostile environment inside an oil fired appliance. Besides being hot, there is oil and soot around which can degrade or damage a sensor. Because of these problems, the photocell is an easily replaceable part in the burner but the cell itself is sold in an expensive housing with wires and connectors to aid rapid replacement (and a good profit). This Instructable shows how to replace the photocell itself at a fraction of the cost of the manufacturers spare part.
Supplies
Hermetically sealed light dependent resistor replacement.
Removing Photocell
Other types of burner may use a different style of photocell. A little detective work may be required to gain access to the cell.
Choice of Cell
The cell in this part had no manufacturers logo or identification number making it more difficult to find a replacement. Maybe they are trying to hide the source of the device or just bought a cheap job lot? Whatever the reason, we can decide on a suitable replacement by making a few assumptions.
A hermetically packaged cell is required such as a TO-18 outline. The epoxy sealed types are not robust enough for this application.
Electrically, the cell must have a high dark resistance because the burner module checks for leaky cells as part of the start up routine.
In operation, the resistance needs to fall to about 1k by the light from the flame. This is a simple on / off detection, no special light sensitivity curve is required like in a light meter.
A readily available part on sites like eBay is the NSL-06S53 which has a resistance of 5k at 100 lux and a specified resistance in the dark. There are other unbranded cells available more cheaply but the quality may be suspect if there is no dark current quoted. The data sheet provides more information on this photocell.
Cadmium sulphide photocells are not made with a tight tolerance. Also, the light intensity will vary according to the setup of the burner and cleanliness of the cell window. All these variables lead to a very tolerant flame detection circuit which means the cell does not need to be exactly specified. The cell circuit must also fail safe if there is any defect with the cell. Using an ill chosen cell as a replacement will fail safe as it will be flagged up as a faulty photocell.
Downloads
Final Assembly and Test
The faulty cell is cut from the wiring and the NSL-06S53 soldered in its place as in the picture. Then the housing is snapped shut again and the retaining screws replaced. As a final check, the cell resistance was compared to a genuine replacement part at several light intensities.
The chart shows the NSL-06S53 resistance against the original part with the orange line showing an exact match. At low light levels the original part is more leaky than the new cell. At about 300 Ohms there is a good match and then the new cell has an even lower resistance compared to the original part. As the new cell ages, the match could become much better.
The proof of the pudding lies in the eating... replacing the repaired photocell made the burner fire up perfectly.