What incandescent bulb to use for replacing an indicator light

[Patratacus]

My rice cooker that I bought 20 years ago finally stopped working so I decided to take it apart to see what's wrong. I found that it uses a simple mechanical spring that latches the switch when the weight of the bowl is applied to the spring. When the weight is released (when the water evaporated and the rice is cooked, the weight is lighter) the button unlatches.

There seems to be nothing wrong with the latching mechanism itself except for the little particle that got jammed between the lever and the casing. Once I took that out it's working as it has before. However, when the latching mechanism failed, the rice cooker was overheating since the button never dislatched when the rice was done. I think this over-heating caused the indicator lights to fail.

The indicator lights were simply incandescent bulbs in series with a resistor. I took the resistor out and measured 47k ohm. The bulbs seemed to be dead since the resistance between the terminals wan over 10Mohm (~essentially open). I think a tugsten filament bulb has resistance lower than 1k (and it's temperature dependent).

So, the actual question is how do I figure out what bulbs to use for a replacement of the burnt out bulbs? The rice cooker seems to use a straight 110 VAC coming from the wall since the plug splits into 2 wires that go into each of the heating plate terminals. The light bulbs draw some current from the terminals. I'm not sure whether I understand it correctly, but I think I'm trying to figure out the correct bulb rating that will draw the current that matches with the 47k resistor. Any pointers would be appreciated.

 

[grahamit]

Those incandescent bulbs you refer to are actually neon lamps designed to work at mains voltage, or there abouts, through the dropping resistor. They'll always test as open circuit because they work similarly to a florescent tube and have no element.

If you can, measure the voltage where they used to be soldered on the board, it should be close to 110V AC. So be careful...

I'd be surprised if the neons were blown, more likely the voltage to drive them has gone missing somewhere along the line.

 

[KJ6EAD]

I agree with the above assessment of the situation. The switch is the likely culprit. It is probably a single pole, double throw (SPDT) switch that sends power to the heating element and a "Cooking" indicator in the down position and then to a "Done" indicator in the up position. It probably got stuck in a nearly closed position and arced, destroying at least one contact internally and weakening it's contact operating spring.

Neon lamps such as the one you have (probably an A1C on the chart or generically an NE-1 or NE-2) start conducting when they have ≈70V-100V across their terminals. Older, worn ones will have some sooting from the electrodes deposited on the inside of the glass. Take a look at the chart linked below to see the various types and their specifications. They are readily available from electronic parts suppliers. This is your opportunity to change the color to green if you like. Just be sure to use the correct resistor value for any new bulb. How about orange for cooking and green for done!

**broken link removed**

As for the suspect switch, some testing may be in order.

 

[Patratacus]

Thanks all for the answers. I was hoping to not take the switch section apart since it's pretty deep inside the casing. I was suspecting that the bulb was a neon bulb also since all the miniature bulb that looks like these ones were all neon bulbs.

Just to be sure, to use the correct resistor I just take (110 - [bulb voltage ~80] / [current draw ~ 0.8] ? I'm not sure whether that's correct because if the old bulb had a similar rating I would be drawing nearly no current and that's unlikely. I use that for calculating resistor on LEDs, but that's in DC. I'm not too familiar with AC calculation. Do I need to do VRMS rather than use the 110 AC?

 

[chuckey]

Yes your calculation are correct, in my head, 30V/.8mA ~ 33k, orange/orange/orange resistor. (whats fitted, could be 27K or 39K)
Frank

 

[KJ6EAD]

Just use the chart in the link in my previous post or the data that comes with a new bulb to determine the resistance needed. The chart is on two facing pages so the data on the lower half has to be placed to the side of the upper half to be complete or you can just count the rows like I did.

 

[Patratacus]

I took the cover apart and got to the switch. Turns out it's actually an SPST switch. I drew out the diagram and I think that the switch and the bulbs are ok. I think the voltage is missing elsewhere since either one of the bulbs should be on no matter what since the "Warm" bulb (as opposed to the "Cook" bulb) should be on when the mechanical switch is not depressed. I have not yet taken the whole thing apart yet to see where all the wires go inside the insulating fiber cloth layer. I'm assuming that there must be side heaters (at least 3 of them) around the side of the body and they are likely connected in series. I think one of the heater went bad after the traumatic overheating and it went to open.

At this point I'm not going to try to fix it since I just found a new rice cooker on Amazon that's quite a bit better than this one. Nevertheless, for my educational purpose, I'm going to take the entire casing apart and trace out the wires and post it here for other people to see the mechanism of this rice cooker. It seems like a pretty arcane technology but it's still in use even in my new rice cooker.

Thanks all for the help. I think I learned a lot about an AC appliance taking this thing apart.