Wiring an indicator lamp to glass stove top - Page 1
Forum section: electronics / beginners

author: edy
author_rank: Super Contributor
timestamp: March 30, 2020, 08:44:54 pm
content: Hi folks,I have a glass stove-top oven that basically looks something like this:I want to wire in a box with 4 indicator neon lamps that will tell me when each coil element is actively getting current.Normally, when I turn on the stove element to high, it will start glowing RED. After a few seconds it will appear to turn off. I am assuming there is a temperature switch that sees the set-temperature was reached and turns it off. A few seconds later it may go on again for a few seconds, then turn off again. I am assuming that the range keeps the proper temperature using this sort of "pulse width modulation" and a thermostat which opens/closes the switch based on what it senses, but I could be wrong. I am not sure how sophisticated the control elements are within this range.In an older design, I would expect that if it operated by simply having a thermostat set-point and having the element coil either get FULL CURRENT or NONE, and pulsing based on temperature, the indicator neon lamp would be an easy hook-up in SERIES with the coil itself, allowing me to see when the coil was on or off. I am relying on some information reviewed from this site as well:https://www.intl-lighttech.com/specialty-light-sources/neon-lampsA neon indicator lamp rated at 240V should be able to handle the stove coils which I assume to be 240V (the entire appliance plugs into a 240V plug). I don't know the resistance of the coils but they are probably high enough ohms that the current passing through the coil-neonlamp circuit would be low enough to keep the neon lamp in the "glow range". Also the neon lamp itself, once activated by the high breakdown voltage, would likely operate at a fairly low voltage and therefore not affect the power going to the coils much.Any thoughts or suggestions before I start doing anything? I am mostly concerned about whether I can simply connect the lamps in SERIES with each coil, and what kind of temperature control is actually happening (on/off or something more complex). I can use a multimeter to check the resistances of the coil elements, and also see what voltage is going to each coil when the burner knob is turned up. Is it controlled by temperature feedback or is it a PWM-type control that varies the duty cycle depending on the temperature knob setting? Thanks again for any help that could be provided.

author: KL27x
author_rank: Super Contributor
timestamp: March 30, 2020, 08:57:24 pm
content: QuoteNo. Parallel.

author: edy
author_rank: Super Contributor
timestamp: March 30, 2020, 09:22:29 pm
content: Thanks, my bad. I was wondering how much current would be passing through the bulbs if they were hooked up in series and the voltage drop plus the huge resistance of the coil likely would not be enough to do anything. Thanks!

author: gcewing
author_rank: Regular Contributor
timestamp: March 31, 2020, 02:06:58 pm
content: Also be aware that a neon bulb needs a current limiting resistor. If you're buying a neon indicator designed for mains use complete with a bezel it will probably have a resistor built in, but you should check. If you buy just a bare neon bulb you will need to add the resistor yourself. The value needed will depend on the current rating of the bulb, typically it will be a few hundred k.

author: SeanB
author_rank: Super Contributor
timestamp: March 31, 2020, 03:09:36 pm
content: Normally those glass top stoves have a red neon lamp in the glass top, which turns on when the plates reach around 50C, telling you the glass is hot. If it is not working then the most common suspect is the thermal switch failing, which is cheap to replace, or the neon lemp sputtering itself black, also cheap.Those controls are panastats, which are basically a form of relaxation oscillator, having a snap action switch and a heating coil, where the heater coil is wired on the output, and heats a bimetal strip to eventually break the output contact, which then disconnects the plate and the heater, till it cools down and the bimetal strip allows the switch to close again. control is via the knob adjusting the amount of tension on a spring that biases the bimetal strip, allowing you to control the power setting ( the PWM of the oscillator) from a low setting of around 10% to full power. There is also a contact on there connected to the pilot light per plate, which closes when the setting knob is turned from the off position.The power output is typically a 30- 60 second pulse of power, spaced any time from 20 seconds to a minute apart, at low settings, gradually changing to near full time at high, and then permanently on at high, and is only approximate, varying with mains voltage, temperature of the control area and pretty much phase of the moon, but is over a 10 minute period going to be within 10%, good enough to cook with.There is no real temperature control, merely a variation of the pulse width, except for certain special controls that are only on solid plate non glass top stoves, where they have a sensor bulb and a switch in the actual solid plate, and you need a pot on them to power the plate, otherwise it remains cold. this is there for simmering things like soup and such, which requires lower power, and you need to keep it below boiling temperature, so the plate power is limited to around 500W, and the switch disconnects power when the pot is lifted. those are rather inaccurate, and fail regularly, plus are expensive to replace, as opposed to the simple spiral element under the glass and the Panastat, which are under $20 each as service spares, and which in many cases are identical from different manufacturers in stoves, as they all use the same suppliers of them.

author: edy
author_rank: Super Contributor
timestamp: March 31, 2020, 09:34:35 pm
content: Thanks. To answer your questions, this application is NOT to determine the temperature of the element coils at all. It is simply to have a visual indicator of when the "pulse" of the PWM (full current ON) is passing through the element. I want to be able to see the ON/OFF current cycles or duty cycle visually to know when the element is being energized and when it is not.I still have not determined whether the coils get 120V or 240V. I'm looking at indicator lamp options like this from this local shop:https://secure.sayal.com/STORE2/View_SHOP.php?SKU=196037https://secure.sayal.com/STORE2/View_SHOP.php?SKU=245619https://secure.sayal.com/STORE2/View_SHOP.php?SKU=241465https://secure.sayal.com/STORE2/View_SHOP.php?SKU=241457I don't think any of these contain a built in resistor. I also don't think this shop provides much detail as far as the specs on those indicators. As far as resistors go, there are 2W and 5W ceramic options, and I found also a 47K 7W option as well:https://secure.sayal.com/STORE2/View_SHOP.php?SKU=199230https://secure.sayal.com/STORE2/View_SHOP.php?SKU=142056https://secure.sayal.com/STORE2/View_SHOP.php?SKU=199032https://secure.sayal.com/STORE2/View_SHOP.php?SKU=225775https://secure.sayal.com/STORE2/View_SHOP.php?SKU=226355I will need to check the voltage going to the coils first and foremost and see if I can find any specs on those neon lamps and get back to you.

author: james_s
author_rank: Super Contributor
timestamp: March 31, 2020, 09:59:09 pm
content: The coils are 240V in every stove I've ever messed with. It's all pretty low tech stuff as already nicely detailed above. Even those fancy modern looking glass top stoves are electrically about the same as your basic 1950s electric range or cooktop. They have nichrome elements that are pulsed by a simple control, it's entirely open loop. The glass ones just have the bare element in a ceramic former under the glass instead of a metal clad Calrod burner element above the surface.

author: edy
author_rank: Super Contributor
timestamp: April 02, 2020, 03:36:17 am
content: Ok, so I had a poke around the inside of the oven and looked at the circuit diagram (see attached "oven3.jpg"). I didn't place the schematic image inline because it would be too large, but the smaller photos (see below) are inline... if you look at it I will explain what I was measuring across and what I found.First have a look at this image:I placed the probe measuring AC volts with one lead going to the brown wire (or it could have been "blue" wire on the next element knob) which is what goes out to the coil itself..... and the BLACK wire which runs common along all the elements control knobs. When the element was turned on, whenever the coil was actively energized it would show 247 V AC on the multimeter, and when the element coil turned off it was 0 V.If I measured volts across to the common RED wire (running between all control knobs) it was exactly the OPPOSITE. It would show 247 V AC when the element was off, and then when the element was on it would drop to 0 V.So what I figured is that RED was the actual "live" feeding the coil and would show a large potential difference when the knob was not letting the current through. As soon as knob thermostat kicked in to activate the coil, it would short the RED with the particular color wire (that went to individual elements) hence the potential difference became 0 V. The BLACK wire is the return or grounded to the coil normally. When the knob thermostat activates, it disconnects the BLACK (because it connects the RED instead) and the potential becomes 247 V from the element coil wire lead to the BLACK common.Here you can see another close up view.If you look at the schematic/wiring diagram it becomes fairly obvious to most of you (see "oven3.jpg" attached), but I'm not as experienced so I need your guidance.Have a look at the LEFT SIDE of the wiring diagram. You will notice there are wires BR-10 (brown) and BL-10 (blue) going off from the control knobs "H2" connection to the coils. You have R-2 (RED) wire connected common to L2 on all the control knobs, and the BK/BL-1 (or BK-1) connecting all the L1 commonly to all control knobs.You will also notice that all of the coils (on the other side) eventually all connect up to BK-10 everywhere which ends up being joined to BK-1 at the bottom near the appliance plug. So that's what drives the coil to energize... it is the RED/BLACK difference.So if I decide to wire some neon indicator bulbs across the H2 and L1 for each control knob, bring all the wires out into a little indicator box where I put my 4 lamps, that should work. I just don't know what resistors I need to limit current across the bulbs, as I had a hotplate before where the indicator lamp burned out (the one installed from factory) and I simply wired in a 125V replacement without any resistor and it works fine to this day. I am also wondering if I should install a switch or get a "button" type indicator lamp so that if I don't want to use them I can just press the buttons and have them turn off so I don't see anything.I also want to make sure I use the appropriate wire types to handle the current to the neon bulbs. The ones in the oven seem to have a special high-temperature insulation on them. I see mostly 16 and 20 AWG gauge wires being used at the control knobs, and some are 105 C and some 200 C (like the ones going to the coil). I could probably get away with 105 C, and I figure there isn't going to be much current to the neon bulbs but I could always get 16 AWG just in case. Any help would be appreciated.

author: james_s
author_rank: Super Contributor
timestamp: April 02, 2020, 04:02:00 am
content: Red and black are both live, each will have 120V between itself and neutral (white) and 240V between the two live wires, these are standard colors.Standard neon indicator lamps use a NE-2 bulb which is a standard type made by lots of companies. These require a series resistor to limit the current, going from memory it's typically about 100k for 120V and 220k for 240V. Integrated indicators normally have the resistor built in but you can add an additional external resistor to a 120V indicator to run it from 240V.

author: edy
author_rank: Super Contributor
timestamp: April 02, 2020, 05:14:10 am
content: Thank you. Here are the 250v indicator bulbs available at my local parts shop:https://secure.sayal.com/STORE2/View_SHOP.php?SKU=192886https://secure.sayal.com/STORE2/View_SHOP.php?SKU=137580https://secure.sayal.com/STORE2/View_SHOP.php?SKU=241458https://secure.sayal.com/STORE2/View_SHOP.php?SKU=241466https://secure.sayal.com/STORE2/View_SHOP.php?SKU=241465https://secure.sayal.com/STORE2/View_SHOP.php?SKU=241456https://secure.sayal.com/STORE2/View_SHOP.php?SKU=241457I have no idea if there are integrated resistors or not. Doesn't seem to give enough information on the website. I would ASSUME that if they are labelled as either 120v or 250v, they contain the appropriate resistors for each of those applications. Probably exactly the same bulbs in all of them but the integrated resistor makes them suitable for the 120v or 250v application.

author: james_s
author_rank: Super Contributor
timestamp: April 02, 2020, 05:46:40 am
content: Yes that's exactly what they will be, a NE-2 bulb and a resistor appropriate for the rating printed on the side.

author: SeanB
author_rank: Super Contributor
timestamp: April 02, 2020, 09:36:39 am
content: Integrated indicators will have the resistor, only bare neon lamps with solid core wire ends do not. A series resistor is not needed, just wire rated for 1kV withstand, basically ordinary household double insulated wire that you use in appliance cords. Get a length of 6 core cable, 0.7mm cores, and use it, or some sleeving and 5 double insulated cores stripped out of old cables will do. Just remember that both wires will be hot all the time, as they are connected to one half of the supply cable.You connect between H2 on the switches to the indicators, and the common wire connects to Bk-7 in the rear of the stove control panel, or to one of the L1 terminals on the controls. you can sleeve the wiring inside with an extra covering of heatshrink tubing, just do not bother to shrink it. to connect you will find that getting in a pack of tap in spade connectors is best, as you simply pull off the connector, push the wire into the male end of the spade tap and push the connector back.this type of connector is about the easiest to usehttps://www.ebay.com/itm/Lot-100pcs-Connectors-Terminals-Set-Male-Female-Spade-Electrical-Wire-Crimps/133244513604though they will be best also with an extra cover of heatshrink over the whole lot after you install it in the stove.

author: edy
author_rank: Super Contributor
timestamp: April 02, 2020, 01:23:50 pm
content: Thanks... I figure these piggyback connectors will work best. I can just replace the existing ends with these and it will provide me with another blade to patch into to connect the indicators:https://secure.sayal.com/STORE2/View_SHOP.php?SKU=131673I have a bunch of cables for 120V (the standard ones used for computer power supplies). I assume that would be capable and appropriately rated. I would need 4 wires... one to each control knob, and 1 wire to only one of the black commons (since they are all joined common across all knobs anyways). So only 5 piggybacks are needed. The indicators will have one side going to each knob wire, and the other side commonly connected.As far as heat protection, this is the back of the stove where the circuit control board and behind the knobs. I doubt it gets that hot back there. I figure 105 C wire is good enough, but if I can find something better insulated I will try to use it.If I install a switch on my indicator light box it will be on the common side of all the indicator bulbs... that connects to the black common. One switch to toggle the entire thing to either work or not. I have to find a robust switch that will handle 250V. To be extra safe and avoid sparks, will also want to switch on/off only when all indicators are OFF knowing that there is no voltage going through at that moment.

author: james_s
author_rank: Super Contributor
timestamp: April 02, 2020, 03:44:14 pm
content: There's no reason to be that cautious, it makes no difference if the indicators are on or off when you switch a switch, any switch rated to handle line voltage is designed to be able to switch a load.

author: edy
author_rank: Super Contributor
timestamp: April 03, 2020, 12:53:48 am
content: Ok I wired up everything in a box as can be seen below... there are 4 indicators and a switch that is supposed to shut off all the indicators from having a connection back to the oven through the common black wire. However the switch behaves strangely... more on that later.Here you can see the piggyback connectors that I will be attaching in place of the 4 connectors that go to the coils on each knob (H2 as designated on the schematic). And then the original coil wires connect back on to these piggybacks. So imagine each HALF of the box goes to one half the oven. There is a blue/green and brown wire on each side of the box. The blue and green attach to the H2 of the knobs in the oven and go to the red and green indicators, with the other lead of the indicators going to the switch. The brown connects to the other side of the switch, and goes to the black L1 knob connector in the oven.Here you can see the wires hooked in "piggybacking". This is showing just one side of the oven... front and back right side knobs, BROWN and BLUE (on the H2 connectors of each knob), and the common BLACK at the bottom on all the L1 of the knobs. On the left side was YELLOW and ORANGE to the coils from the knobs H2 connections, also BLACK common bottom connecting to L1.Here you can see the box as placed on top of the oven.Ok, so the strange thing... everything works fine when the switch is in the ON position, as expected. When the coils activate and get current, the corresponding indicator light turns on. I can now visually see exactly what the duty cycle is on each coil, when it turns on and off.However, when the switch is in the OFF position, I am still getting the indicators turning on when the stovetop is in use... and not just the coil in use. All indicators light up to some extent. There is some leakage current going back through the switch. Also, the switch itself has an indicator built in... but rather than light up when it is in the ON position, it lights up in the OFF position if I am using one of the stovetops along with most of the indicators. So that switch is definitely messing something up with the system (although the stovetops function completely fine as the current is very small).For now, I will leave the switch ON. If I get another switch it will have to COMPLETELY DISCONNECT all the indicators (and not just the common black) and I'll get a switch without an indicator built in.

author: james_s
author_rank: Super Contributor
timestamp: April 03, 2020, 01:05:40 am
content: Those crimp terminals look way too big for the wires that are in them.What is the strange behavior you're seeing? It's hard to see how everything is wired from a photo since the wires are all black.

author: IanB
author_rank: Super Contributor
timestamp: April 03, 2020, 01:08:26 am
content: Quote from: SeanB on March 31, 2020, 03:09:36 pmI've always known these assimmerstats. I've not heard of panastat (turns up nothing on Google).

author: edy
author_rank: Super Contributor
timestamp: April 03, 2020, 01:41:31 am
content: Quote from: james_s on April 03, 2020, 01:05:40 amYes the crimp terminals were made for 10-12 gauge wire and I am using 18... I didn't realize that until after I got home!I will post a wiring diagram that explains how things were done:Please excuse my crude drawing. As you can see, each indicator goes to the corresponding knob H2, and the other sides of each indicator all join up commonly and go to the common L1 connecting all knobs (when the switch is closed). When each element is active, the H2/L1 voltage difference goes to 250V. The switch is completely unnecessary.... if I just connected all the indicators together and to a single L1 on one of the knobs it would have been sufficient. And as I mentioned before, it works perfectly fine when the switch is not there.However, I thought by adding the switch I would "break" the return (L1 black) common and inactivate ALL the indicators. However, what is happening instead is when the switch is in the OFF position, the switch lights up and most of the indicators light up a bit if I turn on any of the stove elements... but unevenly... the right indicators (knob1/2) light more more and left indicators (knob3/4) light up less when a right part of stove is used, and vice versa if left side of stove is used.Somehow the fact that the switch "breaks" the circuit creates a voltage difference to the other knob on the same side (since if it is off but connected through the coil to ground), and there must also be some leakage through the switch light and to the other half. Basically, I think that is the WRONG SWITCH for this application.What I think it is happening here is that when a knob gets activated, it makes it's H2 go to 250 V. All of the other indicators are still connected through to ground via the stove-top coil itself (the other H2 connections) even though the switch being OFF is disconnecting from the L1 black. There must be a small enough current still able to pass through the other indicators and their respective coils to the common black that it can keep all the indicators lit up. Without the switch, or when the switch is closed, the L1 black connection has the least resistance and strongly favors just that single indicator light to go on. As far as why the switch light goes on when in the OFF position, and is off when in ON position, must have to do with the way it is internally wired to bleed some current to activate a light inside.I need to use a switch with 4 poles 2 throws... then it will disconnect ALL indicators, like this:

author: gcewing
author_rank: Regular Contributor
timestamp: April 03, 2020, 10:56:10 am
content: Are you sure you need to bother with a switch at all? It doesn't seem to serve any purpose.If you want to be able to disconnect the box for safety reasons, then yes, a 4-pole switch would be needed. But there won't be any power going to the box when all the elements are turned off anyway.

author: james_s
author_rank: Super Contributor
timestamp: April 03, 2020, 06:48:17 pm
content: Are you sure the switch is wired correctly and is indeed a double pole switch? Many illuminated switches have a separate terminal for the illumination.Keep in mind other paths that the current can take. With the switch off the junction between two of the indicators and live will be open, but depending on how everything else is connected you may be able to end up with a potential across the two indicators that are now effectively in series. You really have to look at the big picture to work this out, you have the same sort of problem if you try to multiplex incandescent lamps in a grid without a blocking diode on each one.

author: edy
author_rank: Super Contributor
timestamp: April 05, 2020, 04:44:15 am
content: Thanks for the suggestions. The switch is useless, I am just leaving it in the "ON" position for now, but can remove it later. I think I figured out what is going on here.Remember, one pole of EACH of the 4 neon indicators connect to one of the 4 knobs (piggybacks onto the terminal which feeds one side of the element coil). As soon as you turn on an element, one pole of that corresponding knob's neon indicator will be "hot". The other poles of all indicators are joined together and "sink" to the black wire (through the switch)... thereby creating a 250V difference across the corresponding indicator.When the switch is OFF, the connection to the more direct black-wire "sink" is gone. However, since the other poles of all the indicators are common, there is nothing to stop current from jumping across another indicator and into one of the elements that is "off" (since it can pass through the coil and to "black" that way). I know this is A/C and I am using "sink" incorrectly but you get the point.To confirm this, I turned ALL KNOBS ON when the switch was OFF, and guess what.. all the indicators went off. That's because ALL coils (and therefore ALL of the poles of every indicator) had the same potential. Current could no longer "sink" through another indicator/element coil. So when the switch is OFF and I have thereby disconnected the main "sink" black wire, as long as there was at least ONE element that is still off... it becomes the sink because it is at a 250V difference to any of the ON elements. That's why turning on all elements makes the lights all go out again.I hope that makes sense. The only solution is to use a single switch that has 4 separate poles inside (i.e. disconnect all of the indicators simultaneously so there is no path to a lower potential). However, as mentioned previously there is no need to do this. The switch is simply useless and the primary purpose seems to be EDUCATIONAL for myself because it was an unanticipated result that helped me understand better what is going on.

author: gcewing
author_rank: Regular Contributor
timestamp: April 05, 2020, 10:16:26 am
content: Actually, there is another way that would allow using a single pole switch -- put a diode in series with each lamp. Just make sure you choose diodes with a high enough reverse brreakdown voltage.

author: james_s
author_rank: Super Contributor
timestamp: April 05, 2020, 07:26:40 pm
content: That should be trivial, the bog standard 1N4007 is good for 1kV. Aside from Schottky types you'd probably be hard pressed to find one that won't work for this.There is a pitfall though, running a glow discharge lamp from DC means only one of the two electrodes will glow so the light output is lower and they don't last as long but it does work.

author: Mambo Danny
author_rank: Newbie
timestamp: January 13, 2025, 07:17:24 pm
content: Just want to thank you folks and the OP. I've wired LED indicators to an older electric stove, but this style was new for me. I did this burner as it is the one I use the most. I may do the others if I start using them more.You might ask why (of me, like the OP was asked), and it's to better see that the dial-control is 'On.' The tiny red text next to tiny red text up in the display to tell me a burner is 'On' is not enough. And I leave my cast-iron skillet on that burner, so I won't see that the burner is on from the red lights of these glass-tops.A bright LED is visible from across the room, or across the house (with an open line-of-sight). This is what I want to ensure I safely turn off the burner. I just bought this place two months ago, and only started cooking on the stovetop two or three weeks ago - and I've accidentally left the burner on a few times already. New Wi-Fi smoke alarm installed in the kitchen, and this light got installed today.