Why doesn't this circuit work with a newer relay?

I have a riddle I hope someone here can answer. I have a circuit that is supposed to flash a light and sound a buzzer at the same time. I have two versions of the circuit. One is from probably the 70's or 80's and one is current. they are wired as shown in the picture. The one built in the 80's flashes the light and rings the buzzer as designed. Unfortunately I don't have the 80's component with me so I cannot read the resistance of the coil. I don't have the part number of the relay. So a quick rundown of what I know about the two.

Original Circuit                                                       
Relay: part number unkown                                
Coil resistance: unknown                                    
Capacitor: 12v 1000uF                                         
Flashes once every second or so.                     
Current Design
Relay: R40-11D2-12
Coil resistance: 960 ohms according to printing on relay
Capacitor: 16v 1000uF
Flashes once when power is applied the relay just buzzes light glows dimly. however if the light is removed and the wire goes straight to ground the circuit operates and the relay opens and closes

I realize there isn't much here to go on. So a little more about the story. As I said I don't have the original relay part number and when I did, I searched for a datasheet. Every search came up with the replacement part number (R40-11D2-12). I cannot seem to locally source a 12v 1000uF capacitor that is of the same physical dimensions. A 16v 1000uF is about the same size as the original.
My thought is that there are only two culprits possible here. The newer relay coil has too much or too little resistance therefore not allowing the cap to charge. Or the cap cannot properly charge because of it's specs. Or both. Being that the circuit operates properly with the light removed and wired straight to ground it makes me suspect there is too much resistance somewhere. I know there are other ways of doing this however the results are not exactly the same. Is there a way to do this as is with current parts?

Picture of Why doesn't this circuit work with a newer relay?
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271828-1 year ago

I'm not sure your schematic is correct. Doing a google search "relay capacitor lamp flasher" brings up this page describing the circuit i think you want? You will need a relay designed to switch with a12v signal. The uF of the capacitor determines how fast the relay turns on and off but it looks like you need a resistor to limit the charge rate of the capacitor. The capacitor rated voltage should be larger than 12v.

MDheliMech (author)  271828-1 year ago

Thank you for the suggestion. I have seen that Google result I assure the circuit works as drawn with older components. As stated the relay was so old it's out of production and the 12v 1000uF cap is huge compared to what a current one looks like. As I stated the only reason I can come up with is either the components are more efficient or less efficient than they used to be. The reason the circuit you suggested is not desirable is that the original circuit produced more of a square wave on, off. The circuit you suggested doesn't have that definite on, off. Instead of turning off everything sort of bleeds down and then turns on again. I need the definite on off square wave. If I were to guess about duty cycle is say like 20 on 80 off but that's a guess.

If the circuit operates like you want it without the bulb in place, wire the bulb through the other leg of the relay like this. Try the red or green connection to flip the duty cycle. R1 can be added to adjust the flash frequency and limit the inrush current as the capacitor charges (use a resistor in the 1W range and start with 100ohm)

MDheliMech (author)  271828-1 year ago

Thank you again however this seems to produce the same results as the circuit you had originally suggested. It is missing the definite on / off cycle. Instead of flashing the light seems to turn on and then bleed off instead of the square wave that the original circuit produces. I have added an image to illustrate.


That's interesting. I'm not sure why it turns off slowly as the bulb should now be separate from the capacitor. The only thing i can think of is a hysteresis problem with the relay operating in this manner, therefore the relay does not have a separate and distinct turn on and turn off voltage. It can be "kinda" off for a range of voltage when turning off as a result of ringing in the circuit, mechanical design, ... i don't know. A larger relay with a less sensitive coil might work (the ones with a clear cover covering the innards).

MDheliMech (author)  271828-1 year ago

Thank you. I believe you are probably right about the relay bring the culprit here. Unfortunately I don't have room for a larger relay. Again thank you very much for your help.

The relay you have should work, just don't exceed its current rating.


MDheliMech (author) 1 year ago

This is how I think it works.

1) Power is applied momentarily energizing the coil closing the relay contacts which cuts off the current from the circuit.

2) Relay coil DE-energizes, the contacts open and current flows through circuit again.

3) The capacitor while charging is the path of least resistance. Therefore current flows there until charge equalizes.

4) when the capacitor charge equalizes it then again allows current to flow through the coil momentarily closing the contacts and again cutting off current from the circuit

5) The loss of current causes the relay to open again and the capacitor to discharge lighting the lamp and sounding the buzzer at which point the coil must charge again. (back to step 3)

Or something like that. Which would leave me to believe that due to the suggested made by iceng that my circuit is not drawing enough amperage to fully discharge the capacitor which means it recharges to rapidly to allow the relay coil to energize long enough to close the contacts as needed.

But why?

iceng1 year ago

I would try this circuit.

The fact that it works with out the light suggests you need a higher current

(more watts) lamp or several lamps in parallel.

MDheliMech (author)  iceng1 year ago

Unfortunately I don't have the components right now to actually build this circuit. However as I'm sure you know you are correct about the amperage draw. I added a CPU fan (because that's all I had laying around that was 12v DC) that pulls 1 amp to the circuit as I originally posted it and it seems to work better than anything else I have tried. I will let you know when I get the diode and try it out. So far this promises to be the best solution so far. However I still don't understand why I can use two relays that are supposed to be interchangeable (However different part numbers and manufacturers) and get two different results. Did the old relay somehow have this diode built into it?

MDheliMech (author)  iceng1 year ago

thanks for the suggestion. It is a little different than others I have seen since it has the diode. Do you know if this will give me a square wave with a definite on / off? Also any guess as to why the original older components work as drawn but newer ones don't? Is it a matter of efficiency old vs new? More or less resistance in relay coil old vs new? Capacitors are more or less efficient old vs new? Any or all of the above?