Hello,

I see these forums move rather quickly. Is your problem already solved? If not, it should be easily solvable, but it would help to have a slightly better idea of the overall context. Since you indicate a common ground and the drive circuit's logic components in series with the ground in a low voltage situation, one might think you had an automotive application in mind, but seeing as how you indicate 24 VDC supply that would seem to rule that out. I am guessing that the reason you "can't employ a ground" connection must have something to do with lack of available conductors in a remote control situation. My assumption is that formerly you had perhaps a simple 24 V power source and lamp at one end and just a two wire pair running back to a switch and now you want, say, a pulsing control circuit instead of a simple switch, so that the lamp flashes rather than simply being manually turned on or off.

Your main problem is that the control device needs continuous power throughout the duty cycle, so you can't just close the control loop with the relay contacts or else obviously you also lose all voltage drop that would actually power the control logic (including of course the relay coil itself). Several solutions do come to mind. If the lamp load is of quite low current (say 100 mA or less) and slight diming is not an issue then the circuit can be reworked with a pass transistor instead of a relay and maintain say a 5V drop to power the control components continuously. Neglecting lamp filament resistance change versus applied voltage, your dimming effect would be about (19/24)^2 or down to about 63 % of previous brightness. The pass transistor would also experience significant heat dissipation during the ON cycle. I can provide you with a complete circuit for this if it would fit your application.

The suggestion about automotive flashers is "semi" correct: those flashers are voltage supply sensitive and would not likely work well at 24 Volts. However, their operating principle is simple and can be readily adapted for 24 V. Basically a 24 VDC coil relay with low coil current is driven by a feedback circuit off one of its own contacts, that charges a capacitor. The capacitor charges up, voltage on the coil rises until the relay hauls, and the contacts switch. In that activated state the coil and cap are no longer receiving power from the external loop, but the cap holds the relay closed for a few seconds. Most relays are designed to close at about 2/3 rated coil voltage and drop out at a substantially lower level (often as low as 1/3 voltage), so with a suitably sized cap pulsing as slow as about 2 seconds on and 2 seconds off can be achieved. The exact circuit requires a diode and usually a resistor or two to optimize its performance, and selection of the most appropriate relay does depend on the actual load current requirements. Again, with some external application specifications (lamp type mainly, and desired pulse rate) I can provide you with a circuit. We built something along those lines for testing bells in fire alarm systems (instead of driving building occupants nuts with continuous ringing, our simple little pulser causes them to emit very brief little "dings" continuously).

If you no longer need any assistance then bye and best of luck, but in case you do, I'll check this thread again in a day or two. It just came to my attention in AAC's email newsletter today and I thought "this one oughta be easy enough to solve". Cheers ds

"L" is not correct about automotive flashers being voltage driven, except for those sold for trailer applications. They are current driven so as to immediately let a driver know that a bulb has failed open. At least for many years that was the case, and now the body control computer is able to check each circuit and deliver the same response if anything fails. It was a rule at one time, not just a feature.

 

If the load resistance is much less than 500 ohms that should not matter, but if it is something else then it might be a problem.

The TS did state the maximum load was 5A in post #16, but he didn't give a minimum, so that certainly is a consideration.

Below is the circuit with reduced bias current to minimize the minimum load problem:

And folks wonder why I ask so man questions about the applications.

Well I, for one, appreciate you pointing out the foibles in my designs.

 

I don't know my f250 power stroke starts on the first click. I'm sure post 31 would blink on the first blink LOL If you think about it
The bulb is not that much above ground and a cap could hold enough power between blinks but cruschow done showed that in post 31

What I don't get if this is going in say truck relay box why is it needing a floating ground most newer are 3 pin flashers
and the older ones had a screw that bolted the block to the frame that was easy place to get a ground.

Then there is relay adapters that you plug in that give you a 2 pin to 3 pin to get grounding.

Oh and this can't be to hard just think flashing blue lights, Then think of all the lights they have flashing. I'm sure there flasher is just a addon too.

Oh (slap)! Stoopid me LOL. Thinking back now sure, I remember some standby transmitter diesels being 24 V. My guess is the fellow who first requested a circuit has only two wires available -- maybe on some long cable, and it's not for a vehicle. Who knows. As for inexpensive flashers, the thermal ones default load current on at startup if memory serves, and even with the relay ones the duty cycle is so short that you really don't notice the slight delay of the first cap charge cycle. Nice little circuit from Crutschow.

 

Oh (slap)! Stoopid me LOL. Thinking back now sure, I remember some standby transmitter diesels being 24 V. My guess is the fellow who first requested a circuit has only two wires available -- maybe on some long cable, and it's not for a vehicle. Who knows. As for inexpensive flashers, the thermal ones default load current on at startup if memory serves, and even with the relay ones the duty cycle is so short that you really don't notice the slight delay of the first cap charge cycle. Nice little circuit from Crutschow.

An F250 diesel engine is not large by "big truck" standards, and if it starts that fast from a cold start in really cold weather you may have a system of glow plugs that work very well. And the other reason for 24 volts on big trucks is to support all of those lights. My reference to "trucks" was not about pickups at all, but the ones that haul all kinds of freight and have separate trailer sections connected by the "fifth wheel" arrangement. I should have been a bit more specific.