Hi all, im looking to build a 24v DC timer circuit and could use a diagram if possible. I need to pulse a relay for 1 second with a variable delay between pulses of approx 3-20 seconds. The supply will be 24vdc and 15 amps.

All help appreciated.

 

Have you done a search for "timer", there have been many threads on this topic.

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Len

 

I need a little challange, I'll help you. I would recommend instead of the relay thou, to use either a bi-polar power transistor or a power mosfet, as at the rate you will be pulsing the relay it won't last too long. The mechanical life of good relays is about 100,000 operations. Do you need to pulse the 15 amps or is that the capacity of the supply? I can invision using a 556 timer or a pair of 555 timers and a few other parts.

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The great thing about electronics is unlimited ways to do the job. The only limit is one\'s imagination. I generally think my way is best.
Show me a different way. I have an open mind.

 

100k operations is more than enough, the relay needs to take 15 amps across the contacts if possible . My understanding of electronics is fairly basic, ive not delved into mosfets and such, if you think its the way to go then so be it.

Thanks for your interest

 

Sorry for the delay in responding, I was ill for 1 day.
You say your electonics knowledge is basic. Will be be able to construct the timer from individual parts?, or do you need one that is all ready built?
I can design one for you if you think you can build it, as it will require some soldering and recognizing different components.

__________________
The great thing about electronics is unlimited ways to do the job. The only limit is one\'s imagination. I generally think my way is best.
Show me a different way. I have an open mind.

 

Yes i can create a pcb and build it. I would use a ready built one, but have had no luck in finding a 24v varient. There are plenty of 12v types availiabe as kits and such, and i have tried them using a voltage regulator or dc-dc converter which start off ok but the stepdowns tend to fail after a short while. The timer needs to be able to withstand being switched on and off a fair bit, so im really looking for a stable circuit running staight off the 24v supply.

However, if you know of a good way to convert the 24v to 12v without using an expensive converter, and can take a lot of switching then im open to that. Hope you`re feeling better now.

 

The attached block diagram is for the timer. Most of the timing circuits that I am familiar with use less than 18 volts for the circuitry. The 7812CT set up as shown should work fine. The relay shown is a 12V relay, but a suitable 24 volt relay will work. Of course the important thing is the contact rating it must be at least 15 amps. If a 24 volt relay is used then the 7812CT may not need to be mounted on a heat sink. A typical 12V power relay requires about 100Ma of coil current, hench the 7812CT will be dissipating over 1W of power. The diode across the relay coil is necessary to suppress the reverse emf when the coil de-energizes.
What more can I help you with?

Attached Images

24V TIMER.GIF (3.9 KB, 33 views)

__________________
The great thing about electronics is unlimited ways to do the job. The only limit is one\'s imagination. I generally think my way is best.
Show me a different way. I have an open mind.

 

Thanks a bunch, that looks great. I`ll build it up tomorrow and give it a good test for a few days and let you know how it turns out.

A couple of questions - i understand the caps around the 7812 for smoothing, but what does the diode above it do?

Also i may be able to obtain this item at a good price. http://www.aimtec.com/product_detail...?productId=742

Would it have any benefit over the 7812?

Thanks again for helping me out

 

It's to help discharge the 10uF.

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PIC programmer software, and PIC Tutorials at:
http://www.winpicprog.co.uk

 

Like Nigel says it helps to discharge the 10uF capacitor. One of the requirements of the 7812 regulator is that the output can not exceed the input. If the input voltage is removed the diode(1N4001 series) discharges the 10uF so the output is to 0.
The other circuit might have an advantage but 7812 are so cheep(generally less than $1, and are extremely reliable and have other charastics that make them real good for the price. Like internal current limiting, over temperature shutdown just to name a few.

__________________
The great thing about electronics is unlimited ways to do the job. The only limit is one\'s imagination. I generally think my way is best.
Show me a different way. I have an open mind.

 

Just a little update, the circuit has been tested and withstood all i could throw at it. I`m very pleased with it.

Thankyou very much for helping me out with this

 

Have you tried house bricks?

__________________
PIC programmer software, and PIC Tutorials at:
http://www.winpicprog.co.uk