Borrowed a "10min" timer circuit from a project website and wish to integrate a high current SPDT relay for a home theater test set. I have breadboarded the timer portion of the circuit and it has a range of about 10s to 5min with the components I'm using.

The circuit diagram shows the planned integration of the SPDT, with the relay coil to the 555 pin3 output, and a quenching diode to dampen the back EMF of the coil. I'm using a RS 275-248 SPDT as it has a comparatively (to most 12v Bosch type high current relays) low current draw. Hoping to get opinion on this approach before I start frying my 555s.

 

I've used the same set up but you need a diode in series with the relay coil too. Don't quite know why, just know that it acctually works properly when that diode is added Something like the 1N4001 should be fine.

 

Speaking of diodes. I dissected a couple of basic auto relays recently and they has diodes in them. I take it that these particular relays would not need another one? Perhaps the OP's relay already has one as well...

 

That seems like it could have been the protection diode, did you take note of how it was connected?

 

I'll contact RS and see if I can find someone who knows if their relay has built in diodes. Otherwise the circuit look ok to you guys?

 

Yup
I've never seen a relay with built in diodes, although it is a good idea for some aplications. Just add your own diodes to be safe, it won't cause any harm.

 

I'd go on DigiKeys website, and ask on the live help for a relay with a built in protection diode.

Then make a note of the manufacturers part number, and buy it from somewhere closer/cheaper!

It would be quicker than going to RS.

 

Also, OPs relay doesn't have built in relays.

I'm sure you know this, but putting a diode across the relay will prevent 'spikes' in voltage which will damage the 555 timer.

If you put a diode across it, or buy a new relay, consider changing the 555 at the same time, or buy a couple spare just in case.

 

The 555 circuit, without the relay, is pretty consistent with the timing. However, it doesn't always trigger with application of 12v. Perhaps something to do with decay time on the electrolytic?

And, yes, I've bought plenty of 555 spares.

 

Do you mean to say that the relay does not operate when the 555 output goes high?
That's probably due to a lack of sufficient current, add a transistor to act as a switch to solve this. 555's can usually output 200mA, how much does your relay require?

 

Here it says 30mA.

http://www.radioshack.com/product/index.jsp?productId=2062482#tabsetBasic

 

It's possible that it takes more than 30mA to energize the relay, but only needs 30mA to hold it energized. What is the output voltage of the 555? Did you measure it?

 

I'm sorry for the confusion...I'm now having trouble just getting the timer circuit to work. I'm using a small (2amp) batt charger for voltage source and it actually runs about 18v which may be frying my 555s (?). I'm going to run down to RS and get some more 555s and also a 12v regulator or perhaps just use a 9v battery to test the circuit. Once I get the timer circuit to operate I'll add the relay with a series and parallel diode and we'll go from there. Thx everyone for help.

 

Go with the 9V battery. Messing with battery chargers as power sources can lead to very unexpected results. ...and yes, 18V is on the upper side of it's ability.

 

There must be a transistor at pin 3 of 555 to drive relay ( I think)

 

Go with the 9V battery. Messing with battery chargers as power sources can lead to very unexpected results. ...and yes, 18V is on the upper side of it's ability.

Concur and thx for confirmation. Should be OK if I can get the 12v regulator working.

There must be a transistor at pin 3 of 555 to drive relay ( I think)

Perhaps, but I've seen several 12v relay timer circuits w/o a transistor, but they all seem to use series/parallel diodes off pin3.