ECN Forum Forums General Topics of the Trade General Discussion Area 12 volt motor delay

Joe, what SCR? Our 88-ohm example relay draws only about 150 mA, so the coil can be connected between the output pin and +V. Once the timing cap voltage rises above 2/3 Vcc, the output goes low, the relay pulls in, and it stays that way until power is removed.

A diode from the timing cap to +V ensures that it's discharged at the beginning of the cycle. Depending on the application details, there may need to be a dummy load R from +V to ground to give it something to discharge into.

The .01 uF bypass cap on pin 5 has a 33 microsecond time constant, so it has no effect at all on the relay output.

All of which is purely academic, because I would never connect a 555 directly to vehicle power, which exhibits transients that would knock your socks off. Like others here, I'd prefer a more robust solution.

Honestly, in this case I think I'd avoid silicon altogether and modify LarryC's approach: Use the R-C network on a pilot relay to drive the power relay.

The pilot relay would be a reed type with an adequate contact rating to energize the power relay and a 5-volt coil. The series resistor would then be just slightly higher than the coil resistance, maximizing the "R" (which, in turn, minimizes the expensive "C"), and guaranteeing operation at low battery voltage. It also reduces the voltage across C, further reducing its cost and size. I'd add a diode across the series-R to discharge the cap quickly when power is removed. Since the cap discharges through the low-resistance power-relay coil while the pilot relay is still closed, it happens quickly.

Example: The COTO 9081-05-00 reed relay ($1.66) has a 500 ohm coil. Choose series-R = 620 ohms, 1/4W and C = 2200 uF, 10V (0.4" diameter x 0.8" long, $0.39).

The pull-in delay is 1 second, and the dropout (of the pilot relay) is about 0.7 seconds.

Including a 30-amp automotive power relay such as the Potter&Brumfield 1432782-1 ($1.82), and a couple of 1N4001 diodes, the total parts cost is less than $4.50.

Cheap enough yet?

Edit: The P&B relay has 30A contacts, not 40A.

[This message has been edited by John Crighton (edited 06-30-2006).]

John, It was mentioned earlier that the 555 could trigger the SCR. I thought that you were expanding on that. I agree with you both in that it would work to sink current for relay drive and that I wouldn't do it. By the time we add those things we've mentioned to try to establish an initial condition, I didn't see how we were any better off than charging up a cap in a gate circuit. I built several windshield wiper delays arould 555s in the late '70s. That app worked very well and used the whole chip.
Of course, they spent all but one cycle running between 1/3 & 2/3 Vcc where they really shine.

You mentioned some pretty good ptices on your relay approach. Please give me your supplier info. You never know when that will come in handy. I went with the SCR approach because it could be inserted in the negative path to the relay. I've probably had the parts to build one sitting around for 30 years. I thought BigB could just stop at his local RS and pick up a few Rs, Cs, an an SCR to play with. I'm not so sure anymore. A 30 amp automotive realy was a first hit but "SCR" doesn't show up on an online search of their site.

So let's pretend that we know what BigB wants more than he does. We see that he's planning on switching a pretty heavy load. Maybe he wants it to be on with the engine and not risk running down his battery. So let's design him a circuit that pulls in a relay a couple seconds after it sees >13 volts and drops out the relay about a volt < the pick voltage. Any takers? I just like these challenges to knock the rust off.

Scott, I was thinking of what you said about the Tech ref. section. Could this whole thread be shifted over into the Theory area with drawings in the adjacent section?
Joe

Joe, those parts are all in stock at DigiKey , but there's a $25 minimum order or $5 handling charge.

They're even cheaper at Mouser ($3.42!), but the reed relay is out of stock until late August. Substituting an identical Magnecraft W117SIP-1 (in stock, $2.14) brings the total to $4.55, no minimum order.

I emailed an image of the diagram to Scott yesterday and asked him to post it here.

Thanks John. I use both of those suppliers and think highly of them. The 2N50640S-ND 800mA, 200V SCR is $0.46. Cheaper still.<G>
Joe

I have posted a Schematic regarding this topic in the Technical Reference section.

It may be found under the topic heading of:

Time Delay Relay

Click on the Hyperlink above to open that page.

Scott35

Wow...I am really impressed and humbled by the knowledge of everyone that has responded! I am really fired up now to try some of that stuff!

As far as the project, an engineer suggested that the loads I wanted to switch would probably be fine both starting at the same time. I also have a 145 Amp alternator. I connected the 2 10 amp fans using 30 amp relays and 30 amp fuses. So far they have been running fine for 3 days.

I will try to link the post from the engineer. ]www.mercedesshop.com/shopforum/showthread.php?t=156902&highlight=volt+motor+delay]


also, thanks to all who replied and even sent me schematics!



[This message has been edited by BigB (edited 07-07-2006).]