This is slightly off topic but I can't seem to locate a 12 volt motor delay for a project I am working on. It needs to be rated at least 30 amps 12 volts DC and be able to delay starting for one to two seconds.

How hard do you want to work? You can buy a Potter Brumfeld time delay relay for a one part fix or you can cobble something up yourself.

I would strongly suggest using a more conventional time delay relay or method to drive your final output relay or contactor.
Joe

OK, I might get in trouble for this BUT:
I looked at some of the TDRs in the range that you need and they are very expensive and, I think, overkill. So at the risk of a DIY label, I would probably solve the delay problem with an SCR. I would use a flyback diode across the relay coil and return the coil to the - supply through an SCR. Add 1 resistor, 1 cap, and maybe a trigger diode for style points. The 12 volt supply will charge the cap through the coil and resistor. The SCR fires after a short time delay, flowing enough current to pick the relay. The cap discharges, at least partially, when the SCR fires. When you kill the supply the current from the back EMF flows through the flyback diode, the SCR turns off and you're ready for the next cycle.

I have seen a small, metal encased relay with just 2, .250" quick connect terminals on it. You put it in series with the load for a short on delay. I think that it is for 120AC loads though. I'll post it if I find one that will work at 12VDC.
Joe

I did something like this for the contactor on my spa. I really only needed a part of a second but all I used was a capacitor and a resistor to the gate of the SCR. The voltage resistor to the gate is limited by the charging capacitor.

See, I guess we old computer guys think alike.
Joe

[This message has been edited by JoeTestingEngr (edited 06-26-2006).]

Actually I could easily use a relay, allowing me to get away with a much lower amperage rated delay. I am not sure how to build a delay, I am weak in electronics. I figured to just buy one. Altho seeing what you guys posted has got me fired up enough to look for some good learning resources.

This is a lot easier if you are using a relay with a DC coil.

This is what I did.

http://members.aol.com/gfretwell/Delay_circuit.jpg

OK BigB, I just sent you an Email with a screen capture of a CADD drawing. There are alot of reasonably priced automotive style relays out there that will handle 30A. The time delay is what makes things get pricy. It is easy to tweak this circuit but a couple of things should be considered. If you were to use a very low value resistance and a very high capacitance, it would be possible to pulse the relay with the cap charging current. If you add your coil resistance and resistor value and multiply by the capacitance in Farads, you get a "time constant" in seconds. In theory, the voltage on the gate of the SCR would reach about 63% of your 12 volt supply in this time. But the SCR will fire in < 1 time constant unless you add something to the gate circuit. Why don't you start by grabbibg a 10k & 100k resistor and 100uF & 470uF cap., plug and chug, and see what you come up with. Feel free to Email me with comments, questions, or descriptions of the smoke rings.

My apologies for morphing this into a theory post.
Joe

If you add a 50 cent 555 timer to this you get a lot more predictable time constant. It isolates the gate trigger level from the equasion and makes the turn on a digital event.

With the increase of posts of this nature (TDC's), maybe it would be of some benefit to draw a few simple + safe Time Delay control circuits, along with some other topics (suggestions???), and post these in the Technical Reference area.

I have a few more weeks of "Free Time", so now is the best time to do so!

p.s.: "JoeTestingEngr"

What "flavor" of CAD Application do you use?
I use AutoCAD (R14 and 2000).

If you have anything you would like to contribute to the ECN Technical Reference section, let me know.

Same goes for anyone else!

Contact me via E-mail, or by replying to this thread.
If you do contact me via E-mail, please include your ECN user name, along with "From the ECN Forums", in the "Subject" heading of your E-mail message. Otherwise, the message may be discarded without being viewed (message filters and such).

Send messages to me at this address:

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(just change the phonetic stuff for the "@" _at_ sign and a "." _dot_)

Scott35

It should probably be pointed out that nothing we make up ourselves will be a U/L listed product. If this is a product or service you sell it will be at your own peril.
Hopefully we all understand the separation requirements of 725 well enough to keep line voltage away from low voltage and can do this safely. Personally I like identified optoisolated solid state relays for this isolation, along with appropriate barriers.

Greg, I'm not sure I could go along on the 555. I use them alot but I can't figure an easy way to use one for BigB's application. I have never used one for timing at the same time power was applied with a trigger event. It's the same with the LS7210 that I like to use for switched timing. It too, has a negative trigger and I wouldn't trust it at power up. By the time we add all the goodies to make the timer fit the app, I'll bet the original SCR schemes we used are as good or better.

I thought of other means to do what he wants, using bipolars and TMOS (BS-170) transistors. Then I started wondering how stiff his supply might be and what would happen when the relay picked. Now we have to add hysteresis to account for that. I'm betting that BigB just needs a little time for things to settle down and wouldn't mind a little extra. I keep coming back to the SCR method being cheap, simple, reliable, and probably the best.

Scott, promise not to laugh at me. I use Generic Cadd 6.0 It was the last version before AutoDesk gobbled up Generic Software. I bought 3 packages for work and a co-worker and I bought it for home at $65.00 each. It was our department's first use of CADD. Since then, everyone else has switched to the latest version of AutoCAD. They try to get me to switch over but I resist. The AutoCAD 2004 in my computer isn't even working now because it's confused over a license file that must have been moved. I just export files as DXFs if we need them for AutoCADD. Fills are the main problem. I'll send you a sample so you can see what I mean.

Greg, you make a fine point. I'll freely admit guilt. The NEC or U/L don't even cross my mind when I'm thinking about 12 volt relay TDO circuits. That could certainly come into play, depending on the application.
Joe

I am doing this with a 4060 counter but my delay time is a big number, up to all day. It is a lot more parts tho.
A .001uf cap and a 100k resistor settles things down prety well on POR.
I'm working on a solar pool/spa controller. Folks are saying PIC but I still like CMOS, particularly when it is a fairly simple truth table. Still looking for a clock but I am seriously leaning toward a garden variety alarm clock with 2 events. The more I work on this problem the simpler the logic is actually becoming.

The application is automotive.

"It needs to be rated at least 30 amps 12 volts DC and be able to delay starting for one to two seconds."

If you can tolerate a delay before the relay opens, the brute force method is to hang a large capacitor across the coil terminals with a resistor in series with the wire driving the relay coil.

R(ohms) x C(farads) = Time Constant(seconds)

My guess is that you will need about 1 to 2 time constants to reach the pull in voltage of the relay. So R times C should equal about 1. Try 20 ohms at 2 Watts and 47,000 uF at 63 VDC.

LarryC

http://www.alarmcontrols.com/

Click on products then Timer Modules and look for the UT-1 Timer. Operates 6-18 VDC and has an on board 3 amp relay. Trigger a 30 amp with that and your good to go. I've got some squirreled away in my basement goodies if you cant find a distributor.

LarryC, I agree with your estimate, except that the capacitor will probably want to be closer to 100,000 uF. For the benefit of the curious, I offer a few more details.

A typical automotive relay, Omron G8W, has an 88-ohm coil and an 8-volt must-operate spec, so with the 20-ohm series resistor it will operate reliably down to 9.8 battery volts.

We have to use the parallel value of the coil resistance and the series resistor to obtain "R" (88 &#0124;&#0124; 20) = 16.3 ohms. The actual RC time constant is then 16.3 ohms x 100000 uF = 1.63 seconds. But, the operating delay time will depend on the actual relay pull-in voltage, the battery voltage, and component value tolerances.

With a 13.7-volt battery we get a steady-state coil voltage ("Vcoil") of (13.7 x 88/(20+88)) = 11.2 volts. Assuming nominal R and C values and a typical pull-in value ("Voperate") of 6 volts, the delay time ("T") is calculated as:

T = -RC x ln ((Vcoil-Voperate) / Vcoil)

T = -1.63 x ln ((11.2-6) / 11.2)

= 1.25 seconds

(The ln () function is "natural logarithm." It's on your scientific pocket calculator.)

Here's the bad news:

1) The operating time will vary with battery voltage. At 11 volts, the same circuit will take 1.8 seconds.

2) The capacitor must be rated at least 16 volts. A 100,000uF 16V capacitor is big (about 1.5" diameter and 2.5" long) and expensive (about $10-12 new).

3) As LarryC mentioned: Assuming the relay is released by simply opening the circuit, the R-C product becomes (88 ohms * 100000 uF) = 8.8 seconds. Assuming the relay drops out at about 2 volts, the release time is:

T = -RC x ln (Vrelease / Vcoil)

T = -8.8 x ln (2 / 11.2)

= a whopping 15.2 seconds!

You can see why the solid-state alternatives, such as the one suggested by TwinCitySparky, are so desirable.

JoeTestingEngr, the key to using the 555 in an application like this is to forget that it's a timer and just use it as a really good inverting Schmitt trigger with a nice, high-current output. Tie the trigger and threshold pins together as an input, and leave the discharge pin unconnected. The output goes high when the input is below 1/3 VCC and low when the input is above 2/3 VCC.

Oh, and I used GCADD for years before finally biting the bullet and buying AutoCAD LT Didn't they have a "Miss Manners"-like character answering FAQs in their newsletter?

Edit: Didn't get those italics right the first time... Or the second...


[This message has been edited by John Crighton (edited 06-29-2006).]

John, this is where I run into the problem with the 555 here. If we do it the usual way, 2 & 6 to ground via a cap and + supply via a resistor, skipping pin 7, we power up with a trigger. We won't be at 1/3 Vcc, and might not be at theoretical 0 on the cap either. If there is a cap on the control voltage pin 5, it is charging. If not, noise could be a factor. So we effectively start out high on the OP and are holding off for a low. This is intuitively backwards for our SCR trigger.

So if I want the output to be better for my SCR timing, I would probably hook the cap to the supply and the resistor to ground. The cap will now be charging down to trigger while my control voltage stabilizes up. The SCR will get a clean firing pulse but when? Do we really know the charge on the cap at power on? Won't the supply voltage effect the time. It's not that I'm saying that this won't work, just that I doubt there will be more precision than a simpler circuit.

I started out using Generic Level 3 CADD in the mid-80s. I think I used 5.0 for a little while too. I still love it. Stuff happens if I forget to assign a printer to LPT1 before a dotplot but I'm lucky that ancient printer drivers still work. I just made 4 circuit boards this week, using it and the DynaArt system. But I had totally forgotten that news letter. I'm going to bounce it off of my EE brother, who got me started on Generic Cadd, to see what he remembers.

Joe

[This message has been edited by JoeTestingEngr (edited 06-29-2006).]

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).]