Design Help 2 - Please NO PIC

jamesgangnc · 2009-03-04T14:34:10+00:00

I'm still waiting for my newark order so I went ahead and worked on the 555 solution. Based on the suggestions I got I came up with this. It has a few more components than some of the other solutions but it is working pretty well. A 100uf capacitor is giving me just about exactly 2 minutes of delay before off. I imagine there may be ways to eliminate the front transistor but for me it helped to have each active component doing one task. The transistor on the front deals with my normally closed, open is on, pressure switch. The 555 gives me 2 minutes of delay. And the transistor at the end drives the relay. The release is very clean and consistent. The relay is one of those 12 volt "ice cube" ones and is drawing about 150ma so I suspect I could get rid of the second transistor. The circuit draws about 14ma when "off" and I think I can live with that. I'm still planing to try some of the other suggestions once I get more parts. Thanks to everyone for all the help. I have learned a bit more :-)

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jamesgangnc 17 years ago

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Nope, a whole lot of other stuff like the starter expect ground to be negative.

I'm running the relay directly off the 555 now. With a power supply of 12 volts it's drawing just about 100ma. I'm surprised at the voltage drop on the 555 though, it's around 1.7v. But it does not seem to be getting hot. I'm going to leave it running for half a day or so on the bench. I like less parts but I was getting way less voltage drop across the npn. Just thinking in terms of durability I wonder if I should keep it in.

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jamesgangnc 17 years ago

Thank you. It does work :-)

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jamesgangnc 17 years ago

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The diode does seem to be not required. It works the same with the diode disconnected. I saw a lot of examples of 555 circuits that used a diode here and that's where I got it from.

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ggherold 17 years ago

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Ahh high temperatures, thanks Ben. I'll try a bit of heat on the cap and see if that makes it discharge faster. BTW I left the cap discharging through a 1G ohm resistor over night and came in to find the voltage had dropped from 18.3V at 5:30 pm to 15.6V at 8:30 AM. It's nice to know you can make a cheap RC time constant of a day or more if you ever need such a thing. A few years ago I 'designed' (stole from H and H) a variable ramp generator with periods from 1 ms to 1000 seconds. I used a 100uF tantalum for the long times. I looked at the leakage specs on the aluminum caps and never even thought to try them in the circuit.

George Herold

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John Fields 17 years ago

That was thirty years or so ago, Bill, so while your advice may have been sage then, it seems you believe that aluminum electrolytics are no better now than they were then. JF

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RST Engineering (jw) 17 years ago

Yes, but the tantalums make such nice bullet-sized holes in the ceiling tiles when the student puts one in bassackwards.

Jim

-- "It is the mark of an educated mind to be able to entertain a thought without accepting it." --Aristotle

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Jim Thompson 17 years ago

Indeed.

A client sent me a PCB with the chip I designed loaded in it, for me to generate a test plan.

Powered it up... VDD shorted.

Probably along traces trying to find what was shorted... kablooie!!

Fortunately I was wearing magnifiers... the Tantalum shrapnel scratched the lenses :-(

...Jim Thompson

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Jim Thompson 17 years ago

^^^^^^ |____Probing (spell checkers :-(

...Jim Thompson

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bill.sloman 17 years ago

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Solid aluminium electrolytics may well be better, but I've no reason to suppose that the regular parts are much different. Do you?

-- Bill Sloman, Nijmegen

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John Fields 17 years ago

Sure. The foil\'s cleaner, the anodizing is more precise, and the electrical forming is a lot better controlled now than it was 30 years ago. Just compare package volume VS capacitance voltage product for what was available then and what\'s available now and it should be pretty clear that run-of-the-mill aluminum electrolytics are a _lot_ better now than they were then. For low leakage, take a look at Xicon\'s LLRL line: http://www.tawelectronics.com/xicon/XICON_LLRL.pdf With about 0.002CWV or 400nA (whichever is worse) of leakage current, they\'re, I believe, about the best available. How do your old caps measure up? JF

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bill.sloman 17 years ago

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And why do you think that?

And why is this going to make the leakage behaviour better than that of modern tantalum capacitors?

-- Bill Sloman, Nijmegen

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Varactor 17 years ago

on the 555

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Glad you dropped the output tranny. You misunderstand me. Wire your circuit with -12v where your old ground was and 0 at where v+ was. The circuit does not need to be grounded to the motor at all as its the relay contacts that control it. Do you see?

Cheers

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John Fields 17 years ago

Because, unlike you, I stay informed re. matters technical and don\'t go around spouting irrelevant 30 year old nonsense. If you think I\'m wrong, cite an example to prove it.

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John Fields 17 years ago

Replaced example: "For low leakage, take a look at Xicon\'s LLRL line: http://www.tawelectronics.com/xicon/XICON_LLRL.pdf With about 0.002CWV or 400nA (whichever is worse) of leakage current, they\'re, I believe, about the best available. How do your old caps measure up?" They can\'t, of course, so you decide to snip what you can\'t respond to what will put you in a bad light, and then you change the subject. Typical Sloman chicanery.

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ggherold 17 years ago

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Thanks for the link John, if I ever need a low leakage aluminum I'll try some.

If anyones interested, the heating did increase the discharge rate of the nichicon cap I was looking at. At 60 degree's C the voltage dropped by 10% in one hour. Then at something near 100C (+/-10C) the voltage dropped by 25% in 30 minutes.

George Herold

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oopere 17 years ago

Hmm... Why does he need to connect the 9V battery during a few days???

Pere

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James Arthur 17 years ago

To "form" the oxide on the electrolytic capacitor. That procedure fills in voids and reduces leakage.

Cheers, James Arthur

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jamesgangnc 17 years ago

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Does seem to be working fine without the output transistor. I left it running for 24 hours in both the "on" and "off" states. 555 not even warm.

Don't think I can do that. I'm working with a 12v battery in a boat. The negative terminal of the battery is connected to the engine block. Can't change that. The pressure switch (the nc switch that controls the output) is screwed into the block, can't change either. One leg of the switch is going to be connected to a negative ground. No way around that.

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Jon Kirwan 17 years ago

I take it, this is the HE series 1000uF/50V with leakage spec'd at

500uA.

Earlier, and I gather it was around 20C, "in 20 minutes the voltage has dropped by less than 10%!" I gather that at 60C it remains less than 10% in 20 minutes... in fact, in 60 minutes!

You also mentioned earlier a 'fast droop in voltage after the initial charge', which is a different effect. Is this initial reflex included when saying 10%?

I take it that this works out to just a few microamps, but not yet quite the 500uA leakage spec?

Anyway, thanks for taking the trouble here and talking about it.

Jon

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ggherold 17 years ago

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Hi Jon, I just set this up quickly on the corner of my lab bench. I stuck the capacitor into a white protoboard, attached a 1 G ohm resistor to one leg and measured the voltage from the other side of the resistor to the opposite side of the capacitor with my 10 Meg ohm input impedance voltmeter. This gave me a 1/100 voltage reading and discharged the cap at only a 10-20 nA rate. I applied 20 (something) volts to the cap for a few seconds and then recorded the voltage at different times

Time Voltage (mV)

2:26 213 2:31 202 2:44 195 3:00 190 4:00 185

8:36 AM 156 (So this was the next morning 16 hours later)

I then read the heat idea and taped a 100 ohm aluminum housed power resistor to the top of the cap, tapped a thermal couple to the side, stuck some melamine foam around the outside and top. I also placed a book on top of the foam to try and hold the contraption together. (I don't want you to think this was some precision measurement.) I applied some power to the resistor,

Time Voltage(mV) Temp

10:25 150 54C 11:18 137 61C 11:29 136 61C (I then cranked up the temperature a bit more) 11:40 128 93C 11:45 120 95C 11:52 110 98C Something is beginning to stink I reduced power 12:00 100 95C Heat off, removed the foam. 12:11 98 25C End of the experiment

There was a bit of noise in the measurements as you might expect with the 1G ohm source driving the voltmeter. As I got near it to make a measurement the voltage would jump and then settle back.

The good news is that at room temperature the caps hold their charge for a long time.

George H.

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