Design Help 2 - Please NO PIC

the 555

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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.

Thank you. It does work :-)

he

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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.

ec

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

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

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

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

...Jim Thompson

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

ck

rs

e no

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

on the 555

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tes of delay

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. =A0The

, pressure

stor at the end

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o I suspect

out 14ma when

ry some of

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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

ck

rs

e no

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

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

Pere

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

Cheers, James Arthur

d on the 555

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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.

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

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