555 relay current reducer

Only thing I'm not clear about is if the TRIG input is pulsed low while the THRESH is held high, will the output stay high if THRESH remains high and TRIG returns to high as well? It's been too long since I played with the 555 to recall. If not then it will need modification...

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I got as far as seeing that the power to the 555 is removed as soon as you turn on the relay, and then I gave up.

What is it supposed to do? Give a timed full-voltage pulse to the relay, and then follow it with reduced voltage or something?

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I think so. The idea is that threshold goes high, which drives the output of the 555 low and pulls in the relay. The cap at the bottom then discharges thru the resistor and discharge pin. When the voltage on that cap reaches 1/3rd the voltage at the supply pin of the 555, the output goes high and the 555 stops driving the relay and the relay is held in by the mosfet at right with low current.

When the input pulse goes low, the mosfet disengages and the relay opens. The flyback pulse is then caught by the shottky at top left and charges the cap at top left. The zener across the coil prevents overvoltage on the 555.

The idea is essentially to recover the flyback pulse from the relay to provide the energy for the next pull in cycle.

At least that's the idea; as I mentioned it's been a while since I've played with the 555 so it may not be set up quite right...

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Trig requires a sink, pull to common or in a network below 1/3 Vcc.

The threshold is positive switched and needs to be above 2/3 Vcc.

Threshold will over ride trigger.

Jamie

This one can't be teased by input timing and uses PWM to set the relay current, which is efficient.

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John Larkin         Highland Technology, Inc 

lunatic fringe electronics

I think it's the other way around - trigger overrides threshold which are both overriden by reset

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A capacitor sure is simpler

NT

Yup, u are correct.

Jamie

Romper Room lady...

You beat me to it--a timed full-on section or'd with a PWM'd section.

I was going to use HC132's, a favorite, and hand-roll the one-shot.

You can set the duty cycle off of 50% with the old paralleled R + diode trick, or (dicier) a bias resistor to a rail.

Cheers, James Arthur

A proper retriggerable one-shot makes the relay drive always work, regardless of the input timing. There's a possible hangup potential if you make a one-shot from an RC+Schmitt and tease the input waveform.

50% isn't bad. It will guarantee that most relays hold closed, and cuts power by 4:1.

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John Larkin         Highland Technology, Inc 

lunatic fringe electronics

Guarantee that "most"???????

Yup. I don't think I have ever seen a relay that wasn't guaranteed to hold at 50% of rated coil current. Read your specific relay data sheet to be sure.

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John Larkin         Highland Technology, Inc 

lunatic fringe electronics

Except that the zener is forward biased when you're trying to pick the relay.

You missed the point. How a guarantee that doesn't apply to all, work?

It's a nonsense statement. I'm nitpicking because ill-conceived requirements and vague statements that imply something that really isn't are the thorn in the side of any project.

It's like the newspaper ad: "UP TO 50% off..."

I think you missed it.

it isn't guaranteed to work with all relay _types_ but for most it will

you have to check the datasheet for the specific relay you use

-Lasse

"I guarantee it will work...in cases where it works...you have to check to see if it works...but I GUARANTEE it will work...except in cases where it doesn't work. Nonsense...

Sno-o-o-ort >:-} ...Jim Thompson

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