HELP needed with relay bounce

Hi All

I've been trying to construct a pressure regulating device, using a 12v

latching solenoid and conventional valve, to protect a filter in a home

irrigation system. I am using a pressure switch which goes contacts open at high pressure/contacts close at falling pressure. These two states in turn trigger 555 timers which in turn drive small 12v SPDT relays.

The relays are configured so that in the relaxed state the circuit connects to the 0v line and when exited by the pulse from the 555, switches to supply 12v for each relay in turn. The purpose of this switching configuration is so that the 12v latching solenoid, which operates the valve, can be switched to close the valve when pressure reaches a high threshold, but then by reversing the supply voltage, is switched to open the valve at a low threshold.

This all works perfectly - until the latching solenoid is connected, whereupon the action of the solenoid causes the relays to bounce erratically.

I have tried various combinations of (non polarised) capacitors and R/C

combinations across the terminals of the solenoid, and/or across the relay. I have also tried using seperate 12v power supplies to power the two sides of the device (timing - relay activation side and power supply for solenoid side)but the action of the solenoid still causes the relays to bounce erretically.

I feel I am running out of options to solve this dilema, but am now considering the use of switching transistors or solid state relays. Only I'm not sure how to switch such devices on and off to give the necessary pulse to switch the solenoid on then off etc.

Any help would be greatly appreciated

Reply to
chris.knight
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The above test may mean this won't work, but: I have seen a 555 circuit that behaved badly when switching a large relay. It was supposed to turn on the relay for about 2 seconds when power was first applied, then turn off the relay. When tested, it just sat there and cycled endlessly. The problem was simply that the current initially drawn by the solenoid was dragging down the supply enough to reset the 555; a big electrolytic capacitor across the power supply leads near the 555 fixed it.

Were the two supplies totally separate... no common ground or anything? If you're not sure, you might use a 12 V battery for one supply or the other to make _sure_ the supplies are isolated.

Random thought: is the problem possibly with the _relay_ coils, instead of the solenoid? Have you tried installing diodes "backwards" across the relay coils? In other words, orient the diode so that it's not conducting when the relay coil is powered. Something like a 1N4007 is usually good. You can get quite impressive spikes from a relay coil; I have seen spikes to over 100 V from the coil of a 12 V automotive relay. If this is the problem, I can't quite explain why the relays would behave with no load on the contacts and act up with a load, but it's something simple to try.

Matt Roberds

Reply to
mroberds

On 11 Jul 2005 17:31:38 -0700, "chris.knight" wroth:

The problem has nothing to do with relay bounce. It's due to pressure fluctuations in the system because the solenoid abruptly shuts off the flow and the pressure changes rapidly. Of course the pressure sensing relay sees this fluxuation and appears to chatter.

The usual fix for problems like this is a pressure "accumulator". A "T" fitting is placed in the system between the solenoid and the sensing relay. At the open leg of the T a short connection is made to a bottle of some sort. The bottle is the accumulator. The bottle is placed with the connection to the T at its lowest point. The bottle is half filled with water with air over that. The water pressure compresses the air and a little more water enters the bottle. When the pressure in the system tries to change rapidly, a little of the water flows into and out od the bottle thus not allowing the pressure to change as rapidly as it would without the accumulator. This smooths out the pressure changes without affecting the overall pressure.

Jim

Reply to
jmeyer

On Tue, 12 Jul 2005 01:13:25 GMT, snipped-for-privacy@nowhere.net wroth:

Substitute "pressure switch" everywhere except the first time I wrote "relay".

Jim

Reply to
jmeyer

If the problem is electrical rather than a pressure surge as described in another post, the problem is almost certainly the inductive voltage kick from the solenoid when voltage is removed. In the old days a snubber (R&C) inseries was used but finding the correct values was always a problem. Nowadays a device call a bidirectional transient voltage suppressor or bidirectional Tranzorb does a much better job - see

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for an example.

These devices are widely used to protect electronic wiring from high voltage noise spikes

Dan

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Dan Hollands
1120 S Creek Dr
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Reply to
Dan Hollands

Sounds probable. More here:

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Reply to
JeffM

Firstly, I used two batteries to give totally seperate voltage supplies. The diode concept is probably worth exploring a bit further, particularly the Tranzorb device. I'll try and locate some Secondly, the problem is electrical, not due to pressure fluctuations, as at this stage I am yet to fit the valve in the irrigation line. The comments about pressure fluctuations are well received though. I'll watch out for problems along those lines when I finally (optimisticaly) get it sorted

Reply to
chris.knight

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