will this cause a voltage spike?

You "bench tested it?" the spike aside for a moment, how does one bench test it?

A "pulse" may be any pulse width - needs to stay on long enough to charge the cap to hold the armature down when the pulse is gone - did you consider that? One assumes this is a DC pulse - since it wouldn't work with AC . . .

The output impedance of the pulse generator may be low or high - if high you have a time constant between the impedance and capacitor that may limit charging.

Two of several failure modes is that a spike goes back to the pulse generator and eats it - unlikely because the cap would absorb the energy - there is no spike to speak of, the cap just peters out, it doesn't switch off abruptly. The cap would appear to be a short circuit to the pulse when initially charging and that may eat the pulse generator.

So, without knowing more about the device generating the pulse there's no good way to tell.

If you are controlling an AC device with the relay, you may want to consider a solid state relay. Many take an input of 3-32 volts for the coil at 10 milliamps and will switch the output down to 24 VAC or up to 240 VAC.

The 10 milliamp spec means it wouldn't load the generator as much as any coil you're likely to find, 3 volt spec would take a smaller cap to stay on, and wouldn't generate any reverse EMF when the circuit opens.

One good idea might be to use a solid state relay.

The one second will be highly variable with this jury rig, one assumes that would be tolerable?

The bench testing was done simply by waving a wire past a contact such that it made contact very briefly. The pulse from the machine is 24VDC and 100ms long. The pulse comes from a relay in the machine control that connects to the 24VDC rail. The relay I need to switch on could control an AC load because it's up to me whether to use a DC or AC solenoid. The on time of the relay can be longer than 1 second but not shorter. This is because the solenoid that the relay controls actuates an air valve that then operates a clamping device. This device needs just a little less than 1 second to open completely. The power for the solenoid is separate from the CNC control. I like your idea of using a solid state relay. Since the pulse from the machine is pretty constant doesn't that mean that the solid state relay on time will be as constant? Thanks, Eric

You may want to put in a flyback diode (free wheeling voltage some call it) to absorb the high voltage from the coil in case something happens to the capacitor. Also, you may want to consider using a One-Shot timer so that a small load is introduced from the pulse of the CNC machine. The method you're using now maybe taxing the output circuit of the CNC machine. I can only assume that you're using a rather large value to be able to hold the relay? that is, a relay suited for the load you're applying.

Something else that occurred to me - putting a cap across an ordinary coil type relay will have another unintended consequence - the relay will drop out much more slowly - the contacts will come apart with glacial speed, that in turn, is likely to promote arcing and shorten the life of the contacts.

SSR wouldn't have that problem.

They do make SSR's in a variety of flavors these days normally closed and AC/DC types etc.. But the simplest, least costly, is normally open to switch AC with a DC control of 3-32

If the pulse is generated by a relay in the machine, there's an excellent chance it will work the way you want without problems.

Actually, the way Relays work by nature (most of them) produce their own Hysteresis. I don't think you'll need to work about that how ever, putting a snubber on the contacts for the inductive load that is connected to it will help save the life of the contacts.

For example, a 12 volt relay will pull in at around 9 volts on the average and will hold down to lets say 6 volts. at the point of where it releases from the coil head will start the rapid decay of attraction as the contact level arm then starts to move aways, which also, weakens the attraction along with the coil voltage dropping. Since you have DC with a CAP going to the coil, there will be no vibration effects. Now, if you were applying this theory using AC, this maybe a different story.

Greetings Jamie, I have decided to go with the SSR. It seems like it's the best way if I can still use a capacitor to keep the relay on for the required 1 second. However, I have another application where a relay is controlling a contactor that controls an AC motor. The relay does suffer from burned contacts and sometimes sticks. So I have replaced the relay a couple times. The hot tub place told me the crummy design is why the company that made the controller went out of business. So, to help prevent the contacts burning how do I make a snubber? Thanks, Eric

A snubber is a combination of resistor in series with a cap across the contacts. They can be made or bought in that form, in a single package.

I have no idea how to size the cap resistor combo . . . typical values are 100 ohm resistor in series with a point one microfarad cap. The cap has to be a able to withstand the AC voltage and should be a high quality low inductance type. The resistor should be flame proof.

The switch on my computer speakers was causing my power supply to reset due to a transient it created - a snubber fixed it. (.1uf/100ohm)

Jameco catalog shows six ready made snubber networks as well as some individual caps suited to transient suppression applications. One snubber is a one microfarad in series with a 100 ohm 1/2 watt resistor, the others are all point one microfarad in series with 22,

The cap should be polyester or polypropylene dielectric rated for the AC voltage you will be using or 2-3 times higher if rated for DC.

In article , snipped-for-privacy@whidbey.com (known to some as Eric R Snow) scribed...

You would do better to use an electronic time-delay relay set for a one-second delay-on-release. The problem with a cap across the coil is that you introduce some delay on activation as well. Such delay is not present with a properly-designed electronically-controlled TD relay.

Using such a device will also eliminate the possibility of any 'spike' being returned back to your driving circuit.

Happy tweaking.

Thank You Default. I buy from Jameco from time to time so I'll check out their stuff. Eric

How much delay on activation do you figure will be introduced?

Ed