Relay - What Kind, and Will This Work?

[breister]

All right all you electrical smarties, I know someone knows the answer to this.

I want automatic fuel transfer from an aux tank. Float switches will not handle enough current to power the pump, which means one or more relays. I don't want the pump turning on and off every time fuel sloshes, so I need to be smarter than the fuel (difficult on some days). Finally, I do not want the relay coils to have to be continually nor intermittently (due to sloshing) energized (that's a bad thing, isn't it?) when using a latching relay to avoid burning out the coils.

Here is what I imagine might work:

Questions:

Will this work, or will the current to the coil be interrupted before it develops sufficient momentum to complete the state change?

What kind of relay do I need - is this a SPDT or a DPST latching-type relay?

Has anyone already done something like this, and would you share your solution?

Many thanks!

 

[breister]

I may have already answered part of my own question.

If I reverse the polarities and have the relay control positive rather than ground, I could include a small capacitor between the relay and float switches so that when the float switch activates it continues to power the alternate-side coil for a moment after the now inactive relay side breaks contact, thus assuring enough of a "push" to get the relay all the way over.

Is this correct, and is it even necessary? If so, what size capacitor should I plan on using?

I somewhat prefer having the float switches only provide a ground path...

 

[JonJay]

Try a programmable relay?

There are many excellent choices in industrial control progammable relays. Try Potter Brumfeld. You can program the outputs with any kind of time delay or interval you need.
It is pretty amazing what you can buy for under $100.

 

[breister]

Ok, so Potter & Brumfield is a brand. I was using the DigiKey search tool trying to locate what I want, but the problem is I don't know what it would be called.

For instance, under P&B there are Magnetic Latching and Mechanical Latching. Which to choose, or neither?

A 12v SPDT latching relay controlling up to 5amps is only $3.50, so if the circuit I drew (or that circuit plus some small mod) works that would probably be fine - I do intend to keep circuit options which are "Hard ON" and "Hard OFF" if the relay fails...

 

[JonJay]

There is no such thing as Hard on or Off.

Ok, so Potter & Brumfield is a brand. I was using the DigiKey search tool trying to locate what I want, but the problem is I don't know what it would be called.

For instance, under P&B there are Magnetic Latching and Mechanical Latching. Which to choose, or neither?

A 12v SPDT latching relay controlling up to 5amps is only $3.50, so if the circuit I drew (or that circuit plus some small mod) works that would probably be fine - I do intend to keep circuit options which are "Hard ON" and "Hard OFF" if the relay fails...

Even a latched relay can fail. Most systems I have seen are manually operated and rely on the pilot to manage his fuel.
If you are dead set on an automatic system, search the P&B website directly for a "programmable" relay. They make all types that can be readily programmed via dip switch settings to make up whatever logic you need. Good luck.

 

[breister]

Had to read a bunch of PDFs, but finally concluded that they have renamed "Programmable" to "Multifunction."

You are correct, latching relays are less reliable - what I now think will be simpler will be a timing function called "Interval Off" (meaning that the timer will commence once voltage is released, allowing me to use my mid-level float in a "fuller is closed, emptier is open" configuration that can also provide an indicator light telling me I have "at least" mid-level fuel in the receiving tank. Setting the interval for a couple of minutes should be enough that I don't wear out the pump once I'm out of fuel, negating the need for a separate "kill the power if transfer tank is empty" sensor.

Now all I need to do is order one and decipher the wiring diagram...

Thanks!

 

[cfiidon]

Enable/Disable

You might consider a switch in there that will enable or disable the auto transfer.

Don

 

[breister]

Yep, will have 3-way switch "On-Off-Auto."

Thanks all!

 

[JonJay]

A little off your topic, but it might apply here...

...a fellow I know ferried a DC3 across the pond. They added a large internal tank and cross over system and a pump to allow the continual pumping of fuel out of the ferry tank while they where flying off the mains. At some point they noticed that the fuel level was falling until they realized they plumbed the pump backwards. They did a mid air reconfiguration of the pump however, with the time lost, the pump was not going to be able to keep up with the fuel flow of the engines. They feared they would run out of fuel with fuel still in the ferry tank unable to pump it out fast enough. They made it but it was a heroing experience.

I am not familiar with DC3's fuel systems and this story comes from memory so take it with a grain or two. Bottom line....
Make sure your pump can keep up with your fuel flow!

 

[dpansier]

12 VDC PLC

There are many excellent choices in industrial control progammable relays. Try Potter Brumfeld. You can program the outputs with any kind of time delay or interval you need.
It is pretty amazing what you can buy for under $100.

http://factorymation.info/plr/fmcat_sg2_specs.pdf

I have used this PLC in a 120VAC model, they have recently released a 12VDC version. With the correct programing and sensors this device could be used to control a number of functions in the RV.
VFE control of the flaps
Wig Wag lights
Starter lockout, etc.

 

[breister]

Either left or right facet pump keeps up fine filling the header tank in my <cough*notRV*cough>. Just need a way to automate the transfer for hands-off operation. The idea is universal, so it COULD be useful for an RV, particularly for a long range ferry tank.

 

[AltonD]

I got to crew an old WWII bomber. It had auto transfer. Nobody would use it. Everyone was afraid it would get stuck on and pump fuel overboard. It was just too darn easy to watch your fuel and toggle the transfer pumps. And then, we would put them in auto just in case we lost track of the transfer.

 

[breister]

I think solid state time-delay relays might be just a touch more reliable than the WWII bomber.

But that's just me.



I will most certainly monitor header tank level to prevent overfill. I'm thinking I will add an extra relay to block power to the pumps when the HIGH level switch is tripped.

 

[JonJay]

Carefull with the wording....

I think solid state time-delay relays might be just a touch more reliable than the WWII bomber.

But that's just me.



I will most certainly monitor header tank level to prevent overfill. I'm thinking I will add an extra relay to block power to the pumps when the HIGH level switch is tripped.

When you say "Solid State Relay" it can infer that the load carrying is being done with Solid State components, like an SCR or Triac. I would not use them. Use a good multifunction (programmable) relay with mechanical contacts. Yes, they have solid state components doing the thinking but the load is carried through good old fasioned dry contacts. I am sure you where not infering anything else but if you ask for a Solid State Relay, you will get the wrong thing.
Not arguing your point at all about reliability.

 

[breister]

When you say "Solid State Relay" it can infer that the load carrying is being done with Solid State components, like an SCR or Triac. I would not use them. Use a good multifunction (programmable) relay with mechanical contacts. Yes, they have solid state components doing the thinking but the load is carried through good old fasioned dry contacts. I am sure you where not infering anything else but if you ask for a Solid State Relay, you will get the wrong thing.
Not arguing your point at all about reliability.

As it turns out, I could not find a solid state programmable relay so yes that's exactly what I ordered.

Solid state is not what it used to be - they are making fine solid state relays suitable for your Master, Starter, and other high-load and even continuous high-load uses. Ultimately I have no doubt that mechanical contactors will go the way of the Dodo.