relay to power a 12v bilge pump

[cheagavara]

hi folks, im new in here and its nice to meet you all. i am looking to install a new bilge pump in a house boat. its not practical to use a float switch as there is often debris in the water that sticks the switch open or closed. i am planning on using one of these

http://www.kemo-electronic.de/en/House/Garden/M158-Water-Switch-9-12-V-DC.php

but the raly side is only i believe 5amp and i need 15 or 20 depending on the pump so i was thinking of emplying a relay. this will sound simple to most of you but i don't know if i need to create a current across the coil to activate the relay? any advice on what is probably a simple one would be helpful.

 

[KrisBlueNZ]

Right, you need an "interposing" relay to boost the switching capability of the existing relay in the water detector.

You need to use the existing relay contact to complete the circuit from a power source to the coil of the interposing relay. So you connect your positive supply to one contact of the relay in the water detector, connect the other contact to one side of the coil of the interposing relay, and connect the other side of the coil of the interposing relay to the negative connection of your power supply.

You must also connect a diode across the coil of the interposing relay. This is needed because when the contact opens, and the current in the coil is interrupted, the interposing relay's coil generates a "back EMF" voltage that can be quite high. This generates interference, and also causes arcing in the contacts of the relay in the water detector, which will make it fail prematurely. A suitable diode is 1N5404: http://www.radioshack.com/product/index.jsp?productId=2062579

Connect it across the coil of the interposing relay, with the cathode (the stripe end) to the side of the coil that receives the positive voltage.

 

[cheagavara]

Hey thanks for the quick reply and it does sound like the solution so will look into it
My original idea was to use an electro magnetic relay and a bulb......sounds stupid but I was going to conect one side of water switches relay to poss power supply and on other side of water switch relay to a small bulb and then to negative of power supply so in theory when the water switch closes the internal relay it would create a current through the coil of the second bigger electromagnetic relay and close that so the pump load would then go through that. Is that madness ?
Your suggestion sounds much better and more reliable
Sorry for any typos but trying to reply on pad whilst on a boat

 

[KrisBlueNZ]

I don't understand why you would use a light bulb. You've described my circuit, except that you're switching your power to a light bulb instead of the coil of an interposing relay. I don't know why you would do that.

 

[cheagavara]

sorry i am new to this, i dont think i explained. i was going to use the bulb on the coil part of the mechanical relay to create a load/current through the coil to activate it.
do i not need a current through the coil side of the mechanical relay? i was going to connect positive to one side of the coil, the other side i ws going to connect to a bulb, from the bulb to one side of waterswith relay and then on to neg of power supply, my (stupid) thinking was that when the water switch closed its relay it would complete the circuit and create a current through the coil and this would create the electromagnetic current to pull the relay....like i said i am new to this so will probably be completely wrong

 

[KrisBlueNZ]

A light bulb? That won't help. A bulb doesn't create current. It just glows.

Yes you need current to flow through the coil of your interposing relay. You connect a power source, through the contact on the controller, to the coil of the interposing relay, and return the other end of the coil to the negative side of the power source.

You're right with your description, apart from the part about the light bulb. You don't need a light bulb. All it would do is limit the voltage available to the relay coil.

Applying a DC supply straight across the coil of the interposing relay will make it pull in. So you just need to interrupt that supply voltage by connecting the contact of your controller in series with it.

And you need the diode that I mentioned earlier.

 

[cheagavara]

im obviously missing something...if i connect one contact, lets say the right hand side, from the relay coil to the positive of my 12v battery, the left hand side i connect to one side of my water switch relay, the other side of my water switch relay to the negative side of my battery, when the water switch detects water it will close its relay thus completing the circuit through the coil side of my second relay....wont that create a short circuit ? a direct pos to neg connection through my second coils relay, a bit like connecting a wire direct from pos to neg on the battery?

 

[cheagavara]

im obviously missing something...if i connect one contact, lets say the right hand side, from the relay coil to the positive of my 12v battery, the left hand side i connect to one side of my water switch relay, the other side of my water switch relay to the negative side of my battery, when the water switch detects water it will close its relay thus completing the circuit through the coil side of my second relay....wont that create a short circuit ? a direct pos to neg connection through my second coils relay, a bit like connecting a wire direct from pos to neg on the battery?

 

[KrisBlueNZ]

No. A relay coil is not a short circuit. They have a resistance, usually a few hundred ohms, depending on how big the relay is. Bigger gruntier relays have less resistance but they can still be connected straight across the power source - assuming the coil is specified for that voltage. That's how you use them.

 

[cheagavara]

ok, thanks kris that has enlightened and helped me a lot, will order the relay and diode today this will solve a long standing problem, previously my father in law has used automatic submersible pumps but they don't last long and are expensive to replace whereas a jabscoe self priming pump sits out of the water and has replaceable impeller

 

[KrisBlueNZ]

No problem. Have you chosen your relay? If not, what is the supply voltage, and how much current does the pump draw? You said 15~20A, right?

 

[cheagavara]

no not chosen relay yet, two choices of pump at the moment so max would be 20A and that would also allow for an up grade to some of the bigest jabsco self priming if needed at a later date.

 

[KrisBlueNZ]

What's the DC supply voltage in the installation?

 

[cheagavara]

sorry, should have said its 12v by way of a battery that is connected to a charger, the intelligent type of charger thats safe to leave permanently connected

 

[KrisBlueNZ]

Here are some recommendations for relays available from Digikey.

1. TE Connectivity VF4: http://www.digikey.com/product-detail/en/1432793-1/PB1773-ND/1236843 USD 2.83; 30A; automotive; chassis mount (via a tab with a hole for a screw); includes the diode.

2. Panasonic CB: http://www.digikey.com/product-detail/en/CB1A-M-12V/255-1828-ND/647020 USD 5.51; 40A; automotive; chassis mount (via a tab with a hole for a screw); fully sealed.

3. Panasonic CB: http://www.digikey.com/product-detail/en/CB1AH-M-12V/255-2079-ND/1242027 USD 7.03; 70A; automotive; chassis mount (via a tab with a hole for a screw); fully sealed.

Items #2 and #3 are the 40A and 70A versions of the same relay. Neither of them have a diode built-in, like #1 does, but they are both fully sealed.

Even if you don't buy from Digikey, these suggestions will give you an idea of what to look for.

 

[cheagavara]

thanks for that, on #1 what do they mean by
Turn On Voltage (Max) 7.8 VDC
Turn Off Voltage (Min) 1.2 VDC
sorry to apear bumb

 

[KrisBlueNZ]

Those are voltages across the coil where the relay is guaranteed to turn ON or OFF.

If you put an adjustable voltage onto the coil and increase it slowly, by the time it reaches 7.8V the relay is guaranteed to be activated, i.e. its contacts will be closed. Of course you're supposed to put 12V across the coil, but if the 12V supply is a bit low, the relay will close, as long as it's at least 7.8V. It may not close with much force though, so there could be arcing in the contacts. That's why you should use 12V.

Once the relay is closed, if you decrease the voltage you're applying to the coil, at some voltage the relay will drop out or "release". That specification says that when the voltage has reached 1.2V, the relay is guaranteed to have dropped out.

I just realised. If your load is a pump, you should have some kind of suppression across the relay contacts, as well as the diode across the coil. The pump and the wiring going to it will have inductance, which causes a "back EMF" when the current is interrupted. This back EMF can damage the contacts as they open, and eventually wear out your interposing relay.

You could just buy a few spare relays, but this is a bit of a critical application. If the relay fails and you don't know about it, your boat could sink, right?

I'm not an expert on this high-current stuff. My best suggestion would be a high-current diode across the contacts, cathode to the positive supply side and anode to the pump side. This one looks suitable: Fairchild FFH75H60S: http://www.digikey.com/product-detail/en/FFH75H60S/FFH75H60S-ND/3855010 USD 3.67; 75A. This would not need a heatsink because the pump only turns off periodically.

IF ANYONE ELSE
HAS ADVICE ON SNUBBING RELAY CONTACTS FOR USE WITH A DC PUMP, please let us know!

 

[cheagavara]

yes you're right about it being a critical application, the pump is on for around 4 hours then off for around 6 then back on. this cycle lasts around 5 days then is dormant for 9 days. one other trhing, do you know of anything i can use to step 12v dc battery down to 9v dc regulated? as the water switch will work at 12v but prefers 9v
just ordered this
http://www.thetoolboxshop.com/0-727-14-durite-12v-30a-mini-make-and-break-relay-with-diode-2155.html

was going to order from digikey and the relay was only £1.88 gbp but shipping was £12 gbp so found alternative

 

[KrisBlueNZ]

one other trhing, do you know of anything i can use to step 12v dc battery down to 9v dc regulated? as the water switch will work at 12v but prefers 9v

The simplest option is a 7809 three-terminal regulator. See http://www.maplin.co.uk/p/ts7809cz-1a-positive-fixed-voltage-regulator-to220-case-n37ca
That page claims "no external components required" but you should have decoupling capacitors on the input and output as described in the data sheet at http://www.fairchildsemi.com/ds/LM/LM7805.pdf.
Suitable capacitors are http://www.maplin.co.uk/p/resin-dipped-ceramic-01uf-capacitor-ra49d (you need two).
The 7809 has a "dropout voltage" of up to 2.5V. That means that the input voltage needs to be at least 11.5V otherwise the output voltage will start to drop. Can you guarantee this?

just ordered this
http://www.thetoolboxshop.com/0-727-14-durite-12v-30a-mini-make-and-break-relay-with-diode-2155.html

That looks good.

was going to order from digikey and the relay was only £1.88 gbp but shipping was £12 gbp so found alternative

Yeah, their international shipping is a bit prohibitive. I had assumed you were in the USA. It would save time in future if you put your location in your profile. There is a field for it.

 

[cheagavara]

ok, thank for that i will have a read. given that the power supply for everything is a 12v lorry battery i don't think i can guarantee the 11.5v though