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Float Switch for 240V pump using a relay

Ben Fairall

Sep 28, 2015
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Hi all,

This is my first attempt at anything involving electronics since my school days so please forgive any ignorance.

Challenge: I have a 240V, 2Amp Pump which I would like to pump once a tank is full of cider, so it pumps when it is at or above that level but not otherwise.

Due to having water with electrics I figured having 240V inside the acid liquid (well 3.5ph cider) was not the smartest of moves. So I planned to have a low volt switching circuit and a higher volt circuit.

Equipment:
Float switch: http://www.ebay.co.uk/itm/Vertical-...loat-Switch-/271665233459?hash=item3f40827e33

Relay: https://maplindownloads.s3-eu-west-1.amazonaws.com/n13aw-6706.pdf

Have some step down transformers, batteries and wires also and an extension lead I plan to use to plug the motor into.

I had it quite clear in my head that the float switch would switch the relay at a low voltage, which would then engage the higher voltage circuit. However now the relay has arrived it has 8 pins and none say what I expected on - so am stuck.

any help would be greatly appreciated!

Thanks
Ben
 

AnalogKid

Jun 10, 2015
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First, you don't say what you did expect, so it's hard to know where to start. Also, the data sheet covers about a dozen different parts. Please give us the exact part number for the relay you have. Without that, there is no way to recommend a circuit.

ak
 

Ben Fairall

Sep 28, 2015
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Sep 28, 2015
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Thanks for your reply,

I have attached the picture of the relay I have, it is the BMY5-2C5-S-VW-COIL 240VAC 50/60HTZ

I expected to have a 5 pins. 2 for the 240V pump circuit and then use 2 of the remaining pins for a lower volt circuit depedning if it should activate on pump switch enabled or disabled.

Was trying to copy this video:
 

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AnalogKid

Jun 10, 2015
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Your relay is DPDT which is more than enough switched contacts. However, it has a 240 VAC coil. Not much current, but still you will have to have 240 VAC on the level switch contacts and the part you have is not rated for that, so the relay you have will not work reliably. What step-down transformers do you have? Best would be one with a secondary rating equal to the relay coil rating. Once we know the control voltage we can determine the correct relay.

ak
 

hevans1944

Hop - AC8NS
Jun 21, 2012
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Once we know the control voltage we can determine the correct relay.
This is the key requirement. You purchased a relay with a coil voltage that is too high. Relay coils come in two flavors: AC and DC. If you are using a step-down transformer to get low-voltage AC from 220 V AC choose a relay with an AC coil that matches the secondary voltage of the transformer, but no more than say 24 V AC. Your float switch only has a 100 V DC rating for the contacts, but 24 V AC (or less) it will probably be okay. For the same style relay, however, lower coil voltage means more coil current. It is unlikely you will need to consider that as a factor unless you choose a voltage less than 12 V AC for the relay coil of the series relay you cited above. Note that a 6 V AC relay coil from this series requires more than 0.6 A, which exceeds the 0.5 A current rating of your float switch.
 

Ben Fairall

Sep 28, 2015
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Thanks,

I assume it is not the amps but the volts I want to avoid going through the liquid in the float switch.

So I think I need a new relay.

I have a 300mA unregulated AC/DC adaptor which has different voltage options from 3 to 12V DC. I have a few others from phones/routers/set to box's etc. But they all seem to be DC.

Would you be able to suggest the best kind of relay and I will have a search on maplin.

Thanks for your help
Ben
 

hevans1944

Hop - AC8NS
Jun 21, 2012
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From the series of relays you cited, one with a 12 V DC coil would work well with your float switch and a 12 V DC adapter. The coil current requirement is only 0.075 A, well within the current capability of your AC/DC adaptor.
 

AnalogKid

Jun 10, 2015
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Unless your float switch is unlike most others, there will be no voltage or current "in" the liquid. Usually the switch is immersible but fully insulated.

ak
 

Ben Fairall

Sep 28, 2015
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Yes it is insulated and working normally it should be okay in the liquid with 240v and amps for the pump (wiring with no relay). I am just worried how long it will last insulated understanding the liqid is very acidic and will be going up and down multiple times a day.

Hence the aim to go for a lower number of volts. Hopefully will be slightly safer?
 

Minder

Apr 24, 2015
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Do you have an auto wrecker near you? the 12vdc coil relays are plentiful and the vast majority have 500vac insulation/dielectric rating between the coil and the contact.
M.
 

Minder

Apr 24, 2015
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Yes probably, There are also many that state the contact voltage insulation rating.
You may find a salvaged one from a typical auto may be better than from China, although many do profess the high voltage on the side.
M.
 

AnalogKid

Jun 10, 2015
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Yes it is insulated and working normally it should be okay in the liquid with 240v and amps for the pump (wiring with no relay).

No, it won't. It is rated for 100 V, not 240 V, and only 0.5 A, not 2 A. Without some kind of interface device or circuit in between the switch and the pump, the switch will fail quickly. Think solid state relay.

ak
 

hevans1944

Hop - AC8NS
Jun 21, 2012
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Great cheers.

Unfortunately it dis still not appear they sell that one in 12V DC.

Have found 2 others that are 12V dc however unsure if the

http://www.maplin.co.uk/p/round-base-10a12v-dc-dpdt-relay-jg58n

http://www.maplin.co.uk/p/12v-dc-2a-dpdt-bt-type-47-equivalent-relay-n17aw

Would either of them work. They also still seem to come with a large number of pins
The first one with an 8-pin octal tube socket is ideal for your purpose. Easy to solder wires onto the tube socket terminals. Your should mount the tube socket inside a protective plastic or metal box, protect the solder connections with shrink tubing. The second one is designed for printed circuit board mounting, nor are the contacts rated for 240 V AC at 2A. I would not attempt to go this route. Coil current requirement for the octal-based relay is well within the current limits of your 300 mA AC/DC adaptor. A diode across the relay coil will help to protect the magnetic reed switch in the float switch. Just make sure the polarity is correct: anode toward the negative supply terminal, cathode toward the positive supply terminal, float switch in series with relay coil and power supply. The diode doesn't normally conduct until the switch opens and the collapsing magnetic field of the relay coil generates a voltage spike that will forward-bias the diode, which upon conducting will limit the spike to about 0.7 V and protect the switch contacts.
 
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