Solving the problem of 'Back EMF'

[cygnusv]

After changing my Johnson fresh pump multiple times under warranty I got sick of doing the job and bypassed the the pump's on-board pressure switch that was the cause of the failure every time.

I fitted an external (Whale) pressure switch and ran it through a 24 volt (our system voltage) 40 amp relay.

After burning out the contacts of the Whale switch a few times I was advised (by friends on YBW) to fit a diode across the relay's + and - contacts. This has completely cured the problem of the micro-switch in the Whale switch burning out, but every 2 to 3 months I have to replace the relay as its internal contacts burn out. We use the pump many times every day, as live-aboards.

Can I fit another (perhaps more heavy duty) diode across the pumps power supply,or is there another wheeze I can use to prevent the relay burning out?

Edited to add.

Has this problem with fresh water pumps been solved? It was endemic a couple of years ago when I first had it. So a question for high volume users of the water pump.

Do you own a current model water pump that you've used A LOT that has remained fully functional? I don't mean the 20 year old stuff, I mean the 'off the shelf' stuff available today. I'll keep changing relays on mine until I find a new pump that has had this problem addressed and cured.

 

[cliff]

Dump the diode and fit a capacitor across the switch or relay input/output.
Think old fashioned points in a car and the little suppressor fitted across the terminals to stop the points burning away

 

[cygnusv]

Hi cliff

What sort of spec capacitor would you suggest on my 24v system? How would you wire the capacitor? On a car a capacitor can produce a 25,000 volt spark Where does that go?

As you may have guessed, I know a bit - but not enough! Thanks so far

Edited to add:

Will this protect the relay and the micro switch? Thanks, Stu

 

[Norman_E]

On the car it was the coil, not the capacitor that produced the high voltage spark. You need someone who knows the formula to relate the current involved to the size of capacitor but cliff is right, it is a capacitor not a diode that you need to suppress arcing at the relay contacts.

 

[cygnusv]

The pump is a Johnson 24 volt x 6-8 amp (I think......) 5 US gallons per minute @ 42 PSI. Fuse rated at 10 amps max. Does that help anyone in doing the math?

 

[JumbleDuck]

On the car it was the coil, not the capacitor that produced the high voltage spark.

It's both. You get an LC resonance in the coil and capacitor, and that's what gives the spark.

 

[lw395]

On the car it was the coil, not the capacitor that produced the high voltage spark. You need someone who knows the formula to relate the current involved to the size of capacitor but cliff is right, it is a capacitor not a diode that you need to suppress arcing at the relay contacts.

Diodes are frequently used for this purpose.
An ignition coil is different, a diode would stop the spark at the plug.
You can use a capacitor, something like 0.1 to 0.5 microfarads (uF) should be OK, but it needs to be rated for quite a lot of volts, 50v might be OK, more is better.
Too big a cap and it causes arcing as the relay closes discharging it.
A cap across the points and a diode across the motor preferably.

 

[nigelmercier]

After burning out the contacts of the Whale switch a few times I was advised (by friends on YBW) to fit a diode across the relay's + and - contacts...

The diode should go across the coil, not the contacts. It should be connected so it doesn't normally conduct when the relay is energised.

 

[William_H]

Back EMF is a function of the magnetic field collapsing when current is turned off. It always appears in the direction that will continue the magnetic field.
The motor will act in a similar way except that you will also get the inertia of the motor and pump continueing to turn after disconnecting power. The motor will act as a generator producing a voltage. As said a capacitor may soak up this excess voltage which can appear across the contacts of the power switch/ relay that provides power to the pump.
You want a ceramic capacitor rated at 200 v working and about .1 ufd. Microfarrad. If that does not do the job a diode across the switch/ relay contacts connected such that it will not pass current to the motor when the switch is open. It is unlikely that any of the normal power diodes rated at several amps and 200+ volts will be overloaded. good luck olewill

 

[Beyondhelp]

Dump the relay totally, and use something like an IRFZ44N - A Power Mosfet. No more contacts to fail, no more problems. Relays are old skool!!!

 

[nigelmercier]

If that does not do the job a diode across the switch/ relay contacts ...

As I keep repeating, the diode goes across the coil, not the contacts.

 

[cygnusv]

Dump the relay totally, and use something like an IRFZ44N - A Power Mosfet. No more contacts to fail, no more problems. Relays are old skool!!!

Hi Beyondhelp. I found some good info on this at www.petervis.com. Maybe you could confirm but I read that the IRFZ44N was only good up to a max of 20 volts. As we're 24 volts we can expect 28 volts and a little more when running. Can you suggest a Mosfet that would be suitable for the higher voltage. Also Peter Vis mentions the need of a 'pull down' resistor between the Gate and the Source. Can you suggest a suitable value for the resistor?

This does sound very attractive. Is there a special holder for the Mosfet or can I solder the wires directly to it?

Many thanks.

 

[lw395]

As I keep repeating, the diode goes across the coil, not the contacts.

Repeat it as often as you like, it won't make it right.
Or at least it's not the whole story.

A diode across the coil can help to protect whatever switches the coil on and off. Specifically off.
The coil is an inductor, changing the current it in produces a voltage -Ldi/dt.
So reducing I to zero instantly produces a large +ve voltage.
If you're driving a relay with a transistor or puny switch, put a diode across the coil.

But the relay contact burning is caused by the inductance of the load the relay is switching.
A diode across the motor gives a current path.

The Back EMF is a different thing, it I the motor acting as a generator because it is still spinning when you stop powering it. The open circuit volts remain high until the motor slows. A diode across the contacts dumps the energy
charge into the battery and anything else that's connected.
The back emf is a much slower effect than the inductance, seconds perhaps for a motor to stop, vs microseconds or milliseconds for the inductance effects.

I hope that helps?

Optimally fit a transorb or two.

 

[maxi77]

Arcing and as a result spark erosion are a reality in switches and especially so in dc switches. AC switches of the current flow itself every half cycyle but dc just tries to keep going. This is why most switches and relays are derated for dc from their ac rating. Most specifically dc switches use mechanical means to break the contact as fast ascpossible to creat a gap the spark cannot cross as quickly as possible. Either change to a solid state relay or use a higher rated conventional relay togive you longer life. I suspect no additional circuitry will suppress the basi spark caused by interupting the current flow.

 

[nigelmercier]

But the relay contact burning is caused by the inductance of the load the relay is switching.
A diode across the motor gives a current path...

So if this is the problem, the diode goes across the motor, as you say. Still not across the relay contacts.

 

[lw395]

So if this is the problem, the diode goes across the motor, as you say. Still not across the relay contacts.

It can go across the contacts.
But a transorb or back to back zeners to clamp the voltage across the contacts to less than, say twice the operating voltage is better than just a diode.
If the issue actually is motor back emf, that's where a diode is what you want across he contacts.
A single zener is sometimes used as a compromise.

I'm using these:
http://www.te.com/catalog/pn/en/RF1256-000

to stop things being twatted by static.

The FET alternative is OK, but note most of them have an inbuilt 'diode across the contacts'.
Go for more than 2x operating voltage as a rating.
Plenty of choice in RS.

 

[Beyondhelp]

Hi Beyondhelp. I found some good info on this at www.petervis.com. Maybe you could confirm but I read that the IRFZ44N was only good up to a max of 20 volts. As we're 24 volts we can expect 28 volts and a little more when running. Can you suggest a Mosfet that would be suitable for the higher voltage. Also Peter Vis mentions the need of a 'pull down' resistor between the Gate and the Source. Can you suggest a suitable value for the resistor?

This does sound very attractive. Is there a special holder for the Mosfet or can I solder the wires directly to it?

Many thanks.

Actually, it would work fine, The maximum Gate or switching voltage is 20, which does appear a little low, however they draw for practical purposes in this context almost no current to switch on, this means via a simple resistor voltage divider network you can drop the gate voltage to anything above 4v (the upper minimum guaranteed fully switched on voltage), this will take care of the pull down resistor required on the input as well.

The Gate to Drain max voltage is around 55volts, so plenty with headroom for your application

Out of interest what currently turns on the relay you have now? Also its worth knowing that this type of MOSFET is an N-Channel, this means that the device to be controlled by it is switched on by the negative side of the supply, known as a pull-down. If that's not very clear, you would in this instance require the pump to be fed via a fuse a constant 12v supply, and the MOSFET sits on the negative side of the motor coil. Don't be alarmed if this sounds odd, its actually incredibly common and pretty much all solid-state (transistor/MOSFET) controlled devices are driven this way.

If you are seriously considering this, I'll scribble down a little diagram, if your capable of assembling it yourself, I suspect you could make it for around £10 parts inc a box to mount it in. Unlike a power transistor, it wont need any cooling the on resistance of this FET is so small that it will dissipate almost no heat.

 

[nigelmercier]

It can go across the contacts...

It can, but it will work much better across the motor coil, and as close as possible to it.

 

[JohnGC]

It's standard practice to use diodes in DC systems.

For AC the standard is a series resistor and capacitor (often in a single assembly that looks like a capacitor).

A capacitor on its own will not dissipate the energy sufficiently quickly to suppress a spark and may actually make matter worse.

Nigel is correct; it is better to fit the suppression across the coil because that is the source of the energy when you open the switch (or relay contact). If you fit the suppression components across the switch (or relay contact) it will still work but the currents will circulate around a much greater length of cable. The energy will dissipate more slowly and the long wires make good aerials so you're more likely to hear clicks on your radio.

There is no need for anything more than a diode across the coil.

A MOSFET requires protection also, without it will fail sooner than a mechanical switch. Of course there are MOSFET "relays" which include protection within the module.

The most important criteria for a protection diode is its peak current handling (which will be several 10s the continuous rating). If the motor continues to run for a while after power is removed then the diode will need to dissipate more energy.

The diode reverse voltage should be substantially higher than the maximum supply voltage (spikes included).

Given that diodes are cheap I would simply buy a big one. This one is over specified but inexpensive;

http://uk.farnell.com/multicomp/ug4d/diode-ultra-fast-4a-200v/dp/1625277

 

[Beyondhelp]

The FET I suggest above is used to drive inductive loads, is fully avalanche rated and is thus designed for the job already.