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Using a Varistor to solve a back EMF problem

Discussion in 'General Electronics Discussion' started by cygnusv, Oct 7, 2014.

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  1. cygnusv

    cygnusv

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    Oct 7, 2014
    Hi everyone, I'm new here and looking to solve a problem I have with burnt out contacts on my boat.

    The problem is with a 24 volt fresh water pump that was continuously burning out pressure switches. I partially solved the problem by bypassing the pump's internal pressure switch and using a simple external one attached to a 30/40 amp relay that has a diode across it. The diode has protected the simple micro-switch that operates the relay, but the relay's contacts fail with monotonous regularity, maybe lasting 6-8 weeks with daily use.

    I've decided to get proper water pressure regulation by installing a mains operated Square D switch. This will ensure a usable pressure range of 20 to 40 PSI. However, this switch also has contacts and I want to protect them with an MOV. I have found that Littelfuse have a V20E25P that will do the job nicely, but their postage costs are ridiculous! I'm having problems though in finding a similar spec MOV. Can anyone suggest a more readily available alternative. Spec below.

    Manufacturer: Littelfuse
    Product Category: Varistors
    RoHS: RoHS Compliant Details
    Brand: Littelfuse
    Product: MOV
    Voltage Rating DC: 31 V
    Voltage Rating AC: 25 V
    Clamping Voltage: 77 V
    Peak Surge Current: 8000 A
    Surge Energy Rating: 120 J
    Capacitance: 12000 pF
    Operating Temperature Range: - 40 C to + 85 C
    Packaging: Bulk
    Case Diameter: 23 mm
    Case Width: 5 mm
    Current Rating: 1 mA
    Dimensions: 23 mm Dia. x 5 mm W
    Lead Spacing: 8.5 mm
    Mounting: Radial
    Series: LV UltraMOV
    Factory Pack Quantity: 500
    Varistor Voltage: 39 V

    Littelfuse part# V20E25P

    Many thanks for any advice. Stu
     
  2. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

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    Hi there and welcome to Electronics Point :)

    Were you trying to buy that varistor directly from Littelfuse? If so, you may find Mouser is a better source, though their shipping rates can be pretty high as well. They have that part in stock: mouser.com/ProductDetail/Littelfuse/V20E25P/?qs=%2fha2pyFadugKxbsAh3dnggXNec4CddfFPzGprCeXQz4%3d
     
  3. cygnusv

    cygnusv

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    Oct 7, 2014
    Hi KrisBlueNZ

    Many thanks for the reply. In fact when I pressed the 'buy button' it went straight through to Mouser. If I'm forced, I will buy from them and pay their price - £12 for postage - for something that can be delivered by post for less than £1, but I don't want to let them get away for what is little less than 'theft by post'! If I pay their price, I encourage them to screw others too. This is why I'm looking for advice. It may be the case of course that the extortion is due to minimum order costs. I can understand that but it won't stop me trying to avoid it!

    So if anyone can suggest a suitable alternative....?

    Thanks again, Stu
     
  4. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

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    You didn't say where you're located, nor who your local retailers are. So I checked Radio Shack; they only have one varistor and it's designed for 115VAC mains protection.

    If you have any other local retailers, check their web sites to see what varistors / MOVs they have. I doubt very much that you'll find a suitable replacement anywhere other than the big e-tailers such as Mouser.

    While you're at it, you might as well order some spares. That might make the postage seem less exhorbitant in comparison!
     
  5. Arouse1973

    Arouse1973 Adam

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    Dec 18, 2013
    Although an MOV will protect the contacts from the flyback voltage produced by inductive loads it won't protect against inrush current. This may also need to be considered if the failing contacts are due to inrush currents from the capacitance of the load.
    Thanks
    Adam
     
  6. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

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    Also, I have no experience with this, but I suspect the best way to increase the contact life would be to reduce the rate of switching, by using a pressure switch with a wider hysteresis deadband. I think this is what Stu meant in post #1: "Square D switch. This will ensure a usable pressure range of 20 to 40 PSI"
     
  7. Arouse1973

    Arouse1973 Adam

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    Dec 18, 2013
    Yeah I also wondered what that was :) Square D switch.
     
  8. Bluejets

    Bluejets

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    Oct 5, 2014
    Assuming your 24v is DC, you will have problems using AC relays or whatever.
    Simply put, they are not suited to the arcing that dc loads produce, especially inductive loads such as motors, as the contacts do not switch quickly enough.
    Design of the contacts and arc barriers are different also in many instances they use different contact materials.
     
  9. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

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    Is the pump AC or DC? If it's DC, just bung a big diode across the relay contacts.
     
  10. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Or even better, the pump.
     
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  11. cygnusv

    cygnusv

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    Oct 7, 2014
    Hi Guys

    I must say I'm very grateful for the help, and your collective interest.

    The pump is a Johnson 24 volt DC rated at 10 amps max. Its design pressure is 42 PSI and the (AC) Square D is factory set for on/off 1.4BAR and 2.8BAR both top and bottom range is adjustable. As noted above this will result in much lower contact operation.

    As the pump is DC would you think maybe big diodes, which I've already got, would be better than MOV's? Either way, I was thinking either a diode or MOV across the pump's power supply and the Square D contacts in an effort to reduce arcing. Would there be any point in fitting both together. (Asks he,
    naively)

    Thanks again, Stu
     
  12. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

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    Right, well, connect a big diode (rated at AT LEAST 10A) across the pump, with its cathode to the positive side. Something like http://www.digikey.com/product-detail/en/BYV79E-200,127/568-3440-ND/1127193 will do, and you won't need to heatsink it, but the TO-220 package isn't easy to tack onto a circuit. Other packages are available but the smaller cylindrical type only goes up to 10A rating, and the large metal stud mount ones are more expensive (see http://www.digikey.com/product-detail/en/VS-1N3210/VS-1N3210GI-ND/16597).

    It's also a good idea to connect a film capacitor across the motor to slow down the voltage transitions and protect the diode from overvoltage due to inductance elsewhere in the system. Something like http://www.digikey.com/product-detail/en/MMK5105K63J04L4BULK/399-5443-ND/1927388.
     
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  13. cygnusv

    cygnusv

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    Oct 7, 2014
    I'm not rushing to fit the new switch, so 'cleaning up' the motor first has got to be the logical first step.

    Am I right in thinking that the capacitor should be connected across the +/- closest to the pump with the diode further away from the pump?

    If I use a large metal stud mount diode, presumably I could break the + pump supply cable, ring tag the 2 ends and fasten them together on the diode while earthing the diode mounting plate?

    As far as the switch contacts are concerned would you recommend a similar large diode from the contact to earth?

    Sorry to be a nuisance, ;), Stu
     
  14. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

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    Yes, that's probably best, but don't put the diode very far away from the pump. Each time the pump turns off, there will be a heavy current spike in that loop, so its area should be minimised.
    Yes, but you would have to use the "R" version of the diode if you want to earth the stud. With the standard, non-"R" version, the stud is the cathode. You need to connect the anode to 0V.
    No, that's not necessary. The capacitor and diode across the pump will be plenty.
     
  15. cygnusv

    cygnusv

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    Oct 7, 2014
    [QUOTE="KrisBlueNZ, post: 1625992, member: 22166

    Yes, but you would have to use the "R" version of the diode if you want to earth the stud. With the standard, non-"R" version, the stud is the cathode. You need to connect the anode to 0V.
    [/QUOTE]

    Thanks KrisBlueNZ

    Oops! I'm a bit confused (and new to this)

    [​IMG]
    Looking at the image, the bottom nut and washers appear to be fasteners to mount the unit, the ring tag at the top appears to be the business end where I would fit my 2 ring tags of the (cut) pump power supply. Is this not correct?

    Are you saying that the bottom mounting should be isolated from the negative ground?

    Sorry to be a bit slow on this.

    Just out of interest, the avatar is the boat I'm on now.

    Thanks, Stu
     
  16. Kiwi

    Kiwi

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    Jan 28, 2013
    Stu,
    What is the pump model?
    Have you measured the pump current to confirm that 10A is its maximum?
    Is the pump wired with suitably sized cable to minimise voltage drop?
    How many times per day is the pump operated?
    How long does the pump go each time it is used?
    Looks like you have been using a 30/40 changeover relay. A relay with a single normally open contact set might be a better option than a changeover type.
    I would suggest that you try a 70A relay. Some come with a resistor or diode protected coil. Fit a diode across the coil if it doesn't have any protection.

     
  17. cygnusv

    cygnusv

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    Oct 7, 2014
    Thanks Kiwi

    The pump is a Johnson 24 volt rated (on the box) at 10 amps max. It delivers up to 5.1 gallons per minute. I'll have to try to borrow a clamp on ammeter to check actual current. We protect it with a 15 amp fuse.

    We live on board all year round and the pump is well used for drinks, washing, showers and auto washing machine etc - so gets the same usage as a domestic cold water supply. The only exception is that water is not used for toilet flushing. It is safe to say that we work the pump hard.

    The supply cable is well up to the task, and it is a dedicated circuit.
    You're right about the the relay, it is a standard automotive changeover type. When supplied, the relay comes with a small harness and there is a small diode across the +/- but this does not prevent the relays contacts burning out.

    The Square D pressure switch is due maybe today, but I'm not going to fit it until I can cure the source problem which is from the pump itself I understand. I understand the capacitor connection suggested by KrisBlueNZ but am unsure about correctly wiring the stud mount diode pictured in post #15. As soon as I know how that connects I'll get the bits ordered and fit the Square D switch then.

    Just to clarify. The Square D will turn the pump on at 1.4BAR and off at 2.8BAR. We have accumulator tanks on both the hot and cold water supply so this should massively reduce contact wear. Hopefully, this, along with cleaning the pump's electrical supply will put this annoying problem to bed.

    Thanks all for the help, Stu
     
  18. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

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    Sorry for the delay.

    270677 schematic 16.png
     
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  19. cygnusv

    cygnusv

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    Oct 7, 2014
    Excellent! Thanks KrisBlueNZ. Got it!

    The Square D arrived yesterday and after a lot of looking I was able to find postage free and no minimum order here in the UK, so ordered the non-R version plus a couple of capacitors and got change out of a tenner.

    Your diagram makes the fitting of the bits a piece of cake.I'll post an update next week when everything is fitted.

    Thanks all for your help. Don't you just love forums like this one! Stu
     
    Last edited: Oct 10, 2014
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  20. cygnusv

    cygnusv

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    Oct 7, 2014
    Hi everyone

    Many thanks to all and particular thanks to KrisBlueNZ for the advice and the life saver diagrams. I fitted all the bits and pieces over the weekend and operated the pump in pitch black with the switch cover off. No sign of any spark at all at the contacts!

    Interesting to note is that for less than the cost of postage quoted by Mouser, I was able to pay for everything (including cost of parts with free postage) from ebay. The parts are original so hopefully, after way too long, we should have a properly working pump!

    It's nice to find a bunch of experts who are prepared to take the time to help a beginner.

    Thanks again, Stu
     
    KrisBlueNZ likes this.
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