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  1. #16
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    Nov 2007
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    Quote Originally Posted by jack620 View Post
    So cunning you could put a tail on it and call it a weasel?
    As cunning as a fox who’s just been appointed Professor of Cunning at Oxford University?

  2. #17
    Join Date
    Sep 2012
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    York, North Yorkshire UK
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    Default Parts collected !

    Hi Guys,

    Thanks for your posts I am going to keep my cunning plan to myself for a little while, suffice to say that I think I have a solution, but I want to test it out before I make an ass of myself !

    Right I've been salvaging some components from a scrap computer power supply.

    18-12-2020-001.jpg 18-12-2020-002.JPG

    The components in the pictures were salvaged from a dead computer power supply unit. In the first picture, two of the three large cylindrical items are the main smoothing capacitors. I already had one capacitor that came from another power supply that had its twin go bang. Some clown had moved the voltage selector switch on the back to the 110 volt position ! Only one of the two had blown its top.

    In the second picture is a diode, type 1N1007 which will work OK even though its only rated at a 1000 volts and 1 Amp. The power resistor at the top of the picture is the load resistor for the 5 volt rail, its 47 Ohms 10 Watts. You can see that I had to prize it out of the circuit board because it was crimped into the rivets. The green disc shaped item is the Thermistor mentioned in the circuit diagram. Its 5 Ohms cold and drops to about 1 Ohm when hot. It is just used in series with the mains supply for surge suppression. Basically protecting the capacitors and switching devices from damage at switch on.

    The last item, bottom right, is a power FET. I only salvaged this because it doesn't appear to have been damaged but needs testing. The other one on the heat sink was blown to bits.

    So there are some if not all the parts needed to build a prototype.
    Best Regards:
    Baron J.

  3. #18
    Join Date
    Nov 2007
    Location
    melbourne australia
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    3,228

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    Quote Originally Posted by BaronJ View Post
    Only one of the two had blown its top.
    I wonder how much internal damage was done to the other one though? It might be compromised.
    Chris

  4. #19
    Join Date
    Jul 2010
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    Melbourne
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    Hi BaronJ,

    Just looking over your first diagram to see if I can follow along, either I can't or you have a small error.
    You are switching "Mains Supply" and have it connected to the motor.

    Have I fixed it? Or am I missing something? '

    edit: I'm not saying it wouldn't work as show. But why both?
    Attached Images Attached Images

  5. #20
    Join Date
    Aug 2019
    Location
    Revesby - Sydney Australia
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    56
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    1,183

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    Quote Originally Posted by BobL View Post
    reading the fine print in the data sheet says it should not be used on an AC motor that draws more than 500W. I have several relays rated at 10A/240V and they have a max inductive (motor) loads of of 375W
    Interesting. I have never seen that sort of fine print !


    After the first few relay failures, I would probably have modified something. Either added some protection (thermistors/diodes/suppression caps?), or stepped up to a contactor. Note last page of this data sheet:

    https://www.sparkydirect.com.au/asse...pdf?9720157859

  6. #21
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    Aug 2019
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    Quote Originally Posted by Stustoys View Post
    Just looking over your first diagram to see if I can follow along

    It made me scratch my head for a while also. The only thing missing was a wire at lower right corner of the (original) circuit. Lets call that Neutral.


    Circuit shows the relay in the braked position.

    When the switched power (top left) becomes Active, it energises the relay and pulls the top half of the relay (and the top half of the motor) to that switched Active.

    When the switched power is de-Active'd, the relay goes to the resting/braking position, and dumps the capacitor charge onto the motor.

  7. #22
    Join Date
    Sep 2012
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    York, North Yorkshire UK
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    Quote Originally Posted by jack620 View Post
    I wonder how much internal damage was done to the other one though? It might be compromised.
    Hi Chris,

    Non as far as I can tell ! All three have been reformed and measure close to the marked value.

    More than likely one capacitor was or became bad and got hot, blowing the end out. Its very possible that the cap giving out was what destroyed the switching transistors. The power FET I showed in the picture tested dead. Drain source short either way round.
    Best Regards:
    Baron J.

  8. #23
    Join Date
    Sep 2012
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    York, North Yorkshire UK
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    Quote Originally Posted by Stustoys View Post
    Hi BaronJ,

    Just looking over your first diagram to see if I can follow along, either I can't or you have a small error.
    You are switching "Mains Supply" and have it connected to the motor.

    Have I fixed it? Or am I missing something? '

    edit: I'm not saying it wouldn't work as show. But why both?
    Hi Stuart,

    I am going to give an explanation of how the circuit functions later ! But for now to answer your question !

    The relay is shown in its un-powered state, when power is applied the relay coil is powered, connecting power to the motor and its own coil thus locking the relay on and ensuring that the supply is broken when the mains is switched off.

    Breaking the connection as you have shown will apply DC to the motor resulting in it not starting.
    Best Regards:
    Baron J.

  9. #24
    Join Date
    Sep 2012
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    York, North Yorkshire UK
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    Quote Originally Posted by nigelpearson View Post
    Interesting. I have never seen that sort of fine print !


    After the first few relay failures, I would probably have modified something. Either added some protection (thermistors/diodes/suppression caps?), or stepped up to a contactor. Note last page of this data sheet:

    https://www.sparkydirect.com.au/asse...pdf?9720157859
    Hi Nigel,

    Those contactors suffer from the same problem switching DC ! Note that they don't publish DC ratings for the contacts.

    DC switching is a major problem as can be seen in the car starter motor solenoids. Just look at the size of the contacts there.
    Best Regards:
    Baron J.

  10. #25
    Join Date
    Sep 2012
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    York, North Yorkshire UK
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    Quote Originally Posted by nigelpearson View Post
    It made me scratch my head for a while also. The only thing missing was a wire at lower right corner of the (original) circuit. Lets call that Neutral.


    Circuit shows the relay in the braked position.

    When the switched power (top left) becomes Active, it energises the relay and pulls the top half of the relay (and the top half of the motor) to that switched Active.

    When the switched power is de-Active'd, the relay goes to the resting/braking position, and dumps the capacitor charge onto the motor.

    100% correct !

    Yes the wire is missing at the bottom right out to neutral. The text box sits on top.
    Best Regards:
    Baron J.

  11. #26
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    Sep 2012
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    York, North Yorkshire UK
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    Default

    Hi Guys,

    Thank you all for your comments !

    It doesn't leave me too much more to say about the circuit and how it functions !

    The relay switches power to the motor and the charging circuit for the capacitor. Note that the thermistor is also permanently connected to the motor at one end and the negative end of the capacitor at the other. This means that the top relay contact is not switching the whole of the capacitor discharge current into the motor. Only the bottom one is handling the bulk of the discharge current. A suitable capacitor would probably suppress the arc across the relay contact, reducing contact damage.

    This also allows the Thermistor/Resistor value to be adjusted to control the severity of the braking effect. A higher value reducing the braking ! This give you the ability to set it to suit your requirements.

    WARNING: Do not attempt to use a variable resistor here !

    The wire to the top contact of the bottom one can be omitted since the relay and motor are both permanently connected to the neutral supply. I've only shown it because it was done in the original circuit.

    All of this circuit should be in a suitable protective enclosure, but for the purpose of illustration I am going to use a piece of copper clad printed circuit board to build the prototype onto. I should be able to do this later today.
    Best Regards:
    Baron J.

  12. #27
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    Jul 2010
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    Melbourne
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    I realised the Mains Supply was neutral but used it to match the diagram.

    Yes when the relay is powered it connects the Switched Mains Supply(active) to the motor.
    But isn't the neutral already connected directly to the motor?

    Don't both blue circled contacts go to neutral, the motor, one side of the diode and one side of the relay coil? i.e. all the purple is neutral. Are you not switching it to itself?
    Attached Images Attached Images

  13. #28
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    Quote Originally Posted by BaronJ View Post
    The wire to the top contact of the bottom one can be omitted since the relay and motor are both permanently connected to the neutral supply. I've only shown it because it was done in the original circuit.
    We were typing at the same time, I just type slower lol. That clears that up

  14. #29
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    Sep 2012
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    Default Prototype build started !

    Hi Guys,

    I've found a piece of double sided copper clad RSPB (resin paper board) to build the prototype onto. I propose to use the "Manhattan" style of construction so it will more closely follow the circuit diagram.

    19-12-2020-002.jpg 19-12-2020-001.jpg

    These pictures are of the copper clad board with the relay mounted and a cup glued to the board to support the electrolytic capacitor. Care needs to be taken here as the capacitor outer can is very often connected to one of the terminals, usually the negative one. These capacitors are well insulated but the nylon cup that I turned has a 1 mm thick bottom so an UN-insulated capacitor could be used.

    I'll take some more pictures as I progress, and hopefully reveal my cunning plan !

    Thanks all.
    Best Regards:
    Baron J.

  15. #30
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    Aug 2019
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    Quote Originally Posted by Stustoys View Post
    Don't both blue circled contacts go to neutral, the motor, one side of the diode and one side of the relay coil? i.e. all the purple is neutral. Are you not switching it to itself?
    In the Active position on the relay, yes. We could save one wire to the relay



    I was thinking of doing a version of this with a large bulb or heating element. I wonder if it would slowly brake a 3ø motor?

    (pretty sure it would do nothing on a single phase)

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