Function Generators

Discussion in 'The Projects Forum' started by Rittter, Jan 11, 2016.

  1. Rittter

    Thread Starter New Member

    Dec 5, 2015
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    I am using a function generator to pulse DC current in a coil. I have noticed that the unit is getting extremely warm after 20-30 min. of use. Does anyone know if this is normal. Are they not meant to run for an extended time ? Could this be flyback voltage creating the problem ?
     
  2. zenerz

    New Member

    Dec 22, 2014
    13
    1
    Put a diode across the coil - make sure you connect the correct way.
    Ps. If the Gen gives out a voltage that swings above and below 0V DON'T use this method!
     
  3. #12

    Expert

    Nov 30, 2010
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    No information at all about the size of the coil or the frequency or the current it needs, or the capability of the function generator?
    Better questions get better answers.
     
  4. BR-549

    Well-Known Member

    Sep 22, 2013
    1,987
    388
    We don't know. Which "unit" is getting warm?... the coil or the generator?

    Diagram of generator? Of coil? What's on other side of coil? Power supply?

    Pulse rate?

    We can't tell.

    You go first. What do you think it is?
     
  5. zenerz

    New Member

    Dec 22, 2014
    13
    1
    If you suspect the flyback voltage creating the problem clamp the max feedback voltage with 2 zeners as shown zener.JPG
     
  6. Rittter

    Thread Starter New Member

    Dec 5, 2015
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    Thank You. Details: I am pulsing a coil at 10-100Hz, 10-27VDC PP, Voltage does not swing +/-, The generator is getting very warm where the power plug hooks up in the back, but not the power plug, I do not have any information on the coil other than what I could measure. The coil has a resistance of 125 Ohms, inductance of 134mH, number of turns etc., unknown. It is a pre-manufactured coil. The function generator is a EM1656 from China, 1Hz-10MHz, DC level -10V to +10V. The coil is connected directly to the leads of the generator. I don't know what the current needs are. Is there a way to calculate that for a coil ?
     
  7. AnalogKid

    Distinguished Member

    Aug 1, 2013
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    With the resistance and inductance of the coil you can calculate the impedance at any frequency. With that you can calculate the peak current out of the function generator (assuming it can make the amount of current required without its output voltage sagging).

    Separate from that, if the AC input connection is getting warm enough to notice, that probably is a different issue. It might be a bad line cord or bad IEC input connector, or it could be that the internal power supply hot parts are mounted on the rear panel and are heating up the area without there being an actual problem.

    ak
     
  8. #12

    Expert

    Nov 30, 2010
    16,284
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    Xl = 2 Pi F L
    Z = sqrt (R^2 + Xl^2)

    If you exceed the magnitude of current the coil was designed for, the magnetic core will saturate (unless the core is air) and the inductance will change drastically. Also, constantly using the same polarity of charge will leave residual magnetism in the core and change the behavior. Most signal generators are very feeble current drivers. They only generate the signal. Most of the time, you have to attach an amplifier to get any useful power. Failing to do that sometimes smokes the output stage of the signal generator. Be sure to double check that you still have an output.
     
  9. crutschow

    Expert

    Mar 14, 2008
    13,009
    3,233
    What is the purpose of this continuous coil pulsing?
     
  10. Rittter

    Thread Starter New Member

    Dec 5, 2015
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    It is for PEMF. If you are familiar with a bone stimulator, it is the constant emf production at a fixed or varying frequency which promotes healing. I am finding it very difficult to find applied electronics rather than theory. AAC Fanatic above (and I thank you for you input), has provided equations, and the math is not a problem, but what do those numbers tell me ? Do the numbers tell me I am going to melt the coil, burn out a circuit, etc. And by the way, I am working with an air coil. This topic, I believe, is similar to a metal detector without the receiver.

    Can anyone suggest a site, publication,etc where someone builds circuits and then explains the components used with the math to back it up. I am really hooked on electronic now but can't get out of the starting gate.
     
  11. AnalogKid

    Distinguished Member

    Aug 1, 2013
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    Curezone is a site with many fora in various traditional and alternative health areas. You might search there on this topic. It is more of a discussion site than a tech site, but techies abound and explain.

    A work friend had a spiral fracture of a leg bone, and wore a magnetic coil gadget for a few months. Unfortunately, he wouldn't let me take it apart. What I know is that the energy level was not very high, the coil barely got warm. This suggests a lot of turns of fine wire and a high frequency.

    ak
     
  12. #12

    Expert

    Nov 30, 2010
    16,284
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    Do the math.

    From Xl = 2 Pi F L
    The reactive inductance of the coil at 10 Hz to 100 Hz is 2 Pi F .134H
    So, 8.4 ohms to 84 ohms.
    The ohmic resistance is 125 ohms, therefore the impedance the generator sees is 125.28 ohms to 150.71 ohms.
    Throw 27 volts at that and the current will be 179 ma to 216 ma.
    That seems a little high for the maximum a signal generator can do, the kind of, "little bit" that would slowly heat things up.
    So, yes, I think I would add a single stage transistor driver.
    I started with a bipolar transistor for immunity to static electricity, but the math goes wonky if you turn the generator voltage down. With a mosfet, you need most of 10V to run its gate and you adjust the power supply voltage to control current through the coil. As long as you're using monopolar square pulses, some feeble thing like a 2N7000 MIGHT work, but if you want to do sine waves, this design is all wrong.

    ps, My moniker is pronounced, "Number Twelve".

    @AnalogKid
    Please use the size of the coil you saw to guess how many watts were making it slightly warm.
     
    Last edited: Jan 14, 2016
    absf likes this.
  13. Rittter

    Thread Starter New Member

    Dec 5, 2015
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    If you could take one apart, could you build one ?
     
  14. Rittter

    Thread Starter New Member

    Dec 5, 2015
    21
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  15. Rittter

    Thread Starter New Member

    Dec 5, 2015
    21
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    Again, thank you for your info. I am pulsing with a square wave. I've gone through the math and I come up with your numbers. I need to look into a singe stage transistor driver. I am very limited on electronics. On your diagram, you are showing a generator and a power source. Arn't they one and the same?
     
  16. AnalogKid

    Distinguished Member

    Aug 1, 2013
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    From the coil's point of view, yes. But at the system level, the generator modifies the system power (DC) into what the coil needs to function (pulsed DC, AC, whatever).

    99% yes.

    ak
     
    #12 likes this.
  17. #12

    Expert

    Nov 30, 2010
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    No. My entire point is that the signal generator probably can't provide enough current to drive the coil. It can drive a transistor, but you need a different, separate, power supply to provide the current which the signal generator was not designed to provide.
     
  18. #12

    Expert

    Nov 30, 2010
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    I would bet the best 100 people here can do that...and a few more. Personally, I had a TENS machine apart and traced out in 3 hours. I had the schematic because I made the schematic. Therefore I could build one. A fair number of other people on this site can do the same. This is a pretty classy website in spite of the fact that it is on the same Internet with things like cat cartoons, actors giving each other awards, and idiots posting their personal failures on YouTube.
     
    JohnInTX likes this.
  19. Rittter

    Thread Starter New Member

    Dec 5, 2015
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    Mod edit: snipped email address in quote.
     
    Last edited by a moderator: Jan 16, 2016
  20. Rittter

    Thread Starter New Member

    Dec 5, 2015
    21
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    99% yes.

    I live in the Charlotte, NC area. I can send you a unit if you would like to look at one. My email is
    We can discuss details from there.


    Mods edit:
    Please don't shows the Email address on the forum, it could be bring the spammer to you and forum.
    Send a PM to a member by clicking 'Inbox' then 'Start a New Conversation'.
     
    Last edited by a moderator: Jan 16, 2016
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