Potentiometer Question...HELP

Discussion in 'The Projects Forum' started by ClipCord, Jun 1, 2013.

  1. ClipCord

    Thread Starter New Member

    May 26, 2013
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    I am trying to wire a simple variable voltage power source. I have a 9V battery connected to a switch, then to a 15A25K potentiometer that I had laying around from an old guitar pickup. I get power to my device when the knob is turned all the way off. Once I turn the knob anywhere else I get no power to my device. What am I missing?

    Thanks
     
  2. wayneh

    Expert

    Sep 9, 2010
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    Your device (it would help if you explained what it is) needs more current than you can draw through that resistor at any setting other than near the zero ohms end. The current at that point is probably too high and may be ruining your pot.

    A pot in series with a load is generally not going to be a good way to control the load, if it requires much power.
     
  3. LDC3

    Active Member

    Apr 27, 2013
    920
    160
    It sounds like the potentiometer may be broken. It could also be that you have wired it in incorrectly. A picture would help.
     
  4. ClipCord

    Thread Starter New Member

    May 26, 2013
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    Well my device is a battery powered tattoo power supply. It needs to run from 1.5v to 9v. I also would like to wire two 9v batteries in series and up the max voltage. I will be going to radio shack today, so if I need an actual rheostat or different pot I would like to know which.
     
  5. sheldons

    Well-Known Member

    Oct 26, 2011
    616
    101
    ditch using that pot-go and buy an LM317 adjustable voltage reg,have a search on google and you will come across quite a few schematics to show how they are used. The way you are trying to do it is totally wrong........
     
  6. wayneh

    Expert

    Sep 9, 2010
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    9V batteries likely won't have enough power for your tattoo supply. That's the first problem. It would help if you have any specs for the required current in amps, or watts.

    The next problem is that controlling by voltage may not work really well. If it does, the LM317 approach will give you voltage control and is easy to try. Read and follow the data sheet schematic. Note that the power dissipated by the regulator can be as much or more than the power being used by your device, so it will be wasteful of battery power.

    It's the same problem with a pot. To work without burning up, a potentiometer needs to be rated to the same power as the tattoo supply. And it would need to be in the very low ohm range, maybe 0-10 or 0-100Ω or so, depending on the load. Pots tend to be expensive, although they have the advantage of being simple.

    The best approach might be pulse width modulation, or PWM. It's great for giving smooth control of power over a wide range, for lights, motors and so on. It's also more efficient than linear control such as the LM317 regulator. If you search this forum for "PWM motor control" or such, you'll find lots of discussions.
     
  7. ClipCord

    Thread Starter New Member

    May 26, 2013
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    I apologize for sounding like an amateur. I enjoy tinkering with things. The average tattoo power supply has an input voltage of AC 220/110V and an output voltage of DC 1.5-18V. There are two different types of tattoo machines that we would be powering. The traditional coil machine which uses two magnetic coils (same principle and operation as the old door bell) and the newer rotary machines which use electric motors and a cam to drive the needle bar. All machines have to have the voltage adjusted. The voltage is increased to make the needle hit harder and reduced to obviously hit softer. This is for different things we do. Lining, shading and coloring. I am not trying to reinvent the wheel but I do want to construct my own power supply just to say that I've done it. I have seen one video on YouTube where an artist breaks down the components in one of the original Spaulding Power Boxes. He said that a 17V2A transformer, 2A fuse, 3300uF capacitor, bridge rectifier, rheostat or potentiometer with certain resistance built in and other capacitors to step down the voltage before it gets to the output. He suggested stepping to a 100uF and then a 1uF. He however did not really get too involved in the assembly. As I am new, I came to allaboutcircuits for help.
     
  8. wayneh

    Expert

    Sep 9, 2010
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    Well you've come to the right place. ;)

    Do you know how much current (or power) your machines require? It's not hard to build a variable voltage, 2 amp supply but you don't want to start before you know the details of what you are supplying power to.

    I'd definitely recommend a LM317 circuit using a cheap small potentiometer to adjust the voltage, over one using just a potentiometer. I think a pot able to handle the power would cost more than the rest of the project combined, and wouldn't give as good control either.
     
  9. tubeguy

    Well-Known Member

    Nov 3, 2012
    1,157
    197
    If wanting to use batteries, and the max current required is around 2 amps, you will need much larger current capacity than a small 9 volt battery can supply. Batteries are rated for voltage and amp-hours. An amp-hour is the current it can supply for an hour. Raising the capacity increases the 'run' time. So, a 2 amp-hour battery would supply 2 amps for 1 hour, a 5 amp hour, 2 amps for 2.5 hours etc..
     
    Last edited: Jun 2, 2013
  10. ClipCord

    Thread Starter New Member

    May 26, 2013
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    The LM317 looks interesting. However, you mentioned pulse width modulation. Can that be built to handle 110V input voltage? I have seen a few YouTube videos that deal with much lower input voltage, some no higher that 18V.
     
  11. ClipCord

    Thread Starter New Member

    May 26, 2013
    13
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    Well the usage of a battery for power is not essential. All of our power supplies are plugged into the wall. I am more concerned with a traditional power supply that will be plugged into 110V AC.
     
  12. ClipCord

    Thread Starter New Member

    May 26, 2013
    13
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    Is there a way to start the circuit with the 17V2A transformer and then send power into the pwm circuit. This capable of adjusting voltage from 0-17V DC. I am excited to build the pwm circuit, however none of the tutorials I have watched explain how to integrate transformers.
     
  13. wayneh

    Expert

    Sep 9, 2010
    12,153
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    Yes! This should be a good approach. I assume the transformer is just that, and is rated 17V AC (not DC) at 2A. If it is already supplying DC, skip the next 2 paragraphs.

    You'll want a rectifier rated to at least 2A (5 would be good). This is an arrangement of 4 diodes and converts AC to DC. You can use 4 individual diodes or there are pre-made rectifiers with them all built in. I recommend the pre-made rectifier if you haven't bought anything yet - they're simpler to work with and you won't get confused about which way the diodes go.

    The next component of a "standard" power supply would be a large electrolytic capacitor. The rated voltage should be at least 25V and the capacity is optional, big is better. For a 2A supply, I think you'll want at least a 1,000µF but I haven't done the math. The voltage at the rectifier is pulsing DC and this capacitor acts like a battery to supply the load more smoothly in between pulses. The capacitor charges during the peaks from the rectifier and discharges into the load during the valleys.

    Now you've got a filtered DC supply at about 22V (or maybe 17V if the supply is a DC supply). The positive +22V will connect to your tattoo "gun" (is that the word?). The return line to ground will be put through an n-channel MOSFET such as IRF540N (just an example, I have them around, there are MANY other options). This MOSFET will be taking the signal from the PWM controller onto its Gate pin. The Source pin is connected to power-supply ground, and the Drain pin connects to your gun.

    The PWM signal will control the on-off status of the MOSFET. A long on time with a short off time - a high duty cycle - will give you high speed. There are many PWM circuit choices but one you can find in this forum is based on the 555 timer IC. It should be fine for your application.
     
  14. ClipCord

    Thread Starter New Member

    May 26, 2013
    13
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    Thanks Wayneh. That sounds great. I need to pick up a breadboard and the necessary components and attempt to put together the circuit rough before transferring it to a PCB. I will need to find a schematic of the specific PWM circuit that I need. I am so new to all of this, but I'm not afraid of trying and failing.
     
  15. wayneh

    Expert

    Sep 9, 2010
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    You might take a look here and here for background. There are many other threads on the topic as well. Anywhere you see a motor is where your device will go. The same type of circuit is used to dim LEDs as well, but you don't need the MOSFET for those low current applications.
     
  16. ClipCord

    Thread Starter New Member

    May 26, 2013
    13
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    How would I incorporate a voltage meter display? Something simple like an LED display.
     
  17. wayneh

    Expert

    Sep 9, 2010
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    I've seen posts in this forum for cheap voltmeter modules from China. The Shack might even have one. Another approach would be to use an entire multimeter like the cheap or even free ones you can get at Harbor Freight.

    Don't forget to get hookup wire for your breadboard. Different colors are very handy and be sure it's the right gauge for your breadboard. A proper (not el cheapo) wire stripper will make working much easier and will make a nice addition to your toolbox for years to come. You'll obviously need all the parts on the schematic you want to build; the 555 timer and a MOSFET. For resistors I'd look for an assortment rather than specific values. The Shack may have stopped selling their big assortment. Look into Mouser, Newark and Digi-key as alternatives to the Shack. Much better in every way except for the immediate convenience of a trip to the Shack.
     
  18. ClipCord

    Thread Starter New Member

    May 26, 2013
    13
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    So what voltage/wattage Mosfet and 555 should I get. Also, what voltage would you recommend for my bridge rectifier. I am going to get a 3300uf 35V capacitor to follow the bridge rectifier for the power supply. Also, will i be able to put my transformer circuit and PWM circuit on the same breadboard?
     
  19. wayneh

    Expert

    Sep 9, 2010
    12,153
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    Did we ever establish what the tattoo gun requires? Assuming the power supply is rated to 2A at 22V or similar, you'll want a MOSFET rated to, say >4A continuous current and to at least 30V or so. You'll have no problem finding an N-channel MOSFET meeting these minimum specs. There's no problem with going larger, except a modest cost increase.

    Same deal with the rectifier: It should more than meet your power supply rating, so for instance 3A and 30V would be fine.

    Hmmm, I don't think breadboards are rated to 2A, so I'll guess NO unless you can verify it's OK. If your load is actually much smaller, maybe 1/2A most of the time, then you can likely get away with it. The rectifier pins may not fit the holes.
     
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