Reduce 12V 300mA to 3V 5mA? /for touch switch/

Discussion in 'General Electronics Chat' started by Ash945, Oct 22, 2015.

  1. Ash945

    Thread Starter New Member

    Oct 22, 2015
    5
    0
    Hey guys!
    I m pretty new here and lack of knowledge in the theme, but I did researches, 3 days now, still nothing, so.. The problem I faced with my project is that I had no idea how to reduce the current from 300mA to any lower.
    The project is a 12V LED lamp which turns on and off by touch. Literally /see below/. The input is AC but I put a 12V 300mA adapter there, so I got the voltage for the LEDs. I M CLEAR, THAT THE LEDS DRAW THE AMOUNT OF CURRENT THEY NEED! The reduction of the current is not even required on the full circuit. I only need it for the touch switch. Basically what I wanna build, is a FET (IRFZ 44) controlled line of LEDs, and the FET Gate source would have been the 3V /required by the FET to switch is Min.: 2, Max.: 4 Volts/ 5mA. 5mA, to prevent any damage to my body, cuz my fingers would close the circuit to the Gate, which then would allow the power to reach the LEDs. The negative side of the Gate would have been also cut and also closed by my fingers, but the switch loaded with only 250uA, as I understand /I dont mind if anyone takes a look at it/ so it wont harm me.
    So. Now that You get my problem, I ll ask it again, can I reduce the current allowed to flow through my 'touch switch', to 5mA from the adapter 300mA's?
    Thank You for any kind of helpig hand!
    (My english might suck, sorry about that)
     
  2. #12

    Expert

    Nov 30, 2010
    16,261
    6,770
    First, your, "see below" didn't come through. I asked the Moderators to see if it got stopped by a spam filter.
    Second, the 300 ma source is not going to force 300 ma through your finger and 12 volts is not going to knock you on the floor. It is much more likely that very little current will flow through any skin you connect to it, but the exact configuration is necessary to suggest any improvements. I await the, "see below". Want to try to send it again?
     
  3. Wendy

    Moderator

    Mar 24, 2008
    20,765
    2,536
    LEDs are not voltage devices, so mere voltage is not enough. The current draw is entirely dependent on your circuit configuration. A resistor is always needed.
    ------------------------------------
    How to upload an attachment.

    To attach a document or picture to a post

    Click "Upload a File" at the bottom of your post,

    Browse for your file,

    Select the file, then open it.

    Schematics should be .gif or .png format, pictures should be .jpg. .jpg formats will fuzz out schematics, and should not be used for that purpose.

    ---------------

    If you want to display this file there is an old thread I made,

    How to Display Attachments Full Size

    ---------------

    A tutorial for LEDs I made a long time ago.

    LEDs, 555s, Flashers, and Light Chasers

    Wendy's Index


    _________________

    Without schematics, which are the language of electronics, there isn't much we can do to help.
     
  4. Ash945

    Thread Starter New Member

    Oct 22, 2015
    5
    0
    The 'see below' part was a hint to the part where I wrote that literally my finger would close the circuit.
    And about the safety part. So You are saying that the 300mA not necessarily will flow through my finger, but I dont get it. I mean.. if there is example LEDs in the circuit, all the 300mA running around, but only the used x mA ll be drawn by them. So if my finger would be in the circuit, then why wouldnt it flow trough me? I dont have an exact draw of current.
    Maybe I m wrong, I just wanna understand it, You know, its not a game. :)
    About the schematics, I didnt draw it yet, cuz I wasnt clear about that current ll hurt me or not part, and didnt knew what I should put there. But I ll draw one in the afternoon /its 3:16 there and I ll have to go to work/ and I would be pleased if You would check it.
    Thy again!
     
  5. dl324

    Distinguished Member

    Mar 30, 2015
    3,242
    619
    Post a schematic.

    A power source of 12V won't be able to force 300mA through your finger; unless maybe you have a bleeding wound.
     
  6. #12

    Expert

    Nov 30, 2010
    16,261
    6,770
    You did say, "FET". You can use your finger to control that gate with microamps of current and the FET can control the LEDs, but there is no way 300 ma is going through your finger from a 12 volt supply.
     
  7. Ash945

    Thread Starter New Member

    Oct 22, 2015
    5
    0
    I was clear about the current that the FETs Gate need, what is unclear its if I stand with bare feets on the ground, after a bath, would I get shocked or not. Extreme conditions, I know, but it can happen. I know I m bull-headed about this thing and sorry if its annoying but still.. I dont wanna risk.

    dl324
    I ll do the schematic asap, but I just wont get home before 8 hours from now.
     
  8. Dodgydave

    Distinguished Member

    Jun 22, 2012
    4,974
    744
    12V wont kill you, Fets are high resistance anyway, so to put the fet on you can power it with your finger touching two contacts, with a couple of 10K resistors to the contacts, one going to the gate to source to turn it off, and one to the 12dc,

    Can you post a diagram.
     
  9. #12

    Expert

    Nov 30, 2010
    16,261
    6,770
    If you want to make this idiot proof, just say so.
    (It is difficult to overestimate the ingenuity of an idiot, but we can try.)

    First, start with a low voltage. Twelve volts has been used in cars which break down on rainy days and are fixed by people with wet shoes, standing in the mud. Twelve volts is probably a safe number. Just don't test it with your tongue.
    Second, use a transistor with high gate resistance, like a j-fet or a mosfet. You can put a million ohms between you and the gate if you are using finger hum to turn on the transistor, but people generally can't detect any less than one milliamp. For that limit, you would need a 12k ohm resistor between the circuit and a finger made of solid gold.

    I just washed the dishes and measured my left pointer finger while it was still dripping water. It varies from 100k ohms to 400k ohms, depending on how close I hold the probes and whether I'm facing the morning sun. That means a quarter inch of very wet skin already has about 8 times the resistance you need from your pointer finger to your wet feet. That would seem to indicate you already have over two thousand times as much resistance in your skin as you would need to not feel any current.

    Can you make this a little more difficult? Perhaps require it to be undetectably perfect while you are soaked in salt water and standing in the ocean? No. I already covered that. A 12k ohm resistor would take care of that.
     
    Ash945 likes this.
  10. Ash945

    Thread Starter New Member

    Oct 22, 2015
    5
    0
    I laughed so hard.. Thank you :D
    So, basically you said, that even trough the adapter could give 300mA, only the 250uA ll flow in the circuit to the Gate of the FET and doesnt matter if i stand in the ocean or not. Did I get it?
     
  11. #12

    Expert

    Nov 30, 2010
    16,261
    6,770
    Basically, yes. I figure 100k ohms in 1/4 inch of very wet skin reduces 12 volts to 120 microamps. That's plenty to run a touch-switch but far below anything you can detect.
     
  12. Ash945

    Thread Starter New Member

    Oct 22, 2015
    5
    0
    Okay, I just came to a final solution, there wont even be 2 touch switch, but I ll put an impulse relay after the FET, on the Source line, which will.. darn, its better to draw it, just a minute guys and Thanks fo Everyone.
     
Loading...