From: jasen@xnet.co.nz   
      
   On 2010-03-13, thermo102  wrote:   
   >   
   >   
   > Well, I tried!  The power resistors I installed are acting as bleed off's I   
   > guess.  A waste of money, but they don't seem to be harming anything, so I   
   > plan to  leave them there.   
   >   
   > The new motor did run, but with exaxtly the same current draw as the old   
   > one.   
   >   
   > Hopefully the following is meaningful, and will help answer my question:   
   > What do I have to do to supply the new motor with approximately 2.0 amps   
   > operating current?   
   >   
   > Pins 2,3,4,&5 of the parallel port go to a DM7 400N Logic NAND gate.   
   >   
   > A trace goes from the NAND gate to another transistor which I cannot   
   > identify, and then on to a large condensor.  There is a trace branching off   
   > (through a 1K resistor) to the 'base' of TIP 120.  5 volts from the parallel   
   > port here?   
   >   
   > Am I correct that this 1K resistor is the actual current limiting resistor?   
   >   
   > I 'think' there is a 42 Volt DC applied to the 'emitter' of TIP 120.  Then   
   > the 'collector' goes to the motor.   
      
   > Of course I have four TIP 120's with the same configuation. (one for each   
   > 'coil' of the motor).   
   >   
   > The OLD motor had 133 ohms resistance across the coils, and the above gave   
   > approximately 0.29 operationg amps.   
   >   
   > The NEW motor I'm working with has 2.2 ohms resistance across the coils.   
   > What value resistor should be used to replace the existing 1K's to achieve   
   > approximately 2.0 operating amps?   
      
   Is your 42V supply capable of producing the extra 70W needed?   
   (140W if you need to energise two coils at the same time)   
      
   The 2.2 ohm motor is designed to run on a 5V supply with a darlington   
   transistor (like TIP120) to ground,   
      
   If your 5V supply can spare another 10W run the motor from that.   
      
   Connect the motor common wires to 5V and   
   the branch wires to the collectors   
      
   Ahe TIP120 transistors may need heatsinking as they'll dissipate 1.5W   
   of heat (and all through one of them) when the motor is stopped.   
      
   The TIP120 has a gain of about 1000 so it needs atleast 2mA input to   
   turn on enough to conduct 2A for switching applications (like this) it's   
   recommended to exceed that by a factor of ten (so 20mA)   
      
   The parallel port won't be able to produce enough 5V curren to turn the   
   chips on.   
      
   you mention 'another transistor' is that a branch off the path to the   
   TIP120 or in the path to the TIP120   
      
   Because if it's in the path you probably just need to reduce the 1K   
   resistor to 180 ohms.   
      
   Else you'll need to replace the DM7400N with someething stronger;   
   the DM7400N chip can only produce 0.4 ma so it's not going to work,   
   A SN74AHC00N chip can procude 8mA on the outputs which is probably   
   close enough to 20mA for it to work, replace the 1K resistors with   
   wire links.   
      
   either way you'll probably need a 5V supply, this can be taken from   
   a keyboard socket, game controller port, or usb socket. (if you're not   
   intending to run the motor from this 5V supply, if you are you could   
   try using one of the hard drive connectors if your PC can spare the power)   
      
      
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