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I have a circuit that must provide PWM to a motor which is expansive in current (around 1.2 A for 4.5V). For the moment I use 100% duty, but I cannot reach the same efficiency as the one I get when I plug the motor directly on the batteries (3x1.5V = 4.5V).

The circuit I used firstly is this one :

+12V ---------+---------+
              |         |
             / \        |
             |M| motor ---  1N4001
             \ /       / \
              |         |
              +---------+
              |
            |/
CTL -/\/\/--|    2N2222 NPN
      1k    |\>
              |
             ---
              -

but then I cannot reach enough voltage so I made a Darlington transistor :

enter image description here

and I replaced the transistor of the first picture by this one. But the current was still under what I wanted (around 0.6A while I want 1.2A), so I tried to put two Darlington transistors in parallel :

+12V ---------+---------+
              |         |
             / \        |
             |M| motor ---  1N4001
             \ /       / \
              |         |
              +---------+-----------+
              |                     |
            |/                    |/
CTL -/\/\/--|         CTL -/\/\/--|    <-- Darlington
      1k    |\>             1k    |\>           
              |                     |           
             ---                   ---        
              -                     -                 

(each "transistor" here is made with two 2N2222A transistors)

But the current stays at around 0.3-0.6A... Why the current isn't added to provide me 1.2A, but stays at the same level as before ? Is their any other way to proceed to reach 1.2A with 2N2222A transistors ?

Thank you in advance.

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  • \$\begingroup\$ First, you're going to have continuous frustration trying to distribute the current between multiple 2n2222 transistors for this amount of load. You are partially on the right track to use a darlington arrangement, but not with this transistor as the main driver. Second, if ever there was a good time to make the jump to using a power MOSFET design, this is it. Only a MOSFET will maximize the power to your motor circuit. And if for some reason your control voltage is too small to control the MOSFET gate, you can always use a transistor there. \$\endgroup\$ – Randy Jan 12 '17 at 1:12
  • \$\begingroup\$ Use the first circuit, get a transistor with a higher Hfe. Darlingtons are on the right track. \$\endgroup\$ – Bradman175 Jan 12 '17 at 3:51
  • \$\begingroup\$ increase the mechanical load on the motor. \$\endgroup\$ – Jasen Jan 12 '17 at 6:30
  • \$\begingroup\$ @Randy : The only MOSFET I have is the IRFZ44N, but the result are not really better... Are 3.3V in input, and 4.5V for the motor enough? \$\endgroup\$ – tobiasBora Jan 12 '17 at 10:13
  • \$\begingroup\$ @Bradman175: Do you have any good model of transistor ? I'm a bit lost in all the options. \$\endgroup\$ – tobiasBora Jan 12 '17 at 10:13
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It sounds like you want to be able to control the 12V motor with a low voltage (your comment mentioned 3.3 to 4.5V), and you also want to eliminate the lost motor voltage that is unavoidable with a transistor or darlington pair. I suspect you also need to consider the current capability of your control circuit. This certainly is an ideal time to consider a MOSFET, to ensure maximum efficiency (least lost voltage) driving the motor, but the low control voltage means you'll need a second transistor to act as a voltage amplifier/switch. Since you mentioned having an IRFZ44N, which is an N-channel mosfet, I'd suggest a circuit like this, using the IRFZ44N for Q2. The 3.3V should have little trouble driving this circuit, even if its just a low power microcontroller I/O pin. That MOSFET has some internal protection, but since the motor is inductive, it still would make sense to keep that reverse blocking diode across the motor. You didn't mention having a PNP transistor available, but you should be able to find a small common PNP transistor like a 2n2904 for less than a buck. (your local radio shack currently sells a package of 15 of those for about $3).

enter image description here

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