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I am working on a project that will ignite a match on command using Arduino. My external power supply, when connected to a thin strand of wire, heats up a wire causing ignition. To control this I want to use an NPN transistor, with my external power supply connected to the emitter and collector, and when I want to heat the wire I will supply power to the base of the transistor, allowing the circuit to close and the wire should heat up.

My problem is that there doesn't seem to be the same current flow through the wire when the transistor is in place and power is supplied to the base, compared to when I remove the transistor from the circuit. In this closed circuit (with the transistor in place), the wire doesn't heat up, which confuses me because I thought that when power is supplied to the base, it is as if there is a free flow between the emitter and collector. I measured the voltage when power is applied to the base of the transistor, and this voltage is identical to the normal voltage of the power supply (without the transistor), which makes me think somehow the current flow is being reduced.

Is it possible that the transistor is reducing the current flow when power is applied to the base? Why is this? Any suggestions to fix this?

My external power supply is a 3.7v 500mAh battery, and the power to the base of the transistor is about 40mA. The transistor I am using is NPN transisto

schematic

simulate this circuit – Schematic created using CircuitLab

r PN2222.

Thanks in advance.

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closed as unclear what you're asking by Andy aka, RoyC, Voltage Spike, Finbarr, Lior Bilia Mar 1 '18 at 13:59

Please clarify your specific problem or add additional details to highlight exactly what you need. As it's currently written, it’s hard to tell exactly what you're asking. See the How to Ask page for help clarifying this question. If this question can be reworded to fit the rules in the help center, please edit the question.

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    \$\begingroup\$ amp or ampere is a unit of measurement of electrical current ... your term amp flow should be current flow or just current \$\endgroup\$ – jsotola Feb 24 '18 at 6:28
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    \$\begingroup\$ since you did not include a schematic diagram, i am forced to use my imagination ... my imagination tells me that you do not have it connected correctly \$\endgroup\$ – jsotola Feb 24 '18 at 6:30
  • \$\begingroup\$ Possibly you didn't hook it up correctly. Or maybe you chose the wrong transistor. Since you shared no information about either your circuit design or your transistor choice, you're not going to get a more detailed answer than that. \$\endgroup\$ – The Photon Feb 24 '18 at 6:38
  • \$\begingroup\$ @ThePhoton I made a diagram can you please take another look? \$\endgroup\$ – J. Doe Feb 24 '18 at 7:13
  • \$\begingroup\$ What is the resistance of the wire, how much current does it draw when connected directly to the battery, and how much when connected through the transistor? When the transistor is turned on, what voltage do you get between the Base and Ground (Arduino/battery negative), and between the Emitter and Ground? Are you sure the transistor is wired correctly? el-component.com/bipolar-transistors/pn2222 \$\endgroup\$ – Bruce Abbott Feb 24 '18 at 7:32
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  1. NPN transistors are mostly used for small signals and low currents these days. Use an N channel mosfet instead. Many off the shelf transistors will not be able to supply enough current for your igniter.

  2. You need to switch the ground, not the high side.

The reason is that the voltage at the gate is in reference to the transistor emitter, so it needs to be connected to ground (to put it simply). Switching the GND instead of the V+ seems weird, but it works fine and is a very common arrangement.

The circuit (with mosfet) will look like this:

schematic

simulate this circuit – Schematic created using CircuitLab Edit: Added a pull-down resistor to make sure mosfet stays off when control is disconnected.

Note: you will need to pick a mosfet with a low Vgs(on). Get something less than 2v Vgs(on), and which can handle your current and you should be golden. Example: IRLI2910PBF

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  • \$\begingroup\$ do you mean logic level MOSFET ? \$\endgroup\$ – Long Pham Feb 24 '18 at 11:30
  • \$\begingroup\$ Long: Yes, that's what I mean, but I've never had much luck searching for mosfets using that term. Better to use the Vgs, and even better to use the Rds(on) vs Vgs chart in the datasheet. \$\endgroup\$ – Drew Feb 24 '18 at 18:52
  • \$\begingroup\$ There are plenty of high current NPN transistors around still. On Semi have a great range. I suspect MOSFETs are used a lot for switching, but BJTs are still used a lot in linear applications such as high end audio AFAIK. \$\endgroup\$ – DiBosco Feb 24 '18 at 19:39
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You could use your original diagram to control a relay which puts the current through the load - relays can handle higher currents but you may have other considerations and it is another component and its size may be a consideration.

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