Simple Transistor Switch - Am I doing it right?

Other Hardware General Electronics
1

Hi,

I have found a schematic online that I want to adjust, but wanted to check I was doing the right thing before trying it and possibly causing damage.

Schematic:

The only difference is that instead of a motor, I want to connect an electric match: http://easypyro.com/e-match-2m

  • All Fire Current: 0.5 Amps.
  • No Fire Current: 50mA
  • Recommended Voltage: 6v +

My power source will be an 11.1v lipo battery.

My main questions:

  • Can I just switch out the motor for the electric match?
  • Will the resistor that is in the image be enough for what I want to do?
  • Do I need the diode?

Thanks in advance.

2

Hi, before we go any further, how old are you and what do you plan to do with the circuit?

Paul

3

A fair enough question.

I'm 29. I run an Airsoft site where we are making some electronically controlled pyrotechnics that will be detonated in a safe area away from the players.

4

How much current does the electric match require?
You need that to estimate the base current.

5

For 0.5A output 2k2 is fine. A darlington consists of two transistors, the second cannot saturate so retains a reasonable gain (100 or so), so the total device has perhaps 1000x gain for switching, 100 x 10 (10 being the normal sort of current gain in saturation for a BJT transistor).

You could also use a modern super-beta NPN like a ZTX851 (saturation gain more like 50), or a logic
level MOSFET, but the TIP120 will be quite able. I presume the load 'goes away quickly' so you don't have
to worry about power dissipation...

6

MarkT:
[...]
the TIP120 will be quite able. I presume the load 'goes away quickly' so you don't have
to worry about power dissipation...

Correct, the circuit only needs to be 'on' for a split second. The program behind it will simply send the signal, delay for a few ms, then the software will stop.

Thanks for your help. I will go with the 2k2 and see how it goes.

Is the diode important, as the match is not polarity conscious? (or do I have the wrong idea about what the diode actually does?)

7

Scalamoosh:
Is the diode important, as the match is not polarity conscious? (or do I have the wrong idea about what the diode actually does?)

You do have a misunderstanding about the diode. It is not about polarity at all; it is only relevant to an inductive load, which the match is not, so no requirement at all for a diode.

8

In an inductive load any attempt to cut the current abruptly causes a massive voltage spike unless
you provide an alternative path for the current to flow and decay. The current flowing is linked to
the magnetic energy in an inductor, and that energy has to go somewhere (warming a diode is
better that causing reverse-breakdown in a transistor!)

9

MarkT:
warming a diode is better that causing reverse-breakdown in a transistor!

Humorous.

The diode (rated only to the same current as the coil) does not absorb the energy in the coil. The coil itself does.

10

The diode (rated only to the same current as the coil) does not absorb the energy in the coil.

Yes it does, maybe not all of it but some of it. The forward voltage drop on the diode sees to that.

11

Grumpy_Mike:
Yes it does, maybe not all of it but some of it. The forward voltage drop on the diode sees to that.

Oooh, we are getting picky now!

Only you could - would - have come up with that. :grinning: Obviously I knew it, but 0.7 V out of 5 is pretty trivial in this context and 0.7 V out of 12, 24 or 60 really is.

12

Only you could - would - have come up with that

Thanks :slight_smile: I thought you might appreciate it.