Transistor base current and general electrical questions

Thread Starter

00Mowgli

Joined Aug 16, 2017
6
As a preface I am relatively new to electrical circuits, and while I have tried to do my homework in terms of learning the basics certain concepts are still hard for me to grasp. I apologize ahead of time for asking very basic questions.

That being said I am currently working on a Nerf gun with the goal of integrating a 12 vdc tire inflator and 3S LiPo battery. The intent was for the air compressor (tire inflator) to fill a silicone air bladder which expands down the length of a plastic tube. Once the air bladder has extended the length of the tube a momentary switch at the end would then break the circuit, turning off the compressor. My attempt at wiring is as follows:

The reasoning behind using a transistor to break and make the circuit was that space inside the tube (housing the air bladder) is at a premium. While a simple 6 amp momentary switch would be ideal, they just tended to be too big to fit. Using the transistor allowed me to use a smaller 3 amp momentary switch that had a lower actuation pressure.

I have three questions:
1. Is this wiring diagram sound for what I am trying to accomplish?
2. Is there a better solution (than a momentary switch) for stopping the compressor once the air bladder has reached capacity?
3. If a transistor is the proper solution, how can I go about determining what resistance the resistor going to the momentary switch should have and do I need further resistors elsewhere?

Thanks to anyone who can share their expertise on this matter.
 
Last edited:

wayneh

Joined Sep 9, 2010
16,390
You need a high ohms pull-up resistor from the PNP base to V+, say 10K-100K. This will turn it fully off when the switch is open. The switch then goes from the base to ground through another resistor to limit the base current to about 60mA (the load current ÷ 100). I figure 180Ω or 220Ω would do nicely.

When closed, the switch will allow the base current to flow and turn on the motor. You should place a reverse-biased diode across the motor poles. It should be specified to at least 24V (which will be very easy) and 6A or more (which will require a bit of cost). This will protect your transistor against an inductive spike of reverse voltage when the motor is turned off. Hmmm... Maybe it only needs a peak current spec over 6A versus a continuous rating for that. If that's the case you could just use a 1N400X series diode. Anyway, look up how to size a snubber diode or wait for an expert to chime in here.
 

ebeowulf17

Joined Aug 12, 2014
3,274
You need a high ohms pull-up resistor from the PNP base to V+, say 10K-100K. This will turn it fully off when the switch is open. The switch then goes from the base to ground through another resistor to limit the base current to about 60mA (the load current ÷ 100). I figure 180Ω or 220Ω would do nicely.

When closed, the switch will allow the base current to flow and turn on the motor. You should place a reverse-biased diode across the motor poles. It should be specified to at least 24V (which will be very easy) and 6A or more (which will require a bit of cost). This will protect your transistor against an inductive spike of reverse voltage when the motor is turned off. Hmmm... Maybe it only needs a peak current spec over 6A versus a continuous rating for that. If that's the case you could just use a 1N400X series diode. Anyway, look up how to size a snubber diode or wait for an expert to chime in here.
I'm glad you answered before me - I didn't notice that it was missing the reverse diode.
 

ebeowulf17

Joined Aug 12, 2014
3,274
Wait, doesn't that need to be an NC switch?

When nothing is touching the switch, we want it to be closed, pulling the transistor base low, turning on the motor.

When the switch is pressed, we want it to open, leaving the transistor under the control of the pull-up resistor, which will then be deactivated, turning off the motor.

Pretty sure that means NC switch, unless I've just gotten myself turned around somehow.
 

Thread Starter

00Mowgli

Joined Aug 16, 2017
6
Thanks for the replies guys, I was sure that I was overlooking some aspect and I'm glad I checked before slapping this together. I'll start looking into your suggestions and will probably post up an updated wiring diagram.

On a side note, right after I posted this thread the thought occurred to me that I could also probably just T off the tube from the compressor to bladder and run a standard pressure cut-off on the extra leg. This would allow me to regulate the bladder based upon pressure rather than size and I'm pretty sure they make ones that can easily handle 6A which would remove the need for the transistor and switch all together.

Thanks again and I will update further once I've done a bit more research.
 

Thread Starter

00Mowgli

Joined Aug 16, 2017
6
Hey guys, I had to take a break from this little project due to real life stuff. However, after re-evaluating what I am trying to accomplish I think I have come up with a much easier (and simpler) way to actuate the motor. Rather than trying to house a push-button switch inside the gun, I could use a pneumatic-pressure switch which has two big advantages. First it would allow me to house the pressure switch outside of the gun which removes the size constraints I was running into with a push-button. Secondly, as the size constraints are no longer there I can find one that can handle the amperage requirements of the motor fairly easily. This would remove the requirement of a low-amperage section of the circuit and the use of transistors. My re-evaluated circuit diagram is as follows; I would appreciate anyone taking a look to see if this would be feasible and if I have overlooked anything.


Thanks again for anyone willing to take the time to help out a newbie.
 

wayneh

Joined Sep 9, 2010
16,390
Looks OK as long as you're confident about the spec of the switch being able to handle your inductive load.

I doubt that it matters in this case, but you could switch the ground line of the motor instead of the V+ line. There are some advantages of doing it that way but I can't think of one that's relevant to this project.
 

Thread Starter

00Mowgli

Joined Aug 16, 2017
6
Thanks for the reply, the switch is specced out to handle 8 amps at 12 vdc or 4 amps at 24 vdc. My understanding is that the maximum draw of the motor should only be 6 amps at 12.3 vdc. I would think that should be enough breathing room, but let me know if I'm mistaken.
 

ebeowulf17

Joined Aug 12, 2014
3,274
I was wondering about the inductive nature of the load too. Some sort of snubber might improve the life of the switch contacts.

A reverse biased diode in parallel with the motor would eliminate back-emf when the motor switched off, but wouldn't do anything to prevent arcing when the switch closes to turn the motor on. I believe an RC snubber would provide some protection for both opening and closing the switch. Not sure how much of an issue this is at these power levels, but I thought it might be worth mentioning.
 

ebeowulf17

Joined Aug 12, 2014
3,274
I'm not familiar with snubbers, but I'll start investigating their potential use now. Thank you for your input!
No problem. They're a good thing to be familiar with, even if they end up not being necessary for this project. I'm still a little fuzzy on when I really need them and when they're optional, so don't feel bad if you feel there's uncertainty even after you do your research.

Realistically, the worst case scenario in this simple circuit is just that the switch might wear out a little faster without snubbers than it would with them (I say might because I'm not sure how much impact there would be.) So, if it's not terribly difficult to replace the switch if/when that happens, you could just ignore the snubber idea and wait to see if it becomes an issue.
 

Thread Starter

00Mowgli

Joined Aug 16, 2017
6
The switch itself is going to be stored in an external vented and zippered pouch next to the compressor itself. Swapping them out shouldn't be too much of a hassle and the gun will not see too much use anyways (its for office nerf wars, and the compressor being loud as hell will probably limit its use anyways. More than anything this is kind of a joke project that should also function as deterence :p ).
 

ebeowulf17

Joined Aug 12, 2014
3,274
The switch itself is going to be stored in an external vented and zippered pouch next to the compressor itself. Swapping them out shouldn't be too much of a hassle and the gun will not see too much use anyways (its for office nerf wars, and the compressor being loud as hell will probably limit its use anyways. More than anything this is kind of a joke project that should also function as deterence :p ).
Sounds like a fun project! I'd say enjoy your nerf wars and don't worry about snubbers unless you just feel like researching them for fun and education.
 
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