Transistor isn switching enough current to drive solenoid!

[Rambo]

transistor problems

I am building a electronics board for my paintball gun but I am having some problems using the transistor to switch. For some reason the transistor isn't switching enough current to drive the solenoid. I made a small schematic to show what the problem is. If it helps, on the top schematic when the solenoid actually does work the voltage across the battery is about 2.5 volts and on the schematic that I try and use the transistor the voltage across the battery is about 8 volts. Any input would be helpful.
**broken link removed**

 

[nand_gates]

Re: transistor problems

Please upload the circuit diagram and part details i.e. transistor no. resitor values
etc. Without this we wont be able to help you!

 

[IanP]

Re: transistor problems

From what you said the voltage drops from 8V (no load) to 2.5V (with load): looks like the battery can not supply enough current.

On the diagram with transistor you should saturate transistor to 100% (or close) and this can be done without voltage divider - only 1kΩ (or les)s resistor from + to the transistor's base. This will depend on the type of the transistor and its β.
The transistor itself should be rated for currents at least twice as much as your load.
Also, in parallel to the load (solenoid) you should connect a 1N4004 or similar diode with its anode at the transistor side and cathode to the +battery.

 

[Rambo]

Re: transistor problems

The voltage on the battery is 8v with the load, but that is on the bottom schematic which doesn't work. The battery I am using is about 10 volts. The transistor is 2N4401. It says the max collector current is 600mA and the battery is only supposed to be 170mA. It just doesn't make sense to me because I thought that once current was flowing from the base to the emitter, that it would flow from the emitter to the collector. And if that were true it should work the same as that top schematic. I dont know though....

 

[IanP]

Re: transistor problems

Don't worry, it will work.
Just connect 1kΩ resistor between bat+, switch and base, and remove this one between base and bat-.

Can you measure current that flows through the solenoid (picture without transistor)?

 

[Rambo]

Re: transistor problems

Well, I just blew another fuse, but it looks like it settled at about 200mA right before it went to zero. That would make sense because the current from the battery is supposed to be 170mA when the battery is 9.6v but it is fully charged to about 10v so 200mA would make sense.

 

[IanP]

Re: transistor problems

So, it looks like the solenoid is to big load for this battery.

Maybe you should consider a power supply instead of a battery..
Something like 9V@500mA?
It doesn't need to be regulated..

 

[Rambo]

Re: transistor problems

The stock board that comes with the gun uses the same battery so that battery will work if I do it right. I just dont understand why if the solenoid fires on the top schematic fine at 200mA, it seems like it should fire fine on the bottom schematic.

 

[IanP]

Re: transistor problems

From what you have said so far it is possible that something is wrong with the transistor (if you have spare, try the other one).
If we assume that the β-factor of your transistor is around 100 then to drive 200mA load you will need more than 2mA pumped into the base.
Can you measure what current flows through 1kΩ (or close) resistor into the base?

 

[Rambo]

Re: transistor problems

yeah, 9.5mA.

 

[IanP]

Re: transistor problems

In this case the transistor should be turned ON and voltage between collector and emitter should be below 0.5V. If it is not then the transistor is faulty.

I would try BD135 or similar (1A current rating) and don't forget about this diode in parallel with the solenoid..(cathode to Bat+). This diode is important as switching solenoid On and OFF creates positive and negative impulses which can damage transistor..

 

[Rambo]

Re: transistor problems

The diode works nice, thanks. I noticed that without the diode, the transistor went bad so now I have a diode hooked up in parallel with the solenoid and the transistors are working fine now. Here is what really confuses me. I measured the current going through the solenoid and it was like exactly 200mA while at the same time the solenoid was not even activating or making any sort of noise like it was trying. It honestly makes no sense. Meanwhile I can take the solenoid and hook it directly up to the battery and it works perfect. This makes no sense because the battery is supposed to be putting out 170mA. So my thought is why does the solenoid not activate when the same amount of current is being pushed through it.......

 

[e_eja]

Re: transistor problems

use nmosfet, trust me it will work, i often use nmosfet olmost of my design when attemting to operate transistor as switch (mosfet had very little r0)

 

[Rambo]

Re: transistor problems

Do you think that this transistor is going to make the difference between not working at all and fully working??

 

[IanP]

Re: transistor problems

In this circuit type of transistor (FET, MOSFET, BJT) doesn't make much difference: if all components in a circuit are OK it will work OK.

 

[Rambo]

Re: transistor problems

Hmmm.... I'm really stumped then. Any other ideas?

 

[XNOX_Rambo]

Re: transistor problems

Are you sure you don't mean that the battery has a capacity of 170 mAH?
Which sounds about right for a rechargeable type. It will be able to deliver a much
higher current though.

Have you at any time actually measured the current through the solenoid when
you use the original circuit without the transistor?
I'm just guessing, but I wouldn't be surprised if it would be more than 1 A.

/Rambo (the other one...)

 

[Rambo]

Re: transistor problems

Wow, I never thought about that, mostly because I don't know that much about electronics. I have never actually measured the current going through the solenoid without the transistor. And the transistors I'm using can only handle a current of 600mA. I'm going to get a transistor that can handle more current and I'll let you know how it turns out. I knew my problem would be something simple I overlooked. Thanks a ton.

 

[IanP]

Re: transistor problems

why if the solenoid fires on the top schematic fine at 200mA

So, did you measure it(the current) or was it your guess?

 

[Rambo]

Re: transistor problems

Well, I just blew another fuse, but it looks like it settled at about 200mA right before it went to zero. That would make sense because the current from the battery is supposed to be 170mA when the battery is 9.6v but it is fully charged to about 10v so 200mA would make sense.

Stupid mistake.