Incorrect implementation of an NPN transistor?

[jozaeta]

I've been trying to drive a 12V inkjet valve (specs)with a 5V digital output from a National Instruments controller (PCI-7334). I'm using a Radio Shack 13.8VDC 3A power supply to power the injet head. Based off the numbers I have of the injet valve, I've picked this NPN transistor.

I tried hooking up the ink valve (with a diode parallel the valve of course) to the transistor, connected the power supply's positive lead to the positive end of the valve and diode, and the negative end to the emitter. I used a 100 ohm resistor to connect the digital out to the base.

So on powering the 12VDC power supply, my injet valve starts dispensing ink. My NI controller isn't on, so the base shouldn't be getting any voltage. It would seem the transistor is already allowing current to flow from the emitter to the collector.

Could the problem be that I'm using the black terminal of the power supply as the ground connection for the emitter?

 

[BrianG]

You may have several problems here. Assuming that the transistor is not faulty, did you actually check if there is any voltage on the NPN base with respect to the emitter? Is the negative side of the Radio Shack PSU connected to the same ground reference or 0V of the controller? If not then a difference in ground potential or earth leakage in the Radio Shack PSU could cause base current to flow in NPN tansistor. Check this out first.

Secondly, looking at your spec for the valve it has some strange (to me) parameters like it says the coil is "4100 LOhms". I am not sure what the resistance actually is from this figure to check if your transistor is man enough for the job. Also "51 volt zener" across the coil, which seems a bit strange. Is this zener actually 5.1V?. Don't forget that zeners conduct like a normal diode one way and with the zener voltage the other way, thus the "positive" end marking is for the zener function connection. If the zener is 5.1V, on 13.8V you may get some problems when the NPN transistor conducts.

 

[IRstuff]

>> Your description is woefully lacking, since you failed to mention the collector connection altogether.

>> You've connected, apparently, the NPN emitter to the 12V through through the coil with the base at 0V. If you had bothered to look at a real datasheet for this transistor, you would have noticed that the emitter-base junction is only rated for 6V.

>> The transistor you've chosen is rated for 200 mA, yet you are using a 3 AMP power supply???

>> Ditto and your valve requires 0.5 AMP at 12V???

TTFN

 

[Skogsgurra]

jozaeya,

If I read your posting correctly, you have connected +13.8 V to the valve/diode positive end. You have also connected the power supply minus (the black terminal) to the valve/diode negative end. So it is only natural that your valve delivers ink. The transistor and its connections has no influence on the valve.

I have a feeling that you should get hands-on help from someone that understands these things. There are so many things wrong here (component choice, circuit topology, incomplete connections etc) that trying to solve it via postings seems futile. Don't you have a technician at your institution?

Gunnar Englund

http://www.gke.org

 

[dino40]

Jozaeeya,
Sounds to me like you want the trans to work like a switch.
If this is indeed the case try configuring the circuit as a basic common collector. This will give you a switching transistor with no phase shift.

 

[cbarn24050]

You should have connected as follows. emmiter to negative supply, colector to valve/diode (anode end), cathode end of valve/diode to posative supply, base to resistor ( a 1k would do)

 

[ScottyUK]

If you are using bipolar TTL, consider using a PNP and active low output. TTL has better sink capability when 'low' than source when 'high'.


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