Transistor as a switch

[alejo]

Hello, I'm new to the forum.

I'm having trouble using a transistor as a switch to activate a solenoid electrovalve when an inductive sensor detects metal.

I've attached the circuit

Here are the details.

The sensor is a HT-P12NA PNP Inductive sensor (24V, 200 mA) that has 10V in the output when is sensing metal

The electrovalve has a 24V, 300mA Relay that activates it (it's a pneumatic valve)

I've tried two different transistors: 2N2218 (Vce max=30V, Ic max = 0.8 Amp) and BC639 (Vce max=80V, Ic max = 1A)

The problem is as follows:

With out the electrovalve (using a multimeter as the load) it works just fine, when the sensor is sensing metal, the transistor goes to saturation and I get 24V DC where the valve should be. But, when I put in the valve, and the sensor is on, I only get about 5V, so the relay on the valve never activates.

I tried a lot of Resistor values with no luck. I aldo checked the transistor placement a million times.

Any advice?


Note: As a power supply i'm using an 220V to 24V AC transformer and a bridge rectifier. The DC voltimeter shows a 24 V DC output, but, of course, the supply it's not perfect DC, could that be causing any trouble with the electrovalve internal impedance?

Thanks in advance.

 

[ElectronWorks]

Here goes..

Is there a smoothing capacitor on your 'dc' output? I am assuming this, but if you just have a transformer and bridge rectifier, you will get a half sinewave dc. It will look like dc on your voltmeter, but will upset any analysis you try to do.

Your circuit looks fine and I see no reason why it will not work. Personally I would change the 50 ohms for a 1k to limit the current from the sensor into the base of the tranny. You will still get enough volts into the base to turn it on though.

Try replacing the relay with a 100 Ohm resistor. This will test if you are getting enough current in the collector of the transistor to drive the relay.

Apart from that, check the circuit as I see no reason why it will not work. Keep the flyback diode in too, otherwise things will go up in smoke!

Bill Naylor

 

[neon]

to get 5 v I hope it is at the collector means one thing the transistor is not saturatedbut it is in a linear region. You must use the rule of a beta of 10 when apply to a switch. if the relay is 24 v and you say 300 ma /10=30ma from the source. put a 820 ohms to 24v ito the base it should close the operate the relay or saturate the transistor with the sensor removed. if ok add the sensor and replace the 50 with 330 ohms it should work if not the sensor has not 30ma cabability. and go from there.