Basic Stamp 2 to activate a 6V solenoid actuator

Vincenzo1309

Dear all

I am now working on a project, and I am using one of the I/O pins of the Basic Stamp 2 to activate a solenoid actuator. My supply is a 6V battery pack with 4 AA batteries. I think a transistor is needed to activate this solenoid, since the logic supply from the Basic Stamp 2's pin is only 5V. How do I determine which transistor to use?
The measured resistance of the solenoid is 8 Ohms. Therefore the current needed to activate the solenoid actuator from a 6V supply is 6/8 = 0.75A.
So I need a transistor which can supply me with minimum collector current of 0.75A, am I right?

Kindly advise.

Thanks and Regards,

Mike Green

It sure sounds like 0.75A is what you need. You usually add a "fudge factor", so I'd use a transistor rated at 1A. Look at Nuts and Volts Column #6 for several suggestions for transistors. It also discusses the use of Darlington Arrays like the ULN2803. The outputs of these can be paralleled. Two (of 8) can be paralleled (with the inputs connected to the same Stamp pin) for 1A output current. The ULN2803 also has the protective diodes built-in that you'll need for controlling a solenoid. You'll need to connect the free end of the diode(s) to the +6V supply as shown in the Nuts and Volts Column.

Vincenzo1309

Hi Mike,

If I use a 9V battery pack, darlington transistor BD679 works perfectly. However when I decided to change the supply to a 6V battery pack, the transistor cannot turn on the solenoid actuator. Any reasons for it?

My battery supply are also used to supply power to 2 DC motors.

Kindly advise

Regards,

Vincenzo1309

Hi Mike,

I have looked through the Nuts and Volts column. Thanks a lot for your information.

I have shortlisted 2 transistors what could be suitable.

Load Current C-E Voltage Drop
ZTX689B 2A 0.5V
ZTX605 1A 1.5V

It can be seen that ZTX605 fits the 1A requirement, however it's C-E voltage drop is too large, since I am using a 6V supply, therefore the voltage across the load (solenoid actuator) is only 4.5V, which is insufficient to drive the solenoid actuator. Am I right in this case?

For ZTX689B, it provides more current than I needed, which is 2A. One good thing is that its C-E voltage drop is only 0.5V, however the voltage across my load would only be 5.5V, which is insufficient as well. Does that mean that if I was to insist on using the 6V supply, I can't use these 2 transistors?

By the way, does Load current also means Collector current in technical terms?

Kindly advise

Thanks and Regards,

Mike Green

Any transistor will have some C-E voltage drop. The minimum is usually around 0.3V. 0.5V is pretty good. If your solenoid is rated at 6V, it should work fine at 5.5V. A solenoid rating is usually a nominal voltage and solenoids are often at the far end of long cable runs where the voltage drop might well be 0.5V and power supplies for solenoids usually are unregulated and may well supply more than 6V anyway.

Make sure you have adequate base drive from the Stamp. The 10mA mentioned on the Zetex datasheet looks like it would be quite adequate. Remember that the Vce is at a collector (load) current of 2A. You'll be drawing about half of that and the Vce saturation voltage will be lower at the lower current.

Vincenzo1309

Hi Mike,

Thanks a lot for your prompt reply.
So do you mean that ZTX605 is not suitable, therefore I should purchase ZTX689B.

"Make sure you have adequate base drive from the Stamp"
The source current of the Basic Stamp 2 is 20mA, therefore it should be sufficient right?

From the datasheet, the base current of ZTX689B is 11mA, and the base resistor is 390 Ohm. what does it means?
Does it mean that 11mA is needed to operate this transistor? why is the resistor value = 390 Ohms?

Also is load current = collector current?

kindly advise

Thanks and Regards,