Switching from positve or negative voltage to zero

[johnsold]

Parallel ports are probably easier because they use nominal 5 V logic (which might not be enough to drive a 5 V relay), but they are also harder to find.  You can use USB to serial adapters on computers with no RS-232 ports.

We are trying to help you learn something, but not to do the project for you.

Lynn

[AustinB]

New problem.  I have the labview software outputting a positive 12 V through the serial port.  This 12 V signal is being used to flip a relay.  I measure the voltage coming from the computer serial port before I insert the signal into the circuit and it measures +12V.  When I put the signal into the circuit, it measures +2V.  Thi sis not enough to voltage to flip my relay.  What is causing this? What are solutions?

Is it because serial port on computer does not supply enough current?  Do I need to build a small current amplifier?

I can e-mail the labview file if needed.  Thanks

[johnsold]

First.  Since you have a new problem, it would be better to start a new thread.

You are right.  The serial port cannot supply enough current to drive a relay.  So you will need some kind of amplifier or driver (and a power source).  Also, the port should swing to -12 V when in the other state so make sure your driver can handle the negative voltage at its input.  Look at the ULN2000 series devices.

Lynn

[AustinB]

Thanks for the advice.  I considered a USB adapted already(i'm not retarded) but was concerned that a USB would not output a current high enough since USB only has max output of 5V and 500mA.  After further review, I think I am going to pull the relay out of the circuit and hook it directly to the signal + and - to make sure I am not losing the ground from the signal in the circuit. If this doesn't work, I will build a current amplifier and juice this up a little bit.  I really appreciate the help though.

[smercurio_fc]

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@AustinB wrote:

After further review, I think I am going to pull the relay out of the circuit and hook it directly to the signal + and -

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Ever hear of back-emf? Perhaps you should do a little more review in your electronics books.

[SteveChandler]

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@AustinB wrote:

Thanks for the advice.  I considered a USB adapted already(i'm not retarded) but was concerned that a USB would not output a current high enough since USB only has max output of 5V and 500mA.  After further review, I think I am going to pull the relay out of the circuit and hook it directly to the signal + and - to make sure I am not losing the ground from the signal in the circuit. If this doesn't work, I will build a current amplifier and juice this up a little bit.  I really appreciate the help though.

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A serial port cannot supply the current to drive a relay. It does not matter if the serial port is on your motherboard or on a USB adapter. Even though USB has the power available the output driver in the chip does not. You need an amplifier. Even if you got lucky and it did work with your particular port and relay it will likely work once. See the response from smercurio_fc as to why that would be.