Thread: Please look at these two relay scheme let me know if they are right

I think what you are getting at is a diode across the the coil contacts on the relay to prevent a voltage spoke back to the ps/2 port when the electromagnetic field collapses?

Transistors and ICs (chips) must be protected from the brief high voltage 'spike' produced when the relay coil is switched off. The diagram shows how a signal diode (eg 1N4148) is connected across the relay coil to provide this protection. Note that the diode is connected 'backwards' so that it will normally not conduct. Conduction only occurs when the relay coil is switched off, at this moment current tries to continue flowing through the coil and it is harmlessly diverted through the diode. Without the diode no current could flow and the coil would produce a damaging high voltage 'spike' in its attempt to keep the current flowing.

So basically all that I really want to do is protect my ps/2 port, LCD screen, and the power switch from the inverter. So to do this would I be safe to put a diode on the 5v coming from the ps/2 port before it gets to any of the relays. That way if there where a voltage spike then the diode would not allow any thing to flow back towards the ps/2 port. I am getting kind of confused because when you say put the diode on the relay (pins 85 and 86) if there where a voltage spike wouldn't the voltage be able to travel back through the coil? Also the diode that you show in your picture which direction is that? When looking at a diode there is the white band around one side and the rest of it is black. So when I look are your scheme and you have the trianlge and the straight line, does this mean that the white line is the striaght line connected to the small side of the triangle?
Thanks for the help!


-haryy

When the current flow through the relay coil is interrupted, it will momentarily generate a voltage that's opposite to the one originally applied. If there's no where for the current to go during this time, it will generate a huge voltage in an attempt to keep flowing. In theory, this voltage can momentarily reach hundreds, or thousands, of volts. In practice, it will reach as many volts as necessary to break something down so it can find a path.

The bar on the doide picture is the same as the stripe on the diode package. Put the diode across the coil (as shown) so it blocks current when your output is on, and "shorts out" the coil when the voltage reversal occurs. This will provide a path for this reverse surge to releive itself and also protects your driver circuit at the same time.

To put it into perspective, this characteristic is the same one that takes 12 Volts from your battery and creates thousands of volts to fire the spark plugs!

So I redid the scheme that I had and just added two diodes in each scheme. I am not really sure if I need to add one more on the 12v relay in each one or not. I put the diode on the reed relay so that if there where a voltage spike it would not be able to go back towards the computer's ps/2 port. I also added one that has a question mark beside it. The reason that I put the diodes on the negative of the 12v relay is because one the reed relay triggers it will be sending 5v to the other side of the reed relay. Since the neg of the reed relay is also connected to the neg of the 12v relay would the 5v coming from the reed relay try to travel through the 12v relay or not? Please let me know and thanks for the help!


-haryy

The diodes you have shown with the question mark aren't helping you. They need to go across the relay jusk like you show on the 5v reed relay.

I just got done builing and testing both of these at the same time. I do not have my carputer setup yet but have all components in the house and testing inside right now. I put two diodes in each scheme and put both between the relay's. Once I got done I double checked my work (everything looked good). So now its time for testing. I have a 120vAC to 12vDC adapter that outputs 800mA (just so you know where the 12x source is coming from). I hooked the ground to the ac/dc adapter because I didn't want to use the neg coming from the ps/2 port on the PC incase if something backfired. So I got it all wired up turned the PC on and everything worked perfect, the screen turned on fine and everything. Then I started checking voltage. So I check the 12v going to the screen and it is a negative 12vDC. I checked over everything and everything looks perfect. So how am I getting the -12v to the screen (I am also getting -12v from going to the 12v relay)? I dont understand this. Can somebody please explain what I need to do to fix this, or isn't this a big problem?

Other thing that I noted is when I was checking the +5v coming from the ps/2 cord it was a yellow wire and the neg was the white wire. So I plugged in the cord just to check with my multimeter and it should that it was a -5v. I am getting confused as to why it is negative.


PLEASE HELP!!


Thanks!

-haryy

And just to note.....the yellow wire is pin 4 and the white wire is pin 3, as pointed out above coming from the ps/2 port.

It sounds like you've got the Volt Meter probes reversed. Red probe is "+" and black probe is "-"

Find a 9v battery, read the writing on the sides to know which is "+" and which is "-" and this will tell you what's up with your meter.

Maybe you should draw the schematic one more time. Without the diodes, the setup will work -- for while! The spikes that happen when the relays turn off are powerful but short. Most things will survive for a while with the voltage spikes.