12VDC Relay Question, Is 1N4148 necessary on relays powered shortly off cap

[goldfist]

So I have a circuit that I wrote up on my lunch break for a project in which I am taking an old 1981 model IBM 5150 and installing modern guts, and I want it to look all original on the outside and to use the original power switch on the back right side to turn on a new motherboard.

The circuit I drew up gives the PWR SW on a new mATX motherboard a 1 second connection via a relay for both the ON and OFF power switch position by use of 4 relays. 2 of the relays are for the ON and OFF state and flip-flop depending on switch position to charge 2 capacitors in alternation. When Relay 1 or 2 is deenergized the capacitor is then brought to the 3rd or 4th relays depending on which state the power switch is switched to ON or OFF. This capacitor causes the relay #3 or #4 to energize for about 1 second. ( Relay# 1 is the primary for relay #3, and relay #2 is the primary for relay #4, and Relay 1 and 2 flip flop between energized and deenergized depending on the power switch position, and relays #3 and #4 supply the 1 second connection to motherboards PWR SW pins when energized to simulate a user pushing the power button of a new computer for 1 second ).

So the question I have is that I am using the 1N4148 diodes on Relay #1 and Relay #2 to protect my contacts from the high voltage spike of the coil EMF collapse. But on Relay #3 and Relay #4 I would like to just keep the electrolytic capacitors there without the 1N4148 diodes because they seem not necessary. Are they unnecessary as I believe they are when the electrolytic capacitor when brough across the coil will just drain to 0 volts?

The contact to the coil will only open when the capacitor is to be charged in which there is no potential because it has been drained out.

I generally put diodes across all relay coils to protect contacts as well as blowing out the relay drivers from spikes, but back in college the teacher pounded it in as a rule for all relays to have this diode across the coil, which I dont believe pertains to the use that I have intended for relay 3 and 4.

 

[davenn]

the diodes dont protect the relay contacts
the diodes protect the circuitry connected to the relay coils
you should really be using 1N4001 or 1N4007 diodes rather than 1N4148 's

maybe you should show a schematic of what you are doing to make sure you have the diodes in the right place

Dave

 

[goldfist]

Schematic to share... removed K4 which was redundant when looking at knapkin

Here is the schematic which is pretty rough since I only have MSPAINT to create schematics. I modified this since I wrote it quickly on a knapkin at lunch. I found that I didnt need 4 relays, but instead 3 were fine and the 4th was redundant.

Voltage is 12VDC. Resistors are all 1K 1/4watts for the LEDs to operate in correct current range to indicate whats enabled and for how long when looking at D5. Diodes D3 and D4 will be replaced with 1N4001's vs 1N4148's. The rest is pretty straight forward although the 2 connections at the bottom of the schematic out of K3 will go to PWR SW pins of a modern motherboard to turn it on via a momentary connection. For C1 and C2 electrolytic capacitors I have not come up with values yet since I need to find a datasheet that will specify the current draw on the K3 coil to know how many uF for them to be rated to hold this relay energized for about 1 second. Relay I may go with is a low cost Axicom D2N Relay V23105 DPDT since the load is very small in the mA for it all. And this is going to be powered off of a 12VDC power supply that is always on when the power cord is plugged into the rear of this computer.