Choosing right diode..?

[Dragiin]

hello, found this forum researching diodes for my project and hoping to get some help..

what im after is to stop current from going back through a relay effecting other things in line of a switch.. im using microswitches in a 12 volt system so i have 2 relays per switch.

so far ive found:


1N4148 Logic Diode but in all my research ive only seen people using diodes for charging batteries, nothing of what im trying to do.

ok, i know its still vague what im after so ill try to describe in detail whats happening. i have 6 switches and 4 dual function valves.(fast and slow) In order:
SW1 (Front,..fill,fast/dump,fast)
SW2 (Front,..fill,slow/dump,slow)
SW3 (Front/Back,..dump,fast)
SW4 (Front/Back,..dump,slow)
SW5 (Back,..fill,fast/dump,fast)
SW6 (Back,..fill,slow/dump,slow)

ok, now if i was just using switches 1,2,5,and 6 ,this project would already be done as i understand relays and already have it working with that basic setup,,but when trying to add a switch to use both front and back valves simeltaneously it is causing other switches to act just like 3 and 4..so i know i need to stop the current somewhere in line to the relay for 3 and 4 but
i cant find anywhere that tells me how to choose the right diode to restrict 12 volts from passing...
am i on the right track with said diode above??

i have sketches and fotos of my setup if needed..hope someone can help, thanks!

 

[(*steve*)]

I think we'll need a circuit diagram, but my first instinct would be that you should use something like a 1N4001 (assuming the voltages are well under 50V).

 

[Dragiin]

thanks ill look that up,, its running off a 12 volt car battery..

 

[hevans1944]

What you are trying to implement is called diode/relay logic. The basic idea is to use a forward-biased diode to "steer" current to the desired relay. This is often implemented as a "wired OR" circuit where one relay can be energized from circuit 1 OR circuit 2 OR circuit 3 ... etc. The AND function is normally implemented with normally-open contacts connected in series on two or more relays, so circuit 1 energizes relay 1 whose NO contacts accept a signal from circuit 2 that then energizes relay 2. Logically inverting a signal (a NOT function) requires a relay with NC contacts that open when the signal energizes the relay. Any combinatorial logic equation can be implemented with combinations of AND, OR, and NOT circuits.

Diode/relay logic is fun to design. I once designed and built an X-Y stage that moved in a raster scanning pattern, all under the control of diode/relay logic. I would not do this today because semiconductor logic families are readily available and inexpensive. And for a raster scanner, I would probably go straight to a microprocessor.

If you can write Boolean equations for what your six switches will do for all combinations of the six switches, you should be able to translate those equations into diode/relay logic. Try starting by constructing a "Truth Table" with all 64 possible combinations of six two-position switches in the first column, and then next to that fill in four columns for the four valves. Fill in each valve column with a "1" if it is actuated, or a "0" if it is not, for each of the 64 switch combinations. Each valve column represents a Boolean equation involving the six switches. With some practice, you will be able to draw up diode/relay circuits to operate each valve just by inspecting the ones and zeros in each column. To get you started, here is a Google search results page with links to diode relay logic design.

 

[gorgon]

What type of diode you can use is depending on the current drawn by the relay. 1N4001 is a 1A max diode, 1N5400 is a 3A max, both 50V. If the problem is that one or more switches activate the same relay, you can use a diode for each relay, from each of those switches.

As said, a diagram or schematic is best, if you want a certain answer.

 

[Dragiin]

good read and info. thx for that hevans.
@gorgon.. my microswitches are 3 amp..so im guessing a 1n5400 would be fine..wish i would have thought of that last 2 days as ive gone to every radio shack in the valley looking for 1n4001 with no luck...
anyways i ordered the 1n4001's online and they will be here saturday..
while im waiting i re-drew my diagram with the new information given and im hoping im on the right track..without the diode here to test it out im stuck with paper theory for now lol

I prolly should have spaced things more but essentially this is the whole setup as i think how it should work based on the new info....

this drawing is just switch 1 and 3 to space out and better see layout

so far its all making sense to me but until i actually get the diodes... im not sure on position either. does it matter if its going into the relay or off the switch? also,,would it matter if i daisy chained all the powers and grounds? im thinking only risk would be if one goes bad everything in line after the bad one is bad too....

well i get the diodes saturday,, ill hook it up as i drew it but if anyone sees something please do let me know.. thanks

 

[Kiwi]

This might help you. Uses 8 relays, instead of 12.

I only use 1000v 1N4007 and 1N5408 diodes. Don't cost much more and you don't have to worry about voltage.
To protect the relay contacts I have added diodes to the solenoid valves.
The relays also have resistor protected coils to protect the switch contacts.
Terminal 30 is the common, so is usually connected to the power source. 87 would go to the valve.
Also I haven't included any fusing.

 

[Dragiin]

wow! that is awesome...and i think that will be exactly what i want it to do. i like how the diodes can be right after the switch and of course less relays. i have limited space as it is..thanks for that diagram! im going to stop at radio shack on way home and see if they have a couple of said diodes so i can start mocking something up before saturday. sure they are stupid expensive but atleast i dont have to wait lol

 

[Dragiin]

Thanks for all the help!! just wanted to post the results of a successful project

 

[Kiwi]

I wondered what you were up to. Glad to see it worked out ok.

 

[(*steve*)]

You wouldn't want to knock some of those switches while you're driving...

 

[Dragiin]

oh i dunno....could make for an exciting ride..! heh