Help with ellectrical issue

[Bill Musarra]

I am in the process of rewiring some of my dash switches. The problem I am having is with my electric water pump and dual front fans.
What I have:
Three pole switches with lites on the end of the switch stalk. The poles are power, load, and ground. The pump and the fans are controlled with relays. The load from the switch, goes to the coil of the relays.The thermostat is a two pole unit, which connects the two poles when the temp is met, and it turns on the warning lite.
What I want:
I want the switches to control the appliances. The easy part.
I want the thermostat to turn them on if I forget to turn them on. I also want the warning lite to turn on when this happens(only) as a visual reminder. It will also alert me that the temp of the water is rising and is approaching a danger point.
Here are my thoughts on the matter.
The wiring from the thermostat needs to have a diode or two in line to keep the system from back feeding and turning the lite on when the switch is thrown. Here is my diagram to illustrate the point. The diode is represented by the two arrows. Looking at it after I drew it I can see one doing the job.

My question is, will this work or is there some easier way to achieve it. My next question is if this is the way, which diode is better served here. I got several to try them out. What I am afraid will happen, is the back current from the switch being thrown will cook the diode and lite the lite. I have some 6 amp rectifier diodes. Its values are:
Vrm(wkg)Vr-50VPK
Vrm(non-rep)-100V
Vr'-35V
Io'-6amp@60C
Ir'-25uAdc(u is micro)
Ifm(surge)-400APK
Electrical characteristics
Vf(AV)-.9VPK
Ir(AV)-25uAdc

Some In 4001 rectifier diodes (silicon 1.5A-1000V)
and some IN 5401 rectifier diodes(3A-50PIV)

Any thoughts on this idea??? Will I have to put up with the lite being lit every time I flip the switch??

Bill

 

[Seymour Snerd]

I have some 6 amp rectifier diodes. Its values are:
Vrm(wkg)Vr-50VPK
Vrm(non-rep)-100V
Vr'-35V
Io'-6amp@60C
Ir'-25uAdc(u is micro)
Ifm(surge)-400APK
Electrical characteristics
Vf(AV)-.9VPK
Ir(AV)-25uAdc

Some In 4001 rectifier diodes (silicon 1.5A-1000V)
and some IN 5401 rectifier diodes(3A-50PIV)

Bill

Bill --

I haven't studied your concept or your diagram yet, but just a couple things about diodes to make this easier:

For the kind of thing you're doing (using a diode in an automotive context to allow multiple current sources to drive one or two relay coils) there are really only two things you need to consider about the diode:

Vr
Io

Vr needs to be higher than the highest voltage it will ever see on its cathode (the striped end) relative to its anode. The 1N400x series has the handy property that every increment of x represents a higher Vr, typically by a couple 100 volts. However, a 1N4001 is only a 60v, not 1000. I would use a 1N4003 just to be safe. You can see 50-100 v spikes in automotive systems if something happens to the alternator (like the battery is disconnected with the alternator running). Chances are really high a 50V diode will work for years, but when you have a choice for not much money it's better to be safer. And 1N400x's are incredibly cheap. Even at Radio Shack you can get three 1N4003s for a dollar, and they're good for 200V Vr. (# 276-1102 )

Io is the continuous current the diode can conduct. Just make sure it's higher than worst case for all the loads together. Here we're talking about max three relay coils, so it's a non-issue even for the 1A diodes I think, since a typical car relay coil draws about 1/6 of an amp.

Once you get over an amp with diodes you have to start thinking about power dissipation and heat, so other parameters come into play.

Hope that helps.

 

[Seymour Snerd]

If the little circle to the right of the thermostat is your warning light and the little in-line > symbols are diodes, your circuit works. However, depending on how your switches work internally, the thermostat might also light up their stalk lights. You could tell by taking a switch that is off and putting 12V on its load terminal. To prevent that just put another diode on the output of each switch. That way the loads are always fed by a diode which means none of the switches nor the thermostat/lamp can be fed by something else.

 

[Jim Cowden]

Have you considered an earth trigger.
I like keeping things simple.

Jim

 

[Seymour Snerd]

earth vs live triggering

Have you considered an earth trigger.
I like keeping things simple.
Jim

Earth triggering per se is not simpler than V+ triggering; it's just different.

Independent of that, in your design you've discarded a couple features Bill's original design had:

1. you no longer have separate control over fans vs. pump
2. you no longer have a separate indication that the thermostat has taken over, vs. those being turned on by the user.

Bill can still earth-trigger his relays; that's just a question of moving the switches and diodes to the other sides of the relay coils. But it will still take the same number of parts and wires.

There is one property in favor of earth triggering: it puts most of the wiring downstream of the load, providing the benefit that if any of that wiring contacts the chassis it simply turns something on rather than blowing a fuse. However, in a properly done wiring job that's not much of a likelihood anyway so to me it's somewhat of a toss-up. The primary situation in which I prefer earth triggering is when I have multiple switches in different locations turning on the same load (eg a dome light in a four-door). In that case you avoid running "live" wires all over the place and perhaps really do reduce fuse-blowing opportunities in case of a pinched wire or something.

 

[Bill Musarra]

Alan,
Thanks for your response. It was as I thought. I wasn't aware there were other 400x diodes. I will go back to Radio Shack and search them out. I think you are right about the stalk lighting and will have to add a diode or two.

Jim,
With a three pole switch wiring to ground is where I got into trouble(had to rewire all six switches). For the stalk to light there has to be a load on the switch. Your method will work, but you just wouldn't have a light on the switch, which is what I was aiming for.

Bill