[Who's Me]

I have a small submersible 120v.AC pump (2.5W 0.03A) running the cooling system for my pc, but sometimes I forget to turn it on.

I would like to switch it on and off with a 12V automotive relay (30A) triggered by the PSU.

I Know a little bit about electronics but probably just enough to be dangerous.
Could someone look at my schematic and make sure I have not missed anything that could kill my PC?

I tested for continuity between the relay coil and the switch contacts and there was none, so I assume the 12v and 120v will be isolated.

The Diode I am not sure about, It's there to prevent feedback into PSU when relay shuts off, should it go across the relay coil as shown?

I was planning on using 1N49003 Diode because that is what I have on hand. Will that work?

[Frank4d]

It looks OK to me (electronics engineer with 31 years experience). Except, I have never seen a 1N49003 diode. Did you mean 1N4003?

Make sure the diode polarity is correct so the PSU doesn't smoke the diode.

[Who's Me]

Sorry, yes I did mean 1N4003

[JohnWill]

I'd be happier with a 1N4005 or 1N4006, the spikes there may be too large for your diode.

[Who's Me]

I was looking through a box and found this,
http://www.harborfreight.com/manuals...1999/91813.pdf
Here is a pic of one of the same style,
http://ep.yimg.com/ca/I/campingsurvival_2069_489876502
If I could use that it would make for a easier installation, and I could get rid of the pump power cord coming out of the pc.

I think the Modified Sinewave should be fine on the pump side but I don't know if it would send back any interference on the 12v side.

Any thoughts on going this route?

[cwwozniak]

The inverter may draw quite a bit current when first turned on and may trip an overload shutdown as soon as you as you turn on the computer's PSU. As you mentioned it may also dump noise back into the 12 volt supply.

Have you considered using a smart outlet strip that only turns on the other outlets when the computer is turned on?

http://www.smarthomeusa.com/ShopByMa...td./Item/SCG3/

[Noyb]

Get one of These and have it turn on and off with the computer

[cwwozniak]

Originally Posted by Noyb

Get one of These and have it turn on and off with the computer

Sure looks a lot like the item I linked to.

Not sure how the prices compare when you add shipping and handling.

[Noyb]

I spotted that after I posted ... I think the Newegg prices are much better.
I can also find them at Menards home improvement stores at about the same price.

I would have used an optically isolated relay .. Then you wouldn't have to worry about spikes or load.
But I think the smart strip is the best option here.

[Knotbored]

How about plugging that tiny punp in-line with the monitor?
If I'm not wrong that outlet on the PC is inactive when the PC is off-perhaps also when the PC goes into sleep mode.

[Who's Me]

Thank you everyone for your opinions/suggestions I decided to stick with my original plan.

Frank4d, I made sure the polarity was correct and checked for heat while running, there was none so I think I'm good.

JohnWill, reading the specs. on the Diode It looks like it should be able to eat anything my relay is going to throw at it. but thank you for making me check.

cwwozniak, Not being sure about the power inverter current draw and possible interference killed that idea. The smart power strip would be a good idea except that it is a little big to fit in my tower and I have all the parts for the relay on hand without spending any money. plus I really enjoy being able to say "I built that" instead of "I bought that". (I even milled my own waterblocks)

Noyb, Optically isolated relay, None on hand plus I think they are a little pricey anyway, arent they?

Knotbored, I think you are talking about a PSU with an extra 120v outlet for the monitor plug, but mine has none.
However, you did give me a good idea. If I get some time this weekend maybe I will open the PSU and solder the female end of an extension cord to where the 120v comes in. That way i can still get rid of the extra cord coming out of my tower. (Don't worry I am very aware of the dangers involved with opening the PSU.)


So, I built my relay circuit and it works great. I plugged in the pump and cycled the relay a bunch of times with a 12v plug-in transformer then left it on for a while to make sure nothing got too hot. I mounted it in a small Radio Shack project box for a nice clean look.

Now, Since I have an extra pin on the relay I want to add a buzzer to detect a failed relay coil. I know the circuit is a little unconventional, but if you consider that all grounds go to the same place I think it is safe. I tested the buzzer through the unplugged pump and it works fine with the pumps resistance. I attached another schematic with the new parts in blue. If anyone notices any problems with adding the buzzer as shown please let me know.

[cwwozniak]

I never suggested putting the smart outlet strip inside the computer case. It would sit outside of the computer. The computer power cord would plug into the controlling outlet and the pump would plug into a controlled outlet. The other switched outlets could be used for the monitor, amplified speakers, etc.

As for your adding a buzzer idea, can I please take out a $1,000,000 life insurance policy on you with me as the beneficiary? I'll pay all of the premiums. I don' t think I will be paying them for too long before I collect. For starters, the motor would connect between the 120 volt line and neutral, not ground. The neutral and ground connect together at the breaker box. You may see some voltage on the neutral line relative to the computer ground due to the currents flowing through the neutral wiring in the house. If you use a two prong plug for the pump supply a reversed plug is going to dump 120 VAC into the 12 VDC line of the supply.

EDIT: Should you decide to add the buzzer anyway, and if you have GFI protection on your AC outlets, you will probably be tripping the protector due to any small currents trying to flow from the neutral line to the computer's chassis ground connection

[Who's Me]

cwwozniak,

I know you wanted the smart strip setup normally but I wanted to keep the cord for the pump in my tower and not spend any money on the project.

I think you may be slightly over estimating your chances of collecting on that policy, BUT I do see your concern. I was going on the Knowledge that the neutral and ground are connected at the breaker box. I tested for voltage between the tower ground and neutral wire in the wall outlet and found ~200mv. Not much, but enough to convince me that something is happening between ground and neutral before they get to the box. I will look into another way to design the relay failure circuit...maybe.
Thank you for bringing that to my attention.

[Who's Me]

P.S I learned the hard way about the reversed two prong issue, I use plugs with one wider plug. The first time I hooked up my pump I cut the wire so it could pass through the reservoir lid and I hooked them up backwards. The pump being, A/C didn't care about the switch and ran fine but one day, A couple of months later, I stuck my finger in the coolant while touching the case and found out that I was running 120V AC through my cooling system. (Which means Bolted directly on top of my CPU Northbridge and my GPU) Somehow my pc was ok.

[paisanol69]

Originally Posted by Who's Me

cwwozniak,

I know you wanted the smart strip setup normally but I wanted to keep the cord for the pump in my tower and not spend any money on the project.

I think you may be slightly over estimating your chances of collecting on that policy, BUT I do see your concern. I was going on the Knowledge that the neutral and ground are connected at the breaker box. I tested for voltage between the tower ground and neutral wire in the wall outlet and found ~200mv. Not much, but enough to convince me that something is happening between ground and neutral before they get to the box. I will look into another way to design the relay failure circuit...maybe.
Thank you for bringing that to my attention.

....definately listen to the advice that cwwozniak has given you. Are you even aware of how dangerious what you have drawn is?? You NEVER, NEVER mix what is considered high and low voltage circuits together on a single control element, and then you are trying to "cheat" your low voltage ground path , by tieing it to a high voltage neutral return path.

One other thing to consider...are you aware of how little space/distance there is between the Normally open, and the Normally closed contact inside of that mechanical relay? every time that relay energizes/de-energizes, there is going to be a small spark/arc between the Normally open and the Common contact, and at the time of a coil collapse, or a mechanical failure of one of the contacts, (which I have seen in my experience) you will be "welding" inside of the relay, and the potential for all 3 of the contacts being a single connection is very likely.

If you are lucky, you will ONLY have to replace your computers power supply when that happens.

If you choose to ignore what advice you have been given in this thread, then best of luck to you, but if you do ignore the advice, then I sure hope you don't start wiring/ re-wiring anything else in your home, since it could be very dangerious for you and your loved ones..