relay question

[jason2012]

hello, I have a question about relays:

I have a 19v / 330 watt dc power supply providing power to a computer motherboard
I have a second 19v / 330 watt dc power supply providing power to a graphics card, on a different circuit.

If the power to the graphics card is disconnected or turned off before the motherboard power, it can cause damage to the components, which I want to prevent.

Therefore, I would like to use a relay to solve this problem by cutting power to the main board if there is no power to the graphics card.

However, I have read that when the relay is de-activated, some current can flow in the reverse direction, and I am concerned this could cause damage to the motherboard.

Also, the 19v input would require quite a large relay.

Does anyone have any suggestions / circuit diagrams etc that would be a safe solution of which relay to use, or what other components could be connected to prevent current flowing in the reverse direction and damaging any of the parts.. Is it possible to seperate the relay from the motherboard circuit, and use a voltage regulator to reduce its voltage as well, say down to 12v? I only need to sense whether the main power input is plugged in or not, so could a parallel circuit work in this case?

Please bear in mind I am quite a novice at electrical work!

Thanks, appreciate any help you can give.

[pebe]

It's difficult to understand why you are using such powerful supplies for relatively small loads. At full power they would supply over 17A each, and that's an awful lot to switch - it would need a pretty beefy relay.

Can you say what the mother board and graphics board each take in terms if volts and current? There may be a better way than using relays.

[jason2012]

Hi, thanks for the reply.

Well, the loads are not so small, and a bit of an unknown factor that is the reason. I purchased a DC adapter that would continue to serve my needs if I decide to add more power hungry parts in the future, and also to avoid the risk of overload, as I wasn't sure what my final power needs would be.

The graphics card for example, pulls 200watts at peak usage on its own.

The motherboard is quite a lot less, but then there are all the other bits it has to power as well, such as the monitor, graphics adapter, hard drives, cd rom drive, and about a zillion internal fans as well.

I did a measurement once a while ago, and i think the motherboard load was about 10 amps in total.

I understand that a beefy relay may be required, but trust me on this one, it wouldn't be the beefiest part in the whole system. The 19v to 12v dc converter for example, is massive.

[pebe]

You are actually only using 12V for the supplies, so there may be enough voltage overhead to feed the motherboard through a series FET controlled by the power supply of the graphics board. Then in the event of the graphics board losing power, the power to the main board would be shut off.

Can you give any more details of your 19V to 12V converter? Is it DC to DC?

[jason2012]

yes the 19v to 12v converter is dc-dc. That's only on one of the circuits, its the one i'd like to require to be on for the motherboard to continue recieving power.

The motherboard is a mini-itx style, that one runs on 19v.

also, the dc adapter is connected through the motherboard via an external jack so there are a couple of 19v wires I can splice into, before the motherboard power connection is made. that's why i was wondering if i could use a resister plus a lower power relay on a seperate circuit to detect the presence of the dc power jack being plugged in.

I can make a diagram if that helps.

[pebe]

Yes, a diagram would help.

[jason2012]

done in MS paint, apologies

diagram.png (44.38 KiB) Viewed 22679 times

[pebe]

Thanks. If you can confirm that the motherboard and the graphics card share a common negative rail, then it can easily be done with a FET.

[jason2012]

i'm not sure... If you mean the ground connection, then some of them are connected, yes. what is a FET and why would it be better than a relay in this case then?

[pebe]

A FET is an electrically controlled switch that can be fitted in series with the main board supply to disconnect it if the supply to the graphics card fails.

Thinking more about it, both boards should share a common negative terminal (ground) so I will draw up a circuit for you.

[pebe]

Here is the circuit that will disconnect the supply to the motherboard if the supply to the graphics card fails, or is switched off.

The LH side shows the power supply to the graphics card. The RH side shows how the positive lead from the power supply goes to the motherboard via a FET that acts like a switch. The circuit needs a common 0V rail from both supplies – I think that almost certainly the graphics card wiring will provide one.

When operating normally, the 12V supply to the graphics board is fed to the base of TR1 via R3 and turns it on. That gives about 19V across R1 and R2, so the gate of the FET is about 13V negative relative to its source pin. That turns on the FET, which acts like a switch. With a rated maximum current of 31A flowing through it, its resistance would be about 0.06ohms – a negligible value that won’t affect the supply to the board.

If the supply to the graphics card is switched off, TR1 will have no base drive so will go o/c. With no gate voltage, the FET will also switch off and go o/c.

I don’t know what the consumption from the RH power supply is, but you may be able to use a P channel power FET with a lower current rating. TR1 can be almost any small NPN transistor.

Best of luck, and if you need any more info, just come back.

[jason2012]

wow, thank's a lot!

I will endeavour to connect the bits in the order that you have shown in the diagram.

Looking at the diagram, there is really only one question I have at this moment, which is where the negative rail should be connected on the two sides.

let me buy the components first and i`ll post some photos

[pebe]

The emitter of TR1 should be connected to one of the negative leads coming out of the 19V-12V converter (I have shown it as 0V).

With power switched off, check for continuity between 0V and the negative lead of the right hand 19V 330W power supply, ie. where I have shown the green lead. (Note: you do not need the green lead - it is only in the circuit to show the exiting connection).

It should show very low resistance. If so, they are common and the circuit will work OK
If they are not, will you let me know what the ohms reading is?

[jason2012]

ok, thanks for that clarification, now I have a pretty good idea how to connect everything up, the green wire was confusing me, now I know it just shows a theoretical route to negative on the motherboard power supply.

fortunately , I already have the device for measuring voltages and all that, so I will be able to check the resistance when i'm done.

I'll get the bits today and start soldering them together tonight.

[jason2012]

hello, all the parts have arrived now.

I'm still unsure about where to connect the "common ground".. I suspect it must be in the connection that runs from the motherboard back to the graphics card. this has the positive power +12v provided from the 19v-12v converter, and several ground connections that go back to the motherboard, which then return eventually to the negative connection of the motherboard's psu.

I'll try connecting the circuit onto the top of one of the ground pins on the pcie connector that run between the motherboard and the graphics card, and see how that works.

So, I have to test for a low resistance between where the wire is connected on the top of that pin, and the negative wire of the power supply (the 19v end)?