Simple serial port controlled switch

[obsidience]

Hi all, I've been searching google all of this morning to no avail. I have a color LCD display that I'm attaching to a computer case. The LCD requires 12v and 500ma. I'd like to be able to power up and down the display using a serial interface. I'm a VB/C++ programmer so the programming aspect I can take care of. My problem is that I'm clueless with electrical component design :-\

I found one google post that is close to what I'm looking for:



<<
1N4148 diode, qty: 2
2N3904 transistor
4.7 K ohm resistor
External 12V supply
relay with 12V coil

Connect the anode of the diode to the RTS line.
Connect the cathode of the diode to one side of the resistor.
Connect the other side of the resistor to the base of the transistor.
Connect the collector of the transistor to a +12V supply.
Connect the emitter of the transistor to one side of the coil of the relay.
Connect the other side of the relay coil to ground on the external power
supply.
Connect the power supply ground to the computer ground.
Connect the second diode's cathode to the collector of the transistor.
Connect the second diode's anode to the emitter of the transistor.

Turn everything on, and toggle your RTS bit. You'll find that when the bit is
high, the relay is off. The transistor should be able to drive a 12V coil of up
to 200mA. If you're certain everything's working, you can use the computer's
power supply instead of an external power supply.
>>



The problem with this solutions is that its 200mA not 500mA. Is there anyway I can adapt this to make it work or does anybody have another solution?

I appreciate your time!

Obsidience

 

[Superdoopercooper]

Ok... I'm assuming that the current is limited to the power handling of the diodes & Transistor. So, I would propose that you use 1N4001 diodes. They are slower than, but larger (more power handling) than the 1N4148. This slowness shouldn't be an issue.

For the transistor... a minimum of a 2n2222 (or the equivalent PNP). These are beefier than the 2n3904, but are functionally equivalent. Any metal can or T0-3 packaged NPN (if that is what the 3904 is -- there is a 3906 which is the other version... can never remember which is which) should be able to handle the power requirments. Maybe a T0-3 would be best.

Ok... wait... what you are failing to realize is that the current that their talking about in the google article is the current for the RELAY.... not for your LCD. Imagine that the relay is just a manual switch. When off, no connection. When on... there is a solid metal contact carrying power from your power supply to your LCD (needing 12V @500ma). The 200ma that they're talking about is flowing through the relay coil... the think which connects or disconnects the metal contact between the supply and LCD. So, as long as you ahve a relay that requires 200mA or less in order to throw it, you shouldn't need anything else more than what the google thing said. However, if you have a heaftier relay, then you'll probably need more coil current, and then you'll need to use the stuff I mentioned in paragraph 2.

Make sure the relay you buy can handle a contact current (probably wont' be called that specifically) of >500mA. 1A or more would be preferrable, to give you some margin.

 

[blahblah99]

Not sure if this would work, but try using an NMOS with low Ron resistance and use the gate as an "on/off" switch. I suppose that lcd isn't too picky about getting exactly 12V right? If not, that NMOS might work and would simplify things greatly, plus you won't hear the click click as the relay switches


A 1 component solution .

OR

do a search at digikey and see if it comes up with a 12V, 1A power supply IC that can be switched on/off with a logic signal. I'm pretty sure they have those. It can be as simple as a 4-pin IC with 1-ground, 2-12V power, 3-output, 4-control signal.

EDIT: a quick search at maxim yielded this baby. That is EXACLTY what you want for your lcd.

 

[obsidience]

Cooper, Blah,

I appreciate your replies! I took a look at the MAX618 like it was made for exactly what I'm trying to do - I'll order a sample tonight.

BTW how loud do relays get when then switch? Could they be heard from within a case?

Best regards,

Obsidience

 

[Superdoopercooper]

Obs...

Relays of the size you're seeking do have an audible switching noise, but you are unlikely to hear it over the noise of the case/power supply fans. There is that possibity though, if you have a very very quiet case.

 

[highwire]

I must have missed something, even in re-reading the thread.

Normally, to power-on a device requires a connection from the device to the +(12v) source. This is usually accomplished using a PNP or PMOS on the + rail to pull up the load. I don't see anything inside the MAX618 to do that. It could be used in the ground rail, but that is usually not allowed or possible.

Two transistors and three resistors will do the job in style.