Need help with regulator IC and how to make the control pin low when input is high

[shahabmusic]

Hi guys,


I have a question please help. I did a google search but could not find anything easy to understand.
I have a 12V voltage regulator that has 4 pins. pin 4 is on/off control. when pin 4 is high (open) then there is regulated voltage on output pin (pin 3).
Now I need this for a car project. this IC is going to be connected directly to the car battery and has to regulate the voltage 24 hours except when the car is turned on(when ignition key is in on position). so I have to somehow make pin 4 low(connect it to ground for example) when I have power coming from ignition key.

First I was going to use normal mechanical relay instead of using regulator IC but since it is mechanical, it has a bit of delay.
oh and by the way I want to reduce the delay as much as I can.

using solid state relay was my other option which I couldn't find a suitable one in my local shop... I found few but they couldn't handle 2Amps...

I am from New Zealand and would greatly appreciate any helps.


cheers

 

[(*steve*)]

The on/off control does not carry the full regulator load. It may require a small fraction of a milliamp.

I'm not sure what voltage you get on what when the key is in the ON position, but you can probably connect this to a transistor and use that to pull the control pin low.

If you need isolation (which I doubt you do) then you could use an opocoupler.

Tell us more about how you can sense the key is in the ON position and also tell us what the regulator part number is so we can ensure we're giving you good advice.

 

[shahabmusic]

first of all thanks heaps for the reply and your time.

part number is (these are what printed on it) :
278RL12
S
8G1 C14


car battery is usually 14.4v and I don't have to exceed 12v... thats the reason I am using this regulator.
and when ignition key is in ON position(car is running), there is a wire called accessories wire that has 13~14v that provides power to car stereo and other stuff in the car...and is 0volt when car is off.... this is the wire I am going to use to control the regulator.
so when there is no voltage on the wire, regulator should start providing regulated power.

 

[(*steve*)]

I couldn't find the specs on this chip, so I'm not sure exactly what it's capable of.

Presumably you have checked and this regulator can supply the current required, and has a low enough dropout voltage to continue to provide 12V when the battery drops below 14.4V (does this device actually require a good 12V rail, or can it cope with a lower voltage?

Car electrical systems are very noisy and you may need to take precautions to prevent your regulator from being damaged by high voltage spikes.

But getting back to the issue at hand, all you need for the basic functionality is a small NPN transistor (such as a BC548) and a 10k resistor. The 10k resistor is connected between the accessories rail and the base of the transistor. Ground the emitter, and connect the collector to the control pin.

Practically you may wish to also protect this from spikes, and to extend the off time a little so that your device is not powered down and then up quickly (which it may not like).

If you can confirm some of this stuff then I should be able to draw you a simple circuit diagram after I've had dinner

 

[shahabmusic]

I wish I could upload files more than 97kb then I could upload the datasheet which is around 120k.
this is the link anyway http://doc.chipfind.ru/kec/278r12.htm

I am not too concerned about the spikes and rest of it. all I need is a regulator that shuts off immediately when there is voltage on my car accessories wire and turns on as soon as voltage goes to 0v on that wire.

By immediately I mean no delay at all.


thanks and enjoy your dinner... ;-)

 

[shahabmusic]

Sorry a question just raised. if I put voltage on pin 3 which is output when the IC is off, does that damage the IC?

so I am basically trying to make a relay using IC to reduce delay.
A relay like this

 

[(*steve*)]

Not concerned with spikes? So you don't care if it stops working within a week or so? <shrug>

If you put a voltage on the output when the input voltage is zero, you may well damage the device. It's far less likely when the device is powered but disabled, in fact I'd say that is safe.

Mmmm, nice dinner, thanks

Here's a very rough schematic without any protection.

 

[shahabmusic]

please look at the attachment. this is the diagram if I was gonna use normal relay....
so I just simply want to replace that relay in that diagram with that regulator to get zero delay....
do you think it is possible? or do you have any advice?

 

[(*steve*)]

You can use a relay if you like, but it will be pretty slow. It may take a couple of milliseconds.

 

[Rleo6965]

Here's a very rough schematic without any protection.

[/QUOTE]

Do we need to place pull up resistor on pin 4 then connect other end of resistor to +14V?

 

[(*steve*)]

No, I don't believe so. If you leave this pin open it floats high. Adding a pullup resistor would work though. 10k would be fine.

Just check the specs to make sure the voltage doesn't have to be TTL compatible. If it does, then you may not want to pull it higher than about 5V

 

[drno503]

dear newzeland,,,, voltages from battery can range from 5,, dead battery,,, to 13.8 charging
y not just use a zener diode for your application
gerald florida

 

[(*steve*)]

dear newzeland,,,, voltages from battery can range from 5,, dead battery,,, to 13.8 charging
y not just use a zener diode for your application
gerald florida

Oh, I don't know.... Perhaps the 30 watts or more of constant load (and therefore heat) might be the reason.

Also, it's a lot harder to switch it off without adding the extra overhead a LDO regulator will add anyway.

 

[shahabmusic]

awesome. thanks a lot man you are life saver. all sorted now.

 

[shahabmusic]

dear newzeland,,,, voltages from battery can range from 5,, dead battery,,, to 13.8 charging
y not just use a zener diode for your application
gerald florida

yes zener was my other option but it can't handle 2amps... my load is about 1.5A.

its all sorted now anyway.