Electronics Help for a newbie?? 12v down to 5v

hi i have a problem i need to sort, i need to fit a five volt back lite LCD display but the only voltage i have to play with is 12/13v (as in car battery supply). this supply runs a few other gadgets inc a mini laptop which i am not about to take apart to hook up the LCD to as it will void the warrenty, the display shows temp and has a remote pick up which monitors the temp one of the gadgets is working at to make sure its within it op range.

so to cut short. how do i drop 12/13 volts to 5 volts using either resistors or diodes or anything alse you might know of (space isnt that great either so cant use one of those nasty plug in car voltage regulator things in a cigar lighter socket.)

many thanks for any help

 

The cheap and dirty way that will actually work would be diodes. Two ways you can go about this using diodes is using rectifiers to drop the voltage or Zener diodes to "regulate" it. A typical silicon diode, like the 1n4001, will drop about .75v per diode, and handle up to 1A. Slap 9-10 of these guys in series and you're good to go. 10 sounds a lot, but they are cheap, especially here. The Zener method obviously reduces the part count down, as all you need is one diode rated at your intended voltage (5.1v would be most suited for you) and the correct wattage. Since you're dropping 7v, the max current you can pump through a Zener would be 1/7th of its wattage rating.

However, because what you need is 5v, you can get a voltage regulator, like the 7805. As long as input voltage is ~1.5v more than the intended voltage, and you don't try to draw too much current, you'll get 5v with a minimal part count. The one big drawback though is that you might need a heatsink, so you'll need proper ventilation so that your regulator doesn't shut off all the time (most have thermal protection, which is a hell of a lot better than simply dying).

So, to sum it up, you should use these guys for the following situations:

Recitifier diodes - Close to 1A draw, no space for regulator and heatsink, love to solder lots of tiny bits together
Zener diode - Low to moderate power draw, need to be frugal with cash and/or space
Regulator - High power draw, have space for heatsink, require a fairly accurate voltage output.

 

I'd second that 7805 recommendation. Though some of them only need heatsinking if supplying over an amp. I would think the LCD isn't drawing much so you could probably forgo it.

 

The 7805 has a no-sink thermal rating of 65°C/W so 1W to waste will take it to 90°C or more in a hot car. If it's dropping 9V from an auto alternator that only allows 1/9A, 110mA, before you need a sink. Won't need much of one for an LCD though AFAIK.

Still the best solution, thirded.

 

Thanks for pointing that out. Any detailed online resources to learn more about how to calculate this?

For instance, on my car project, I'm using the equivalent of a 7812. It can deliver more than your vanilla one, but I don't have the part# on hand but I believe up to 1.5 amps with a heatsink.

I don't have a heatsink. If all the rgb lights are on (white), it should be pulling around 1080 millamps max. Is this ok you think? Trying to fit any type of heatsink would be tough in the small space. I might be able to do it if I cut it down, but I'd have to at least coat it in some dielectric paste or something to avoid a short.

Thanks,
-Lee

 

Most datasheets will give a value for thermal resistance of the device, and in the case of power devices a couple, junction to ambient (no sink) and junction to case (for sink calculations).

Junction to ambient is the easy one, if it's 65°C/W, like the 78xx, the maximum voltage you expect to drop x current = watts, junction temperature rise will be (65 x W)°C above ambient. Another number on the datasheet will give the maximum safe junction temperature, so you can work out the risks involved.

The thermal resistance figure given usually only applies if the device is soldered to a pcb (which helps remove some heat) so check the small print if you're working near the limits.

Using a sink, you need to take the junction-to-case figure and factor in the TIM (case-to-sink) and sink (sink-to-ambient) ratings, example calculation here. There's a more accurate method in Micrel's LDO datasheets that includes in the load the extra current used by the device itself, but they only reckon about 1%.

So if you're using a low-drop-out 12V regulator on a car alternator, the maximum voltage drop will be about (14.5-12) = 2.5V; 2.5W at 1A load. Best figure I can find for a TO-220 case is 50°C/W so with no sink the junction would get to 125°C over ambient - only use in winter.

Good news is, you may not need much of a sink, doubling the tab area will help a lot. And check if the tab is connected to anything dangerous, IIRC 7812 tab connects to ground pin so can often be bolted straight to a chassis. Or the insulating TIMs for TO-220 come as a dry rubbery washer with a nylon bush, so it's easy to bolt the case to some other metal to keep it cool & electrically insulated.

 

Thanks for taking the time to write that all out. I appreciate it.

I might be able to wedge in one of those small "U" sinks if I cut off one side. I'd probably coat the bottom near the PCB with liquid electrical paste to be safe. It's such a tight as it is. I used the mini-boards from ExpressPCB and while I made the circuit and components fit, it was quite a challenge.