Build me a circuit please

Suggestion... (assuming you just want "one" of these)

Buy a "vehicle to 1.5v" cigarette lighter adapter and tear it apart.

Use the circuit to do what you need.

Take the 12v under the dash via a fuse and run the 1.5 to wherever you need it.

(Those adapters are available for under $10)

If you need multiple copies for a "product" then it's a different story.

Jim

 

What's your tolerance on the 1.5v output. Does it need to be regulated, or can it swing with the 14.2?

If it can swing, then a simple voltage divider circuit with two resistors will work, and be more tolerant of underhood heat than any IC.

Put a 10K on the +14.2 and a 1.2K in series with that to ground, then you'll have 1.5v from the junction of the two resistors to ground.

Vout = Vin*(R2/(R1+R2)) where the 10K is R1 and the 1.2K is R2.

But since it's a direct ratio, the output will vary just as the input varies, and your 1.5v load may not like that.

 

John -- You are ignoring the relatively low impedance of the load (1.5V drawing 12 mA implies a 125 ohm load) which is much lower than your 1.2K resistor so it significantly changes your voltage divider calcs when the load is placed in parallel with the 1.2K ohm resistor. Any current in the little resistor is just wasted heat, so, if this scheme works, better to just hook the 14.2V -to- a ~1050 ohm resistor -to- the (125 ohm) load -to- ground.

The power dissapation in the 1050 ohm resistor is just R x I^2 = 150mW

 

An easy way to acheive your need is in DC to DC converters. very small packages with no heat to speak of, because it is not simply sinking the excess power.

National Semiconductor LM3671 miniature DC-DC converter with 95% efficiency, is available in the following output voltage options: 3.3 V, 1.875 V, 1.8 V, 1.6 V, 1.5 V, 1.375 V, 1.25 V, 1.2 V as well as an adjustable option for other voltages.

 

I'd investigate that approach too Bill (before doing a funky resistor thing) -- but isn't the 14.2V input voltage outside what the LM3671 can accept?

 

http://www.powerconversion.com/products/cat/38/Low-power-isolated-dc-dc-converters-for-industrial-applications

 

Circuit: A 78 ohm resistance in series with a 10 ohm resistor.

Place your device (1.5V 12.5Ma) across the 10 ohm resistor.
The realized resistance of this circuit leg will vary from 10 ohms (at no load) to 9.25 ohms (at full 12.5 MA load).

Performance:
1.61v at no load (14.2 *[10 / 88])
1.50v at full load (14.2*[9.25/88])

2.3 watts power dissipation (14.2 squared / 88 ohms) which fulfills your low heat requirement. Use 10watt resistors to insure very little temperature rise above ambient.

Notes
1) Real world: To obtain the 78 ohm resistance use a 68 ohm resistor in series with another 10 ohm resistor. 68 is a standard value.
2) This circuit has two 10 ohm resistors.
3) While 10% tolerance resistors can give slightly different voltage, making a 'one off' means you can easily tweak the final voltage.

 

Seems to me the easiest way to do this is a LM317T from Radio Shack for 2 bucks and 20 minutes.

 

So what are you using this for Chubba?

 

Yeah, that part is sort of important.

If it's a one-off it's one thing.

If you need these in quantities, a board has to be made/etc.

(Again, if it's a one off - get an adapter - and break it open - the little circuit board inside will be the cats meow for what you need - regulated - for under $10)

 

I've been thinking about your request. The simpliest why to build a step down converter is to use a simple zener diode of 2.1V as illustrated here : http://www.zen22142.zen.co.uk/Circuits/Power/dcdc_con.htm.

But then, I read again your post and saw that you want this working in a very hot environment of up to 95°C ! The above schematics wouldn't work as the zener diode current will change by approx 0.3V per 10°C. Also, such schematics will drain approx 20mA even if no current is used and output voltage will never be perfectly at 1.5V.

I would advise you to use the LM117 which supports an operating temp of −55°C ≤ TJ ≤ +150°C.

Page 9 of the LM117 datasheet contains a simple schematic on how to use this converter:
http://www.national.com/ds/LM/LM117.pdf

I would go for the LM117,
Hope this helps,
Stef

 

Last but not least, a very good source of information, all about DC/DC converters/regulators

http://www.siongboon.com/projects/2005-08-07_lm2576_dc-dc_converter/

 

If your circuit was made for an AA battery, batteries vary 1.1 to 1.6v, so you might not need extreme voltage accuracy,

Since a silicon diode junction will drop about 0.75V forward, you could put a 1Kohm (half watt) resistor in series with two silicon diodes, and run your load parallel to the two series diodes. A selenium power diode can drop 1.4 to 1.7v forward, so you might use one in place of two silicon diodes, but selenium diodes tend to have large heat sink cases. Which would be bad where a heat sink would be picking up engine heat.

The total circuit will draw about a quarter watt off your car system.

The trick is that 95C is industrial grade temperature specs, so you're probably going to have to mail order the parts. Stuff from Radio Shack may not have the temp tolerance. (And I don't think there are any Radio Shack outlets in PRK.)