Using a 5.1V Zener as a regulator ?

[iso9001]

Bad idea ?

I have an automotive application that requires a pic and circuit that will run at 5V and draw MAYBE 50ma alltogather.

I was thinking I could use the 12v in, tie it to a resistor (4.7kish) then reverse the zener to ground, to that between the diode and the resistor I have a 5V source.

This is buffered 12v so I am not worried terrible about spikes, but they may still occur duing startup.

This zener says its good to 1W. I figure my deal will be using ~.3W.

This is mainly to get away from using a 5V regulator. Which I have niether the money or space for (mainly space, so please dont tell me how cheap they are). Another reason for not going with a regulator is that this unit will always have power running to it, day and night, the pic will sleep most of the time but I have a feeling the regulator will draw more power then I'de like. (If possible I'de like to be at or under 1ma when sleeping)

 

[k7elp60]

Should work okey, but the resistor needs to be about 100 ohms @ 1 W.
12V-5.1V= 6.9V drop across the resistor. Add about 10 Ma for zener current. 6.9/.06=115 ohms. Go with about 100 ohms. P=IxIxR
I=6.9/100 or 69Ma. P= .069 x .069 x 100 =0.47W. Double the power rating for safety.
Make sure the cathode of the Zener is + and the anode is ground or -, the resistor between +12V and the cathode of the zener. The 5.1 volts will be across the zener.

 

[Russlk]

The zener is not going to sleep, so it is drawing current all the time. Check out Texas Instruments part TPS76350DBVT, it is a 5 volt regulator in a SOT-23-5 package that is much smaller than a 1 watt zener and it draws only 0.14 mA while supplying up to 150 mA. It is available from Mouser for 78 cents.

 

[iso9001]

I like zeners, never let me down. If the PIC isnt on, wont the zener only draw in a little, oh, wait thats what that resistor is gonna do. Yea I see what you mean.

The problem w/ the regulators I've been using (stupid radioshack parts)

Is that the size is 1 issue, but also, I can ever trust them. I've had a ton fail and I'm always double checking them. PLUS, thier is the whole capcitor issue.

Does the TPS76350DBVT need caps on the + and ground lines ?

Thanks,
rhet

 

[Juglenaut]

You will want to drive the zener to stay inside its characteristic curve.

 

[Russlk]

The data sheet says that 4.7uF is required at the output, but 10uF, 16V cap from Sprague (196D) is only 2.5X7.6 mm on the board.