Poor man's voltage reference?

[kpatz]

In poking around on the 'net I see some folks have used LEDs as zeners for voltage references. Any thoughts on this, or are they also too sensitive to temperature?

 

[Roff]

In poking around on the 'net I see some folks have used LEDs as zeners for voltage references. Any thoughts on this, or are they also too sensitive to temperature?

On p.3 of this article, the tempcos of various LEDs were measured:

IR........-2.3mV/deg C
Blue.....-5mV/deg C
Green...-3.8mV/deg C
Red......-1.5mV/deg C

 

[kpatz]

(I'm not giving up easily...)

How 'bout this? A JFET current source, and tap a 2.5V VRef off the resistor as shown in the attached screenshot.

In the simulation, it is stable down to about a 3.1V supply voltage, which is more than sufficient. But is it (reasonably) thermally stable?

 

[Roff]

(I'm not giving up easily...)

How 'bout this? A JFET current source, and tap a 2.5V VRef off the resistor as shown in the attached screenshot.

In the simulation, it is stable down to about a 3.1V supply voltage, which is more than sufficient. But is it (reasonably) thermally stable?

Add this to your sim:
.step temp 25 50 5
or whatever you want. This will run 6 sims, starting at 25C, ending at 50C, in 5C steps.
You can also do a DC sweep instead of a transient sim. Set the supply voltage to some arbitrary voltage, then sweep it in small steps. I did 0 to 5V in 1mV steps. You get a nice plot of Vout vs Vcc.

 

[kpatz]

Even better than SPICE is an actual breadboard. With an actual MPF102 and using a 3.9k in parallel with a 180K resistor (giving me an actual value of 3.77k according to my multimeter) I get 2.500 volts on the dot.

As for temperature stability, I only saw a change of 1mV when I touched an ice cube to the top of the FET. I think it's good enough for my purposes.

EDIT: I spoke too soon. I had 2.5V at the output when I fed the circuit with 5V. When I feed it with 9V, I get 2.532V. Not exactly self regulating voltage wise, and not what I got with Spice...

 

[Roff]

Even better than SPICE is an actual breadboard. With an actual MPF102 and using a 3.9k in parallel with a 180K resistor (giving me an actual value of 3.77k according to my multimeter) I get 2.500 volts on the dot.

As for temperature stability, I only saw a change of 1mV when I touched an ice cube to the top of the FET. I think it's good enough for my purposes.

EDIT: I spoke too soon. I had 2.5V at the output when I fed the circuit with 5V. When I feed it with 9V, I get 2.532V. Not exactly self regulating voltage wise, and not what I got with Spice...

Why would you test over such a wide range? Can't you power it from your 78L05?

 

[kpatz]

Why would you test over such a wide range? Can't you power it from your 78L05?

Well, if you built a voltage regulator or reference wouldn't you test it over a wider range than you'd see in the actual circuit? My simulations showed it to have a steady voltage over a wide input range.

Also, one of the purposes of this reference is to be able to detect a low battery by detecting sag on the 5V rail. So if I run this reference off the 78L05 and it sags as well, it won't work too well for that purpose. Otherwise, I'd just use a voltage divider off the 5V to get my reference.

But that is a good point... I should test it between 4.5-5V to see how much the output varies over that range. But, in theory at least, the output voltage shouldn't change, if you run a constant current through a fixed resistor, you should get a fixed voltage, right? (as long as the input voltage is high enough). Ohm's Law at work.

 

[Roff]

Well, if you built a voltage regulator or reference wouldn't you test it over a wider range than you'd see in the actual circuit? My simulations showed it to have a steady voltage over a wide input range.

Also, one of the purposes of this reference is to be able to detect a low battery by detecting sag on the 5V rail. So if I run this reference off the 78L05 and it sags as well, it won't work too well for that purpose. Otherwise, I'd just use a voltage divider off the 5V to get my reference.

But that is a good point... I should test it between 4.5-5V to see how much the output varies over that range. But, in theory at least, the output voltage shouldn't change, if you run a constant current through a fixed resistor, you should get a fixed voltage, right? (as long as the input voltage is high enough). Ohm's Law at work.

Yeah, that was my point. Test over the range that you expect to see.
A single transistor "constant" current source will never be absolutely constant as a function of Vcc.

 

[Noggin]

How much current do you need? Will your PIC run at 3.6v? If you can get it down to about a 100uA, a Li-SOCl2 AA battery will last you 5 years. Their voltage output is very flat until they're dead (so little warning before they die) but they have a pretty good energy density and VERY low self discharge.

Do you really need much stability? If you want to know when the 5v line starts to sag, just measure the voltage across a diode as has already been brought up. There seems to be a lot of talk about how accurate it is and how stable over temperature it is, but it doesn't seem like any of that matters. You just need to know when the 5v starts to sag, and you would do so by seeing the apparent forward voltage across the diode rise. As long as its not linear and you chose a good threshold, I'd suspect you'd be ok. Is 2mV/Deg C (or whatever was stated) a big enough deal to make this a burden? I'm guessing this will be used where temperature can range 30 deg C over the course of a day (60mV swing, or 30mV around a center point). I'd just try a 100mV threshold.

Only problem is that once the 5v starts to sag, you probably don't have much time left anyway. Of course that all depends on the current you're drawing, and all the diode detection is going to do is waste additional power.

 

[Roff]

Noggin, you seem to be barking up the wrong tree. He needs a stable reference for his A/D, so he can accurately measure temperature with a temperature transducer which he already has.
If that's the issue you were addressing, it was not evident to me.

 

[Noggin]

Oh, I guess I didn't read well enough. I thought his A/D was for watching the power sag on his 5v rail and assumed that he had a temp sensor with a digital output. I pulled the datasheet on the part he listed and it has an analog output.

Well, then I would suggest he switch to a sensor with a digital output

Sorry for making a useless post 'cause of my inability to read properly.