***power-hungry zener***

[Jules]

I'm using basic op amp comparator to switch on a LED when battery voltage falls below a set point (11.8 V) and switch it off when battery rises to 13.8 v (so I've got some positive feedback hysteresis from pins 3 to 6). The problem is that the 5V6 Zener I'm using as a V reference at the input is taking nearly all of the circuit current! I have to use a lowish K resistor to bias the zener (it needs about 10mA to work correctly) but this is using more mA than I want. Has anyone got another lower current option? TL431....? NPN transistor...? Is there a very low current zener available?
I only want to use a simple 741..TL082....etc. op amp circuit, but with a stable low current (uA) reference. The battery voltage is not regulated, and falls with use, so can't be used as a stable reference.
Thanks

 

[jnegowski]

just some ideas i would think all zener's use as refs or power sources need a current limiting resistor in series. ref.diodes/devices may not i have used them before they are pricey but i don't rember using a resistor with them. and yes you can use your battery voltage as a reference, use a voltage divider to get 8 volts then use a zener ref from that source. scale both inputs to your opamp.

 

[k7elp60]

You may want to consider the LM336-5.0 reference diode. It will work with less than 1mA of current, and it is a 5.0 volt diode, but it can be trimmed to 5.6 volts.
I have had similar problems and so I use the LM336-2.5 and use appropiate resistors as it is 2.5 volt reference diode.

 

[Nigel Goodwin]

The TL341 should work better, I feed it at about 1mA or so in my analogue tutorial, as a precision reference. It's also adjustable from about 2.5V upwards, plus it's a far more accurate reference than a simple zener.

 

[Jules]

Thanks for the idea. I couldn't find the part of your site which lists this device. Which tutorial is it?

 

[Nigel Goodwin]

Thanks for the idea. I couldn't find the part of your site which lists this device. Which tutorial is it?

It's used on the analogue input board, try for the exact page. You can wire the 'gate' via two resistors to set the exact voltage, a quick google will find the datasheet.

 

[Jules]

Thanks for the link. I've found it now. (You have it listed as TL341, and it should be TL431). I'll give this IC a try. I've never seen a simple op amp comparator circuit using this as a V ref. They always use zeners and a series resistor. Perhaps you or someone else reading this knows different!

 

[Nigel Goodwin]

Thanks for the link. I've found it now. (You have it listed as TL341, and it should be TL431). I'll give this IC a try. I've never seen a simple op amp comparator circuit using this as a V ref. They always use zeners and a series resistor. Perhaps you or someone else reading this knows different!

Slight touch of dyslexia :lol:

It's basically just a 'super adjustable zener', you can use it just like a normal zener (as my example does), or with two resistors to make it variable. Presumably it's not often used in simple comparators as it's more expensive than a zener?, and the precision probably isn't required?.

 

[Roff]

Have a look at Linear Technology's micropower regulator-comparator combinations. Total current draw for the LT1020 or the LT1120 is only 40ua.

 

[Mike - K8LH]

Nigel (and gang),

Please forgive me if I'm straying off topic here...

I was just looking at your Analogue Board Tutorial... Very nicely done Sir, as are all of your tutorials that I've stumbled upon this past year...

Admittedly, I don't know a great deal about op-amps, but may I ask if this circuit could be simplified using 'single-rail' op-amps? Might this eliminate the need for a negative voltage rail...

Thank you again for your time and kind consideration...

Regards, Mike

 

[Nigel Goodwin]

Admittedly, I don't know a great deal about op-amps, but may I ask if this circuit could be simplified using 'single-rail' op-amps? Might this eliminate the need for a negative voltage rail...

I decided to do it that way for three reasons:

1) So you didn't need specific opamps.

2) To guarantee that the output goes completely down to 0V, I'm not 100% confident a rail to rail opamp would achieve that. Doing it this way gives a reading of 0x000 with no input, and I really wanted to ensure that it did.

3) A single 5V supply is rather low for most opamps, a +/-5V (giving 10V) supply is better suited.

Also, using the TL431 as a reference means the opamp output doesn't need to reach the +5V rail either, again I'm not 100% confident that a rail to rail opamp would give a full 5V output. Plus it's a far more stable reference than using the 5V rail.

 

[Russlk]

An LED is a better voltage regulator than a low voltage zener, and does not require as much current (you don't need any light). The voltage ranges from 1.5 volts to 3.5 volts, depending on color.

 

[FRIED]

Should look at LM10. Low power, builtin reference