Anyone see anything wrong with this EDN TL431 battery monitor circuit?

But it got in EDN, that's the important part, right? :) Since the TL431 can be treated simply as high power zener in many circuits, I bet you can design something that works.

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The R4/R5 ratio is wrong, and (perhaps) R6 is too large.

This should be simple enough that the resident engineers could fix in minutes.

I'll wait, but I won't hold my breath :-) ...Jim Thompson

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It should work with a micropower XX431 that has some tens of uA minimum cathode current.

I did.

This works and it only consumes less then 800uA at my worst case line in 25Vdc. I'm using an LMV431.

Simulated results

I wont have time to breadboard it until later tonight.

The BJT is used to short out another BJT driving a pfet turning off the PFET and blocking voltage to the circuit until it reaches about

11.7V. There's about 1V of hysteresis; once the fet closes a 470uf cap gets charged causing a line sag. I have to play around with the resistor values but it's a lot more reliable and lower current consumption then the EDN one.

Tried it if you increases the R it increases the drop turning the BJT on.

You would kinda think it should work and it may with the right values of resistors and the right temperature, and the right LED,and 431.

Maybe not work is to harsh it just wouldnt be very consistent.

A regular 3 cent (or whatever) TL431 needs around 1mA to regulate worst-case (from memory, check the datasheet), so it won't be regulating properly at the 60uA where the transistor turns on.

You need a different DESIGN of reference (perhaps with a part number which looks like xx431) wot is designed to work at very low cathode currents (usually has a lower reference voltage such as ~1.25V as well).

Otherwise you'd have to decrease the resistor values by something like

20:1 which would quickly drain the battery it's supposed to be measuring.

No, wouldn't really work properly. A reference that needs 1mA to regulate is generally inappropriate as a low battery indicator for a small battery unless it's pulsed or something.

My need isn't monitoring a battery it's for UVLO for a flyback smps. You don't have to convince me the circuit isn't really the best for its intended purpose far from it.

For monitoring batteries I've used the LTC1440 which is a micropower comparator with integrated reference.

I'm using a LMV431 see my post above for my UVLO. The LMV431 needs min

80uA for guaranteed regulation and can withstand 30V cathode to anode. The flyback input is 12V to 25Vdc . The input has a TVS and fuse so if anyone does go above 25V the TVS will go low impedance and vaporize the fuse Minimum breakdown is 25.7V max is 27.9. Not overly eloquent but it works I've done it before.

I just posted the circuit because it doesn't work with the values shown. I swept the supply from about 5V to 25vdc and the cathode wouldn't saturate the LED starts to emit faintly though.

This is another bizzare circuit from IR a neg output post regulator useing the tl431 and a PFET?

"Hammy" wrote in message news: snipped-for-privacy@4ax.com...

I think you typed it into spice incorrectly. When I simulated it, it appeared to work properly. So, thinking there must be some effect I was missing, I built it, and it works properly. The LED snaps on/off at about

10.5V.

The main issue I have with it is that it sucks power through the 1k resistor. I'm guessing they are doing this to prevent any 'premature elucidation' from the LED.

I included the simulation I did, and another one I hacked up using a couple of NPNs.

Regards, Bob Monsen

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Your right is does work. I found a dead track on my breadboard which would have been the ground of the anode on the 431 that might have been the source of the problem.:-)

All the same I'm using this one it only uses a couple hundred micro amps. In the breadboarded cct I'm using a LMV431.

Here is a scope shot

CH2 is the input 120Hz FW rectified,CH1 is the 2N7002 gate. The 2n7002 gate is being discharged through a 1M resistor. The fet still turns off in around 600uS which is good enough for this. I just wanted to keep Q1 off until the supply voltage gets around 12V.

For comparison here is the original cct scope shot.

CH2 is the input signal and CH1 is the cathode of the 431.

Yep, like many circuits published in EDN for the past 30 years, it=20 does not work right. That is part of why i quit my subscription, also=20 there is the huge jump in advertising pages. Needless to say, the=20 editorial standards morphed back then and again since.

No it does work. I made a boo boo;-)

There is a dead track in the breadboard I used. I figured that out when I tried the other cct. I posted and it did the same thing. The

431 wasn't grounded it was floating. But still it uses to much current.

I have 5 Breadboards and they are all full with works in progress. Even to do up the little 431 cct I had to cram it in an unused section of one breadboard. Time to make up another batch of PCB's.

It's almost down to a newsletter in size..