How would you increase hysteresis in this simple batt cutoff? - Page 1
Forum section: electronics / projects

author: mcinque
author_rank: Unknown
timestamp: October 04, 2014, 06:08:20 pm
content: I want to prevent overdischarge on a battery pack that powers a circuit. Of course, I need ZERO quiescence current when the battery is disconnected.I thought to use a programmable reference like the TL431 (that can't works below about 2.51V) so that when the input goes below 2.5V it turn off and change the gate voltage of the P-Channel mosfet, that drives a relay.The voltage on the input pin cut can be adjusted by a trimmer that, toghether with R1, acts as voltage divider.Setting the voltage at about 2.5V when the battery pack is 0.4V close to the undervoltage limit works fine.The issue: this configuration has a small hysteresis, but when the battery pack is disconnected, its voltage slowly starts to grow (autocharge phenomenon), so the voltage can potentially reach the ON state again, turning on the circuit again, and this is not what I want.How would you add more hysteresis to this circuit?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 04, 2014, 06:50:21 pm
content: Decrease R3.If you're actually literally using a TL431 there, you're never going to win... R1-POT1 need to be on the order of 10k, 100k max to get accurate voltage sensing, in the presence of "REF"-pin input bias current. R2 needs to be sized so that P-CHANNEL_FET is guaranteed off (Vgs > Vgs(th)min, -0.8V or whatever it is) at the minimum bias of 1mA(!), a massive amount of bias for a battery disconnect.What you need is a proper battery monitor IC, or something similar. On a recent project, I didn't use a monitor, but I built one with a micropower reference and voltage comparator (total ~12uA supply load). Better than self-discharge.The relay is fine, but draws a whole lot of current while on; why not use a large MOSFET?Set enough hysteresis that it satisfies your battery's specs on discharge and recovery and stuff. It's always going to bounce back; if it bounces back into the 'green' zone, who's to say it's not supposed to run again?Tim

author: Seekonk
author_rank: Super Contributor
timestamp: October 05, 2014, 06:55:37 pm
content: It could be simple. A TL431 can easily drive a relay directly. They are rated up to 100ma. I would say forget the feedback. Power everything off the NO side of the relay. If your battery is say 12V, powering the relay with only about 3V should hold it in. Size a resistor in series accordingly. The big secret is use a push button to power the relay and to get it to latch on. Then when you reach the low voltage level it turns off never to go on again till the push button is pressed.

author: rs20
author_rank: Super Contributor
timestamp: October 05, 2014, 11:29:31 pm
content: Quote from: Seekonk on October 05, 2014, 06:55:37 pmOr, substitute a FET instead of a relay so that your circuit doesn't burn power holding a relay closed... the circuit in this video just needs a comparator after the switch so that it turns itself off below a certain voltage:Or, use a dedicated IC designed for this task. If you measure the battery voltage before the switch, you're almost inevitably always drawing current from the battery, even when it's supposed to be cut off. If you measure the voltage after the switch, you have to let the after-switch voltage rise for a little while to make sure it's at the true battery voltage before you measure it. Basically, it's a pain to get it right.

author: mcinque
author_rank: Unknown
timestamp: October 06, 2014, 09:05:42 pm
content: Thank you Tim,Quote from: T3sl4co1l on October 04, 2014, 06:50:21 pmIt's not his proper useQuoteI'm using a cermet to find the right voltage and it's very stable.What you need is a proper battery monitor IC, or something similar.Dp you suggest a particular IC number? I could spot some example circuit in his datasheet.QuoteYou're right, that was just an example.QuoteIndeed.Thank you, Seekonk,Quote from: Seekonk on October 05, 2014, 06:55:37 pmThis is a nice idea!Thank you, rs20,Quote from: rs20 on October 05, 2014, 11:29:31 pmExactly.I think I'll try the Dave's power latch circuit you linked.

author: lowimpedance
author_rank: Super Contributor
timestamp: October 06, 2014, 11:56:13 pm
content: One other part that would be well worth a look for this is the MAX8211, 8212 series 'uP' voltage monitors, micro amp supply currents and operating range of 1.5v to 16.5v. easy to implement hysteresis and drive external FET's etc.A very useful part to keep in the back of your mind for quite a variety of applications

author: mcinque
author_rank: Unknown
timestamp: October 07, 2014, 07:31:00 am
content: Nice advice! I've ordered a couple of them. Thank you