POST ISSUE 16 of 2016_12_03 (also see the schematic of post #37 for version 2 of the cutoff circuit using a single shunt reference)
Hello,
I've been browsing around trying to fine a schematic for a 48VDC low volt disconnect. So far I've only turned up LVD's for 12 & 24v schematic.
It would be super if the dropout voltage was adjustable with a pot between 40~50vdc.
From the circuits I've seen they look like pretty simple things to design? But I'm a little lost and would greatly appreciate the assistance of some individuals more knowledgeable in this field than myself
(while I'm aware there are a limited amount of 48VDC LVD available to buy on the market, for the project I'm working on I already have to get a custom PCB made, so I might aswell integrate everything on the single PCB)
Thanks in advance for any assistance
Hello EF,
Which part of New Zealand are you from. If you put it next to 'Location' on your user page, it will show in the box at the left of your posts. That will help us give you replies to your questions.
Below is a precision battery cut-off circuit that has been configured for 48V nominal. I have made various assumptions, but most parameters can be adjusted to suit your requirements as they develop.
Although, strictly speaking, you need to specify the following:
(1) Maximum current that the cut-off circuit is required to supply
(2) Absolute maximum battery voltage
(3) Cut-off voltage
(4) Re-entry voltage
(5) Cut-off voltage adjustment range
(6) Maximum allowable current drain of cut-off circuit
spec
ERRATA
(1) Change LED to type Vishay TLLK4401
Circuit Operation and Performance
(1) The cut-off voltage can be set from 46V to 50V by adjusting RV3. This range can be changed if required.
(2) This is a precision circuit and will have negligible voltage drift (at a guess Vcoff +-10mV from 10 Deg C to 50 Deg C)
(3) The hysteresis is presently set to 10% so, if the cut off voltage were set to 48V, the circuit would not cut on until the battery voltage had increased to 52.8V. The hysteresis can be altered by changing the value of R18
(4) The circuit will switch 5A easily (I chose the PMOSFET before knowing how little current you intended to switch) and can switch more current, but I haven't bothered to worked it out. Just a thought but, depending on what you are switching, perhaps you could eliminate the relay.
(5) The LED illuminates when the PMOSFET is turned on. The LED can be removed if not required but it is advisable to still have a 47K resistor connected from the PMOSFET drain to 0V.
(6) The current consumption of the circuit is 48uA (micro Amps) when cut off. This could be reduced. The current consumption will be essentially the same when cut on, except for the additional LED current of 1mA.
Construction and Components
(1) The LED is a high efficiency type (2mA current). The LED brightness can be increased by reducing the value of R19
(2) The schematic shows the pin numbering for the TLV3701ID (SOIC-8 package)
(3) This is a low current, high impedance circuit so the physical layout is critical.
(4) This is a low current, high impedance circuit, so it will need to be protected from contaminates, especially condensation. A varnish conformal coating may even be required, depending on the environment.
(5) This is a low current, high impedance circuit and is susceptible to electromagnetic interference (EMI) so it should not be place in electrostatic or magnetic fields. A metal screening case is advisable.
(6) All resistors, unless otherwise stated (UOS), are metal oxide, 0.250W or more, 5%, or better, thru hole (not surface mount).
(7) RV3 is a multi-turn potentiometer and may not be available in metal oxide (to be advised)
(8) All capacitors are ceramic X7R dielectric UOS, +- 10%, or better. C5 is 10V working minimum. C6 is 60V working minimum. It may be necessary to parallel a number of capacitors to achieve C6 value. C6 can be polypropylene dialectic. Do not be tempted to omit these decoupling capacitors- they are essential for the frequency stability of the circuit.
(9) A snubbing diode is not required for protection to switch inductive loads (relay) because the PMOSFET has a built-in diode that will do the job.
(10) The physical layout must be as shown in the schematic, with special reference to the battery and PMOSFET. Heavy schematic lines indicate heavy wire or printed circuit traces.
(11) R14 is a gate stopper to reduce the chance of the PMOSFET oscillating. R14 also isolates the output of the opamp from the large effective input capacitance of the PMOSFET gate.
Datasheets
https://www.ti.com/lit/ds/symlink/tlv3701.pdf
https://datasheets.maximintegrated.com/en/ds/MAX6006A-MAX6009B.pdf
https://www.vishay.com/docs/62971/sqm100p06-9m3l.pdf
https://www.vishay.com/docs/83343/tlle4401.pdf