5V Battery elimination circuit - Page 1
Forum section: electronics / projects

author: Electro Dodo
author_rank: Contributor
timestamp: August 26, 2025, 08:50:40 am
content: HelloI want to design a 5V BEC to use in robots powered by Lipo batteries.I use the STMicroelectronics L78M05ABDT-TR with two input capacitors, an output capacitor, a resistor to maintain min current requirement of the LDO and a ZVS diode to prevent over-voltage at the output.The LDO is rated for 35Vmax and 500mA, so my assumption is that it will work for up to 8S Lipos (max voltage 31.2V) and 200mA.I'm not really sure how I would calculate the max current.For 200mA the max power dissipation would be 5.25W which sound reasonable to me.The LDO has OTP, and I don't really need the whole 200mA so I'm not really worried.However if anyone has some tips how to calculate the max current I would be really grateful.I chose the components based on being basic parts by JLCPCB, so the assembly is cheap.I also tried paneling for the first time, so if anything seems off it would be great if you could tell me.Any tips are greatly appreciated

author: moffy
author_rank: Super Contributor
timestamp: August 26, 2025, 09:03:53 am
content: At 200mA and a 30V drop or roughly 6W you need a substantial heatsink and ventilation, are you sure you don't want to go with a switching step down regulator?

author: 5U4GB
author_rank: Super Contributor
timestamp: August 26, 2025, 09:12:58 am
content: And if you're going to go with a switching regulator you can get UBEC modules off your favourite crapvendor for less than the cost of the parts if you were to make one yourself. It really depends on the specific requirements, do you specifically need a linear regulator for low(er)-noise purposes or similar?

author: Electro Dodo
author_rank: Contributor
timestamp: August 26, 2025, 09:16:30 am
content: The use case I want to use it for right now is 4S Lipo (16.8V) to 5V at roughly 100mA.The run time only a couple minutes at a time.But I wanted to be able to use it in future projects too, so I want to find out how far I can use it.

author: Electro Dodo
author_rank: Contributor
timestamp: August 26, 2025, 09:19:04 am
content: The cost for one of the PCB+Assembly is ca 30ct a piece, so the cost is reasonably, but half the reason I want to do this is to try stuff out and learn from it.So yeah, buying would probably probably be a better option if I only cared about getting a functional part.

author: 5U4GB
author_rank: Super Contributor
timestamp: August 26, 2025, 09:33:27 am
content: Quote from: Electro Dodo on August 26, 2025, 09:19:04 amThat's always a good reason for DIY-ing :-).

author: Ian.M
author_rank: Super Contributor
timestamp: August 26, 2025, 10:24:03 am
content: Caution: If you use low ESR ceramic input capacitors, they can ring with the cable inductance when you connect power to the input, and as their capacitance decreases with voltage, can also act as parametric multipliers resulting in a transient spike of several times the input voltage. This can easily exceed your L78M05's max input voltage (35V), blowing the regulator, possibly with as little input voltage as a fully charged 4S pack!https://www.analog.com/media/en/technical-documentation/application-notes/an88f.pdf

author: Electro Dodo
author_rank: Contributor
timestamp: August 26, 2025, 10:56:52 am
content: Quote from: Ian.M on August 26, 2025, 10:24:03 amOK, I read the linked document, and I have to say I understood maybe halve of it?As I understand it, I should put a high ESR, high capacitance capacitor parallel to the ceramic one, or use a TVS diode.Right???

author: xvr
author_rank: Frequent Contributor
timestamp: August 26, 2025, 11:09:10 am
content: If you plan to dissipate 5.25W on the IC, you need a heatsink attached directly (if possible) to the pin of the IC. But you can't attach it directly, so you have to attach it to the PCB underneath the regulator IC. This requires a ground plane on the bottom of the board, and LOTS of vias on the pad (and nearby PCB area) to transfer the heat to the copper layer the heatsink is attached to.Thermal resistance of heatsink (and PCB layer) should be less than 11 (to maintain junction temperature below 125 °C).

author: Terry Bites
author_rank: Super Contributor
timestamp: August 26, 2025, 01:44:53 pm
content: By the time you've spent your hard earned cash on the parts and a pcb and a heatsink you might as well buy a ready made buck converter module. Say "DC-DC Step-down Module 60V48V36V24V To 12V5V3.3V Car Power Mini Buck Converter" on ebay or ali for a couple of bucks.If you're really worried about over volting add a good old fashioned crowbar circuit with a polyfuse for an extra buck. Use a TL431 and a BT151 or 2N6397, any pnp.

author: fchk
author_rank: Frequent Contributor
timestamp: August 26, 2025, 05:06:58 pm
content: Quote from: Electro Dodo on August 26, 2025, 08:50:40 amLDOs are not a good choice here.I used a step down converter module.Module is this one:https://www.allegromicro.com/en/products/regulate/dcdc-regulators/clearpower-regulator-modules/apm819113.60€ on mouser.comAnd I've added undervoltage lockout. In order to prevent deep discharge the converter turns off at a setpoint defined by a resistor divider. I choose the turnoff point to be at 3.0V per cell = 24V for an 8S pack.fchk

author: Zero999
author_rank: Super Contributor
timestamp: August 27, 2025, 11:23:03 am
content: If you want to learn something and the voltage regulation requirements aren't too stringent, then you can make an efficient switched mode regulator with a couple of transistors.https://romanblack.com/smps/smps.htm