Help a newbie understand voltage step down - Page 1
Forum section: electronics / beginners

author: pixelsafoison
author_rank: Newbie
timestamp: August 31, 2019, 09:31:38 pm
content: Hello there,I was browsing the web for answers but could not find any definite answer, so I decided to subscribe here since I know that this place is full of people that truly know their S - I WILL watch any dedicated videos on the subject but I just need to be pointed in the right direction.I have an e-bike that runs on 42V (a standard 18650 battery pack with BMS), and I would like to be able to divert some of that power to various LED contraptions in order to not get killed on the way back home at night (where I live everyone is drunk past 6PM).I have purchased a few modules on aliexpress (do not kill me yet) that DID step down the voltage to 12V, but dissipated a lot of heat while doing so. As such my mind went "so that's where the voltage difference went then...?"But I then thought about modern switching power transformers, and as they do not heat up.Could anyone point me in the right direction for me to dig up info on how to step down a variable voltage (42-34 depending on battery level) to a stable 12V with as little heat dissipation as possible? (if any)Ideally, I would need to draw 10 to 20 Watts.I feel very bad asking something that in appearance looks so simple, but I truly lost.Thanks in advance and sorry ^^"

author: LateLesley
author_rank: Frequent Contributor
timestamp: August 31, 2019, 10:16:25 pm
content: Hi, What you are looking for is a buck converter. it's a type of switching power supply which will step down the voltage, and can be highly efficient.You can get them pretty cheap from the usual suspects, here is one for example -https://www.ebay.co.uk/itm/DC-DC-Buck-Converter-Step-down-Voltage-Regulator-24-60V-to-3V-5V-9V-12V-19V-10A/362740059754?hash=item5474fe0e6a:g:I5UAAOSwsXpdaN8BThey can take in a variable voltage, and step it down and regulate the output voltage. You can get fixed ones, variable ones, dual rail ones. They are highly efficient, as rather than varying the current flowing through, a pass transistor, which drops voltage, and thus dissipates heat (P = I x V), switching supplies send pulses of power, where the transistor (or FET) is fully on or fully off, which means ther is either very little voltage across it, or very little current through it. An inductor and cap smooth the pulses into a steady output voltage, with very little ripple. The switching circuit controllng the transistor, monitors the output voltage, and increases the pulse length if more power is needed, and reduces it if the load reduces. this is how it keeps the output steady (PWM - Pulse Width Modulation).Hope that helps.

author: Brumby
author_rank: Unknown
timestamp: September 01, 2019, 12:52:29 am
content: ^ ^ ^ Excellent summary ^ ^ ^The only point I would like to address is "with very little ripple". This, together with radiated EMI needs to be understood in the context of the application.Because this type of device operates using on-off switching, therewillbe transients and thesemaycause problems. I won't go into details at this point since you shouldn't have any problems with LED circuits on an e-bike.Nevertheless, I would be inclined to mount such a board away from any e-bike control electronics and run its own power cabling (input side) from as close to the battery as practicable. (Sharing power supply wiring to anything sensitive is not a good idea as the switching nature of these devices can introduce noise on the power rail.) I would also try to avoid long cable runs on the input side.For your application, don't take my "inclinations" as essential steps - but if you have a choice....