This is an issue that has been ongoing for a few years and some of yall may be aware of it. In a nutshell, packs smaller than about 600mah are a common denominator to "infantile death" or other very untimely failures of many direct power servos. Some models of 3bros dont get killed because they have higher voltage ratings, but finding good 35v driver circuits is tough. There are a good bit of 25v components and a huge plethora of 16v from the origional 8.4v servo designs. Pretty much all 4s direct power servos are 25v drivers.
So what is happening? In a nutshell, whenever a servo or motor stops suddenly, there is a burst of back EMF. Amperage doesnt want to stop, so the voltage starts to climb. In a "normal" system the battery shunts the current and recharges just a little, and the voltage doesn't spike above pack voltage. But when the pack is so small that the ESR is very high, that amperage has nowhere to go and the voltage can climb very high. With a normal pack ive never seen above 20v. With tiny packs ive seen close to 30v.
I have addressed this issue already with TVS and zener diodes to absorb the spikes, so why did it come back? It appears to be two fold.
-- First is the servos and motor systems getting more and more power dense. As the resistance of the systems get lower, the spikes get faster and go higher. So a zener has to work harder, and might not be fast enough. You may see people blaming this on "drone ESCS" but the problem happens with all types of systems. Its just that everybody running the small packs are also running am32 escs right now.
--The second issue is that battery sizes have gotten smaller and smaller,. And even though the discharge rates are very high, the ESR is still too high to absorb the spikes. Older batteries also go up in ESR which makes the issue worse.
So what is the solution right now ? Either larger batteries, running a BEC powered servo, or adding a lot of capacitance in parallel with zeners or TVS diodes. We are talking 1000uf with an ESR of 50mohm or less. About 10 grams of components, the same volume and weight as a BEC. And another 1000uf will destroy your plugs, its a LOT of inrush current.
Im looking for 35v drivers, but in the mean time the solution is going to be either going back to BEC units or installing a snubber thats about the size of a BEC. I would cram it into the servo if I could, lol! But not enough room, no way no how.
So thats whats happening in the direct servo world these days
So what is happening? In a nutshell, whenever a servo or motor stops suddenly, there is a burst of back EMF. Amperage doesnt want to stop, so the voltage starts to climb. In a "normal" system the battery shunts the current and recharges just a little, and the voltage doesn't spike above pack voltage. But when the pack is so small that the ESR is very high, that amperage has nowhere to go and the voltage can climb very high. With a normal pack ive never seen above 20v. With tiny packs ive seen close to 30v.
I have addressed this issue already with TVS and zener diodes to absorb the spikes, so why did it come back? It appears to be two fold.
-- First is the servos and motor systems getting more and more power dense. As the resistance of the systems get lower, the spikes get faster and go higher. So a zener has to work harder, and might not be fast enough. You may see people blaming this on "drone ESCS" but the problem happens with all types of systems. Its just that everybody running the small packs are also running am32 escs right now.
--The second issue is that battery sizes have gotten smaller and smaller,. And even though the discharge rates are very high, the ESR is still too high to absorb the spikes. Older batteries also go up in ESR which makes the issue worse.
So what is the solution right now ? Either larger batteries, running a BEC powered servo, or adding a lot of capacitance in parallel with zeners or TVS diodes. We are talking 1000uf with an ESR of 50mohm or less. About 10 grams of components, the same volume and weight as a BEC. And another 1000uf will destroy your plugs, its a LOT of inrush current.
Im looking for 35v drivers, but in the mean time the solution is going to be either going back to BEC units or installing a snubber thats about the size of a BEC. I would cram it into the servo if I could, lol! But not enough room, no way no how.
So thats whats happening in the direct servo world these days
