I'm powering a 5g 5v servo with a 2 cell LiPo battery. I understand that I need to reduce the voltage from 7.2 to 5 volts, and I've found a regulator (7805 TO-220: 5V 1.5A Linear Voltage Regulator - 7805 TO-220 : ID 2164 : Adafruit Industries, Unique & fun DIY electronics and kits) that I think might work. The servo will only need activate for less than a second (it is deploying a parachute for a rocket, so it only needs to release the parachute once). I have read that I may/may not need a heat sink for this regulator, and that I might also need capacitors? I'd like to avoid these components to keep the rocket lightweight and simple as I'm a beginner when it comes to working with circuits. The servo has a stall current of ~500 mA, which I don't think it will ever reach given the way that the deployment mechanism is designed. Are the capacitors necessary in this use case?
- Suggest you use two series silicone diodes, can’t get much lighter.
- Better still, a Buck or Buck/Boost converter would be best.
The servo will draw the 500ma whenever the motor starts to move, not while it is moving. So, yes, the capacitors are necessary to support the motor starting to move. Why are you using a servo when a solenoid would do the job?
I'm very new to this, but looking at a voltage regulator and a buck converter, what's the difference between them? It seems to me that they both take an input voltage and drop it to an output voltage (obviously there is a key difference). What makes the buck converter different such that it won't require capacitors?
I don't know much about rocketry but a servo seems overly complicated.
I like the diode idea.
The forward voltage drop across each diode is around 0.5 to 0.7V. Of course the result isn't regulated.
With 7805 (or any linear regulator) and the actual current limit is determined by heat which is related to power dissipation. Power is calculated as Current X Voltage, and it's the voltage "dropped" across the regulator. You'd have 2.2V x .5A = 1.1 Watts. That's not bad and short duration means it doesn't have time to heat-up fully. The rule of thumb says if you can hold your finger on it, it's OK. If it's too hot to touch you probably need a heatsink.
1.5A is rather "optimistic" for these things and think it requires and "infinite" heatskink keeping it at room temperature (although it will be heating-up inside).
Linear regulators also have a "dropout voltage" which is 2V for the 78xx. That means if the battery drops below 7V it will drop-out of regulation and the output will drop below 5V.
A buck-boost regulator is a switching regulator which can regulate up or down... To a point, it could put-out 5V even when the battery is below 5V. And switchmode operation means that it the transistor or MOSFET inside only "sees" voltage or current at any moment so it dissipates and wastes very little power. (The output is filtered DC so your circuit doesn't see the switching.)
- A Buck Converter (~ $2.00) is a small lightweight voltage regulator; has all the necessary components on its PCB.
Their efficiency is such that a heat sink is not needed.
You usually set the output voltage (ex. 5v) which needs to be less than the input voltage.
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A Boost/Buck converter is similar.
It can boost a low voltage OR reduce a higher voltage to a set point output voltage. -
Two diodes is a reasonable solution for this application. Each diode reduces a higher voltage by about 0.7v.
If your battery is 7.2v the two diodes gives you, 7.2v - 1.4v = 5.8v, most hobby servos work from 5v to 7v.
A 1N4007 diode is about $0.50
I think you may be ok using the servo without the regulator as it will be on for a very short time. A trick with the linear regulator is put it and its capacitors in a small container, paper is OK. then fill it with wax,It will adsorb the heat for a short time. This link may help. https://www.electronics-cooling.com/wp-content/uploads/2023/08/Paraffin-Wax-PCM-Heat-Sinks.pdf
Now that I think about it...a solenoid would simplify things greatly. The only potential issue I see is possibly weight? The design I have right now requires two pins to be pulled simultaneously (this is a function of the materials, dimensions, and overall geometry of the nose cone). The lightest solenoid I've been able to find so far is 12.5 grams, which would be 25 grams for two. The servo method then saves 20 grams, but there are likely lighter solenoids out there that I haven't found yet.
Your most important parameter is the force required!
5g servo likely have stall current less than 500mA and nominal current likely 1/3 of that. Bare linear regulator without caps is not ideal, tiny caps don't add weight too much. For <1 second operation heat sinks are not required.
It seems to me that one advantage of the two diodes is that if you need to draw a lot of current suddenly, the diodes will give you whatever the batteries can supply, almost instantaneously. A regulator might not respond as quickly. But I can't say I've ever tested that.
Probably not. The servo has a geartrain inside that increases the torque. A solenoid with the same "torque" as the servo will always be heaver. Stay with the servo,
What is the make and model of the servo?
Without the data for this we are all guessing.
How will you activate the servo?
If the current is only applied for a very short time its highly unlikely you will need to reduce the voltage, and with a lipo battery why would you need capacitors?
I'm inclined to believe that for a given force and distance, most solenoids will be significantly heavier and more power-hungry than a small hobbyist servo motor. (the servo is slower...)
You MIGHT be able to get a very carefully designed solenoid (mechanical and electrical) to work better (and simpler) at the same weight as a modern micro-servo, but I think it would be a lot more work.
You ought to be able to get 1n400x diodes for a lot cheaper than $0.50.
Digikey's price is $0.10 each (q1 - $0.73 for 10), and I once bought a box of 1000 (?) for a ridiculously low price (want some? If you're in the US I'll mail some to you if you PM me an address.)
OTOH, I think the 7805 would work fine too. It SHOULD have input and output caps (both of which are small an light), but they probably aren't necessary in your application (but: test, test, test!)
Finally, in the RC communities where you probably bought your servo and batteries, you can find off-the-shelf "battery eliminator modules" that are designed for exactly this purpose - supplying lower-voltage electronics and servos from the beefy 20+V power used by the high power motors, like Amazon.com: QWinOut Mini RC 3A U-BEC UBEC 2-6S Input: 5-23V Output:3A,5V (1pcs) : Toys & Games