## 7.2V To 5.8V

Hello everyone, I have a question.

I am continueing my work on Hank, and I am back to needing a seperate power source for the high torque servo's. Allthough Hank is no longer a hexapod but a quadruped instead, I still have 12 high torque servo's to power. Ofcourse I do not want to exceed the 6v power limit and have been looking for a LiPo battery to power them. Unfortunaty there is no 6v LiPo battery but with 12 high torque servo's i will be needing quite some amps if i want it to work for some time without having to charge every 15 minutes, or so.

I have a hard time understanding the options that I have. Hank isnt al that big and it wont be able to carry a huge lead acid battery on its back. Im not farmiliar with the functioning of a bec or esc and i want to avoid using nimh or alkaline batteries. There is this idea that keeps popping up in my mind and I just have to ask..

A diode will generally cause a loss of power by approximatly 0.7v, right? So if i were to place 2 diodes in series on a 7.2v plus lead, would that not come to about 5.8v? Its seems so simple to me and im wondering if maybe it is a bit too simple and may not work as I hope it would.

What do you guys think? Will using 2 diodes in series reduce the voltage of a 7.2v battery, to properly power the hightorque servo's?

Kristian.

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United Kingdom
#1

Yes in principle that would work. You just need to make sure that the diodes are capable of the current load. I would get a cheap multimeter just to measue the volts to see if you get what you expect. Calculate the maximum current load by looking at the spec of the servos then choose an appropriate diode then you will be good to go

Netherlands
#2

Awesome! I am using 12 times towerpro (or towardpro) mg996r. I cant really find a datasheet on these servo's but according to someone's comment on some webpage they draw about 937 mah - 4.1w under load. So I would need diodes that handle at least 12 amps right?

This is great, now I have an idea how to tackle this, thanks!

United Kingdom
#3

937ma seems high maybe max load I suppose. So if they were all on max load you would need 12amp diodes. They could get hot so be careful

Netherlands
#4

Yes, I believe that was under pretty much a Max load. Im going to assume that all of them are likely to be running at the same time at some point. And most of them may also be likely to get near of reach a max load and draw about 1 amp each. The 12 servo's alone do weigh quite a bit, including 2 or more batteries and the rest of Hank, it all adds up quickly and it'll be power hungry. So the diodes are likely to get very hot and I will keep that in mind, thanks.

I have just one more question. To have a decent run time I will probably end up using 2 batteries in parrallel. But 3 batteries would be too much and was thinking that maybe I could add 2 or more solar panel(s) on its back to give it that litte extra bit of juice. What kind of a voltage would I need to actually charge a 7.2v LiPo battery? Anykind of charge I'll be more then happy with as litte as it may be, assuming ofcourse that it is even possible to charge a LiPo battery in such a manner.

I have little experience with both components, so any information woud be very helpful.

United Kingdom
#5

Most Solar Panels are for charging lead acid cells (Car and Boat batteries). I have a charger that takes mains or a 12 volt car battery as an input that is designed for LiPo batteries so you can charge RC models in the field. So 12 volts and a special charger would be needed but I think the size of panel you would need to produce enough amps to make it worthwhile would be prohibitive. Is it that sunny in the Netherlands? take a look on RC model sites and see if there is anything there. Found this on You Tube should give you some sort of idea on the size of everything

Netherlands
#6

Haha alright, you dont need to say any more. That pretty much rules that idea out and I have a clear answer to my question, thanks.

Netherlands
#7

Alright, so I've bought a couple of 7.4v LiPo batteries. Ive charged them and soldered the wires together with the diodes to bring down the voltage to just below 6v. I measured the voltage and to my surprise the batteries are actually 8.33 v. this is not a problem as i will simply add more diodes to bring it where i want it to be.

Still though, is this normal? I was sort of counting on it to be around 7.4 volts lol

#8

The diodes only drops voltage under load, if you have no load connected that's your problem, 2S lipo charged to full 8.4v, 4.20v per cell.

The better and more efficient way would be using a BEC as commonly used in R/C car/heli applications. This is a very popular and powerful BEC: http://www.aprohobbies.com/castle-creations-switching-regulator-lipo-input-p-104.html

Use two of them in parallel will provide 14amp of continuous current, 20amp peak. The default output is 5.1v, output voltage can even be custom set (4.8v-9v) using a PC software if you also purchase the Castle Link device.

Netherlands
#9

Alright, thanks. I'll look into that.

Netherlands
#10

@Louis T

I find this strange though, I mean the batteries are labeled as 7.4 volts. But if it is really 8.4 volts, would it not be more senseable to label it as infact 8.4v? I have little experience with LiPo batteries but when I get a 12v lead acid battery, i can pretty much assume that it will infact produce around 12 volts, not 13.5v right?

Wether there is a load on it or not, this LiPo battery is most solidly 8.4v which means I have to add a volt to the entire equation.

When I measure the battery directly, it produces 8.33v. When I apply the diodes and then measure without a load, it comes down to 7.5v. If I then connect the EZ-B to it without any functions or other hardware, I then measure 6.5v. So the theory of using a diode to reduce or lower the power of a power source does work. Evidently though there is little to no stabilisation. The drop voltage of a diode (or a regular diode) is variable according to the power or current that runs accross them. Chances are that even if I add enough diodes to initiatlly lower the voltage to around 6 volt, as soon as the servo's start doing their thing the voltage will probably rise and fall exponentially. As you say, this is obviously not a very reliable way of controlling the power.

The problem is that although I understand the theory behind the functioning of an electrical component, I lack the practical experience to really know how to apply it or know what to really expect. I still have a relativly simple mind when it comes to electronics and I would prefer to keep things simple for my own understanding. Also, my robot is not all that big, I have tried to keep things as compact as possible, which I have to say I did rather well. Unfortunatly though there is little to no room left to be adding much of anything. All I need now is to round up the power issue for the 12 high torque servo's. Running a few extra wires is still possible, but there is absolutly no room or places to mount added hardware. I was already horrified by the thickness of the wires included with these batteries since they have a max discharging current of about 160 Amps.

The BEC seems simple enough but will quickly start costing money. Which I would also like to keep as low as possible since I've already spent quite some money on this bot. As you say I'd have to use 2 of them. What I read is that this BEC will provide 5.1v of stable power. I will need to also purchase an instrument to set them to the ideal power level, I mean I can get 5v from the board itself so without that instrument, using 2 BECS seems like overkill for something that really hasnt changed much about the power level. The point is to bring it at or just below 6 volts, stable.

Spending another considerable amount of \$\$ on 2 BECS, including the instrument to set the appropriate power level, that I really dont have the space for...I'm hesistant. I'm sort of a perfectionist and I do not dig things dangeling around or seemingly being out of place. Heck, even if the color of a component is al wrong it might realy eat at me, so I realise i'm picky and kind of limiting myself at the same time..

I'm not sure, I would like to stick with the concept of using diodes or something similar to reduce the power in a simple way. Unfortunatly, that which is simple and basic in electronics will not provide something very sophisticated either. Maybe I can consider using capacitors to produce a certain outgoing voltage, or linear components that will provide more precise results. The concept of using diodes would work fine and I would prefer to build on this realtivly simple concept if I could somehow stabilise the voltage.

If I am going down that route of simply spending money on sophisticated already functioning equipment, rather then developing my own system, I feel I might as well go to a toy store and buy a complete robot n skip the entire process of building anything at all. At the same time, if 1 small piece of equipment can solve a particular problem in this build, that might be my best option. But in this case it would be 2 pieces of equipment. Plus I'd have to buy the instrument to set the ideal voltage....which i would probably end up using only once. Though i'm sure its a very useful intrument to have, it's just so easy to thrown around with \$\$ to simply buy something already functioning and then just slap the entire colection together without ever having to think about much of anything or learning anything from this. But then again, I'm probably going overboard in trying to develope somekind of voltage regulator myself..

I have a tendancy to think out loud, as you can see. It helps me in getting a clearer picture, and I will have to think about this more since I'm clearly not going anywhere with this. Is it really so hard to power something of 6v without the need for sophisticated or expensive hardware? If there are more suggestions I would appreciate it very much. If not then feel free to disregard all this yadeya haha, I will have to get more information first.

Kristian.

Netherlands
#11

..I guess the real point here is to provide sufficiant amps for the servo's, externally from the board. Be it 5 or 6 volts, its the current that is probably most important. In which case, a BEC (or 2) is probably the best and easiest way to solve this. Using alkaline batteries wont last very long and a large lead acid battery is far too heavy.

So the power source is 2 times 7.4v (8.4v) LiPo batteries. I need to power the servo's externally to avoid browning out the board due to large current and higher voltage consumtions of the high torque servo's. 5 or 6 volts, they both work. So yeah, a BEC is the logical choice to use. The only problem then is where to put 2 BEC's in my bot. They arent all that big but its just too much for my bot and really doesn't seem to sit well with me. Using just a couple of components like diodes or transistors would be ideal for me.

But is that possible?

#13

Lipos are labeled as 7.4V because the sustain typical voltage is 3.7V/cell, 8.4V is fully charged. Diodes are good for low current device with fixed source voltage. For high current devices and variable source voltage, forget about using diodes...Period!

A switching BEC or DC-to-DC converter is what you need, takes variable input voltage, more power efficient and fixed output voltage. What you shown me is NOT a BEC, DO NOT BUY THAT! That is a brushless speed controller (ESC) for heli/plane. The built-in BEC of these ESC is only 2-3A at most. The size is much bigger than the BEC i shown you.

The Castle Creation BEC are back by manufacturer warranty, if something goes wrong you contact them they will take care of you. I found you something on eBay that may work, don't know about reliability of these Chinese BEC, cheaper but no warranty obviously:

Personally i think using two BECs in parallel is better, should one fail, you still have control over the robot on the one BEC left than completely lost control.