View Full Version : Operating the Hitec 5990 TG with 8.4v peak?

07-26-2008, 04:48 AM
Hi has anyone ever used a 2S lipos (http://www.rcuniverse.com/buynow/keywordclick.cfm?bid_id=6088) pack without a voltage regulator on servos?

I have heard that the Hitec (http://www.rcuniverse.com/buynow/keywordclick.cfm?bid_id=151) 5990 TG can actually operate from 6-7.4 V. I wanted to see if anyone has had any first hand experiance running Hitec (http://www.rcuniverse.com/buynow/keywordclick.cfm?bid_id=151) 5990 TG on this 2S lipos pack.

The thing is that you will get around 8.4v peak, around 8.2v under load until it starts to decrease from drain although 2S lipos pack should actually give 7.4V.

By the way, does anyone know a good regulator which takes input from a 2S lipos (http://www.rcuniverse.com/buynow/keywordclick.cfm?bid_id=6088) pack and gives output around 7.4 V and up to 10 A in order to operate with the Hitec (http://www.rcuniverse.com/buynow/keywordclick.cfm?bid_id=151) 5990 TG ?:tongue:

As far as I know , Hitec (http://www.rcuniverse.com/buynow/keywordclick.cfm?bid_id=151) 5990 TG is the most highest torque servo comparing to its size and weight, right ?

07-26-2008, 05:00 AM
LM317 adjustable voltage regulator all the way. Probably more than one depending upon how many servos you're going to run (~2A load). Make sure you heatsink it well and you can push it a bit.

Some 5990TG's may be able to run at 8.2-8.4v for some limited period. Some will self destruct immediately. You're pushing it too far out of spec and the majority are going to fail. Electronic components are rated by how they fail, and how many samples fail over some period at some specification. You're talking about pushing it something like 20% out of spec and that's rarely a good idea unless they're severely under-rated.

The LM317 will deliver a good regulated voltage. Make sure to add filter caps rated appropriately (25v would be comfy, 18v would be minimum I'd advise - always at least double).

Datasheet for the ones I have on the shelf here: http://www.fairchildsemi.com/ds/LM%2FLM317.pdf

07-26-2008, 05:34 AM
Thank a lot : ) . So, If I want to use about 20 servos. So I need 7.4 V constant and about max 2 A for each servo = 7.4v, 40 A. Is it enough to use only one LM317?

07-26-2008, 06:28 AM

Not even close. I wouldn't know off the top of my head what adjustable regulator could carry 40A @ 7.2/7.4v... That's a toughie. That's probably a big MOSFET. Later today when I'm awake I can check my parts catalogs. No number off the top of my head, but I think I remember seeing a 3-40v @ 20A not that long ago. I'll have to pour over the catalogs.

Now that you know basically what you're looking for, you probably have the keywords to try searching Digikey and Mouser - let me know if you find something, otherwise I'll look after a nap (4:30am here)

07-26-2008, 06:51 AM
Thank a lot ... good nite : )

07-26-2008, 07:02 AM
Amos: The Lm 317 is capable of running at 1.5amps continuous, If it exceeds 2amps the thermal shut down will take place and switch off, The specs for the HSR5990 are:
The chip can be pulsed at 14A but continous current max rating is 3.5A. Given the servo data below, under continuous heavy loads the chips might well eventually overheat.

Current Drain (6.0V): 300mA/idle and 4.2 amps at lock/stall
Current Drain (7.4V): 380mA/idle and 5.2 amps at lock/stall

So, if under the worst conditions at stall 4.2 and 5.2 amps the lm317 will shut down, This is based on 1 servo and 1 lm317 and you are looking at 20.

Lm317 is rated at 1.2-37v @1.5amps continuous. You could use a pass transistor which will give you more amps. Here are some details.


07-26-2008, 08:49 AM
That's waht I thought. I thought that iit was closer to 4 amps.

07-26-2008, 11:09 AM
The upshot is still, even at idle, you're not gonna run 20 on an LM317 without supercooling, and even then...

07-26-2008, 12:05 PM
You may want to keep in mind that with an LDO such as the LM317, even if it could handle the power, you're using loads of the battery power just as a heater.

For instance, one servo at 7,4V with a current draw of 5.2A. Now feed that with the 8,4V from the battery pack through the LM317.

That means you have 1V drop over the LM317 at 5.2A, that's 5.2 watts of pure heat nibbling away at your battery...

That being said, switching regulators are usually more trouble to handle since they require quite a lot more external components, and even if they don't they usually still require a PCB because of the package of the component itself.

So, with that in mind, and for the sake of simplicity, the LD1084V (http://www.st.com/stonline/products/literature/ds/9035/ld1084xx.pdf) from ST will allow a 5A current draw with a voltage dropout of 1.3V, while the output power and current is internally limited to protect the component itself.

In theory you could use one of these LDO's per servo, but it would turn out rather costly, then again, it's cheaper than replacing a burned-out servo

/edit: I'm not sure how these digital servo's react to switching power supplies though, anyone with an idea on this?

07-26-2008, 03:27 PM
Not sure, I have not used voltage regs on servos before, Worth a try to just see what the results are, I would sooner protect the servo that burn it out at $124 a pot, could always use a 2amp fuse inline to protect it, But that would mean 20 if you are using 20 servos, One or two or even a higher amp rating would not cut it as you do not know how many servos are going to be used at any given time.

07-27-2008, 02:57 AM
Could someone tell me as example. So, how do you actually supply about like 18 servo motors for your robot?
Do you use directly 6.0V NiHM?

I actually use Lipo 8.4 V and convert the power to 6 V 8 A via parallel 4 regulators where each of them gives output 6 V and 2 A.
But I want to change the system by using HSR5990 to obtain moe power. Therefore I have been lookig for the convertor giving 7.4 V output and enough current for all 18-20 servos (HSR5990).

Or do you have any suggestion for supplying these 18-20 servos at 6-7.4 V with enough current (around Max 10 A from the test beased on the existing system)?

07-27-2008, 03:08 AM
What regulators are you using? Seems like you already have your answer staring you down. At 6v it sounds like you're using an adjustable regulator.

07-27-2008, 04:11 AM
LRP LiPo RX Regulator 6V/5A