R/C Tech Forums

Go Back   R/C Tech Forums > General Forums > Electric On-Road

Reply
 
Thread Tools Search this Thread
Old 05-11-2005, 07:31 PM   #1
Tech Fanatic
 
hawker's Avatar
 
Join Date: Jan 2003
Location: Brisbane, Australia
Posts: 751
Default Electrical Gurus - Reducing 6v

Ok heres my situation -

Buiding up an RC18T, I'm running HRS 3pk, so I wanna stick with HRS so I need digital servos. Try to find a digital servo small enough is not an easy task! I've found a futaba slimline that will fit however Futaba say that it must run on 4.8 volts not 6V. Bummer

Answer? I want to put a resistor in line with the positive lead on the servo that will drop the voltage slightly. That way I figure I'll be protecting my servo and because the amp draw is only very minorI shouldn't have a problem?

But what resistor do I use and is my thinking correct?

Thanks
hawker is offline   Reply With Quote
Old 05-11-2005, 07:42 PM   #2
Tech Master
 
burbs's Avatar
 
Join Date: Feb 2004
Location: wisconsin
Posts: 1,152
Trader Rating: 1 (100%+)
Send a message via AIM to burbs Send a message via Yahoo to burbs
Default

ask someone at an electronics store.. they should know..and prolly have what you need there..
burbs is offline   Reply With Quote
Old 05-11-2005, 07:50 PM   #3
Tech Elite
 
kufman's Avatar
 
Join Date: Feb 2004
Location: Elburn, IL
Posts: 3,428
Trader Rating: 6 (100%+)
Default

The problem with a resistor is that you need constant current to get a constant voltage drop. This isn't the case with servos. They only draw current when they are trying to move or hold . You need to find a device like the mc7805 except for 4.8 volts. The mc7805 is a 5 volt regulator and may be close enough for what you are doing. The first sample circuit on page 21 of this datasheet is what you need.

http://www.fairchildsemi.com/ds/MC/MC7805.pdf
__________________
Brushes? We don't need no stinking brushes!!!
kufman is offline   Reply With Quote
Old 05-11-2005, 11:03 PM   #4
Tech Fanatic
 
hawker's Avatar
 
Join Date: Jan 2003
Location: Brisbane, Australia
Posts: 751
Default

legendary, thats exactly what i was looking for, best thing is, my local electronics shop has them in stock only $1.20 :P

I assume the ground point can just be wired into the negative (black wire) on the servo lead
hawker is offline   Reply With Quote
Old 05-12-2005, 12:10 AM   #5
Tech Adept
 
nimble's Avatar
 
Join Date: Jan 2005
Location: Sacramento, California
Posts: 188
Default

Note that voltage drop of a 7805 regulator, it needs 2volts more than the regulated voltage to function, that is 7v for 5v regulation. There are other regulators that are better (more efficent), but usually can't handle a lot of current, such as 1amp, what amp load does the servo draw? I think they can draw much more than 1 amp.
nimble is offline   Reply With Quote
Old 05-12-2005, 12:21 AM   #6
Tech Elite
 
Jack Smash's Avatar
 
Join Date: Aug 2001
Location: Reno, NV
Posts: 2,897
Trader Rating: 29 (100%+)
Default

You need a voltage dropping diode. We used to have to use these back in the mechanical speed control days to run without an onboard reciever pack. We would put 2 inline to drop the 7.2v main pack down to 4.8v. Again, local electronics store can help you with the exact specs.
__________________
Kyosho / Orion / Amain.com / Protek / MIP /
Jack Smash is offline   Reply With Quote
Old 05-12-2005, 05:56 AM   #7
Tech Elite
 
kufman's Avatar
 
Join Date: Feb 2004
Location: Elburn, IL
Posts: 3,428
Trader Rating: 6 (100%+)
Default

I didn't even think of a series diode, that would work too. Just look at the rated current and voltage drop. They are usually between 0.7V and 1.2V. Put that in series with the red wire of you servo. The end with the White ring around it would go closest to the servo.
__________________
Brushes? We don't need no stinking brushes!!!
kufman is offline   Reply With Quote
Old 05-12-2005, 12:50 PM   #8
Tech Adept
 
nimble's Avatar
 
Join Date: Jan 2005
Location: Sacramento, California
Posts: 188
Default

yeah, diode is a good simple solution and even small ones can handle 5amps and cost $0.10
nimble is offline   Reply With Quote
Reply

Similar Threads
Thread Thread Starter Forum Replies Last Post
Reducing Inventory Riketsu R/C Items: For Sale/Trade 6 04-21-2007 08:30 PM
HELP PLEASE! T-MAX electrical!! DriveToLive Monster Trucks 3 09-12-2006 05:42 PM
Reducing IR in Batteries Paladin Electric On-Road 3 10-29-2005 12:06 AM
Reducing 12v to 5 v SpeedySST Electric On-Road 20 05-04-2005 11:19 PM
reducing slope? jcr Electric On-Road 18 04-14-2003 08:59 PM



Currently Active Users Viewing This Thread: 1 (0 members and 1 guests)
 
Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are On
Pingbacks are On
Refbacks are Off



All times are GMT -7. It is currently 05:09 PM.


Powered By: vBulletin v3.9.2.1
Privacy Policy | Terms of Use | Advertise Content © 2001-2011 RCTech.net