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robotdragon

Hello,

I have a project that uses 10 servos (possibly more in the future).  Four of these servos are the HS-7950TH servos which use more current than the more standard servos.  I have it powered by a 6 volt Lead Acid Battery (http://www.mkbattery.com/images/ES13-6.pdf).  I do not have a good measure on the peak amperage level because my battery is going bad but I would say at least 5 amps.  Maybe someone has more experience with servos and amperage level.

I want to replace the battery with a power supply that I can plug into the wall because my project is not mobile and I would rather not have to charge the battery.  The power supply could be anywhere between 6 and 7.4 volts.

Should I be looking at building my own AC to DC power supply?  I have read a little bit about creating one but haven't seen yet how to regulate the output amps.  Or should I purchase something like this: http://www.amazon.com/12-Volt-Power-Supply-Standard/dp/B00B8TRF0A
and step down the voltage to 6 volts?  Is stepping down voltage with resistors reliable?

If I did buy a power supply and cut off the end, which wire is positive and which is negative?  Will that be the same as the terminals on the battery?

Thanks  :)

MarkT



Should I be looking at building my own AC to DC power supply?  I have read a little bit about creating one but haven't seen yet how to regulate the output amps.  Or should I purchase something like this: http://www.amazon.com/12-Volt-Power-Supply-Standard/dp/B00B8TRF0A
and step down the voltage to 6 volts?  Is stepping down voltage with resistors reliable?



If you think stepping down voltage with resistors is reliable then, no, I don't think you should
be tackling designing a mains power supply. 

A 6V output SMPS (or even 5V) might be the thing you are looking for?  But you will have to measure
your peak current requirement before proceeding.
[ I will NOT respond to personal messages, I WILL delete them, use the forum please ]

Grumpy_Mike

Quote
Is stepping down voltage with resistors reliable?

Yes it is very reliable, but totally useless when you have to draw a significant amount of current from the potential divider. As a rough rule of thumb you need 10 times more current down the potential divider chain than you draw out. So if you draw 5 Amps out you need to supply the potential divider with 50 Amps, even then regulation will be poor.
This means that the resistors will get very hot, 50 Amps at 12 Volts is 600 Watts, most of which is heat in the resistors. So yes if you want a combined electric heater it might be the way to go.

robotdragon

Quote
Yes it is very reliable, but totally useless when you have to draw a significant amount of current from the potential divider. As a rough rule of thumb you need 10 times more current down the potential divider chain than you draw out. So if you draw 5 Amps out you need to supply the potential divider with 50 Amps, even then regulation will be poor.
This means that the resistors will get very hot, 50 Amps at 12 Volts is 600 Watts, most of which is heat in the resistors. So yes if you want a combined electric heater it might be the way to go.


Ha! Thanks for clearing that one, I don't know why I was thinking about the voltage and completely disregarding the current. 

Quote
A 6V output SMPS (or even 5V) might be the thing you are looking for?  But you will have to measure
your peak current requirement before proceeding.


I measured the current and the highest I saw under operation was about 9 amps.  I will be adding 4 more servos so I am guessing around 11 or 12 amps. 5 volts would be too low as I am also using the power supply to power an Arduino Mega and Mini Maestro (The power for the servos is not running through the Arduino board).

I tried looking for a 6V output SMPS but I can't find any.  It seems that the standard output voltage skips from 5 volts to 7.5 volts.  Maybe there are some other suppliers I did not find.  I searched Mean Well and Jameco.  I did find some that were 7.5 volts with higher output current though:
http://www.jameco.com/webapp/wcs/stores/servlet/StoreCatalogDrillDownView?langId=-1&storeId=10001&catalogId=10001&position=1&refineType=1&sub_attr_name=Output%20Voltage%20(VDC)&refineValue=7.5&refine=1&history=qssb6aes%7CfreeText~ac%2Bto%2Bdc%2B7%2Bvolt%5Esearch_type~jamecoall%5EprodPage~15%5Epage~SEARCH%252BNAV%406dxepmf9%7Ccategory~45%5EcategoryName~category_root%5Eposition~1%5Erefine~1%5EsubCategoryName~Power%2BSupplies%2B%2526%2BWall%2BAdapters%5EprodPage~15%5Epage~SEARCH%252BNAV%40616cm1sz%7CrefineValue~10%2B%253C%253D%2B%2B%253C%2B50%5ErefineType~2%5Eposition~1%5Esub_attr_name~Output%2BCurrent%2B%2528A%2529%5Erefine~1%5EprodPage~15%5Epage~SEARCH%252BNAV

I am not sure if the servos will be ok with a 7.5 volt input when they are supposed to be between 6.0 and 7.4 volts.   I also have a couple of micro servos that run a maximum of 6 volts.  I saw a 5 volt regulator from sparkfun, would that work for those?

drewdavis

I would not go over the max Voltage! Although not extremely common 6v power supplies can be found. Look at this one

http://www.alibaba.com/product-gs/970152463/6V_20A_High_frequency_lab_power.html?s=p

The other option would be to use a 6v BEC. The biggest problem you have is the amperage. 6v is easy to supply at 1-5 amps. 6v at 12amps is not so easy supply. Most BEC's stop before 5amps.

robotdragon

Quote
I would not go over the max Voltage! Although not extremely common 6v power supplies can be found. Look at this one

http://www.alibaba.com/product-gs/970152463/6V_20A_High_frequency_lab_power.html?s=p

The other option would be to use a 6v BEC. The biggest problem you have is the amperage. 6v is easy to supply at 1-5 amps. 6v at 12amps is not so easy supply. Most BEC's stop before 5amps.


Would it be possible to connect 2 of these in series to get 6.6 volt output?

http://www.jameco.com/Jameco/Products/ProdDS/693231.pdf

I am talking about the 3.3 Volt version.  If that works then it would be cheaper than the rare 6 volt power supply.

Grumpy_Mike


robotdragon

So if I got two of these:

http://www.jameco.com/webapp/wcs/stores/servlet/Product_10001_10001_201903_-1

and two of these:

http://www.jameco.com/webapp/wcs/stores/servlet/Product_10001_10001_36169_-1

and connect them like this:



I should get a 6.6 voltage output with a current that can go up to 30 amps.  I picked 30 amps just to make sure it is future proof with additions.  There are different types of diodes, does the one I picked do the same thing as a switching diode? 

Is the sense positive and negative standard on these power supplies?  Where do these lines connect on the load?

Is there a major shock hazard when I wire these together?  I never hot wire anything; just want to be aware.

piratebrian

Not sure how those power supply work.  If you call up the manufacturer they may have a terminal on them to connect them together so their individual current monitoring circuit doesn't bounce fight volts and amps among each other.    
On those type power supplies there is usually an adjustment pot where you can add or subtract voltage by small amounts,  The one below says 5v, but I bet you can get 4.1 to 5.9 volts out of it by adjusting the pot up or down.  

http://www.jameco.com/webapp/wcs/stores/servlet/Product_10001_10001_2102490_-1
The wattage is at 225W so that should be enough if each of your servos pull a melting 2.5 amps, that's 2.5amp*5.9v*10servos=147watts. < 225   The power supply is properly supplied with a fan to push through that kind of amperage.    

Grumpy_Mike

The power supplies are fine but those diodes are zenner diodes, not simple diodes. You need a diode rated for at least twice the current you are going to use.

dc42

Why not try a standard ATX power supply? You can use the 5V output to power the servos, and the 12V output to power the Arduino. That's assuming 5V is enough for the servos.
Formal verification of safety-critical software, software development, and electronic design and prototyping. See http://www.eschertech.com. Please do not ask for unpaid help via PM, use the forum.

MarkT

Many industrial PSU's have a +/- output adjustment, its possible some 5V supplies actually can provide nearer 6V if
adjusted (though range is often only +/-10% to +/-15%)

Anyway found a 7V supply with +/-10% adjust I think: http://uk.farnell.com/xp-power/ecm100us07/psu-100w-single-output/dp/2099506
[ I will NOT respond to personal messages, I WILL delete them, use the forum please ]

drjiohnsmith

#12
Sep 14, 2013, 02:14 pm Last Edit: Sep 14, 2013, 02:15 pm by drjiohnsmith Reason: 1
question

will the system work at 5volts ?

I ask as you have a battery, at 6 volts,
    so I guess 4 off 1.5 volt cells,

what I wonder is what the cell voltage is like at the end ?
   I bet near if not below 1.25 volts,
       QED : 5v supply should be OK.


robotdragon

Thanks for the replies everyone! I believe I have settled on this supply:

http://www.jameco.com/Jameco/Products/ProdDS/123415.pdf

There is a pot that can be adjusted from 6.0 - 8.3 volts and a 20 amp current supply.  I measured a maximum current yesterday at 9 amps so with the addition a few more servos then the maximum current may be around 12- 14 amps.  So the 20 amp supply should work great. 

Quote
question

will the system work at 5volts ?


All my components will run off of 6.0 volts.

crazypj

I don't know enough about electronics (yet  :smiley-roll-sweat:) but personally I would use some cheap DC to DC converters
It would be easy enough to connect them to a serial bus at 12v (or higher) then step down outputs to various servos
May not be the most elegant way to do it but should prove reliable as you would have 'personalised' outputs

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