AXE024 isolated power supplies

#1
I am working on a lighting project for a fundraiser banquet for a local non profit organization. I decided to use a 08M & RC servo to provide some random motion. I freeformed a prototype circuit on a 16 pin dip socket.

I was searcing for sources for parts to build quantities of boards, when I found the 08M SERVO DRIVER (AXE024), with all the parts, was less expensive. Since it can run 3 servos, it will further reduce the total costs.
I didn't realize the servo command could be used on all output pins.

My question is about the suggestion in all of the documentation that a seperate power supply should be provided for inductive loads, to reduce noise on the logic power supply.

The AXE024 lowers the logic voltage supply, by one diode voltage drop, connected to the 6 volt motor power supply. I don't believe this provides any noise isolation between the 2 power supplies.

I was wondering if the suggested dual isolated power supplies could be provided by 2 7805 regulator circuits, with seperate outputs, but common inputs and grounds connected to the 12 volt supply for my lighting.

 
#3
I have never used that product but a quick look shows that the capacitors are providing suppression to a degree.
As it says in the DATA SHEET you may need more suppression and remember it's only for tiddly servos and not to control your Dad's Sinclair C5.

Yes, of course separate power supplies would be better. But if you are suggesting 2 x 7805 that means your peripheral (motor?) is 5 volts. Is it?

Also remember this is a low cost board for experimenting. I'm sure you can pay extra and get dual supply, but many hobbyists only want to spend pennies. Some will even drive 50 miles to save 50p.

Anyway, give it a whirl.
 
#4
Since the project board can use a 6v supply, you may want to use a 7806 regulator. The 78xx series regs come is a number of different voltages that are widely available: 7805, 06, 08, 09, 12, 15 and others.

Edited by - inglewoodpete on 21/08/2007 14:21:13
 
#5
Good points from sedeap and Dippy but also note that a 7805 is not going to be 'man enough' to provide current for a servo. Even the small ones will pull over 1A when they start to move and that will collapse the output of the 7805.
Good wiring layout is also very key to avoiding power supply problems.
 
#6
Thanks for the replies. I know caps will help a lot, I'm just not sure how much. My quick prototype doesn't have caps. when I us a diode to drop the motor's 6 volts to drive the 08M, my test program acts like its getting glitches. When I use a seperate 4.5 volt supply for the 08M, it runs as expected.

I guess I knew about regulators with other voltages, but haven't searched for any sources. I thought the servos would still work ok with a lower voltage.

I didn't know the servos would sap a 78xx. Since the AXE08M can drive 3 servos, I planned to try just 2, I guess I'll need more power. Are there any heavy duty regulators, or can you parallel them for more current capacity?
These devices are only going to be used for about 4 hours. The servos only have to pluck a string occasionally to cause ripples in mirrored mylar reflectors, to create the illusion of light reflecting on water.
 
#7
You can't parallel 7805's. You could use a big capacitor to stop the supply collapsing with the startup current eg a 1 farad 5.5V supercap. You could also use one 7805 per servo and maybe 4700uF on each 7805.
 
#8
Have a look at the Fairchild datasheet for 78xx regulators Refer &quot;Typical Applications, Figure 12. <A href='http://www.tranzistoare.ro/datasheets/228/390068_DS.pdf' Target=_Blank>External Web Link</a>. The addition of 1 power transistor and a (? watt) resistor can boost the current capacity to 3 (or more) amps. If the power supply is up to it, of course.
 
#9
Or you could use a nicad pack to drive the servos. Should be OK for 4 hours if you use 1000mAh cells, but would be a more expensive way of achieving the result.
 
#10
I just bought 12 x AA NiMH rechargeables from Hong Kong for $20 including shipping. They are 2500mAH which ought to easily last 4 hours. Rechargeables could well work out cheaper than regulators etc.
 
#11
Ted, Rule No. 1:
ALWAYS get hold of DATA SHEETS and give them a READ. As IP's link shows; you can add to a regulator. The old 78xx is made by everyone and most Data Sheets will show circuit applications.

I realise that everyone goes on about this dinosaur of a regulator because it's cheap'n'cheerful , but there are many others out there.... and varieties of 78xx too.

But guess what? You'll have to READ the Data Sheets before buying.
 
#12
Thanks again, for all the advice. I rarely have time to read everything I need before a project deadline.

This is a non-profit project, so cost is always a factor. After checking a few more prices, it looks like dual battery packs are going to be the cheapest, and quickest solution.

I'll use a 4.5 volt AA pack for the 08M, and a 6 volt 'D' cell pack for the motors.

Thanks!
 
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