Jim VS 7805 regulator

Jim Richey

I decided to make a simple improvement to my track drive machine.The radio receiver /servo battery had been going dead too fast and at all the wrong times.What better than to install a 7805 and run it from the main 12 volt battery.
After removing the 4.8v nicad pack and hooking up the new regulator in it's place,things went screwy.The 7805 got hot and the output spiked all over the place.Looked like it couldn't handle the 3 servos and camera.
So.I carefully made up a perfboard circuit with 2 -7805's in parallel and several filter caps.I powered it up with 12 V on the workbench and the big 220uf cap blew up.OK,so I had something reversed,right? I probed and checked the circuit for a long time,nothing was wrong with it.But I checked all 3 -7805's that I had just purchased from Radio Shack only to find they were defective.Never thought that could happen.
So,back to The workbench armed with a different batch,this time all was well.
I proudly installed the new circuit in the machine and powered it up.Everything worked great with no quirks,but the 7805's were again getting too warm.
I broke open the circuit and wired in an ammeter just to see what was going wrong.It was the servos causing problems because they were traveling too far and stalling out.They were drawing around 500Ma each.Way too much...
Lesson learned,sometimes things are best left alone!

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Thanks, Parallax!

SteveW

That's a sweet crawler!
Are you sure you're allowed to run those 7805s in parallel? I don't think they necessarily current share like you'd hope, and oscillation is possible. One meatier device is almost certain to be better for this.
(However, now you've found out why your batteries were going flat so fast, one 7805 might be enough?)

Steve

LoopyByteloose

Two or more 7805s in parallel is generally a NO-NO.
In theory it will work fine, but as they turn on and off, they oscillate and one will confuse the other into a run away mode. I suppose that a big enough filtering system would settle it down [noparse][[/noparse]for a while].

You really should look at the PDFs for High amp regulation or separate the outputs of each 7805 into localized and different power demands.· You can use a 7805 to turn on and off a 3055 transistor or a big MOSfet.

It may be that the new 7805 have better thermal shut-down than your first batch. Thus you avoid damage, but eventually you may get another thermal shut down.

Tell me, what is the cable winch for? I think many of us admire your hardware as we haven't seen such a beefy crawler system.

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"When all think alike, no one is thinking very much.' - Walter Lippmann (1889-1974)

······································································ Warm regards,····· G. Herzog [noparse][[/noparse]·黃鶴 ]·in Taiwan

Jim Richey

Hey,thanks for the replys,guys. I never was sure about pairing up 2-7805's.Sometimes the real fun,at least to me,is trying new things out[noparse][[/noparse]sometimes zillions of times until I get it right].I'll give the machine some hard use when time allows and let you know how the regulators perform.
The winch serves a useful purpose because of where I live here near Detroit.The woods and hills are difficult to move around on at least in my neighborhood.The machine tumbled down a gulley and was trapped on the bottom.Since It weighs more than I could possibly lift,it got a winch for future assistance in wilderness trecks.
That reminds me,I recently got a GPS and mapping software connected to my laptop.How would someone work out a transmitter on the machine to display it's position to a distant location?Sounds challenging.
I understand that Batman lives somewhere close by,maybe he could get a little surprise heh heh.

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Thanks, Parallax!

Kevin Wood

Jim,

You might want to look into getting a Technician class amateur radio license and setting up an APRS system. APRS stands for Automatic Position Reporting System, and would allow you to pretty much monitor position anywhere you want to. Here is a link with some info from the ARRL site:

www.arrl.org/tis/info/HTML/aprs/

If you are wondering about the license, if you built that thing, you won't have any problems. A good study manual is "Now You're Talking" published by the ARRL.

Jim Richey

That's a great idea,Kevin. I used to live next door to an old ham operator who did fascinating things and was involved with ARRL activities.
As far as the 7805 regulator pair goes,maybe Mythbusters might do a show to study the urban legend of oscillation or not!
Jim

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Thanks, Parallax!