# Power Supply Stumper



## Hornet (Dec 1, 2005)

I got one that puzzles me a bit.

I have a little Pyramid PS9KX 13.8 volt 5 amp regulated power supply.
It's been modified with add-on caps and LM338 regulators,so that i can run 2 or 3 things at once,and vary the voltage output on 2 circuits individual of the main supply.

This is the stumper,when i hook my Hudy Tire Truer up to the stock 13.8 5 amp outlet,it won't turn my tire truer,but when i hook it up to the external add-on circuits it runs fine.
At first i thought it was because the add-on circuits were carry'ing lots of cap capacity,so i hooked my seperate filter box into the circuit,which is nothing more then a Radio Shack project box,stuffed full of caps,figuring it was the extra Oomph from the caps that were doing the trick,but it still wouldn't crank it over.
Then i thought maybe i had a dud power supply,so i took my tire truer over to my buddies house and tried it on the identical power supply,still the same results,no turnie of truer,then i thought bad truer,so i had my other buddy bring his Hudy truer over and we tried his on the same 2 Pryamid power supplies and got the same reaction.
His works on the external add-on circuits just like mine,but won't work on the factory circuit.
So i'm wondering if anybody else has ever ran into this,i'm scratching my head over why a pair of LM338 regulators would change things enough to operate a truer.
I figure i ruled out the caps being the deciding factor,by adding my filter box into the circuit,so that pretty well leaves it at the regulators themselves.
Anybody got any ideas of why they'd change things enough to make this supply work like it does.
Rick


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## Rick Carter (Dec 2, 2008)

Slick, 

Try a Square power supply instead of a Pyramid -LOL!


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## tossedman (Mar 19, 2006)

Squares don't work up here. Snow piles up on them and then you can't find 'em. Need a pyramid so the snow slides off.


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## Rick Carter (Dec 2, 2008)

LOL! Good comeback Toddinator!


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## Hornet (Dec 1, 2005)

LMAO,you guys crack me up:thumbsup::wave:

It's 4 in the morning,just going out to dig out out my pyramid again,now i know why igloo's are shaped the way they are,lol


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## AfxToo (Aug 29, 2003)

Hmmmmm. I would try to measure how much current you are getting from the regular power supply output. From what I've seen with the Hudy comm lathes they really need the full 5 amps. They will run with fairly low voltage, I've seen then work all the way down to around 3V but they need a full 5 amps of current to start. 

To measure the current you will to connect an ammeter in *series* with the load. Also connect a voltmeter in parallel to the power supply outputs. Make sure the ammeter is set on the appropriate range, one that exceeds the maximum expected current, say 10A. I would start with a light load, say a stock magnet car. This should draw about half an amp and if you load the motor by putting some friction on the rear tires you should see the current increase and the voltage stay the same. This will tell you whether the power supply is maintaining regulation as the current demand increases. 

Next do the same thing with the Hudy motor, but first try it unloaded to see if it even spins the motor. When it is not loaded try spinning the motor by hand to get it going. Then try it when the lathe motor is loaded. It will be interesting to see what the current is when the motor is not spinning. I suspect you are not getting the full 5 amps needed to start the motor. The capacitors in the external circuit may be giving you just enough kick to get the motor started. Once the motor is started it's current draw will decrease significantly as the motor spins.


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## Hornet (Dec 1, 2005)

I'd thought it was the extra push from the caps,but even if i add my filter box (approx 12,000uf of caps stuffed inside a RS project box) into the mix,it still won't start it.
I did test it's output,and it has no problem putting out 4.9 amps,and holds a steady 12.70 at that load (the old magic lightbulb tester:thumbsup.

The Hudy starts and runs up to speed if i tie it to a heavy duty 1.5 amp 12volt wallwart battery charger,so i tied the 2 together just to see if the 5 amp Pyirmid supply would run it after it was over the initial start-up load.
Doing this gets it running with no problems,but as soon as i unhook the wallwart and leave it spinning on just the 5 amp supply,it comes to a fairly rapid stop.

That's what's got me a bit confused,as it won't maintain it,even after it's past the start-up load and spinning,but the add-on circuits run it great.

I think i ruled out the caps as the differance,which leaves me wondering what the LM338's do,and why it'll run a Hudy through them,but not on it's own.

If this link works,here's a of a picture of it,it's a bit rude and crude looking,so don't hold that against me,lol

http://www.tossedman.com/images/rick/IMG_1112.jpg

It'll maintain about 10.7 volts on the add-on meters with the hudy hooked up to the extenal circuits while truing a tire.

Dave,it almost acts like it's got a dynamic brake applied when it's hooked up to the stock outlets.
It tries to make a slight revolution then stalls,and even trying to spin it over with fingers is real tough.
It's hard to explain,but i'd swear it acts like a brake style circuit is kicking in somewhere.
Rick


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## AfxToo (Aug 29, 2003)

It sounds like the protection circuitry (crowbar or thermal) is kicking in because the current demand is higher than what the power supply can put out. I'd check the R/C forums to see if anyone has direct experience with your tire truer. It sounds like you are right on the edge of what the truer motor needs and the wall wart is giving it the added push.


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## Hornet (Dec 1, 2005)

That was my initial thought too,that it was kicking in the overload circuit.
But even a 12 volt 1 amp wallwart will free spin it.
I also tried bypassing the overload circuit,by ty'ing the 12v wallwart and Pryamid supply together in parrallel,and getting it free spinning,and then unhooking the wallwart out of the circuit after it was already running.
It still did the same thing,the Hudy came to a screeching halt as soon as the wallwart was taken out of the circuit.
I even thought i had a dud tire truer or a dud power supply,but we checked this with another identical power supply that hasn't been modded,and another identical tire truer,and we got the same results.
So i figure that rules out anything been a dud.
Gotta admit,it's got me scratching my head,i'm not too worried about it,as it runs my tire truer through the extra add-on circuits,more just satisify'ing curiousity then anything.


Vid of a local race


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## AfxToo (Aug 29, 2003)

Aha! Rick based on what you've said, especially the part about the motor spinning on the 1.0 amp wall wart, the only thing I can conjecture is that the motor feedback (back EMF) is causing the protection circuits in the regulated power supply to think there is an overvoltage condition on the load side and it is tripping the power supply's crowbar circuitry to limit output current to protect the load - not to protect the power supply. The crowbar causes the power supply to shunt all of its current internally so the load gets essentially zero current. The fact that the motor stops tells you that it is getting no current. 

The external circuitry that you are adding is not going to do anything to increase the current to the motor but it does act like a filter between the motor and the power supply, dissipating the effect that the back EMF has on the *regulated* power supply. The wall wart is unregulated and thus suffers no ill effect from the back EMF. The combination of tight voltage regulation with current limiting (crowbar) is actually hurting you in this particular case. The power supply is trying to protect the load from what it thinks is an overvoltage condition and is shutting down the output current. Some regulated power supplies use current clamping, which locks the output current at a maximum value under the same set of conditions. I'd bet that your power supply uses a crowbar. This is exactly what I think is happening.


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## Hornet (Dec 1, 2005)

Awright,thanks Dave,that makes sense now,i hadn't thought about back EMF kicking in the crowbar circuit.
I did add diodes into the extra add-on circuits,i never gave them any thought till now,but would they be some of the reason it acts like it does.
Thanks for study'ing this out for me,it's very appreciated.
Rick


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## Hornet (Dec 1, 2005)

Dave thanks,you nailed it for me.
I just tried a s'periment.
I added a pair of diodes and my filter box inline with the truer,and it works to beat heck.
It needs the filter box to have enough Oomph to get past the start-up load with-out kicking in the overload circuit.
I never thought of the diodes being the differance,and cutting down on back EMF,thank-you.
You made my day,and answered a stumper for me:thumbsup:
Rick

The diodes are now a permanent part of my filter box


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