24V 6.5A power supplies now provide 9.6v at a few mA

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I have 2 GlobeTeck RT-3s150-24 power supplies that have both failed within a few weeks of each other. Both doing the same duty, both in the same manner.

 

They've been train supplies, lab supplies, about anything I've needed them to do since June 2, 2005. That's the date I wrote on them when I first got them. They say they passed QC in February of 2001.

 

I put in an invisible dog fence wire for a lady a few weeks ago. Of course, when I was done putting in 1375 ft of wire, there was no continuity. So I made myself a tracer with an AVR processor to generate a 100KHz carrier and 2 tones I could hear with an AM radio. It had an output transistor and pullup resistor to generate enough power to hear the signal a few inches from the wire. I took one of these and my "tracer" to the lady's house and in just a few minutes found where the wire was broken. When I got home, my handy 24V 6.5A power supply would only give me 9.6V at a few mA. 

 

"Rats. My power supply passed away."

 

So this weekend, the lady called back. "The lawn care people accidentally broke the wire. Can you come fix?"

 

I went to look, and, of course, they didn't know where. I had taken apart my "Dog Wire Tracer" to use for another project, so I built another with a tiny 10, an NFet, a 7805 regulator, 2 capacitors and a 1K resistor. I took my remaining 24V 6.5A power supply out, and in a few minutes, found the break, repaired it, buried the wire, to find no continuity, so I dug the spot up again, fixed it again... Now it's working.

 

But when I got home, my second 24V 6.5A power supply gives me 9.6V at only a few milliamps.

 

I took the cover off one of them. Nothing fried, smoked, split or burned. All the big aluminum capacitors are in place and not swollen. No smell. There's a little fuse in here... It's good.

 

Ok, they ARE 13 years old. Perhaps they've just expired? Anything I should check before I consign them to the dustbin?

 

No, the dog did not pee on it. He was outside and the power supply was on a shelf in the garage. Quite a nice dog, actually.

 

Slightly distraught,

 

If you don't know my whole story, keep your mouth shut.

If you know my whole story, you're an accomplice. Keep your mouth shut. 

Last Edited: Thu. Sep 25, 2014 - 07:55 PM
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*scream*

 

Now, my other, completely different 24V 5A power supply has gone "POP!" and gives only 4.3v.

 

It didn't make any smell, but it was a popping IC sound, not a popping capacitor can sound. No brown fuzz.

 

Powerless in Tulsa.

If you don't know my whole story, keep your mouth shut.

If you know my whole story, you're an accomplice. Keep your mouth shut. 

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Stop plugging them in smiley

 

That is odd to say the least though.  Any chance the input voltage select switch got moved to 220?

I would rather attempt something great and fail, than attempt nothing and succeed - Fortune Cookie

 

"The critical shortage here is not stuff, but time." - Johan Ekdahl

 

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Please Read: Code-of-Conduct

Atmel Studio6.2/AS7, DipTrace, Quartus, MPLAB, RSLogix user

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I gather these are switched mode supplies. If so, a common problem is a small electro in the control circuit on the live side. It is usually around 10 to 220uf at less than 63V. Try replacing that.

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Were there any black helicopters nearby? ;)

"Experience is what enables you to recognise a mistake the second time you make it."

"Good judgement comes from experience.  Experience comes from bad judgement."

"Wisdom is always wont to arrive late, and to be a little approximate on first possession."

"When you hear hoofbeats, think horses, not unicorns."

"Fast.  Cheap.  Good.  Pick two."

"We see a lot of arses on handlebars around here." - [J Ekdahl]

 

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Was the Power supply just running a 5 V micro, or was the micro running an NFet and the 24 V signal was injected into the loop?

 

Was the system fused?  You do not want to fry the underground wire with a 6 A power supply, nor do you want to short out the power supply, unless it is designed to handle the load.

 

Maybe you should put a dog collar on the yard guys to keep them away from your wire!

 

JC

 

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Hmmm... could it be that heavy back-EMF from the inductance of the loop and the 100 kHz carrier is to blame?  Maybe next time leave the PS at home and use a couple of 12V lead-acid gel cells.

"Experience is what enables you to recognise a mistake the second time you make it."

"Good judgement comes from experience.  Experience comes from bad judgement."

"Wisdom is always wont to arrive late, and to be a little approximate on first possession."

"When you hear hoofbeats, think horses, not unicorns."

"Fast.  Cheap.  Good.  Pick two."

"We see a lot of arses on handlebars around here." - [J Ekdahl]

 

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One blurry photo of the power supply killer. It took 4 tries and 2 reboots of the stupid phone to get it.

If you don't know my whole story, keep your mouth shut.

If you know my whole story, you're an accomplice. Keep your mouth shut. 

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Maybe if you posted your code we could see where the problem lies........

 

(Possum Lodge oath) Quando omni flunkus, moritati.

"I thought growing old would take longer"

 

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I don't have any code for the power supply.

 

/*
 * DogWireFault10.c
 *
 * Created: 9/22/2014 3:28:10 PM
 *  Author: Tom
 */ 


#include <avr/io.h>
#define F_CPU 1000000ul
#include <util/delay.h>

int main(void)
{
	DDRB = (1<<0) | (1<<1) | (1<<2) ;
	
	TCCR0A = (1<<COM0A0) ;
	TCCR0B = (1<<WGM02) | (1<<CS00) ;
	
	OCR0A = 5 ; // 1MHz / 5 / 2 = 100KHz
	
    while(1)
    {
       for (int i = 0; i < 1000; i++)
       {
	       TCCR0B = (1<<WGM02) | (1<<CS00) ;
	       _delay_us(500) ;
	       TCCR0B = (1<<WGM02) ;
	       _delay_us(500) ;
       }
       for (int i = 0; i<500; i++)
       {
	       TCCR0B = (1<<WGM02) | (1<<CS00) ;
	       _delay_us(700) ;
	       TCCR0B = (1<<WGM02) ;
	       _delay_us(700) ;
       }
    }
}

And now you know how to disrupt radio signals and trace a broken wire using an AM radio.

If you don't know my whole story, keep your mouth shut.

If you know my whole story, you're an accomplice. Keep your mouth shut. 

Last Edited: Fri. Sep 26, 2014 - 03:29 PM
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I vote for 100V spike from loop turning off eating the output transistor in the power supply?

Imagecraft compiler user

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Maybe should have put a BFC on the power.

If you don't know my whole story, keep your mouth shut.

If you know my whole story, you're an accomplice. Keep your mouth shut. 

Last Edited: Fri. Sep 26, 2014 - 07:28 PM
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And/or a flyback diode across the FET.

 

- S

 

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Looking at the schematic, I see a 1k load resistor and a .1 uF bypass cap that forms a low pass filter back to the power supply that is powering the signal injector. Also, given the circuit topology, there is no way for any more than 24mA to be available for the loop, and even then, the other end of the wire and power supply would have to share a common ground   Given that the 7805 and avr lived through the entire process and not having enough facts about the setup, I don't yet see any reason to jump on a flyback effect or inductive kick as the culprit.

 

Torby: Where does the 3rd supply figure into this? Was it used at the customers site? If not, maybe the culprit is at your workbench/home shop. Still too many unknowns to reach a conclusion.

Last Edited: Sat. Sep 27, 2014 - 07:16 PM
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The only thing the 3rd power supply has in common is it was also old and 24v.

 

It wasn't powering anything at the time, but had run a toy train loco a few minutes before the pop.

If you don't know my whole story, keep your mouth shut.

If you know my whole story, you're an accomplice. Keep your mouth shut. 

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jgmdesign wrote:

Stop plugging them in smiley

That's what I call stepping back and looking at the big picture. yes

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kk6gm wrote:

 

jgmdesign wrote:

 

Stop plugging them in smiley

 

 

That's what I call stepping back and looking at the big picture. yes

 

jgm gets directly to the heart of the matter.

If you don't know my whole story, keep your mouth shut.

If you know my whole story, you're an accomplice. Keep your mouth shut. 

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still gives something? is there a fuse in the main linear path? has it blown?

 

Ross McKenzie ValuSoft Melbourne Australia

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Fuse is fine. Don't see any aluminum caps popped or swollen. The transistors are enclosed in greasy heat sinks I haven't disassembled yet.

If you don't know my whole story, keep your mouth shut.

If you know my whole story, you're an accomplice. Keep your mouth shut. 

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Tom,

 

Do you have the schematic? If both P/Ss are outputting the same 9 volts, it has to be a systematic failure that should be possible to diagnose from the circuit.

 

Ross

Ross McKenzie ValuSoft Melbourne Australia

Last Edited: Sun. Sep 28, 2014 - 11:51 PM
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I think (as bob suggested) the output driver has been fried.

as you have a relative very long line you have to consider it a transmission line.

It has been a while, since I played with them, but IIRC:

 

the speed of light (in vacuum) is about 300000Km/s. Through a wire it is about 2/3, so 200000Km/s

from the moment you make the start of the line high it takes about 2us for that high level to reach the end of the line. It then can not go anywhere so bounces back (simplified).

after another 2us it then gets back to the supply, causing the voltage there to temporarilly double.

I am not sure what happens if at that exact time you disconnect the wire+avr. If then voltage then again doubles you end up with the 100V as suggested by Bob, but my guess is that the first during operation will have already seriously damaged your output transistor and that when you powered on the supply again it had to change some buffer capacitance and died in that process.

 

 

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Just found the datasheet for this model. It is a switch-mode supply.

 

Ross McKenzie ValuSoft Melbourne Australia

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Not buying the explanation. How on earth can you pass enough energy through the 1 k ohm resistor and past the .1 uF cap to damage the power supply, which will have its own filtering? Note that the 7805 regulator and avr was not damaged. Also note that the third power supply was not used in the dog wire repair.

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  It is not about energy, it is about voltage. To destroy a FET not much energy is needed. the voltage is what is destroying the Drain-source channel or the gate. as it is to do with distances between gate, drain and source. 

 

What I now do wonder about is at what frequency the SMPS was running itself. If that also was 100KHz it might also be that by adding the 100KHz switch it might have started oscillating and thus be damaged.

 

Or does the SMPS need/needed a minimum load to work correctly and is that not met with as result the failure.

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It's all about Karma.

Torturing poor little doggies with electric shocks.

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hehehe Evil me!

If you don't know my whole story, keep your mouth shut.

If you know my whole story, you're an accomplice. Keep your mouth shut. 

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It is not about energy, it is about voltage.

 Yes, it is about energy, and voltage. for any inductive kick to get back to the power supply, it must pass through a low pass rc filter consisting of the fets drain resistor and the input bypass cap on the 7805. If the current to charge the cap is insufficient to raise the voltage to a lethal level (for the power supply fet) before the flyback pulse ends, no harm done to the p.s. Study the schematic of torbys tiny driven pulser. Also explain how his circuit could survive a flyback pulse capable of damaging the power supply. The t/c of the rc filter alone is 100 uS plus the filtering on the output of the power supply.