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The Dangers of Working with a High Voltage Supply

D-Dog

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I recently acquired a new lab supply which was originally meant for an electrophoresis table. As such it has three modes of output... constant voltage, constant power, constant current. (1000v, 200watts, 400ma)

I understand what constant current and constant voltage are, however, I'm confused by constant power. I'm using the supply to charge a highrate SLA and while 400ma will get the job done (highest it goes on constant current), constant power is adjustable from 0 to 200 watts and being able to charge the battery a little faster would be nice.

I could do constant voltage, however, it is way too hard to modify the voltage with precision being as one pot brings it from 0 all the way to 500/1000 volts... imagine trying to get 13.6 on that :p

As I figure it, constant power outputs the number of watts you specify regardless of output voltage, however, this sounds just like CC which is why I'm confused.

I think if I select 2 watts then it puts out 500ma at 4 volts or 250ma at 8, however, I don't want to risk anything until I'm sure :)

Any help is appreciated as I try to figure this out... and yes, 1000volts is dangerous...
 
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ctom

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Re: Constant Voltage vs Constant Current vs Constant Power

A constant power power supply will vary the output voltage and current to maintain a constant power level to the load (power = voltage*current) while a constant current supply will only vary the output voltage in an attempt to maintain a constant current to the load. If you can get hold of the manual, it should give you a voltage and current curve for a given power output. Without that information, it would be hard to predict what the voltages and currents will be for a particular power output short of mapping out the output with a variable resistor rated for high power.

As I'm sure you are aware, that power supply can be very dangerous! For example, in the constant current mode, if you were to disconnect the load (battery) from the power supply, the output voltage would most likely rise to the maximum voltage (1000 volts). The insulation on your wire leads may not be rated that high...
 

D-Dog

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Re: Constant Voltage vs Constant Current vs Constant Power

Yep, The unit has two modes, 500v/400ma/200watts and 1000v/200ma/200watts. The wire I use is rated for 2000 volts and even then I keep it on the 500volt max setting figuring every little bit of safety helps :)

So the question remains if I connect a 12 volt load to the supply in constant power mode will it feed it 2 watts of power continuous at the battery's voltage or at 500v? I will try to look for a manual which may give these curves.

The supply is an ISCO 493 Electrophoresis Power Supply

/Begin Safety Rant

But yea, just reiterating again how dangerous a supply like this can be. I wouldn't want anyone else to try working with this supply unless they really knew what they were doing. Which equation is the one which tells you power through a resistor? I think it's P = V^2 / R The resistance of skin is know so you can see how each little increase in voltage makes the result much more deadly. Or is it just Ohm's Law rearranged I = V / R ? Either way electricity is not something to be taken lightly. A defibrillator pumps 160 Joules or so into you to try to save your heart. The main caps in this supply along can pump out 10x that amount and I'm sure some supply are even more dangerous. It only takes one mistake to end a life... and there is no fuse inside your body to save you (I can't recall how many times I tried measuring voltage on the current mode of my DMM only to see a spark...)

/End Safety Rant :)
 

ctom

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Re: Constant Voltage vs Constant Current vs Constant Power

Again, without the specifications available (or the circuit schematic), it's hard to predict the output. As an experiment, you could connect an ammeter in series with the battery and power supply. If the supply is outputting 0.167 amps (2 watts/12 volts), you should be OK. A caveat to this method is that you need to check the specs for your battery - if it is a lead acid battery, it may not tolerate a trickle charge rate of 167 ma continuously after the battery is fully charged. Most lead acid batteries are charged with a constant voltage of 13.6 to 13.8 volts with some form of current limit.
 

PeAK

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Re: Constant Voltage vs Constant Current vs Constant Power

I recently acquired a new lab supply .
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I understand what constant current and constant voltage are, however, I'm confused by constant power. I'm using the supply to charge a highrate SLA and while 400ma will get the job done (highest it goes on constant current), constant power is adjustable from 0 to 200 watts and being able to charge the battery a little faster would be nice.

Comment:
As P=V*I is finite at 200Watts, the highest current (400mA) will correspond to a voltage of V=P/I=200/0.4=500V. For a 1000V output, this means that the current must drop (to 0.2A).

While it might be possible that your unit can put out higher currents (same power) at lower voltages (examples: 100V at 2A or 25 V at 8A). The max constant current setting often specifies the maximum current available when used in the dual mode as a current source.


D-Dog said:
I could do constant voltage, however, it is way too hard to modify the voltage with precision being as one pot brings it from 0 all the way to 500/1000 volts... imagine trying to get 13.6 on that :p

As I figure it, constant power outputs the number of watts you specify regardless of output voltage, however, this sounds just like CC which is why I'm confused.
To test how well the power output mode work, you can set the power output to 2 watts and plug different resistor values that differ in ratios of 2 and 4 to see if either voltage changes to put out a different voltage for each resistor value.

Since P=V**2/R, the supply should scale the voltage so that each time the resistor value quadruples; the voltage will double (and the current will reduce at the same time). In this mode, you tax the supply when you add lower resistance values that draw more current. So first you need to do is to calculate the minimum load for the 2 watt setting and the 0.4A current limitation, simultaneously.

Since
Rmin = P/(Imax*Imax) from P=(I**2)/R
=2/(0.4*0.4)=12.5

So try loads of 18 ohms, 36 ohms and 72 ohms to see if the corresponding voltages are

V= sqrt (P*R)= sqrt(2*18)=6V (& 8.4V and 12V)


I think if I select 2 watts then it puts out 500ma at 4 volts or 250ma at 8, however, I don't want to risk anything until I'm sure :)

Any help is appreciated as I try to figure this out... and yes, 1000volts is dangerous, I know what I'm dealing with :)
 

D-Dog

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Re: Constant Voltage vs Constant Current vs Constant Power

Remember what I was saying about the dangers of high voltage? Even if you are usually careful?

I'm typing this with one hand because my other one has two second degree burns. I'm quite ok, however, don't ever assume voltage plays nice.

Usually a catastrophic failure is the result of many small mishaps. What follows is what just happened 15 minutes ago.

I had supply set to constant current 300ma charging small AW CR123A cell. It was totally dead to cutoff so I figured on having at least 2 hrs. I went to take a shower and voltage reading 3.9 volts... all good.

Took longer than I thought and got back saw voltage at 12.64 volts (this is 3.7 volt nominal) :-O Must not have been as dead as I thought or because it's old the capacity is lower as it was only on there for an hour)

Not wanting a fire I immediately go to disconnect leads(secured with magnets) to battery. (aka I panic and make stupid mistake in sequencing)

My hands still being wet from shower I see a small spark and feel electrical jolt as I grab negative lead. (I thought the battery circuitry would cutoff all current to prevent overcharge... wrong) Arm retracts and disconnects from source which is now at 500 volts 300ma (remember constant current). I kill power to source and remove battery which is rather warm. I put battery in fridge to contain explosion however, there is none. I remember Where the leads touched me and notice two small white lumps... oh no :-(



So what did I do wrong/right and why am I still alive?

When I saw high voltage condition I panicked (It's 3am here way to late to charge cells)

I'm very tired and reacted without thinking (If I killed the power first no harm done to anything)

I charged a li-ion cell (protected) without being present in room

I had supply set to constant current which will supply 0-500volts at 300ma
incorrectly assuming a 750mah cell would take at least 2 hrs to charge.


Why I'm alive: I followed a bit of common sense and only used one hand, even in my panic. Such rules can save your life and a circuit through your hand is better than one through my heart. I have 2 blister burns but I'm alive, alert, etc... perfectly fine. I've done this once before around 3 years ago and while not fun (330 volts that time), follow the one hand rule and you will live to tell about it. It only takes 40ma through your heart, I had 300ma through my hand.

So now you can all yell at me and tell me how lucky I am to still be posting here... I deserve it. I made sure my leads were rated for 2000 volts, made sure to calculate with headroom how long the cell would take to charge, then went and left the thing there. If I was charging my 18aH SLA all would have been fine as that can bleed heat a lot better than an cr123a. I thought it would be interesting to see if I could charge it on the supply without thinking everything out... I payed...

Anyways, never assume and remember voltage can kill... it doesn't care who you are, what degree you have or what you ate for dinner...

Also remember current takes the path of least resistance. For me that was my middle finger to my thumb. I urge anyone who is working on similar supplies to only use one hand... it may just save your life :)

-D-Dog aka (Dumb)-Dog
 

IMSabbel

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You have been VERY VERY VERY lucky.

Disregarding the fact that you might be dead, the usage of a PSU of that caliber for tasks like charging a RCR123A is like nuking sparrows.
If _anything_ ever went wrong (and it could, as you used it in absence), you could have easily burned your house down, etc.
 

PeAK

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Re: Constant Voltage vs Constant Current vs Constant Power

D-Dog, good to see you survive the experience and take one for the forum...hope your hand has healed. I had a look at your power supply and do not see any controls to limit the voltage output while in constant current mode:
%21Bc6hOB%21%21mk%7E$%28KGrHqQH-D%21Equ5fmE5eBK2nO,KLmg%7E%7E_12.JPG


Remember what I was saying about the dangers of high voltage? Even if you are usually careful?

I'm typing this with one hand ...
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So what did I do wrong/right and why am I still alive?

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If you provided a path to ground with your feet, the "current" would possibly have travelled from your hand (then through your heart) and then to your foot. The one handed rule means that the path provided will be "local" and the worst that could happen is to lose your hand.

The idea of a safety ground (third pin in a plug) is provide this preferential path (i.e. lower resistance) rather than travel via your heart/foot.


D-Dog said:
I had supply set to constant current which will supply 0-500volts at 300ma
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Most units will have a voltage limit control that is set by open circuiting the output and then setting the limit. Remember that a current source functions by raising the voltage until the current desire current is achieved. With an open circuit it tries to raise it to an infinite number.

D-Dog said:
Anyways, never assume and remember voltage can kill...
When I supervised a lab course, I've always told the students that it is the current that kills...the voltage initiates the current, though. So have your current limit on. If the supply does not contain a current limit...throw it away!!!


D-Dog said:
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Also remember current takes the path of least resistance. For me that was my middle finger to my thumb. I urge anyone who is working on similar supplies to only use one hand... it may just save your life :)
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Correct...raise one to the "one hand rule" lovecpf

PeAK
 

D-Dog

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Just an update after a night's worth of sleep.

While what I did was undoubtedly stupid, what I did before wasn't. The sad part is that if I hadn't freaked about the voltage (being so tired) and only flipped one switch then there would have been no problem.

Now for the dummy proof (probably why the result wasn't a lot worse)

*You are right, no voltage limit in CC mode, however, I did have a choice between 500v and 1000v and went with 500 for obvious reasons.

*I located the PSU on a table above natural hand height with only enough space to the right of the psu for the battery I'm charging and one hand. This was so even if I forgot the 1hand rule it would be almost impossible to stick two in there.

*The table height is so that if you build a circuit through your hand causing the muscles to contract, the hand pulls back and after the jolt returns to an "at ease" position next to your body. Because of this I suffered no internal burns, only 2 little blisters on the surface... speaking of which...

...back to typing with both hands a mere few hours later... short exposure and 1 hand saved me plus 500v probably helped too although if it was 1000v then the current would have been lower and output power the same.

Oh yea, time to thank AW for putting such a circuit on his battery. The cell read 12.6 volts while chaging but once I took it off and let it cool dropped to 4.3 or so. After I put in the refrig to up the internal resistance it went down to 4.2 :) Still don't trust it anymore and will buy another but glad the circuit did its job.

Also thinking of putting in a voltage pot even in cc mode because I agree if I had even crude voltage adjustment (lets say I set it for 10 volts) no shock and just a ruined cell :) If anyone want to help with this it will be appreciated as I have the full schematic, however, don't really have the time right now to look into it.
 
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D-Dog

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I don't know if the :crackup:meant "just kidding" or not, however, this is the first time in 5 years I've injured myself to any extent regarding the use of high voltages and it was a lapse in concentration which did it along with trying to fit too much into one day. My point was that it doesn't matter if you are a power engineer or a DIY flashaholic, this sort of accident can happen to anyone (exploding bulbs, exploding cells come to mind right away). Many people don't want to admit to anyone they made a mistake, however, I decided to share mine to hopefully help others also thinking of cutting corners by using a supply not meant for what they are trying to do.

Also, if you were kidding than :), however, if you were serious then I don't find it amusing to toy with the idea of my life as "one less in the gene pool"...

I've decided come Monday the supply is going back as I don't have the time to add voltage regulation in CC mode and I have a much safer supply I can use in its place. I'm also using high voltage gloves from now on if I ever use CC mode on another charger which can output more than 50 volts...

The blisters last a week, however, the knowledge gained is for a lifetime, right?

Also, some people I know were talking about their night (or what they remember of it) last night. One puked on her phone, another ran into a light and than fell down a flight of stairs...all in one night while drunk. If anything they are more likely candidates... or at least for a liver transplant than the guy who completed a circuit and got 2 blisters. The guys who fell ended up in the ER while the other wished she was dead... I remained alert enough to remove my hand from the supply, kill the voltage, move the battery to a cool location and then brush my teeth :p

I don't drink so "alert enough" was in regards to going into shock or the like...

Perhaps I haven't done a great deal putting out what actually happened, however, when I say blisters I mean one the size on the tip of a pencil, the other about 1/3 the surface area of a dime. No skin was charred and my burns" don't look anything like what you see when you search electrical burn on google images. The perception of pain was very similar to a shock pen which leads me to believe the exposure was so short/ the supply turns off if it detects a short. I still don't know how the battery could possibly absorb even half its capacity in an hour at 300ma at lets say 4 volts, however, the volt meter didn't lie and the voltage of the cell was 4.4 volts resting.
 
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LuxLuthor

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D-Dog, I'm glad you are ok. Honestly, I can't imagine someone using that for lithium battery charging...just thinking casually of all the things that can go wrong...especially with magnets!

And with due respect, in your OP, your asking about constant power, and only getting a few responses before using it is not consistent with "... and yes, 1000volts is dangerous, I know what I'm dealing with :)" I don't believe you understood what you were using, nor the most elementary fundamentals of electricity to have used that PS.

I was like....tell me he didn't touch active, constant output charging leads with wet hands...even as "a being tired mistake." It's up to you, but with those knee-jerk (in a panic) dangerous reactions, I would not only count your blessings as you have....I would make sure to switch to a totally safe setup--no matter how much it costs. No way would I switch to another PS as your charging source which is your apparent next step. Given the plethora of safe lithium battery chargers, what's the point given what happened?

Please don't be offended by my comments, but I'm anticipating there will be another time you are distracted, reacting in a panic, or being forced to deal with a lithium fire, perhaps with wet hands. If for some reason you feel you MUST use a PS, at least switch to safe lithium chemistries as another step.
 

BVH

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Isn't there also a potential of back-feeding into the power supply when the batts are full or near full? Are you using a Schotkey diode to prevent this? Before I knew of this, I blew a $500 Mastec 30V, 50 Amp PS. Not charging batteries, but by making a "hand connection too slowly" to an HID ballast and missing and re-contacting. The spike backfed the PS and killed it. My new one is protected by a 60 Volt, 80 Amp Schotkey diode.
 

D-Dog

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I won't disagree with anything you said as your 100% right. I was using the supply to charge large SLA batteries and that's where I should have ended it. Those batteries (18aH) can absorb overcharge much better (at 300ma), not to mention have a voltage curve which rises much slower. I would have had a lot of time to react and would have been able to shut of the supply. The use of magnets was equally stupid ... it was because of them coupled with the small battery size that cause me to touch the leads. Also, for clarification, I completed a circuit between the anode of the battery and the conducting lead when the magnet snapped back to the battery, not lead to lead as I only had 1 hand in there :)

Don't ask why I charged a lithium battery (it was protected at least :)) with a CC charger... like I said that was really stupid. I did have a friend in the room at the time who knew to turn off the charger if anything looked fishy (by unplugging it from the wall... not touching the leads :)), however, my multimeter shuts off automatically to conserve power and thus the last reading he saw was evidently normal (I think he said 4.15 volts).

My hands were dry when I removed the leads, however, having been in the shower I can imagine the water content of the sub-skin would have been higher. I also have a Pyramid PSU which is 0-15 volts (for the SLA's) so from now on I'm using that, getting rid of this one on Monday.

You right in my stupidity and also in my being naive in terms of electricity. I know the equations and understand schematics, that's what I'm getting my degree in (thesis on the thermal efficiency of variable pulse charging), however, especially with electricity not being something you can visually quantify, until yesterday I hadn't been shocked to any degree for 5 years. That's a while, and in that time I "forgot" just how dangerous the electricity can be (as lame as that sounds). A logger has the spinning blade of a chainsaw to remind him of the danger, a pilot the armed guards and oxygen masks. With electricity you have a high voltage warning and different color wiring. We can't see potential or current, we can't observe electron drift with the naked eye. We know it's there but measuring potential from a DMM gives us a number we plug into an equation, it's not real until the difference is running through you.

Perhaps I thought I had a good enough idea of battery capacity to judge when the charge should be terminated. Perhaps I didn't think of the possibility the magnet woudl try to snap back and my hand would be in the way. Whatever the reason, I made a mistake and payed for it. I'm glad I'm fine and certainly don't understand what I was thinking when I decided to charge one more battery (a small lithium one at that). Like I said, it was a mistake that will stay with me for quite a while.
 

D-Dog

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Isn't there also a potential of back-feeding into the power supply when the batts are full or near full? Are you using a Schotkey diode to prevent this? Before I knew of this, I blew a $500 Mastec 30V, 50 Amp PS. Not charging batteries, but by making a "hand connection too slowly" to an HID ballast and missing and re-contacting. The spike backfed the PS and killed it. My new one is protected by a 60 Volt, 80 Amp Schotkey diode.

Fairly certain there is a backflow-prevention diode inline with the high voltage output... going to check the schematic now
 

BVH

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Seems to me backflow prevention should be a normally "designed in" and included component in power supplies. Wonder why not in my 1st Mastech.
 

D-Dog

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I actually don't see any main diode although there are ones preventing against reverse polarity and ones protecting most of the vital integrated circuit components. Kinda odd that it isn't present on this either although the age of the supply could be a factor. In fact I don't see over-current either although by now I"m fairly tired (the PSU is unplugged tonight ;) )
 

LuxLuthor

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D-Dog, again the most important thing is that you are ok, and we appreciate you sharing the event with us. I tried to word my post in a way that would sound "constructively merciful." ;) What really caught my attention, and still does with your last post, is now switching to a 15V Pyramid PS to charge.

Personally, I would never use my various Pyramid PS's to charge Lithium Cobalt cells. Not even my adjustable Mastec 30V 20A. Could I use them for that purpose, and understand how to monitor the charging? Yes. Will I? No. Why? Because "poop" happens. A family member has an emergency. You hear a loud crash upstairs. You see your dog running out the back door about to get skunked. 15, 20, 30+ minutes go by and you suddenly realize you forgot about your charging.

I don't see why you don't error on the side of safety and just use a lithium charger with CC/CV, and 4.2V termination. Then you are not exposing yourself to a distraction/panic situation where you would perhaps again worry about overcharge. Look at the cost effectiveness of being safe.

Obviously, as you progress in your thesis and education you will have more certainty and build in more safeguards. Please see this as me trying to be helpful and keep all of us safe. :wave:
 

Linger

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Theoretical understanding and practical application are very different.
Regardless of the exact cause, this reads a bit like a horror story. Beside the shock thing, the first 'accident' was spiking the cell above 12v. I can't ever see calculating charge times before hand to be a viable, long-term 'free lumens' plan. Maybe I'm too ignorant of this area to see the deeper implications, but i'm completely unconvinced by any of the 'safety' measures described.
Some places engineer for 4 levels of safety, each action in a sequence works to protect you during the subsequent steps. Even if a single step fails completely it is so contained by the previous actions that it is not possible for any harm to come. A person would have to fail three procedures before the next step even exposes them to personal risk. Ideally the system is designed that you may not proceed unless the previous safeguard has been successfuly initiated. In this ideology, you have to perform a complex set of actions to successfully put yourself at risk.
In your current set-up, you have to perform a complicated set of actions to remain safe.
 
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