Change the trim pot on a charger to get more range?

DanGT86

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So I bought a used EMC-400 charger to bulk charge my 20s lipo pack. Previous owner said it was set to charge a 21s pack so I figured I would just turn down the output voltage. When I adjust the trim pot the lowest I can get is about 87v. I am looking for 84v and wondering if I can change the trim pot out for one with different range to lower the output. Anybody know if that will work without damaging another component?

Its a basic aluminum case bulk charger probably originally from BMS battery
 
It is the one by the output cable exit. I cant read the board in that location but its not VR1 or VR3. I would guess its VR2. It will adjust to the 86 87 threshold and then keep turning but with no change in voltage. I was thinking there might be another resistor in parallel that controls the coarse range and then the pot fine tunes it or perhaps that is a 0-5k and I need a 0-10k pot. Just guessing so I was hoping someone might know for sure before I break this one messing with it. Seems like these charger get a bit expensive past 48v but without the quality to justify the high price.

 
when you get it apart you will see that there is small resistor in series with that trimpot to ground. try changing that small resistor to a smaller value. if it goes up then use a higher value resistor instead to see how much it changes.
 
So that is the right trim pot for the voltage adjustment? I will try taking it apart and looking for that resistor. Looks like that involves getting the board off of the base of the case and probably removing the heatsink paste where components are stuck to the sides right? Is that resistor obvious by looking at the traces on the board or will it require some specific testing to locate it?

Thanks
 
DanGT86 said:
So that is the right trim pot for the voltage adjustment? I will try taking it apart and looking for that resistor. Looks like that involves getting the board off of the base of the case and probably removing the heatsink paste where components are stuck to the sides right? Is that resistor obvious by looking at the traces on the board or will it require some specific testing to locate it?

Thanks

are you capable of following the trace? the resistor is within 3 mm. if this is too difficult maybe you can just order one that is made to your specifications. just specify the voltage you want them to build the charger to.
 
I can probably locate it if it can be done visually. I just didn't know if it would be some random place on the other side of the board. I will have some time too dig into it this weekend. Thanks for the help so far.
 
I finally got a chance to work on my charger. Dnmun, It was exactly as you described. The resistor closest to the trimpot was in fact the one I needed to change. The trimpot was a 0-5k and the resistor was a 4.7k. Trimpot and the resistor appeared to be in parallel. I added a 1k ohm resistor in series with the 4.7k and now I can adjust from about 80-100v. Could be more than 100v but there is a cap that says 100v on it so I stopped there.

I did zap my hand twice with the AC side while plugging it in with the case removed. :oops: I also noticed that touching any single pin of the trimpot with even a plastic screwdriver will cause the charger to make a strange noise and the green LED flickers. Its not a problem in normal operation but its definitely unexpected. Could it really be lack of isolation from the AC and DC sides conducting through me?

Anyway, It seems to be giving me the 84v output I wanted so I'm happy. ES saves the day again. Thanks everyone.
 
dnmun said:
no, do not put glue on the trimpot.

you did good to solve that problem. most people just give up in a few seconds. well done.

Well not glue, but RTV.
Why wouldn't you do that, is there something specicif with this unit that means you shouldn't?

EDIT: just looked at the initial photo again, yeah your fine without glue I thought it was like most other power supplies and use the crummy ones that need glue or they float.
 
I hurried up and put it back together not even thinking about the glue anyway. I have heard that some of these will vibrate the trimpots around when carried on rigid racks on bikes. I am not really worried about it at the moment. I still keep a good eye on this Lipo when bulk charging and I charge through the Cycle analyst shunt so I can see the ending voltage on my screen. Also have cell-logs with high voltage alarms on the pack. If the pot moves I'll have plenty of warnings.

Glad the forum inspired me to fix it.

I actually have another charger that hasn't worked since a reverse polarity event killed it. Its one of the EM3ev 50v Li Ion chargers with the 3 voltage selector switch. It now outputs random no load voltages anywhere from 0-58v and makes a repetitive clicking noise. When its hooked up to a battery pack it just sits at the packs voltage. No current flows and the fan doesn't come on. Anyone care to take a guess at what components are dead in that one?
 
there is a zener diode across the output on most chargers that allows the reverse current to flow through the diode and it blows the fuse on the output to stop the current after a few milliseconds.
 
So my charger adventures never end. I have been using the 83v output KP charger for a while now and all has been well until just the other day. When I plugged it in smoke came out of the fan and it smelled like burning stuff. I opened it up and found the varnish burned off of the wires that are coiled around the magnet ring device. I dont know what it is called. This thing.
ring.jpg
Looks like the glue that supports it has come loose and the resulting vibration must have rubbed the insulation off of the wires. So what is it? Can I replace it? Should I just rewind it? Perhaps separate the wires and glue them to re-insulate?
Could have gotten hot and then melted the insulation but i find it ironic that its flopping around and the insulation is burned where it would rub together. Would that make sense if it shorted before traveling through the whole coil?
 
that is called a choke and it softens the pulses of current coming from the schottky diode. they often burn up. the problem is that there is a trace that runs from those 4 little diodes behind the transformer over to the 12V regulator that goes underneath it right there and when the choke cooks everything off the trace and finally contacts the trace it will make problems for the 12V regulator.

which charger needed more span on the output?
 
It was this charger that I resistor cheated by a couple of volts. Wondered if that created a mismatch with the choke. It also blew the input fuse right before this. I replaced the fuse and it started charging normally at 3 amps for about 5 seconds when i saw the smoke and unplugged it.

So maybe I can desolder it and insulate it from the possibly melted trace burned spot? Will it get to hot again? Would there be a better choice to replace it with or are they not that exact?
 
i do not understand why they get so hot. maybe someone who understands circuits a little better can explain what causes them to get hot. it is possible that changing the feedback bias on the TL494 causes it to run at a different level to push out the extra voltage into the output. the transformers are made to match a small range of output voltages.
 
i would have to speculate. if the output voltages is lower there is larger difference in voltage from the spike induced in the transformer from the amount of current in the primary that is based on the winding ratio. maybe the pulse is shorter to compensate so the schottky has trouble keeping up. i think this is where they use something called a snubber. which is a capacitor and power resistor in series across the schottky to absorb a lot of the harmonics of the diode turning on and off. they get hot too. because the current flows into and out of the cap and as it is absorbed in the resistor it puts out a lotta heat.
 
Some of that makes sense to me. Sounds like this may be the result of venturing a little outside of the design parameters. I think I will try to insulate everything and figure out how to sink more heat off of it. Maybe thermal epoxy and a cooling fin or a second fan will do it. I may just find the next weakest link that way but it seems better than redesigning a cheap charger. Any guess as to what capacitor and resistors I would use for the snubber if I were going to try that?

I took the charger to work a few times in a rack mounted bag. Im hoping vibrations just caused it to rub through that trace you mentioned and arc there heating it up. That would be nice if thats all it was.
 
you can look and even test continuity but you would know if it shorted to the trace. they make high temp silicone too that would hold it. yours does not look as burned as some i have seen.

i once did rewind a choke because a new one was like $5 or so and i bot an 8oz spool of 14 AWG magnet wire on ebay for $10 and it is enuff to rewind about 15 chokes. 16 gauge wire with 200o C thermoplastic shellac.

the choke worked but the charger never worked again even though i replaced almost every part on it.
 
I just de soldered it and put some heatshrink over the burned spots. They were all at the beginning of the coil by the legs that go into the board. I'm pretty sure its just going to melt my heatshrink but it was worth a shot. Good practice anyway. Im bad at soldering. The board looked fine under and around it. We'll see what happens.
 
Hey guys,

I have a problem with my charger EMC-400. I bought it from bms battery few years ago.
Inside of the case are a capacitor I think broken. You can see it at the picture in the red circle. I want to repair it, but I don't know what kind of element must to this place. On the board are RTH written.
I charge nearly 4 years a 16s LiFepo4 Pack with it.

I hope anyone have the same charger and can help me. Maybe you can take a picture of the element.

Thank you
 

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