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2M+ ohm resistors not resisting

KrisBlueNZ

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OMG. LISTEN.

I

NEED

TO

SEE

A

CLOSE-UP

PHOTO

OF

THE

AREA

FROM

THE

CONTROLLER

CHIP

TO

THE

TRIMPOT

PINS.
 

KrisBlueNZ

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... and...

WHAT

IS

THE

RESISTANCE

MARKING

(NOT THE "3296", THE OTHER NUMBER)

ON

THE

TRIMPOT?
 

schmidtbag

Nov 8, 2012
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The photo I supplied IS close up, and I told you I can't get a better one until there's sunlight. The trimpot is on the other side, so I put the green square showing where its normally mounted. You should be able to see the tracks just fine in that image, the only thing you really can't see in that image are the numbers of everything, which I already listed to you, including the 8 pin IC on the back. I tried looking it up myself but the only result I could find was some electronics website in chinese.

I THINK the resistance marking is 103.
 

KrisBlueNZ

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OK. Well, you'll need to wait until there's sunlight then.

The area I need to see clearly is the row of component positions (most of them are empty) that is above and to the right of the green box you drew on the fourth drawing.

I need to be able to make out every track and connection in that area.

Use a bright light but avoid reflections. Use a magnifying glass with the camera if you can.
 

schmidtbag

Nov 8, 2012
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I apparently managed to figure out the problem. The trimpot actually does the reverse of what I expected - the lower the resistance, the lower the output voltage. This makes all of the symptoms I was getting so obvious now.

I found that the 10ohm resistor made the final output voltage about 35V. Then, I just put a 56K resistor at the output terminal and my multimeter reads 24V. The 56K resistor doesn't seem to heat up at all so I'm guessing it should run safely when the power source is at full load but if anyone thinks that continuing to do this is a bad idea, I can try submitting the new pictures tomorrow for further investigation.


I appreciate everyone's effort so far. And Kris, I know I might have been frustrating but for future reference if someone isn't getting the message the first time around, its probably because the person doesn't know what you're referring to. I didn't even notice the 8 pin IC until last minute (I almost argued that there wasn't one), and I had no reason to know that it was a controller. I also didn't know what you meant by the resistance marking on the trimpot until last second. I know you mentioned it in your first post but it meant nothing to me at the time because I tend to only focus on the numbers that give me datasheets (I tend to not understand what the others mean). These are all things I eventually figured out since you were pushing me to find it, but I made my initial disclosure about my crappy electronics knowledge specifically because I'm bound to miss seemingly obvious details like these.
 
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KrisBlueNZ

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You put a 56K resistor "at the output terminal"? I don't know what you mean.

And I appreciate you trying to advise me on how to communicate, but I have some advice for you. If you don't know what someone means, SAY SO. Don't just ignore questions. Especially, don't ignore the same question repeated 2, 3 or 4 times. I don't ask questions for fun, you know. It's because I need an answer. If you don't understand a question, say so, so the person asking the question can rephrase it.

If you're a newbie in a particular field, that is all the more reason to read advice CAREFULLY.

I could have told you that reducing the trimpot resistance would reduce the output voltage. That is why you can't get 24V by changing the resistance in the trimpot position. You can't reduce that resistance below zero.

I'm still happy to try to help, when you discover the mistake you have made and are prepared to listen to advice again.
 

schmidtbag

Nov 8, 2012
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You put a 56K resistor "at the output terminal"? I don't know what you mean.
If you look at the picture of the back side, you can see the in+, in-, out+, and out-. I put the resistor at the out+, and put the + multimeter terminal to that while putting the - terminal to in-.

I could have told you that reducing the trimpot resistance would reduce the output voltage. That is why you can't get 24V by changing the resistance in the trimpot position. You can't reduce that resistance below zero.
Then why didn't you? I didn't need to specify what the timpot value was in order to figure that out. Nobody this entire time (including right now) has explained why I was getting the results I was getting. When I think of increasing the resistance, I think of voltages dropping, not getting higher. Anyways, considering that the trimpot/resistor basically can't single handedly drop the voltage to 24v, I also don't understand what that controller on the bottom can do about it. I'm not saying that it isn't related, but I just don't see how a tiny un-cooled IC could have a direct relation to what I'm trying to accomplish.

I'm still happy to try to help, when you discover the mistake you have made and are prepared to listen to advice again.
What mistake? Putting the resistor at the output? And I'm not deciding to ignore advice, I'm trying out my options to see what works while I wait until the sun comes up, and so far I got desired results on my own (although, I'm not sure how safe it is to have about 120W running through a tiny resistor, but nobody has yet explained if it is or isn't).
 

davenn

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............. I also don't understand what that controller on the bottom can do about it. I'm not saying that it isn't related, but I just don't see how a tiny un-cooled IC could have a direct relation to what I'm trying to accomplish.

Thats because you, unfortunately, are into something that is way out of your depth for your current knowledge and abilities.
It controls the regulation of the voltage... without it there is NO regulation
This type of circuit is called a DC-DC switching boost regulator

You cannot just put a resistor in the output to drop the voltage as this will have the unwanted effect of severely limiting the available current to the device(s) you want to power

Kris and I said to you a number of posts ago why you cant just change the value of the trimpot, as it is only part of the voltage adjusting control. The rest is primarily done by that controller chip

Dave
 
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KrisBlueNZ

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Kris, the controller might be this (UC3843A) or something substantially similar.
Thanks, I'd already recognised that from the pin connections. He said it's a UC8848A, which I can't find anywhere. There IS a 3848, but it has 16 pins. Maybe the 8848 is a third-party 8-pin version of the 3848 with the same pinout as the 3842/3/4/5.

Ebay auction photo shows it as a 10K (103)...
Yes, I saw that. Somehow I got the idea that his board was different from the advertised board in some way. So I wanted to check that the trimpot was the same value.

If you look at the picture of the back side, you can see the in+, in-, out+, and out-. I put the resistor at the out+, and put the + multimeter terminal to that while putting the - terminal to in-.
I THINK you're saying that you put the resistor between the +OUT connection and the multimeter's positive probe. That may make the multimeter SHOW 24V, but it won't make the power supply output equal to 24V. If you connect your load after the resistor, there will be almost no current available, and the voltage will drop to (near) zero, as duke said in the previous post.

I assumed this was your mistake, but since you seemed to want to ignore my advice and waste your time, I thought it would be only fair to let you do so.

Then why didn't you? I didn't need to specify what the timpot value was in order to figure that out. Nobody this entire time (including right now) has explained why I was getting the results I was getting. When I think of increasing the resistance, I think of voltages dropping, not getting higher. Anyways, considering that the trimpot/resistor basically can't single handedly drop the voltage to 24v, I also don't understand what that controller on the bottom can do about it. I'm not saying that it isn't related, but I just don't see how a tiny un-cooled IC could have a direct relation to what I'm trying to accomplish.
Your understanding of this circuit, and basic electronic theory, is a drop in the ocean, I'm afraid. I didn't tell you that decreasing the trimpot resistance would decrease the voltage because I didn't know that you had assumed the opposite. Your explanations are not very clear.

I did however tell you TWICE that you will need to modify the board, and that you cannot get the result you want by playing around with the trimpot connections. You seem to be STILL ignoring this advice. So I don't know whether it's worth trying to tell you ANYTHING.

What mistake? Putting the resistor at the output? And I'm not deciding to ignore advice, I'm trying out my options to see what works while I wait until the sun comes up, and so far I got desired results on my own (although, I'm not sure how safe it is to have about 120W running through a tiny resistor, but nobody has yet explained if it is or isn't).
Yes, that mistake. I suggest you get some sleep. Maybe you will be able to concentrate better when you wake up.
 

CocaCola

Apr 7, 2012
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Until the OP takes some better pictures, sourced from other auctions, likely enough to figure it out if you squint... :D
 

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KrisBlueNZ

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CC, thanks for those... I can't really be sure of the connections to those component positions though. The overlay gets in the way too much. Also, I can't read the resistor values. But I did notice that the controller in that photo is a UC2843. The OP said his was an 8848; maybe it's a 3843 and he just misread it.

If you want to have a go at marking what connects to what, well, voltage feedback is on pin 2 of the controller chip. Good luck!


To the OP. If you can't wait for the sun to come up.

Does your board match the two photos that CocaCola posted?

The board in the photo has two resistors (small black things with three digits printed on them) in the area between the trimpot and the controller IC, and three empty positions side-by-side in between the two resistors. (Don't worry about the brownish capacitor.)

If your board has those two resistors in those positions, and the three side-by-side positions empty, just like in the photo, tell me the numbers on the resistor connected to the trimpot, and the other resistor, and I may be able to tell you what to do without needing a photo.
 

KrisBlueNZ

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To the OP. I'm going to sleep now. I'm sure KJ6EAD or CocaCola or someone else can tell you what to do. Perhaps you don't want to talk to me anyway. You need to reduce the resistance in series with the trimpot and/or increase the resistance from the voltage feedback pin (pin 2) to ground. The controller's Vref is 2.5V.
 

MrEE

Apr 13, 2012
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Looking at the specs on seller's ebay lisintg, it says that the Vout is from 35 to 60VDC.
So I don't think that you can get 24V , at least not using that pot alone.
 

KJ6EAD

Aug 13, 2011
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MrEE, please read the thread before posting. I guess I should have said the same to CocaCola when he posted the pot value.
 
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MrEE

Apr 13, 2012
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Ah yes, I see that now. Sorry about that. I guess I should have my morning coffee before attempting any replies.
The controller IC shows 2842 indeed, which is a current mode PWM controller. It is made by various mfrs under different prefixes. I haven't worked with this type of controllers, but I'll do some research.
 

schmidtbag

Nov 8, 2012
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My board does seem to match what cocacola posted, but the controller chip on my board is a little too difficult to read to confirm this. I'll check the voltage output of pin 2 in a minute.

Unfortunately, now its overcast outside but I tried using a lit magnifying glass and I got better pictures. This time it appears the controller is UC3843, as mentioned before. Here's the best pictures I can supply until there's better light:

http://zicronsoft.comxa.com/temp/100_2376.JPG

http://zicronsoft.comxa.com/temp/100_2378.JPG

Even with a magnifying class, I can't read the resistor values.


Reading back in the post, I do realize I was being pretty dumb throughout most of this process and I can't believe how much I missed, I guess I was just really tired. I was mostly focused on the trimpot, because that's what was giving me results. I know people have said the pot isn't what I need to focus on and "it probably won't let you go over 10k" but nobody explained WHY until I got annoying about it, and by that time I already figured it out on my own. As stated before, to me, it made sense that adding resistance will lower voltage, so I was more hung up on the trimpot than anything because I expected adding more ohms would bring me to the voltage I wanted. Even if I paid more attention to the controller, I'd still get confused (and give unwanted results) because there's 2 sources that alter the output voltage - apparently adding resistance to the controller will lower the output, but so does decreasing resistance of the pot. For someone like me makes no sense.

So, when I realized that the lower trimpot resistance gave me the lowest output, I decided to keep the 10 ohm resistor in place of the trimpot (since I don't intend to increase it until told otherwise). Since I wasn't able to get any further useful results for the rest of the night, I thought I'd try putting a resistor to out+ to see if there was a shortcut really that simple, but I posted the fact that I did this because I figured such a solution was too obvious and there might be something wrong with doing that, which apparently there is.
 

KrisBlueNZ

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OK, here's what I suggest.

Disconnect everything from the trimpot position, so there's nothing connected to those three holes at all. Now measure the resistance of the resistor closest to the trimpot. This will give you a correct reading, because (as far as I can see), there is now nothing connected to the left hand end of that resistor, so it's effectively "out of circuit" and the resistance reading cannot be affected by any other components.

Can you tell me the resistance you measure.
 
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