Help with PSU (Temp control fan, load bank, & PWM circuit)

[ronv]

The part would go in series with the positive lead of the PSU. It starts out at a high resistance (0.5 ohms) This would keep the current to 50 amps even if the supply is shorted (or charging caps like () said). But this current makes it heat up and as it does its resistance drops to 0.01 ohms so you don't loose much voltage. Its known as inrush limiting.

 

[()blivion]

The part would go in series with the positive lead of the PSU. It starts out at a high resistance (0.5 ohms) This would keep the current to 50 amps even if the supply is shorted (or charging caps like () said). But this current makes it heat up and as it does its resistance drops to 0.01 ohms so you don't loose much voltage. Its known as inrush limiting.

jocanon
Remember my fan controller write up? Remember the NTC thermistors I say NOT to use in that write up? These parts are those thermistors.

This image...
View attachment 66551

 

[jocanon]

Ah, ok. I am thinking that at $6 each I may just stick with telling people they need to plug in their charger before turning the power on. Most are already use to doing this anyway. Also, there is not much room inside the PSU to do too many mods. The fan control is going to take up most all the "extra" room there is. I am just not sure how I would run it in series with the positive terminal too easily.

On another note, since there is so little current going through all the logic circuits, I wonder if I could get away with going down to a very small 32 gauge wire to make all the connections between the Op-Amps and gates to the FETs?

 

[()blivion]

A quick search shows that 32 gauge wire can handle up to ~500mA (1/2 an Amp) of current before melting. I'm guessing that the logic's for our circuit will draw less than this. But wiring the power and ground for all the logic's up with the thicker stuff would be best.

 

[jocanon]

OK, I'll just go with 26 gauge. I got some of that from the hobby shop yesterday. I said 22 gauge in my previous post, but it's actually 26 gauge that they had.

 

[jocanon]

For ground, I was thinking of just stripping all the insulation off of some 10 AWG wire and soldering the bare copper wire directly to the PCB in a long strip. Then I can connect all the common grounds to it since the logic and the DUT share the same ground connection. Does this sound like a good idea? Or is there a better way to accomplish the same thing?

 

[()blivion]

For ground, I was thinking of just stripping all the insulation off of some 10 AWG wire and soldering the bare copper wire directly to the PCB in a long strip. Then I can connect all the common grounds to it since the logic and the DUT share the same ground connection. Does this sound like a good idea? Or is there a better way to accomplish the same thing?

Hummm... Ain't rocket surgery... So I would say, do what ever works for ya

 

[jocanon]

cool, thanks.

 

[jocanon]

So if I do decide to do the inrush limiting as ronv described, could I install it in one of these? That would give me the option of putting it in series with the positive terminial, but on the outside the PSU (since there is not a lot of room in there). I imagine something like in the picture would probably be quite expensive as well, but just want to check. I don't even know what it is called to search for it online. What would you call a hard housing like the one in the picture that is in series with the computer wire?

 

[jocanon]

OK, so I answered my own question...it's a ferrite snap bead used to put ferrites in that reduce electrical noise. Hm, I wonder if I would be able to put that inrush limitor inside one...

**broken link removed**

Edit:
Just checked dimensions, that snap bead is way too small for the inrush limitor. If I decide to do that I can look for one large enough, unless it's just an overall bad idea that won't work.

 

[jocanon]

...If you ever forget it is on and just pull the plug on the supply the arc will probably blow stuff up. A single MOV or TVS on the load box from +24 to ground might help...
On another note. It might not be to hard to disable the decimal point on the little volt meters if we knew where that pin was.

To the first point, I haven't ordered an MOV or TVS. I wonder if I can just not unplug it when I have the POT turned all the way up but still turned on, would that fix the potential problem? Or should I just put an MOV or TVS into the dummy load circuit, if so which one?

To the second point, I will try to find the pin by playing around with it tonight. Any pointers are welcome on where I might start, or how I might find which pin disables the decimal point.

Thanks,
Jeremy

 

[()blivion]

To the second point, I will try to find the pin by playing around with it tonight. Any pointers are welcome on where I might start, or how I might find which pin disables the decimal point.

You most likely will have to carefully take the glass screen off of the board, find what trace goes to the decimal segment of the screen, and cut it. If you look at the screen in just the right light you will be able to see the clear conductor they place on the glass. It has a slightly different refractive index than the glass and thus will exhibit total internal reflection at the right angle. This will tell you which pad powers the decimal point. Match that to the circuit board and you have the wire you need to cut.

There may be other ways to go about it too, but that will entirely depend on exactly how they made the circuit. Also the above may be impossible depending on how the screen is attached to the circuit board. I would need detailed pictures to know for sure and walk you through the mod.

-()blivion

 

[jocanon]

I am still at work right now. Hopefully I can get out of here not too late and then I will see what I can figure out and send you pictures if I need help. Luckily I ordered 3 of them and only need two for this project, would be nice to have an extra, but if one does get ruined when I try to take off the screen then I guess that's OK.

 

[()blivion]

If you have three, and you don't mind waiting. You could mail the extra unit to me and I could do the mod for you. Free of charge of course.

Other news
I'm working on my own P-channel version of this circuit now actually. Mine is going to be primarily for computer PSU's, but I am going to have an adapter and jumpers for ganging all the channels together so you could do the same thing your doing. Mine is also going to have a Microcontroller (single chip computer) and custom LCD I'm thinking. Since I can do that kind of stuff

 

[jocanon]

Cool, that sounds like it will be awesome. I'll see what I can do with the volt meter, if I can't get it to work maybe I will take you up on that offer

 

[jocanon]

Now you’re making me jealous wishing I would have waited and built a P-channel version...but then again, mine will work perfectly fine for my purposes (hopefully ...crossing fingers I don't screw it up somehow lol).

 

[jocanon]

I don't see any way to take it apart without damaging it. Funny thing though, I tested them out and one of them already has the decimal disabled! Factory defect I guess but that's what I wanted in this case. The decimal does light up on the other two . Really not a big deal, if it can't be easily turned off I'll just deal with it.

 

[jocanon]

ronv, I was going through the all the parts I ordered last night and there are 10 .1uF capacitors you had me order which I don't see in the schematic attached. Do you know where these are suppose to go?

Edit:
There are 10 of them.

 

[()blivion]

Capaitors
The 10X 10uF caps go across the OP-amps power rails. They are just filter caps to keep noise out.

Screen Mod
As for your screen, I didn't realize they were 7 segment LED displays. I though they were LCD. But that just makes it easier.

Of course, I say that... but as I look at the pics, I count 24 LED's and only 12 pins for them. This means that it is using some kind of multiplexing scheme, and we need to get voltage readings from those pins to ground to know the scheme for sure. Here is what I want you to do. Number all the pins 1-12, Power the screen up and try to get something meaning full on the screen, put the black probe of a multimeter on the ground connection, then start touching the positive probe to all the pins and write down the voltages. Then do this again with some other numbers displayed on the screen. With this kind of information we would be better able tell you what wire to unsolder/cut.

Edit; The obvious most interesting light to get to turn on and off while testing would be the decimal point, but that likely doesn't change.

Do note though that if the decimal point is multiplexed with a segment that we are using, then we may have to put a diode inline with it to block voltage to one LED, but allow the other to work. Or we may not be able to do it at all. Multiplexing schemes can be like that some times.

P-Channel project
Here are some pictures of my version of the project... (unfinished of course)

View attachment 66657

View attachment 66658

Note again that this project is designed for a different set of circumstances than yours. As such it would not be optimum for your needs. First, it is designed to accept more than one voltage from the same supply at a time. It's also going to have to test for a tighter compliance than your system has to worry about. The ATX standard has most rails at no more than -+5% voltage change over the full load range. Your chargers can handle much more deviation than this. This project also has to measure ripple, computers can't have much ripple on the main power rails. I may have provisions installed for temperature monitoring, load and line monitoring, and efficiency measurement capability. But that kind of stuff is external to the dummy load and would require an "apparatus" or "jig" for setting the DUT into. A better overall product, but much more complicated and less user friendly.

 

[jocanon]

Looking good ()blivion.

Did I put the capacitors in the right spot, see attachment, option 1 or 2 (maybe it doesn't matter)? Also, what about the logic circuit Op-Amps, should I put capacitors there too (I know U1 is actually a comparitor).