CurrentLoad Question

I have been wanting to build a Adjustable Constant Current Load project and I have a couple questions ... The schematic is listed below and do you guys see any problems or anything can be upgraded or should ??? My idea was to change the mechanical switch to a soft latching power switch ... Then I will be using dual display instead of using the single display and a switch... I was thinking about using battery power powered but I have not done the math to see how long the battery will last or was it even worth or just build a power supply board ??? Whats your thoughts ???







Here is design I am working off (http://paulorenato.com/joomla/index.php?option=com_content&view=article&id=91&Itemid=4
).............
Also here is the schematic ..........

There are a few things:
FETs don't like to run in linear mode and in parallel like they do as switches due to the range of gate threshold voltages. So each one should have it's own load resistor and op amp driver.
R7 and C5 don't do anything.
The combination of R11, C7 seems large. I would use 75 ohms and no capacitor. You can probably get some ideas by googling driving capacitive loads with op amps.
60 volts at 7 amps is 420 watts. That is a lot of heat to get rid of. I helped a guy build one where he soldered the FETs to a water pipe and let tap water run thru it. He was doing 1200 watts with 10 fets, but with a safety margin.

The end result I am looking for is a 0v to 30v and 0amp to 5amp constant current load ...

Hey Jason,

MJ Lorton did a nice series of videos on DC electronic loads, I found them very enlightening.

HTH Steve.

The trick is to keep the FET cool.

ronv said:

The trick is to keep the FET cool.

Click to expand...

I know and right now I have a heat sink that can handle a 100 w..
Right now I continue the research for more information and the schematic is being modded as I need and will post up later for structure criticism ...







Also I use to in Tucson when I was a kid...

An interesting project, and one that I could probably use in my lab. I'll keep an eye on your progress. Fans, heatsinks, and 5A 12VDC power supplies I have in abundance.

I stared working on this but not sure how it will work ????

Almost there.
Remove the 10Ufd. caps.
Change the 10Ks to 4.7K
You can remove the buffer and just run the wiper of the 50K pot to the + inputs of the 324's.
Add a 75 0r 100 ohm resistor in series with the gate of each fet.
I think I would also calculate the maximum current I wanted and add a resistor to the top side of the pot so it couldn't ask for more.

ronv said:

Almost there.
Remove the 10Ufd. caps.
Change the 10Ks to 4.7K
You can remove the buffer and just run the wiper of the 50K pot to the + inputs of the 324's.
Add a 75 0r 100 ohm resistor in series with the gate of each fet.
I think I would also calculate the maximum current I wanted and add a resistor to the top side of the pot so it couldn't ask for more.

Click to expand...

Thanks for input
The pot after the buffer is to limit the amount of current ...
Would you change anything else ???



Thanks
Jason Sr

Does anyone know where I can get a nice voltage and amp display in a single display ???
I know I could make one out of a pic chip but the programing is way above my head right now ..


Update
Also with the display or displays could I run everything off a 9v battery or would I be better with a wall wart ???
I want to update my version with a power Led and a Load Led too.. But do you guys think the circuit needs anything else like a overload circuit or maybe a fan controller ???


Thanks
Jason Sr

I was designing something similar, but a lot simpler. I basically dropped due to lack of interest on my part. I'm going to be tonight, but I'll study it a bit more when I get up.

You can use one display, with a switch. Most display modules need an isolated power supply, but I remember a design Wookie drew that filled the bill. It used a 555 for a simple DC-DC convertor that was also a power supply isolator.

Bill_Marsden said:

I was designing something similar, but a lot simpler. I basically dropped due to lack of interest on my part. I'm going to be tonight, but I'll study it a bit more when I get up.

Click to expand...

Can you post up schematic later or paper work ???

Bill_Marsden said:

You can use one display, with a switch. Most display modules need an isolated power supply, but I remember a design Wookie drew that filled the bill. It used a 555 for a simple DC-DC convertor that was also a power supply isolator.

Click to expand...

I was thinking about using one of those cheap power meters and opening it up and mounting it inside a case and I will check Wookie's design if I can find it but will look ..



Thanks
Jason SR

This is not done but pretty close I hope ...

Take a look at this schematic.

My Chinese is not so good, but I think this meter would work.

http://www.ebay.com/itm/100V-20A-sh...689?pt=LH_DefaultDomain_0&hash=item2ecc894891

Sorry to bring up a old thread but I am still working on this and learning..
But I have noticed that alot of people use a 10 turn 50k pot for there adjustments.. Why is that and why use that value ?? MJLorton







Thanks
Jason Sr

10 turn pot will allow finer adjustment than a single turn one

R!f@@ said:

10 turn pot will allow finer adjustment than a single turn one

Click to expand...

Sorry
I know that But I would like to know why the value is 50k ???

re post #8 diagram.
the pot after the buffer wont really do anything, anything out of the buffer will go to ground at any position of the pot.

Any value would work there if you know what I mean.
But higher values would not load the PSU...

Wait...Ur regulator is 78L05...means it can supply 100mA safely. So lower pots will load it too much