Current Limiter - Can I use it on 36VDC?

[Mad Professor]

Good Day All.

I have been looking at Alexander's Ultra Simple Current Limiter
I can't see anywhere on the page or in the info file what it's max supported voltage is, But I can see that there is a post here saying that the max is 30VDC.

I have a 36VDC 7AMP power supply that I would like to use.

My stepper motors are rated at 1amp, I know this is going to need some big resistors and give off alot of heat, I am going to be using cooling fans as well.

So can I get away with using Alexander's Ultra Simple Current Limiter with my power supply?

Thanks for your time.

Best Regards.

[PMinMO]

The current limiter will work at 36V, but you will need mosfets on the PIC that are logic level mosfets rated at 100vdc.

[Mad Professor]

Ok thanks, I will order the needed parts today.

But 1st whats the math I need to work out what ohm resistor I need for the sense resistor, and it's wattage?

I am using RFP12N10L N-channel logicFET, 12A, 100V MOSFETs.

Best Regards.

[PMinMO]

The series resistors at the emitter leg set the current based on the BE junction voltage of the BD135 approximately .6 to .7V. So a .9 ohm resistor should give you around .67 to .77A. I=(Vbe/R) Wattage is I*I*R.

[Mad Professor]

Ok Thanks.

So how do I work that out to give me a current limit of 1amp? Sorry my brains on a go slow today.

Best Regards.

[PMinMO]

Between .6 and .7 ohms. One issue with that circuit is it isn't absolutely accurate as Vbe's vary between mfg codes. While 1W should suffice, I would get a 2w. Not wire wound.

[Mad Professor]

Thanks again for your reply.

Why not a wire wound type?

Best Regards.

[PMinMO]

As the pic switches phases, the inductance of the wirewoumd resistor comes into play. It derates the performance some.

[Mad Professor]

I am looking at the Schematic for Alexander's Ultra Simple Current Limiter.

And I must admit I am a little confused.

1st thing I spoted is the POT, all pots I have worked with have the wiper arm as pin 2, the Schematic has it as pin 3.

The next this I noted is that the BD135 seems to be numbered worng, and is shown as.

Pin 1 - Base.
Pin 2 - Collector.
Pin 3 - Emitter.

Looking at the datasheets.

BD135
Pin 1 - Emitter.
Pin 2 - Collector.
Pin 3 - Base.

MJ15003
Pin 1 - Base.
Pin 2 - Emitter.
Case - Collector.

Or

2N3055
Pin 1 - Base.
Pin 2 - Emitter.
Case - Collector.

So with regards to the BD135 it's right but ir's also worng at the same time, depending on how you are looking at it.

But what about the pot?

I ask as I am having problems with mine.

I am using a 0.68ohm resistor, but I have to adjust the pot all most to the full end of travel before I can get any current to the stepper motor, Using my DVM with the stepper motor in a static lock the it is pulling just over 2amps.

Current Limiter Picture (1).
Current Limiter Picture (2).
Current Limiter Picture (3).
Current Limiter Picture (4).

Best Regards.

[PMinMO]

I believe the pins on the schematic aren't the physical pins, they are the symbol pins. Many schematic and pcb tools are generic in usage of pin numbers, it's up to the user if they want to keep the physical pin number the same as the symbol pin number. If you look at the pictures it seems the pcb layout has it correct.

[Mad Professor]

Thats ok then.

It looks like you replyed to my last post just as I was editing it, so you might not of seen the problem I am having with it atm.

I am sure I have done something worng as the current limter does not seem to be working right.

I got no current output unto the pot it almost fully adjusted then the current jumps from 0amps to just over 2amps.

I have added some pictures of my board to my last post.

What have I done worng?

Best Regards.

[Mad Professor]

Looks like I have spotted my error.

Looks like I have got the Base & Emitter the worng way round on the 2N3055, Doh!

[PMinMO]

it happens....

[Mad Professor]

Good Day To You.

I am still having a bit of a problem with this current limter design.

One of the current limters I can adjust the current from 0.40amps to 1.20amps then it jumps to 3.50amps.
The other current limter I can adjust the current from 0.30amps to 0.70amps then it jumps to 3.50amps.

Even with the pots pre-set and the fan on the current read out on my DVM is always changing upto +/- 0.10amp.

When I made my new PCB I tried to keep all the tracks the same, both current limter are using the same make and model parts.

Any ideas what might be going on here?

Thanks for your time.

[PMinMO]

What transistors did you use? You might want to read this thread: http://pminmo.com/phpBB2/viewtopic.php?f=27&t=240