1mA constant current source:

I've got the LM 234. Should be perfect for 1ma but it gives .00266 no matter what I do. I'm working with 12vcc, what am I doing wrong?

Your (Albert Hall) pnp ccs gives me the 1ma but it drops the voltage too far. I'll try it with 2 OR 3 diodes on the base side.

But the LM 234 is hacking me off. Why does it fixate at 2.66 ma??

 

What are you doing with the LM324? Schematic please?

 

LM234 current regulator issue

(Mobile posts not shown here) I'm trying to set up a 1mA constant current source. I cannot get below 2.66 mA with an LM 234.

see the pdf attached. It must be something simple and Hugely stupid that I am doing wrong. any resistor from a jumper to meg ohms gives the same 2.66 ma.

 

see the pdf attached. It must be something simple and Hugely stupid that I am doing wrong. any resistor from a jumper to meg ohms gives the same 2.66 ma.

Your pdf shows 2.66mA flowing through a 47k load resistor from a 12V source. This is clearly impossible.
Please give us the actual schematic of the circuit where you are measuring this current.

 

I certainly appreciate the prompt response. yikes...yes...did the math. here is the set up...something is amiss.

 

.something is amiss.

In this picture you have the 47k resistor and a voltmeter in series with that resistor. A reading somewhere close to the 12V supply is what I would expect.

Replace the 47K resistor with a 1K resistor between the LM234 V- and the supply negative. Then measure the voltage across that 1k resistor.
I can't read the colour code on the current set resistor connected to the LM234, but it should be 68Ω to get 1mA.

Incidentally, the LM234 current is not constant - it is proportional to temperature.

 

one pic is the voltage the other pic is the current. Below is the ma pic in case it didn't come through on the other post. the drawing does match the bread board. when I pull out the set resistor the current reading drops to .19 ma. but I stick any resistor in there and the it settles right back at 00266

Today is 20 degrees cooler I'm my shop. days ago I had the same experience and the temp was in the 90s F.

if I fryed the chip I must have fryed both....I have two and they both do the same thing. I was hoping I had the hook up wrong. the thumbnail expands...looks like all can be seen.

 

It does not make sense to try to do this with a 47k resistor in series with the LM234.

Please try what I said before:

Replace the 47K resistor with a 1K resistor between the LM234 V- and the supply negative. Then measure the voltage across that 1k resistor.
I can't read the colour code on the current set resistor connected to the LM234, but it should be 68Ω to get 1mA.

 

Very good...it measured 1 volt over the 1K. Thank you. At least I have not fryed the chip. The bad news: I put the 47k directly to ground and read the voltage...that reveals the current to be .0003 or so. with 22K no better. Soooo....the load...the op amp divider input shown above is not compatible. I'll have to keep digging.

I can't use a simulator....do you think that inverted follower with a small gain will work just as well with drastically lower resistors around it?

I thank you so much for sticking with me on this. respond at your leisure.

Donald

 

If you want the load for the constant current to be 47k then the current must be much lower. Try a 330Ω as the LM234 current set resistor.
BTW I suspect that your meter does not measure current correctly so use the 1k load resistor and measure the voltage across it as a means of measuring the current.

 

I am trying to create a potientiameter (like a slide pot) from a string of hall effect switches (open collector outputs). I have it fabricated as a series of resistors tapped into by the open collectors. So the ground point moves along the string of resistors. The equivelent circuit is a linear pot BUT the wiper is ground. You can reference either end of the string but thats it.

Using it as the bottom of a voltage divider does not yeild a linear result.

What sort of circuit (op amp or otherwise) will allow it to source linear voltage like a pot?

Donald
new to hobby electronics

You want to build a ladder attenuator. The AD 603 http://www.analog.com/media/en/technical-documentation/data-sheets/AD603.pdf has such an attenuator built in. I think you'll want to use this idea as a template.

 

This Hall switch / slide pot, linear FET, highside motor controller with brake is now working very well. However, I am learning more and wanting to improve the circuit.

The main problem is the OFF position: There is enough current remaining to cause the main fets (4 SUP90s in parallel) to overheat when left sitting with the brake on.

2-3 causes as I understand it: the sat voltage of hall switch #0 is preventing full shutdown, the LM358 is not a good high rail op amp (-1.2V if I am reading the data sheet correctly) And the SUP90 has a -1V minimum threshold.

My work-around is another SUP90 (fet2) in series to open the main drain during brake mode. fet1 at the input helps but, only about 50%

I welcome all recommendations and comments on this circuit. Like - more bypassing caps? a better op amp? a better place to get the small gain needed. How best to get the opamp to take the main gate back up closer to VCC?

 

This Hall switch / slide pot, linear FET, highside motor controller with brake is now working very well. However, I am learning more and wanting to improve the circuit.

The main problem is the OFF position: There is enough current remaining to cause the main fets (4 SUP90s in parallel) to overheat when left sitting with the brake on.

2-3 causes as I understand it: the sat voltage of hall switch #0 is preventing full shutdown, the LM358 is not a good high rail op amp (-1.2V if I am reading the data sheet correctly) And the SUP90 has a -1V minimum threshold.

My work-around is another SUP90 (fet2) in series to open the main drain during brake mode. fet1 at the input helps but, only about 50%

I welcome all recommendations and comments on this circuit. Like - more bypassing caps? a better op amp? a better place to get the small gain needed. How best to get the opamp to take the main gate back up closer to VCC?

Super! Thanks for the update.

 

How best to get the opamp to take the main gate back up closer to VCC?

Try a 2k2 resistor from gate to Vcc.