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How do I create a 5 second soft-start with an LM334Z current regulator? Answered

Hey all,

A bit of a newbie to creating circuits

I have a 12V battery regulated to 2mA with an LM334Z current regulator.
Here are the docs for the current regulator: http://www.ti.com/lit/ds/symlink/lm134.pdf

I would like to create a soft start, so that when I turn the device on there is not a rush of current. 

I’d like it to ramp up over 5 seconds.

I’ve breadboarded a few schematics I found online but I can’t get them working. Even if the battery power is slowly ramped up, the LM334 rams the voltage to get the current at exactly 2mA. (my Rset w/ a 33ohm resistor)

Here’s a couple I’ve tried unsuccessfully.
http://dl.dropbox.com/u/5822/softstart1.gif (might be b/c the 317 is different?)

I even tried this schematic to no avail:
http://dl.dropbox.com/u/5822/Screen%20Shot%202012-04-29%20at%209.16.57%20AM.png

But I was using different transistors, didn’t think that mattered- let me know if it does

Can anyone help me figure this out? I’ve been stumped for days. Starting to pull my hair out :)

Thanks
Nick

6 Replies

user
ggauthier (author)2012-08-29

I see this was a while ago and you might have already built your tDCS unit. Any luck with results?

But to answer your question, you can use a capacitor between the Anode and Cathode, something between 400-500uF. This will not only do a current ramp up, but when you open the switch it will do a ramp down with the remaining store in the capacitor.

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user
steveastrouk (author)2012-04-29

Why are you ramping CURRENT here ?

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user
tontoa4 (author)steveastrouk2012-04-29

Because the abrupt change in current is too much for the load to bear. Any ideas?

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steveastrouk (author)tontoa42012-04-29

Do you mean current, or do you mean voltage ? What are you driving a constant current into ? There are very few loads that need a constant current drive, so I'm curious.

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tontoa4 (author)steveastrouk2012-04-30

It's a medical-type device I've been working on

And honestly the rush of current irritates the body

But it's ok when current is risen gradually

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steveastrouk (author)tontoa42012-04-30

Use an LM200 as the regulator.

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