current source, large voltage over load - Page 1
Forum section: electronics / beginners

author: chrome
author_rank: Regular Contributor
timestamp: May 19, 2012, 08:48:18 pm
content: So I need a current source for 200mA but the load needs to go to about 45V(max).I tried with a transistor (where the circle is) and that works great up to a certain load voltage but then it can't keep up because of the opamp only having +12V (or +15V) power, so is there anything else I can use for this?Maybe some sort of Darlington pair or something that feeds of the +50V ?

author: vxp036000
author_rank: Regular Contributor
timestamp: May 19, 2012, 08:55:40 pm
content: I would try using a common base amplifier after the op amp to increase the voltage swing going to your transistor.

author: jimmc
author_rank: Frequent Contributor
timestamp: May 19, 2012, 09:45:17 pm
content: If the load cannot be connected to the 50v rail then either refer everything to the 50v rail (gives earthed load) or use CB stage to level shift Op-Amp output. i.e.Both use PNP output transistor.Of course if the load can be connected to the 50v rail then connect other side of load to the collector in your diagram and connect the emitter direct to the 10 ohm resistor.Watch the dissipation in the output transistor.Jim

author: ejeffrey
author_rank: Super Contributor
timestamp: May 19, 2012, 10:14:10 pm
content: All good solutions. Converting to a current sink is the simplest, but the other options are fine as well.An expensive but simple solution is to use a high voltage opamp. If the power rating isn't high enough you can buffer it with an emitter follower.You could also look at power regulators instead of opamps. For instance, I think there is a high voltage version of the LM317. If that has a high enough voltage rating you could use it configured as a constant current source. This is only convenient if you want a fixed current. There may be other more modern regulators that would be suitable although high voltage ICs are not that common.

author: vxp036000
author_rank: Regular Contributor
timestamp: May 19, 2012, 10:33:02 pm
content: Actually this is probably the best way to go, using a current sink. I didn't even think about thatQuote from: ejeffrey on May 19, 2012, 10:14:10 pm

author: chrome
author_rank: Regular Contributor
timestamp: May 20, 2012, 11:37:58 am
content: The load can be connected to 50V if that makes it easier.

author: electroguy
author_rank: Regular Contributor
timestamp: May 20, 2012, 02:01:49 pm
content: Quote from: chrome on May 20, 2012, 11:37:58 amthat would make it much easier. in that case, you should be able to use most transistors/mosfets capable of handling the power dissipation and it should work. Doing it the way you had pictured originally is harder.

author: chrome
author_rank: Regular Contributor
timestamp: May 20, 2012, 05:55:55 pm
content: Oh man why didn't I think of that, I do that all the time (on/off though).

author: electroguy
author_rank: Regular Contributor
timestamp: May 21, 2012, 08:52:53 am
content: Quote from: chrome on May 20, 2012, 05:55:55 pmhehe me too!the other advantage is you can also use nearly any op amp, and can use a much lower voltage to power it, say only 5v. There is no need to get a HV op amp or power it with a voltage as high as your input voltage, etc...search the forum for "constant current" and you will find a few other threads with similar circuits (and some pitfalls/improvements)

author: Rufus
author_rank: Super Contributor
timestamp: May 21, 2012, 09:17:50 am
content: Quote from: chrome on May 19, 2012, 08:48:18 pmAn LM317 and one resistor will do that.It will add about 4v to your supply requirement.The standard version will stand 40v the HV version 60v.