High performance current sink using cascoded LM317 and mosfet - Page 1
Forum section: electronics / projects

author: Circlotron
author_rank: Super Contributor
timestamp: February 12, 2014, 04:06:47 am
content: Dave's blog video about a constant current source got me thinking about a constant current sink I developed back in 2002.Basically the circuit relies on the mosfet with a contant gate voltage sinking a constant current over a wide range of drain-source voltage. If the current goes up a little for example, the LM317 allows the source voltage to rise and so the current returns to the original value. The LM317 is wired as a constant current sink and has a slope resistance of about 250K ohms.To measure the performance I got a zero to 60vdc supply and attached it to the cct and put a 100R resistor in series, and put a decent meter across the resistor. Cranked up the voltage and it starts to sink current at a little over 10v. The voltage drop across the resistor (i.e. the current) stayedconstantfrom 20v to 60v (the limit of the bench supply). What do I mean by constant?? I cant remember the exact figure but it was about 51 mA so say there was 5.12345 volts across the 100R resistor. At 20v the reading dithered from 5.12345 to 5.12346. At 60v the reading *still* dithered from 5.12345 to 5.12346 so you see it changed LESS THAN 10 microvolts across 100 ohms, which means less than 0.1 microamps even though the applied voltage changed by 40 volts. R=V/I = 40/0.0000001= 400,000,000 ohms mimimum! I'm satisfied with that.-----------------------Edit -> In hindsight it would have perhaps been better to approximately measure any change in current by monitoring the voltage between mosfet source and ground. The 250,000 ohm slope resistance of the LM317 is 2500 times as sensitive as the 100R resistor when it comes to showing a change in current. 0.1 microamps change would show 0.25mv change instead of just dithering the least significant digit of the DMM which would still probably still occur even with no change in current.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: February 12, 2014, 12:06:53 pm
content: Yup, a very good way to achieve high isolation or output compliance; the downside being awful dropout voltage. Trying to optimize both isn't so easy.Tim

author: dannyf
author_rank: Super Contributor
timestamp: February 12, 2014, 12:13:56 pm
content: QuoteWhat do you think would be the value-add of that mosfet?

author: Jay_Diddy_B
author_rank: Super Contributor
timestamp: February 12, 2014, 12:48:44 pm
content: Hi,The addition of the MOSFET has two main benefits, there may be others.1) The compliance voltage can be increased above the voltage rating of the 317.2) The dissipation is split between the MOSFET and the 317. In fact the dissipation in the 317 was almost fixed. It is the sink current x (Vgs - 1.25V). Reducing the dissipation in the 317 will improve the thermal stability in the circuit.Jay_Diddy_B

author: ConKbot
author_rank: Super Contributor
timestamp: February 12, 2014, 05:36:05 pm
content: Quote from: dannyf on February 12, 2014, 12:13:56 pmThe mosfet will regulate the LM317 'IN' node to ~15-16VDC because if it rises any higher, its VGS drops, and the mosfet starts shutting off. So it finds a balance point, and it doesnt have any leakage current to skew the accuracy of the current sink. Its essentially a voltage regulator, but without any current to ground needed.So with the stated mosfet (IRF540, 100V, 130W max dissipation) and enough cooling, you could make a 1A current sink with a maximum input voltage over 100VDC

author: dannyf
author_rank: Super Contributor
timestamp: February 12, 2014, 10:07:18 pm
content: You may want to google jung's pre-regulator.