EMRFD Message Archive 3336

Message Date From Subject
3336 2009-07-19 13:13:46 Vojtěch Bubník current mirror as a current sink for LM317?

Currently without a bench power supply, I decided to build a simple variable voltage regulator from 1.2 to 16V with LM317 and an old laptop power supply. It sounded trivial to me until I checked the datasheet and found out, that the regulator output must sink at least 5mA. This calls for 1k potentiometer, which RadioShack does not stock. Instead to mail order the right pot, I decided to use 10k pot and sink those required 5mA somehow. The simplest solution is to sink it with a fixed resistor. To sink 5mA at 1.2V with resistor, one would burn 1.1W at 16V with the same resistor. I don't like the idea of bunch of 1/4W resistors in parallel smelling badly.

I decided to try current mirror. I found some NPN transistor pair devices in my junk box. They are made on the same die (at least I hope they are), so they are matched and they shall have the same temperature. The transistor pair I used - KCZ59 is a cold war product of Czech company Tesla. It may be a copy of some western product, but I do not known an equivalent. I drove the source side of the current mirror with about 4mA with 470Ohm resistor from 5V. The controlled side of the current mirror sank the output of LM317. The sink current changed from 4mA at 3V to about 16mA at 12V. It should be constant, right? When the output voltage was 16V, I smelled something to burn, so I probed with my finger I burned myself with the metal case of the transistor pair. I wonder what the problem may be? The transistor pair was certainly not pushed beyond limits. Vce max = 30V, Icmax = 100mA, power dissipation 0.5W.

Any other idea how to sink 4mA from 1.2-16V power supply without excessive heating? I did not find any alternative to the simple current mirror, that would work from 1.2V.

I think I will mail order the 1k pot now, but still I would like to know why I burned my fingers that I will not burn myself again.

Thanks and 73, Vojtech AB2ZA
3338 2009-07-20 12:04:44 n2toh Re: current mirror as a current sink for LM317?
one method I can think of is building a low dropout current regulator using a mosfet, a bipolar transistor , and two resistors. you should be able to get your target current over the range you specified, using a germanium bipolar device. I seem the remember the dropout was around 1.4 volts with a silic