Keithley 195 - regulator getting really hot - Page 1
Forum section: electronics / testgear

author: epitaxial
author_rank: Contributor
timestamp: August 28, 2020, 12:00:06 am
content: Hello all. I just picked up a Keithley 195 bench meter and it appears to work just fine except for one thing. The LM317 on the large heatsink and the accompanying bridge rectifier get too hot to touch after maybe 5 minutes. Its VR101 on the schematic and it supplies +5 for the digital side. The one lone tantalum capacitor in the entire meter is on the front panel board and it's not getting warm or reading shorted in circuit.No signs of leaking filter caps or anything else that appears discolored from heat. Can anyone else with this meter confirm how hot theirs gets? Thanks.

author: coromonadalix
author_rank: Super Contributor
timestamp: August 28, 2020, 05:49:04 am
content: They heat a lot, on a 196 i removed the regulator, redone the heatsink compund and rebolted it and resolded it and still works ok, i would check all psu caps or redo them if possible, theses meters are kinda old lolllBut if it work, leave it this way ?

author: epitaxial
author_rank: Contributor
timestamp: August 29, 2020, 03:31:04 am
content: Did a quick check of the input voltage to the regulator and its 11 volts. No wonder it gets hot they are using it to drop 6 volts! Think I will replace it with one of thesehttps://www.digikey.com/products/en?keywords=%09945-3042-ND

author: tom66
author_rank: Super Contributor
timestamp: August 29, 2020, 07:40:33 am
content: No. Don't do that. Chances are they are concerned about the noise on the output. And, it won't work anyway, you cannot replace an LM317 with a fixed 5V regulator without modifying the circuit.Measure the current going into the regulator. Let's say it is 1A. Size a resistor, in this case 3.3 ohms works OK, to drop half the voltage. Wire that in series with the regulator input - the power is now roughly dissipated in equal parts between the LM317 and your resistor.Mount the resistor far away from the regulator (for instance, use a thermal adhesive to secure it to part of the case.)

author: Kleinstein
author_rank: Super Contributor
timestamp: August 29, 2020, 10:40:41 am
content: A 6 volt drop on the regulator is not that bad. Depending on the current, the LM317 needs at least some 2-3 V. This leaves only 3-4 V reserve for ripple and a possibly lower mains voltage. So this about a correct value if one wants the meter to still work on a low mains voltage. Some 6 W would not even be excessive power for an LM317.Some of the older gear does run quite hot. So this is probably normal.

author: coromonadalix
author_rank: Super Contributor
timestamp: August 29, 2020, 11:28:13 am
content: If the meter works okay, do not fiddle or modify this or that because of the heat, dont search for problems that dont exist(s) unless there is.It was calculated and designed this way ..... you may add problems doing so, do what i said, unsolder it / un screw it from the heatsink, clean the heatsnk compund, put a new layer, re-screw and solder, thats all.This regulator is for the digital section, aka the displays, the mcu, eeprom etc .... there's a lot of stuff connected to this regulator, do not take a chance to blow something if you modify it, and it doesnt work.Theses meters only need a recapping over time and they can outlives us in many ways loll

author: epitaxial
author_rank: Contributor
timestamp: August 30, 2020, 09:22:51 pm
content: I'm aware that those drop in switching regulators will not "drop in" for an LM317. Measured the current and its taking 600ma and the regulator is dropping 6v so its dissipating 3.6w. The parts list says the bridge rectifier is rated for 5 amps but both have discolored the pcb from heat. I wouldn't call that a good design.

author: coromonadalix
author_rank: Super Contributor
timestamp: August 31, 2020, 02:17:26 am
content: You could change the bridge rectifier for a model with longer leads and have a model with an hole in it, and add an aluminum plate on it ?The regulator heatsink could be changed with a thicker one ??But as per design, your meter still work after all these years or not ? some recent stuff would not even last half of its age ....