Best way to test a IRF140 and a C147B SCR - Page 1
Forum section: electronics / beginners

author: PTR_1275
author_rank: Frequent Contributor
timestamp: September 03, 2015, 10:57:28 am
content: Hi all,Please let me know if this should be in 2 separate posts. I don't normally do a lot of work at component level so am needing some assistance to confirm my thoughts. Also please excuse any incorrect terminology.I have a DC electronic load dating back to the 1980s, it has a SCR feeding into 8 x IRF140 TO3 FET's. The drains are all commoned and the source runs from a common busbar through a 15? 25w resistor. The drain is coming from the Bat- and the source bus bar is bat +. It appears that they are switching the FET's on and using the resistors as the load. The resistors aren't mounted on the heatsink, but the FET's are.Some time back the unit stopped acting as a load and when I turn it on with nothing connected to the load terminals I noticed that it was producing 14 volts (It never used to do that). I have taken out one of the IRF140 and am getting 70? between the gate and the drain (with the meter reading the same both ways). From D-S and G-S I am getting an open circuit.What is the best way to test the FET is intact gone before I get more in to replace these damaged ones?The SCR is in the circuit too, but it appears to be between the heatsink (Anode of SCR) which is Bat - and the Bat + input bus. I have no idea how to test a SCR so don't even know where to begin. If someone can point me in the right direction that would be fantastic.The load bank is a 3H industries Swinger 2 300. It was working, then I had to leave the work bench and the boss was playing around with it and now it doesn't work. I have no Idea what he could have done to it seeing as it was connected to a battery with a 50 amp load running through it and he didn't change any cables etc... so it wasn't reverse polaritied. It could just be a coincidence that after the years running before I bought it that it chose that moment to give up and die.Thank you-Chris

author: PTR_1275
author_rank: Frequent Contributor
timestamp: September 03, 2015, 11:02:42 am
content: I should mention that I do have a peak DCA75 component analyser and when I try to test the IRF140 it says there is no component connected.I got the same result with the SCR (I know that this unit can only do SCR's with a trigger current of upto 10mA and the spec sheet is saying upto 350mA, but I thought I would give it a go anyway).Here is the link to the IRF140 data sheethttp://www.irf.com/product-info/datasheets/data/irf140.pdfThis is a SCR data sheet with the C147 on ithttp://pdf1.alldatasheet.com/datasheet-pdf/view/578975/DIGITRON/C147B.html

author: Andy Watson
author_rank: Super Contributor
timestamp: September 03, 2015, 11:32:19 am
content: Quote from: PTR_1275 on September 03, 2015, 10:57:28 amIt's dead. You should read nothing (open-circuit) between the gate and either drain or source. Note, the gate-source can only withstand about 20V maximum - more voltagewillkill it.From your description of the SCR I think it is there to crowbar the supply in the event of a fault - it is unlikely to be causing the problem you describe.

author: PTR_1275
author_rank: Frequent Contributor
timestamp: September 03, 2015, 11:39:20 am
content: Thanks Andy,Now to find some replacements and get them ordered.With 0v on the source and 10V+ on the gate I am getting 10v between drain and source and the resistance is about 4Mohm. I will cut the can open out of curiosity and see what it looks like inside.Any hints on how to test the SCR?-Chris

author: Kappes Buur
author_rank: Regular Contributor
timestamp: September 03, 2015, 05:11:13 pm
content: An easy way to test an SCR or TRIAC while out of circuitEven easier is to set your DMM to diode test and substitute the test leads for the pos. supply and ground.Positive lead to A, negative lead to K.With a wire connect A to G to see continuity on the DMM readout.Of course, in the case of a TRIAC you would have T1 and T2 instead of A and K.Positive lead to T1, negative lead to T2.With a wire connect G to T2 to see continuity on the DMM readout.