failing transistors
failing transistors
(OP)
Newbie here. I repair machines for a living, I'm currently working on an EDM. Normally I would contact the manufacturer but I'm told they are no longer in business.
The machine has a set of 6 transistors (Hitachi 2SC1343). These can be switched into and out of the circuit depending on how much power is wanted while machining. The problem is these transistors are failing (shorted) and I'm not sure why. I had replaced the board which contained these and all 6 have failed again. Another board which had been in the machine had 4 of the 6 shorted.
I'm not looking for specifics on this machine but in general what would cause a transistor to fail shorted.
Thanks
Dan
The machine has a set of 6 transistors (Hitachi 2SC1343). These can be switched into and out of the circuit depending on how much power is wanted while machining. The problem is these transistors are failing (shorted) and I'm not sure why. I had replaced the board which contained these and all 6 have failed again. Another board which had been in the machine had 4 of the 6 shorted.
I'm not looking for specifics on this machine but in general what would cause a transistor to fail shorted.
Thanks
Dan





RE: failing transistors
OK, you are dealing with a power device in some kind of circuit. Transistors fail for the following reasons - too much voltage, too much current, too much heat. Most power transistors normally fail shorted.
Not much information provided. You need to indicate more how the transistor is used in the circuit.
I once permanetly repaired a very old CNC mill that regurarly blew the drive transistors for it's steppers. The design used a mag-amp arrangement, and everytime the incomming power had a short interruption while the machine was operating it would blow. By carefully examinaning the mag-amp transformer arrangement and measuring the voltages, I selected and placed large MOVs to clip any transients from the transformers before they reached the drive transistors. Also replaced the house-numbered drive transistors with some easily available high voltage TV sweep ones. Sometimes a design engineer is called upon to do the unusual.
RE: failing transistors
Here is what I know about how the machine works:
--power is supplied to the transistors in question.
--the transistors are switched on and off rapidly to get the proper burning at the electrode.
--when more power is needed more transistors are switched into the circuit.
--the transistors are current limited by large wire wound resistors. Each transistor has a seperate resistor.
--I measured the voltage from the emitter to the collector at 70-100 volts DC
--the transistors are all switched on and off, whether they are needed or not. When needed, a relay is energised closing a set of contacts in series with the wire wound resistor and collector. This wil then increase the power available.
Sorry for the poor description,
thanks
dan
RE: failing transistors
RE: failing transistors
Well, wire wound resistors can have inductance that then cause a 'kick' which can be very much higher than 100V.
Also you realize that the transistors are depending heavily on good cooling. If a fan is bad, or not spinning at rated speed, or a filter is dirty, or a door is left open, or the heatsink grease is wrong, or different, or the ambient temperature is high, or the electrode is bigger, or the liquid is conductive, or, or, or... You will blow a transistor which will heap the load onto its neighbors which will all blow in sequence faster and faster.
I suspect the design is poor since each T's current is not being monitored to prevent recurring sequential burnouts. If something obvious in my list isn't present you may need to do as Comcokid mentions and actually hunt down the problem and make changes to the design.
Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com
RE: failing transistors
By the way, the machine is 10-15 years old. I'm told it's been working properly until recently.
Thanks for the replies.
Dan
RE: failing transistors
Transistors aren't big fans of operating in parallel... if one burns out, the load is shifted to the remaining operational transistors. Unless they are perfectly matched, one is going to heat up just a bit more than the others, causing a larger portion of the current to flow through that one unit. Thermal overload quickly ensues on that one unit, blowing it, and a dominoe effect is seen on the remaining units.
When it comes to current sharing, FETs are a more reasonable choice. IGBTs are another possible choice, though I have nearly zero experience using/designing with them and could not provide much useful info on them.
I'm curious as to why the original designers kept the transistors turned on all of the time and switched them into the circuit with relays. Seems like adding in an unnecessary step, and the relays are probably bulky/pricey.
Dan - Owner
http://www.Hi-TecDesigns.com
RE: failing transistors
RE: failing transistors
-The transistors are not getting hot, only lukewarm when touched. All cooling fans are working.
-The transistors and the associated switching circuitry are all located on the same board. This board has been replaced and has failed since being replaced.
-There is a single wire with the on/off pulse stream coming into this board, all the transistors are switched on/off at the same time.
-Each transistor has a large wire wound resistor in series with the collector terminal, these resistors are 13 ohm. The voltage between the collector and emitter is 100v DC.
-each resistor has a reverse voltage diode across it, these all are on the board with the transistors, so they have been replaced.
-The only other voltages coming into this circuit board are two lines with +8 volts dc and -8 volts dc. These supply power to a few transistors used to turn on/off the power transistors.
thanks
Dan
RE: failing transistors
Make sure the reverse diodes across the transistors are fast enough to conduct quickly. 1N400x power diodes are fine for 60 Hz rectification, but don't always do the job for fast inductive spikes.
The general specs I found on the transistor indicated 150V. If you're measuring 100 Volts, this doesn't leave much margin. I went to Digikey, searched "transistors" and filtered for the highest voltage and current TO-3 case NPN transistors and saw there were heafty choices available at around $5 each. If you choose this route, do a little more research on your original transistor before selecting a better replacement.
RE: failing transistors
It's sounding like a voltage hit or bad drive. 100V is pretty easy to come up with voltage wise. You can get 250V 16A NPNs.
But the drive problem could easily explain the multiple failures as it would effect all the transistors. It might only be an intermittent drive problem that would toast the transistors in a second or two but otherwise they run cool.
Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com
RE: failing transistors
I would agree that a drive problem could explain the failures except that one of the boards (with the 6 transistors) has 2 of the 6 transistors still working perfectly. Wouldn't all of the transistors fail? Maybe it's justs coincidence that 2 haven't.
dan
RE: failing transistors
"The only other voltages coming into this circuit board are two lines with +8 volts dc and -8 volts dc. These supply power to a few transistors used to turn on/off the power transistors."
It depends on how the drive is realized. Could be that once a few Ts are cooked the weak drive may then be enough to properly drive the remainders.
Can you say more about the drive? Is there a drive transistor per main T or are they all driven by one?
Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com
RE: failing transistors
thanks again
dan
RE: failing transistors
You might also check out the +/- 8 Volt supply caps as well and any electrolytics in the drive circuits, but if the issue was here you would probably be complaining that the transistor were not turning on hard enough rather than blowing.
RE: failing transistors
Dan - Owner
http://www.Hi-TecDesigns.com
RE: failing transistors
RE: failing transistors
The last time I was able to work on the machine I noticed that the negative side of the +/-8 volts was low, about 5.5 to 6.0 volts dc. The transistors (the two left that hadn't failed) still ran cool.
Dan
RE: failing transistors
If drive power is flakey, all 6 running may make for a set of hot transistors. Once a few blow, it all cools down.
Dan - Owner
http://www.Hi-TecDesigns.com
RE: failing transistors
Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com
RE: failing transistors
dan
RE: failing transistors
http://i10
Also I was wrong, the transistors are switched in banks of 3 not 6.
dan
RE: failing transistors
You're saying you have +/-8V but the schematic is calling for 10V that will reduce drive!
Any chance of you putting a scope on the board while it operates? That is where I'd be trying to go.
Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com
RE: failing transistors
I may be able to get a scope on the board. At this point though the customer may not want to spend any more money trying to fix this machine unless a solution appears forthcoming.
I wish this guy was next door, I'd spend my free time working on it. I love a good problem. Thats the reason I lurk on this and similar boards.
dan
RE: failing transistors
RE: failing transistors
RE: failing transistors
Any given transistor has max permissible values of Ic and Vce but none of them can pass max current at max volts for long (micro seconds or less) without giving up on life.
The manufacturers data sheet should provide an indication of how cruel you can be to their transistors.
The instantaneous Vce x Ic product is usualy much lower than the maximum continuous power dissipation and can be a common cause of failiure when power amplifiers are connected to reactive loads.
When this breakdown mechanism takes hold there may often be no apparent overheating or easy to find cause.
The poor driver supply voltage is a very likly candidate for you EDM machines repeated consumption of power devices wether its due to second breakdown or plain old overload.
There are may big fat NPNs out there, For the audio brigade On semi make some low saturation voltage "beta sustain" devices with Vce >250v, Ic max >16A with Hfe >100, and a genrous second breakdown area.
M D lamb
RE: failing transistors
RE: failing transistors
RE: failing transistors
I'll look at the schematic but when was the last time it worked, does it current limit?
RE: failing transistors
RE: failing transistors
Hope this helps,
Scott
In a hundred years, it isn't going to matter anyway.
RE: failing transistors
Bob D
RE: failing transistors
RE: failing transistors
Bob D
RE: failing transistors
To answer a couple of questions:
-The low voltage supply is unregulated. It consists of a transformer, bridge rectifier and a pair of caps.
-There are 2 electrolytic caps on the transistor board.
-I don't know the value of the four resistors in the circuit with each transistor. I did take a picture of the board the last time I was at the customer's shop, you can see the board here.
http://i
dan
RE: failing transistors
The negative voltage is low. With this circuit it seems to rely on the negative voltage to charge the caps in the gate drive circuit to actually turn-on the transistors. So, low -ve supply means less base drive.
The circuit relies on the capacitors in the final part of the gate drive to turn-on the transistors. Failing caps would again mean less base drive.
If transistors fail at random then it points to problem 1 and if the same transistors keep failing it points to problem 2.
It appears the output of the final driving stage approximately swings from -10V to 0V as it is switched.
Overall, troubleshooting it further seems fairly pointless without using an oscilloscope to watch the gate drive circuitry. It's not a complex circuit so once you can find the problem the solution should be easy.
RE: failing transistors
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"It's the questions that drive us"
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RE: failing transistors
RE: failing transistors
RE: failing transistors
Bob D
RE: failing transistors
dan