Return of the humble Thermistor.
Repairing a Daiwa 30 ampere DC power supply.
2020 April 20
The journey starts out with the Daiwa power supply I purchased from VE3QX a few years ago popping the fuse the other morning. I wasn't surprised as the lights always dimmed when I turned it on and sometimes you could hear a loud snap as the power switch contacts closed. Rated at 30 amperes, there is a nasty back-EMF to the hefty transformer when powering it up.
I tried another fuse, just in case. The lights dimmed and the fuse popped again. Okay, there is something wrong.
It did not take long to narrow it down to the bridge rectifier. A dead short across all four contacts. It is as if they were each welded together.
It is a KBPC3502G, rated at 35 amperes, reverse voltage limit of 120V. As the power supply is rated at 30 amperes @ 13.8 volts, this seems a little light for the application. So I went looking in DigiKey. The KBPC5010 looked like an admirable replacement for only $4.65 each. 50 amperes, 1.4KV reverse voltage on the diodes.
You can read all about it at:
https://html.alldatasheet.com/html-pdf/34010/WTE/KBPC5010/47/1/KBPC5010.html if you feel inclined.
It was a bit of a struggle getting it out, and the new one in as all connections were push-on connectors that were soldered in place.
Before I finalized the connections, I fed the voltage regulator circuit with another variable power supply and thankfully, there were no other problems. Nice. I kind of expected the regulator circuit to be fried as well!
As is my habit with high power anything, I like to "ease-up" the input voltage the first time and did so without connecting the primary winding to the rectifier first. My 100-watt Variac was perfect for the job, and I monitored the output voltage of the transformer as well as input current. All went well. This step ruled out shorted windings or some other catastrophic failure in the transformer.
All connections were finished up, and the variac was used again to power it up.
Here it is working just fine. The Daiwa is back in the rack powering my Icom 7300. It is a very stately power supply, isn't it? The Gothic design versus the Frank Lloyd-Wright design so common today with power supply faces.
But the story is not over!!
I got to thinking that this power supply worked for 25 years before the rectifiers failed. Countless ON-OFF cycles. So what may have caused this? I could only think that an overcurrent or voltage would have had to have occurred for the diodes to fail into a shorted condition. The first thing to do was to measure the inrush current. I have a harness with ten turns of the "hot" wire between a plug and receptacle. I rummaged around for my old Fluke 41B electrical meter. It can measure these things. But it turned out not be be "quick" enough to catch the spike of current. Then I turned to my Fluke 189 II meter. Turns out it has MIN MAX readings that are in the microsecond level Perfect. I connected the probe from the 41B to the input of the Fluke and measured MIN MAX AC Volts. The probe generates 1mv of AC RMS per Ampere of current. My ten times coil made it a little easier to measure, as it increased the coupling ten fold. After a dozen or so on-off cycles I measure 36.5 amperes (RMS) on one event. A good starting point.
Then I remembered working on old television sets and hearing the dull thud of the degaussing coil. Didn't they use Thermistors to limit the current? Yes, that is true, and it looked something like this:
Thermistors come in two varieties, Negative Temperature Coefficient, meaning less current as they heat up, and Positive Temperature Coefficient, meaning the resistance drops as they heat up. Degauss coils had both, one is series, the other in parallel. You can read all about them at:
https://www.ametherm.com/blog/inrush-current/what-is-an-inrush-current-limiter-the-single-part-solution/
There is a curious connection to the invention of alkaline Duracells.
It is a little different, but you can see that the thermistor is a PTC version, so the resistance DROPS as it heats up, but the Varistor (sometimes an NTC thermistor was used here) worked in the reverse method, Increasing in resistance as it warmed up.
So, I looked around the web and found Ameretherm and sent a message asking for guidance on choosing the correct Thermistor to help ease the startup current on the Daiwa power supply.
Here is the response from my message to Ameretherm:
Hello David,
Tony is on vacation so I am filling in for him.
Given:
Input voltage = 126V
Output power = 13.8V x 30A ≈ 414 Watt
Max allowable inrush current on primary = fuse rating = 8.0A
Inrush current measured = 35A
Assumption:
The Duration of inrush current = one cycle of 50Hz = 20 Milliseconds.
Calculations:
Energy the thermistor needs to handle repeatedly = Instantaneous Power x
duration of inrush, Instantaneous Power = Inrush current x peak input voltage =
35Ax ( 126Vx 1.414) ≈ 6240 Watts
energy = 0.02 sec x 6240 Joules / sec ≈ 125Joules.
Steady-state = output power ÷ input voltage = 414Watt ÷ 126VAC ≈
3.30Amp
Minimum Resistance = Peak voltage ÷ fuse rating = 126VAC x 1.414 ÷ 8.0A =
22.2 Ω
Selection:
I believe MS22 20005 is a good fit for you, would you like samples?
Mehdi Samii
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VP Engineering &
Sales
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p:
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(800) 808-2434
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f:
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(775) 884-0670
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a:
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961 Fairview Drive,
Carson City, Nevada
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w:
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So I ordered some samples.
It didn't take long after reading this to wonder what the current curve looked like, so I hooked up my OWON "baby scope" (compared to the old Tektronix TDS754 behemoth on the bench) with the Fluke current clamp and set the single shot trigger and proceeded to turn the PS on and off. After a dozen or so tries this waveform was captured:
The CURRENT is 500 millivolts divide by 10 for the number of winds, yields peak current of just under 50 amperes, for roughly 2 milliseconds. The whole spike is roughly 8 msec long. Mehdi's calculations assume a longer duration of the current spike, likely for a worst-case, and it appears the choice of the Thermistor will be just fine. This value corresponds nicely with the Fluke 189 II RMS MAX reading of 36.5 Volts.
My next entry will be AFTER the thermistor is installed. it will be interesting to see the difference.
David Balcaen, VE2GYA. 2020-April 20
73 and good dx
2020 May 4
Despite the Covid-19 pandemic, the UPS truck and driver delivered my samples from Ameretherm. It did not take long to pull a tie strip out of my parts drawers and install the Thermistor in the mains lead. I seem to recall they were always mounted out in the clear in case they exploded or got really hot. A through-bolt on the transformer was quite handy:
The placement could not be simpler, place in the mains supply lead, either leg. For all intents and purposes, the resistance at running current is negligible.
But does it make a difference?
20 mv per division. Peak to Peak, 6 divisions, for 120 mv. And as this is connected to a X10 multiplier for the current rating we divide by 10 for a 12 mv peak. As the probe yields 1mv per Ampere, we have a peak measured current of 12 amperes.
So was it worth the time and effort?
You bet! Inrush current is ONE FOURTH what it was without the Thermistor. The power supply has lost the familiar "THUD" when powered up. Simply, inrush current is REDUCED.
I declare the experiment a Success!!
Every Daiwa 30A power supply ought to have one installed.
Here is the detail about the MS22 2005 thermistor I used.
