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Simple, no-math transformer snubber using Quasimodo test-jig
Simple, no-math transformer snubber using Quasimodo test-jig
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Old 1st October 2018, 05:03 PM   #1351
Mark Johnson is offline Mark Johnson  United States
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Simple, no-math transformer snubber using Quasimodo test-jig
Yes the full EE math with pencil and paper predicts 100V RMS across your snubber resistor and 2 watts of RMS power dissipation.

LTSPICE simulation gives the same result, schematic below.

It's wonderful that your 3W resistor is very efficient at dissipating heat into the surrounding air, without its case becoming uncomfortably hot. Some 3W resistors are not so efficient, and their exterior case does become very very hot.
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Old 1st October 2018, 05:54 PM   #1352
fralippo is offline fralippo  Italy
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Many thanks Mark for the lightning fast check!

The resistor is a cheap Vishay PR03 series 3W metal film one. I think I'll have better to check its temp once again and probably anyway replace it by a 6W one! That would be a much better and suitable ratio IHMO.
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Old 1st October 2018, 06:02 PM   #1353
JeffYoung is offline JeffYoung  Ireland
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Simple, no-math transformer snubber using Quasimodo test-jig
"True RMS" DVM? An "averaging" DVM can't accurately measure any AC that isn't sinusoidal.
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Old 1st October 2018, 06:04 PM   #1354
JeffYoung is offline JeffYoung  Ireland
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Simple, no-math transformer snubber using Quasimodo test-jig
Oops. Didn't see the next page of replies.

I'll take it from Mark's confirming-data-point that you're using a True RMS DVM.
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Old 1st October 2018, 09:01 PM   #1355
BenY is offline BenY
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Quote:
Originally Posted by fralippo View Post
Many thanks Mark for the lightning fast check!

The resistor is a cheap Vishay PR03 series 3W metal film one. I think I'll have better to check its temp once again and probably anyway replace it by a 6W one! That would be a much better and suitable ratio IHMO.
This resistor (PR03) is fusible and it might blow without you even noticing …
You better find a better resistor for the job.
Metal oxide for example.
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Old 2nd October 2018, 09:37 AM   #1356
fralippo is offline fralippo  Italy
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As far as I can understand from the resistor's datasheet the fusing features are only related to safety issues. There are no indications about maximum currents. So it seems to be especially suitable for the specific usage.
Still I'd like to install a 6W one later.
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Old 9th October 2018, 07:22 PM   #1357
fralippo is offline fralippo  Italy
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Today I connected my Quasimodo to a 250-0-250V transformer used for the tube section of my DAC. A 600R resistor is required on each leg. What's the expected dissipated power? Could the math behind be published here please?
Thanks!
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Old 11th October 2018, 12:40 AM   #1358
Mark Johnson is offline Mark Johnson  United States
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Simple, no-math transformer snubber using Quasimodo test-jig
Here is the math to calculate power dissipated in the snubber resistor. It is typical of a homework problem given to 2nd year EE undergraduates taking their first Circuit Analysis class.
  • R is the snubber resistance in ohms. In the example of post #1350, R=4700
  • C is the series capacitor "Cs" in farads. In the example, C = 1.5E-7 (0.15uF)
  • Vin is the secondary RMS voltage in volts. In the example, Vin = 460
  • f is the mains frequency in Hertz. In the example, f = 50
You may decide that the LTSPICE simulation method shown in post #1351, has much to recommend it after all. But either path will lead to your final objective.

Please remember to make the actual resistor in real life, at least 2X and preferably 3X the wattage, compared to the calculated power. This gives a margin of safety for reliability and a long, long service life.

_
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Old 13th October 2018, 01:00 AM   #1359
fralippo is offline fralippo  Italy
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Thank you Mark!
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Old 14th October 2018, 03:34 PM   #1360
dsolodov is offline dsolodov  United States
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Have Quasimodo V4 through hole version kits for sale in stock. The kit is $36.08 which includes free shipping to the lower states and PayPal fees. The kit is standard BOM. The boards are with ENIG finish. Parts are labeled and packaged in small plastic bags.

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Have Quasimodo V4 through hole version kits for sale in stock. The kit is 29USD with free shipping to the lower states. The kit is standard BOM. The boards are with ENIG finish. Parts are labeled and packaged in small plastic bags.
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