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Old 4th Oct 2019, 10:36 am   #1
bennybrc
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Default Bush SRP31D. Series resistor.

Hello
Would anyone be able to tell me what ohm and wattage resistor I need in series with the 1N4007 diode? I only have a 2 watt 270ohm in my spare box but I'm not sure if that would be suitable
Many thanks
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Old 4th Oct 2019, 11:31 am   #2
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Default Re: Bush SRP31D. Series resistor.

Try it and measure the resultant voltage and resistor temperature.
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Old 4th Oct 2019, 3:07 pm   #3
Michael Maurice
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Default Re: Bush SRP31D. Series resistor.

270 ohm should be fine, my preference is 180 ohms
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Old 4th Oct 2019, 5:19 pm   #4
bennybrc
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Default Re: Bush SRP31D. Series resistor.

Hi
Thanks for that! I did a bush a few years back but I couldn't remember what resistor I needed
Thanks again
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Old 4th Oct 2019, 8:36 pm   #5
Radio tirana
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Default Re: Bush SRP31D. Series resistor.

HI Bennybrc
I would recommend a higher wattage resistor by my calculation the dissipation is likely to be nearer 4W. I usually fit 7w wire wound resistors in this application
the exact value is best determined by measuring the resultant HT voltage aiming to get as near to the voltage referenced in the service sheet.

Kind regards RT
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Old 4th Oct 2019, 8:47 pm   #6
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Default Re: Bush SRP31D. Series resistor.

Quote:
Originally Posted by Radio tirana View Post
HI Bennybrc
I would recommend a higher wattage resistor by my calculation the dissipation is likely to be nearer 4W.
Kind regards RT
How do you arrive at nearer 4W?
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Old 4th Oct 2019, 9:15 pm   #7
Radio tirana
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Default Re: Bush SRP31D. Series resistor.

Hi, sorry should have said estimation not calculation, not easy as its not quite a dc waveform. I know that a 4w resistor used here gets pretty hot. I would recomend a higher wattage resistor in the interest of reliability.

Regards RT
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Old 4th Oct 2019, 10:38 pm   #8
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Default Re: Bush SRP31D. Series resistor.

A rule of thumb is to allow for the dissipation due to the ripple current to be three times greater than that calculated using the DC load current, I would then double the total figure for the resistors wattage rating for a happy resistor and any of the resistors neighbours.

In the schematic I'm looking at the no signal DC load current is approx. 45mA (by calculation)

Lawrence.
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Old 4th Oct 2019, 11:03 pm   #9
TrevorG3VLF
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Default Re: Bush SRP31D. Series resistor.

Why are you planning to use a diode and resistor?
I have had a bit of a look at the circuit diagram. You are replacing a STD C2D selenium rectifier with a votage drop of 1V? with a silicon diode with about the same voltage drop so no resistor should be necessary. Remember that the transformer will have a resistance approximtely the sum of the primary and secondary resistances.

A better replacement would be a UF4007 which switches faster and produces less noise than the 1N4007. It would be nice to know if any difference could be ascertained.

A resistor passing current for one fifth of the time will drop five times the voltage and dissipate five times the power compared to when it is passing the same average steady current.

Selenium rectifiers I have seen go high resistance in the forward direction and a diode placed across them will keep them cool and may allow sufficient reverse capability. This is a quick bodge with no extra terminal needed.

Last edited by TrevorG3VLF; 4th Oct 2019 at 11:06 pm. Reason: spelin
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Old 23rd Oct 2019, 8:14 am   #10
bennybrc
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Default Re: Bush SRP31D. Series resistor.

Hi all.

I used a 270 ohm 5watt in the end as I thought 2 watt wasn't enough as they can run hot. I've also changed the 2 Plessey red and black caps and also the .02uF Hunts cap. It now sounds great! Been running it for about an hour and all seems very well indeed. Just needs a new bulb.

Ben
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