Motorola ts907 low hv - Page 51 - Videokarma.org TV - Video

old_tv_nut · #**751** 10-23-2022, 05:07 PM

I don't know where you got the 135v rating, but forget it.

It's a 3 watt 2700 ohm resistor, and will dissipate 3 watts when the voltage is 90 volts DC. (P = (V^2)/R). So, if the specified wattage rating is 3 watts, the circuit was designed to apply less than 90 volts to that resistor. Of course, a circuit fault might put more voltage across a resistor than intended, and that would have to be fixed, and the resistor would indicate that by overheating and smoking.

Edit: resistor voltage ratings are usually for momentary voltage that might cause the resistor to arc over. Typical max voltage rating for discrete resistors might be 200-600 volts, but as you see, for DC, the wattage rating may be exceeded long before the voltage is. AC or pulse current will be different, as r.m.s. current determines the power while the peak voltage determines whether the voltage rating is exceeded.

timmy · #**752** 10-23-2022, 05:22 PM

Quote:

Originally Posted by old_tv_nut

I don't know where you got the 135v rating, but forget it.

It's a 3 watt 2700 ohm resistor, and will dissipate 3 watts when the voltage is 90 volts DC. (P = (V^2)/R). So, if the specified wattage rating is 3 watts, the circuit was designed to apply less than 90 volts to that resistor. Of course, a circuit fault might put more voltage across a resistor than intended, and that would have to be fixed, and the resistor would indicate that by overheating and smoking.

Edit: resistor voltage ratings are usually for momentary voltage that might cause the resistor to arc over. Typical max voltage rating for discrete resistors might be 200-600 volts, but as you see, for DC, the wattage rating may be exceeded long before the voltage is. AC or pulse current will be different, as r.m.s. current determines the power while the peak voltage determines whether the voltage rating is exceeded.

The voltage rating was listed on the mouser site where I got it and this voltage is 275 dc from the power supply so I just want to make sure because these resistors get hot but it is 3 watt same as what was in there. And it’s a wire wound. These resistors are small bearly half inch.

old_tv_nut · #**753** 10-23-2022, 08:40 PM

Was the original wirewound?
Can you post a link to the mouser page?

Yamamaya42 · #**754** 10-23-2022, 08:42 PM

the resistor types I mentioned a few weeks back were 750v type metal film ones.

https://www.mouser.com/ProductDetail...Wwtn4Z2Q%3D%3D

perfect for this situation. :O

which DID you order?

timmy · #**755** 10-24-2022, 06:11 AM

Well it turns out that I never used that site before so the 6.8 and 8.2 were metal film and the oxide the 2.7 is a wire wound.

timmy · #**756** 10-24-2022, 06:12 AM

Quote:

Originally Posted by old_tv_nut

Was the original wirewound?
Can you post a link to the mouser page?

All the power resistors were wire wound.

timmy · #**757** 10-24-2022, 06:31 AM

Here’s the 2.7 3 watt wire wound

Yamamaya42 · #**758** 10-24-2022, 07:14 AM

please post info links of the Mouser parts that were ordered, you should have gotten this info in a confirmation e-mail when the order was placed.

timmy · #**759** 10-24-2022, 07:22 AM

Here’s the resistors I got

Yamamaya42 · #**760** 10-24-2022, 08:21 AM

The only one in doubt is indeed the wire wond one.
https://www.mouser.com/ProductDetail...0BKWoLFQ%3D%3D
https://www.mouser.com/ProductDetail...1SlkNR2g%3D%3D
https://www.mouser.com/ProductDetail...Mm2017zw%3D%3D
https://www.mouser.com/ProductDetail...FaCwg9Xg%3D%3D

The 750v ones I had shown on page 46 were a good match and even cheaper, :/

https://www.mouser.com/ProductDetail...Wwtn4Z2Q%3D%3D

As a general rule, I try to get a voltage rating that is 2x above the circuit the resistor is to go in, not that a lower won't work, it's just for better longevity.

timmy · #**761** 10-24-2022, 08:29 AM

So the 2.7 k will work but may not last.

Yamamaya42 · #**762** 10-24-2022, 08:59 AM

It's not going to immediately explode, throwing fire and sparks all over, it will work, but it will always be a weak point, prone to fail at some point.

timmy · #**763** 10-24-2022, 09:06 AM

I don’t understand what’s going on with this set anymore I put the other 2 resistors in v2 the image is even better but the b+ voltages are dipping again and maybe at this point if the voltages were up where they should be I might have video now. One moment there good the next they drop. I’ll have to change the 2.7k resistors maybe v1 v2 are drawing to much because they are not conducting at full. Who knows.

bandersen · #**764** 10-24-2022, 10:40 AM

You guys need to read the datasheet a little more closely and take a step back.
The limiting factor on the resistor is power dissipation. Nothing to do with voltage rating.
https://www.mouser.com/datasheet/2/3...20-2758246.pdf

If you had 750 volts across a 2.7K resistor, it would be pumping out over 200W!

Old TV Nut had it right, you hit the 3W mark at 90 volts so there better be much less than that across the resistor.

Yamamaya42 · #**765** 10-24-2022, 11:06 AM

Quote:

Originally Posted by bandersen

You guys need to read the datasheet a little more closely and take a step back.
The limiting factor on the resistor is power dissipation. Nothing to do with voltage rating.
https://www.mouser.com/datasheet/2/3...20-2758246.pdf

If you had 750 volts across a 2.7K resistor, it would be pumping out over 200W!

Old TV Nut had it right, you hit the 3W mark at 90 volts so there better be much less than that across the resistor.

It's not more concerned 750 "across" the resistor itself, but more the being at that potential as a whole, and the possibility of there to be a breech of the outer layer if the voltage rating is to high.