03-08-2012, 01:08 PM
(This post was last modified: 03-08-2012, 01:11 PM by Robert Darwent.)
Hi John,
The voltages are low because the 10 uF capacitor by itself is a little low in value. Even with a +/-10% tolerance, which is typical for motor run types, the maximum value at best could only be 10.1uF and the calculations require 10.68uF.
If you change the surge limiter to 10 ohms instead of the 33 ohms you are using at present you should find the voltage across the heater chain much closer to the 70v you are looking for, doing this also changes the capacitor value needed to 10.03 uF bringing it into tolerance.
Since a 10 ohm surge limiter produces a 3v drop instead of the 10v you originally allowed for, the resistor will only need a 0.9W power rating. So using a 3 or 5W type it should run much cooler and you will probably not need to make ventilation holes in the screening can.
Regards
(03-08-2012, 09:29 AM)Geaney Wrote: All my voltages seem just a little on the low side; heater volts = 65V and B+ = 90V. Heater chain current is about 270mA. I've set the supply voltage at 120v on the variac.
The voltages are low because the 10 uF capacitor by itself is a little low in value. Even with a +/-10% tolerance, which is typical for motor run types, the maximum value at best could only be 10.1uF and the calculations require 10.68uF.
(03-08-2012, 09:29 AM)Geaney Wrote: One thing I noticed is the surge limit resistor does get quite hot so I will make sure it is fixed to the case of the can to make sure the heat gets dissipated. I used a 5watt resistor (formula indicates 3watt - hopefully 5watt gives sufficient headroom).
If you change the surge limiter to 10 ohms instead of the 33 ohms you are using at present you should find the voltage across the heater chain much closer to the 70v you are looking for, doing this also changes the capacitor value needed to 10.03 uF bringing it into tolerance.
Since a 10 ohm surge limiter produces a 3v drop instead of the 10v you originally allowed for, the resistor will only need a 0.9W power rating. So using a 3 or 5W type it should run much cooler and you will probably not need to make ventilation holes in the screening can.
Regards
