UY85 Heater voltage
 

Good evening all,
according to the data sheet the UY85 has a heater voltage of 38V @100mA. For a home brew project I have a transformer that supplies 35,6V off load and 34,5V when connected to the valve.

Is it bad practice to under-run the rectifier like this?

Re: UY85 Heater voltage
 

Heater voltages are generally best kept within 5% of nominal. This is low by slightly more. But unless you are pulling near the maximum current, I'd say don't worry.

For maximum life, slightly under is better than slightly over.

Re: UY85 Heater voltage
 

What was the mains voltage at the time of the test ?
If on the low side, then that should be fine since any subsequent variation should be upwards and therefore towards running the heater at nominal voltage, or closer thereto.
If however the mains voltage was on the high side, then that is of greater concern since future fluctuations are likely to be downwards, and thus running the heater further below the design voltage.

Re: UY85 Heater voltage
 

Guys, thanks for your replies. I fed the Transformer from a variac at the nominal 230V. There will not be a great deal of demand on the rectifier, so I think I'll go ahead with the idea. The mains voltage around here is quite stable and normally hovers around 234-236V which I suppose is good.

Re: UY85 Heater voltage
 

It will be fine. These are designed for series operation normally, so in real use would see different voltages for the spec depending on the mains voltage, other valves in the chain and so on. Slightly lower is better as kalee says...

Re: UY85 Heater voltage
 

Since the heater voltage is not the tightly controlled parameter in 100mA valves, you should check the current in the actual circuit. If it's within 5% the valve will be perfectly fine. Even if the voltage is 10% out from nominal, the current might still be within tolerance depending on production tolerances of the valve.