[ARC5] BC-AS-230 Transmitter, part 1 - tube adapters, general clean-up, q...

[Bruce Long via ARC5]

Be careful of the to220 package power resistors.  They are film resistors on a ceramic substrate.   They perform exactly as specified on the data sheet but if you read the data sheet very closely you will learn they have a very limited ability to survive a transient over-current event.  Most leaded power  resistors, wirewound or otherwise will tolerate a quite severe overload if the overload is short.  The reason for this is the overload failure mode arises from overheating and it takes some time for the bulk of the power resistor to rise to damaging temperature levels.  For this reason a transit overload as you might get if you slip with a voltmeter probe is generally survivable.

The to-220 and related type resistors have a rather small ceramic substrate with a very thin, relatively narrow and rather fragile resistive trace that opens circuits extremely quickly just like a fuse in the presence of quiet short over current events.
A number of years ago i designed such to-220 ten watt resistors into a design and had a number of failures.  I was doing a lot of troubleshooting on this new design and the resistors where tucked away in the corner under some other stuff. Several times when i attempted to measure voltage I had accidentally brushed the exposed test probe again another grounded conductor.   Each time I did that the to-220 resistor failed open circuit.
The fuse action of the resistor was so fast i never saw a spark even when i deliberately shorted the test lead and was looking for evidence of a short.
So if i were you I think i would avoid the to-220 power resistor and similarly packaged thin film power resistors and consider instead the possibility of putting a number of conventional, non wire would resistors in parallel to get the required power dissipation.

      From: WA5CAB--- via ARC5 <arc5 at mailman.qth.net>
 To: arc5 at mailman.qth.net 
 Sent: Monday, December 22, 2014 2:50 PM
 Subject: Re: [ARC5] BC-AS-230 Transmitter, part 1 - tube adapters, general clean-up, q...
   
This is a phontom antenna, not a dummy load.  I have a number of different 
WW-II vintage phantom antennas and most have or appear to have wire wound 
resistors.  Only one that I know of for sure doesn't.

In a message dated 12/22/2014 12:40:09 PM Central Standard Time, 
brianclarke01 at optusnet.com.au writes: 
> Hello Ben,
> 
> 8 Ohm is a standard E12 resistor, and so, readily available. Most 8 Ohm 10 
> W 
> resistors are wire-wound. Therefore, they are quite inductive and quite 
> unsuitable as an RF dummy load.
> Resistors designed for Ward-Leonard motor control are claimed to be 
> wire-wound, but with reversed turns to cancel out the inductance. But such 
> a 
> winding does not cancel out the inter-turn capacitance.
> It would be wise to seek a ceramic resistor that bolts directly onto a 
> heatsink - some look like a TO-220 case style, some are smaller. But often 
> 
> they are non-inductive. Ask for the specs.
> 
> 73 de Brian, VK2GCE.
> 
> On Monday, December 22, 2014 9:48 AM , you said:
> <snip>
> 
> I'm thinking that because the ARC-5 sets, the follow-on past the BC-230
> series, were designed for a 5 to 15 ohm load I can't really go too far
> wrong with an 8 ohm, 10 watt resistor.  Thoughts?
> 
> thanks much and 73,
> ben, kd5byb 
> 

Robert Downs - Houston
wa5cab dot com (Web Store)
MVPA 9480
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