[Milsurplus] WTB Manuals and Power Supply Question

[Bob Camp]

Hi

This is one of those really sneaky problems when you troubleshoot an 
older power supply. If the bleeder resistor has gone way up in value 
(can you say carbon comp resistor ...) it will still do a reasonable 
job of discharging the supply. The regulation of the supply won't be 
what it should be and that can cause all kinds of odd things to happen.

It doesn't take to much time looking at boat anchor schematics to 
notice that choke based power supplies were a very popular item. This 
was because they had lower ripple and better regulation than a straight 
capacitor filtered power supply. The odd part is that a choke based 
supply still has at least one capacitor in it and may have several. 
Chokes work on current, capacitors work on voltage. When you get below 
a certain current level the choke stops having much of an effect. 
Essentially at very low current the choke power supply isn't a choke 
power supply any more.

The bleeder plus the minimum load on the supply keeps enough current 
flowing through the choke so that you stay above the point that the 
filter turns into a pure capacitor based filter. Roughly a capacitor 
only filter will put out 1.4 times the RMS input voltage (= peak of the 
rectified input waveform) . A choke supply will usually run around 1.0 
to 1.1 times the RMS input voltage (= the average of the rectified 
input voltage waveform). The thirty percent voltage change as the 
current drops is a pretty big hit to the supply regulation.

If you suspect that this is what is going on in a supply an easy to 
check it is to measure the input waveform with an oscilloscope. You can 
then take a look at the filter output voltage can compare it to the 
peak input. That should give you a pretty good idea weather the bleeder 
is doing it's job or not. It should also give you a quick check of the 
performance of the choke as well.

	Enjoy!

		Bob Camp
		KB8TQ



On Sep 27, 2004, at 11:12 PM, antqradio at juno.com wrote:

> Isn't a bleeder resistor a SAFETY device?  Helps to keep those 
> accidental
> finger contacts with high voltage supplies from being fatal!
> Jim
>
> On Mon, 27 Sep 2004 19:58:16 -0400 Bob Camp <ham at cq.nu> writes:
>> Hi
> snip
>> Most receiver loads do not require much messing around. A CW
>> transmitter might need a bit of thought. An AM transmitter probably
>> puts a stable enough load on the supply to get away without the
>> bleeder.
>>
>>         Take Care!
>>
>>                 Bob Camp
>>                 KB8TQ
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