Rob Flinn

Re: M670 compressor
« Reply #180 on: September 28, 2020, 02:34:31 PM »
I've switched everything over to the full m670 schematic with -12vdc heaters, and the unit works great for 5 minutes, and then the -12vdc supply shuts down. I've used the lm338t in 220 format, and the heat sink gets piping hot. It's pulling 3a with lights, scamp, heaters, and bias. I have a thermistor controlled fan on my lid that isn't installed yet, but something seems to need addressed first. Perhaps a bigger heat sink, or attached to the back case panel? I have a 1"x1"x1" sink currently. Any thoughts? Do I need to go with the lm338k?

It's probably going into thermal shutdown.  How much voltage is across it input pin to output pin ?   The more voltage that is across it the hotter it gets and the less current it can deliver.  Looks at the data sheet there is a graph for this phenomenon.
« Last Edit: October 16, 2020, 02:38:25 PM by Rob Flinn »
regards Rob


duantro

Re: M670 compressor
« Reply #181 on: September 29, 2020, 03:57:35 PM »
It's probably going into thermal shutdown.  How much voltage is across it input pin to output pin ?   The more voltage that is across it the hotter it gets and the less current it can deliver.  Looks at the data sheet there is a graph for this phenomenon.
Thanks Rob,
I'm getting 5vdc on input pin, output pin is grounded per circuit. The data shows up to 10vdc differential from input to output, if I'm reading it correctly. I'm using an Antek 2215 for the heater circuit. https://www.antekinc.com/content/AS-2215.pdf
« Last Edit: September 29, 2020, 06:45:49 PM by duantro »

Kingston

Re: M670 compressor
« Reply #182 on: October 16, 2020, 12:05:51 PM »
duantro,

you have to distribute that heat somewhere away from the regulator. Sure that chip claims 5A regulation, but that is only in perfectly optimal heat-sinked conditions. Ideally that lm338k should be left to drop exactly 3VDC. That ensures minimum heat across the device. 10VDC differential is simply too much for the amps you are drawing. A good way to deal with this is a big-watt wire-wound resistor to drop a good 5-7VDC _before_ it enters the regulator. You have to calculate the exact resistance and required resistor watts based on expected heater current draw. Often it may turn out you never really needed the regulator at all. A perfectly matched big-ass wire-wound resistor (or two in series) can sometimes produce stable enough VDC without noise.

This is in no way specific to M670 but applies to all tube DC powered heaters since it's usually impossible to get a perfectly matching transformer for the job.

duantro

Re: M670 compressor
« Reply #183 on: October 16, 2020, 05:22:49 PM »
duantro,

you have to distribute that heat somewhere away from the regulator. Sure that chip claims 5A regulation, but that is only in perfectly optimal heat-sinked conditions. Ideally that lm338k should be left to drop exactly 3VDC. That ensures minimum heat across the device. 10VDC differential is simply too much for the amps you are drawing. A good way to deal with this is a big-watt wire-wound resistor to drop a good 5-7VDC _before_ it enters the regulator. You have to calculate the exact resistance and required resistor watts based on expected heater current draw. Often it may turn out you never really needed the regulator at all. A perfectly matched big-ass wire-wound resistor (or two in series) can sometimes produce stable enough VDC without noise.

This is in no way specific to M670 but applies to all tube DC powered heaters since it's usually impossible to get a perfectly matching transformer for the job.
Thanks for this info! I'm going to yank the LM338T and put in a LM338K like recommended, I just ordered the wrong part. I see that the LM338T is only rated at 25wattts. I'll explore a wirewound resistor in front of the lm338K also.


 

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