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7.5c62 Selenium rectifer replacement

countryboy07

Registered
Hey guys, so I'm getting ready to replace the selenium rectifer in my 7.5c62/l654 set. It's a exciter cranked manual start unit. I've got a 40a 600v bridge rectifer to replace it with but I was wondering if I needed a resister to account for the voltage drop of the old selenium?

Thanks
 

Vanman

Subscriber
Last Subscription Date
07/10/2019
No resistor needed. You’ll just have ever so slightly better regulation. (y)

The reduced voltage across the rectifier will result in less voltage across the load windings in the transformer, in proportion to the turns ratio. So the load voltage ends up being just a tiny bit higher at any given load.

Keith
 

countryboy07

Registered
No resistor needed. You’ll just have ever so slightly better regulation. (y)

The reduced voltage across the rectifier will result in less voltage across the load windings in the transformer, in proportion to the turns ratio. So the load voltage ends up being just a tiny bit higher at any given load.

Keith
Perfect i need to make a little heat sink to mount it to and I should be in business
 

Zephyr7

Registered
Those heatsinks sometimes need to be bigger than you’d expect. If you are using half of the bridge, you are running two diode drops (which sometimes cancel since only one half is actually energized at any given time), so 0.7v per diode. Multiple the volt Drop times the current to get watts. With 10 amps through half the bridge, you need to dissipate 14 watts. That’s a decent size heatsink.

Oftentimes just mounting the rectifier to the steel chassis provides sufficient heatsinking in practice. Due to the large area of the rectifier block, thermal interface materials (grease, pads) are not usually required. If you do want to maximize thermal conductivity, I’d use a thermal pad such as the silpad material available from Bergquist. Grease is too messy here.

Bill
 

countryboy07

Registered
I'll have to look into sizing then, I've got a piece of scrap 3/4" Al plate out of the scrap hopper at work, I was planning on milling a few fins in it and mounting the bridge to it. I'll have to look tonight if we have any of that material, I know I have the nasty white paste.
 

Zephyr7

Registered
If you mill maybe 8 fins into that piece you’ll probably be ok as long as the total heatsink ends up being about 2x4 inches or so.

You can go to heatsink manufactures like Wakefield or aavid and see their designs that have dissipation ratings. Duplicate something there in terms of size and fin count and you’ll end up with a heatsink with similar thermal performance. Install it with the fins vertical to allow for maximum convection airflow. Don’t paint it.

Bill
 

Power

Registered
The easy way out may be to find an older junked computer and modify the CPU heat sink. The really old ones did not use a fan, but even a newer one if mounted to the steel chassis should have more than enough dissipation to handle 15 watts.
 

countryboy07

Registered
The easy way out may be to find an older junked computer and modify the CPU heat sink. The really old ones did not use a fan, but even a newer one if mounted to the steel chassis should have more than enough dissipation to handle 15 watts.
It's funny you mention that, when I was at work tonight I spied a little automatic battery charger in the scrap barrel. After a quick lunch time surgery I rescued a nice heat sink out of it
 

Zephyr7

Registered
Scrounging for surplus is the time honored way to find heatsinks for power gizmos!

Power, CPU heatsinks, while they often have plenty of thermal dissipation capacity, they don’t usually have a good way to mount the rectifier. If you mount the rectifier on the flat side of the heatsink, there is no where to use to bolt the assembly to anything. You end up having to rig a bracket. Using a heatsink intended for stud mount diodes or power transistors generally allows for much easier mounting.

Bill
 

edisondiamond

Registered
I mounted my stud mount type to a block of aluminum then to one of the towers the selenium was originaly supported on with a thru screw. Guess the only way to check would be to run under load and check with temp gun.
 

countryboy07

Registered
This should be plenty big, and it's already got a diode mounted to to. It's about 3"x4". I'll use my new diode, those little automatic chargers take a beating out in the plant. It's got a little bracket on it, I'm thinking I can use one of the mounts for the old rectifer and bolt it to that.
 

Vanman

Subscriber
Last Subscription Date
07/10/2019
As I recall, the auxiliary field circuit is only a few amperes at full load, so I'd imagine most any mounting scheme would provide adequate cooling?

Keith
 

countryboy07

Registered
As I recall, the auxiliary field circuit is only a few amperes at full load, so I'd imagine most any mounting scheme would provide adequate cooling?

Keith
Just judging by the wire size it dosnt look like it's much more than that. I was trying to look up the specs on the old selenium rectifer but the part number on it dosnt appear to be good.
 

Zephyr7

Registered
If you’re only running a few amps through the rectifier, simply mounting it to the metal control box is probably sufficient for heatsinking.

Bill
 

dkamp

eMail NOT Working
My L600 6.5R22's selenium was retired by a 30A 1kv bridge, and I just greased it and bolted it to the tin... probably have 150 hours of outages on it with no issues.

The selenium's voltage drop accounted for substantially more thermal disspiation. My silicon bridge has no significant temperature rise in operation... so I'm thinkin' the tinwork is enough. Mounting it to a PC processor heat sink is easy... drill a hole from the flat side, and when you get all the way through, tap it to an appropriate fastener side. Drill a hole in one of the fins, and call it a mounting point... ;-)
 

LWB250

Registered
I would agree with the others about just mounting the bridge to the inside of the controller on a flat (metal) surface. The retrofit kits that Kohler sold using a 25A full wave bridge were just bolted to the inside of the controller.

Dan
 

countryboy07

Registered
Well I had some time tonight since we're on a rotating schedule at work now, glad to be Essential, so I was able to get everything converted. I ended up just mounting it to the bracket that held the old selenium rectifer. Looks like it's working pretty good. At fullish load I have about 34VDC on the field and was showing about 12-15 amps. IMG_20200326_213614322.jpgIMG_20200326_213841835.jpgIMG_20200326_213930995.jpg
 

Zephyr7

Registered
That bracket looks like it’s probably big enough to provide sufficient heatsinking to that rectifier.

Do a good, heavily loaded run with your generator. Immediately after the run, feel that bracket. If it’s hot to the touch, but not so hot that you can’t keep your hand on it, then that’s ok. If you can’t hold it in your hand because it’s too hot, then it’s TOO HOT and you need a bigger heatsink.

Bill
 
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