LM7815A and LM7915A metal tab or no metal tab?

I presume, however, that the tab connects to the chassis for heatsinking?

Best practice if the strain on the component is breaking wires, is to use a strain-relieved wiring connection, like a few-inches pigtail wiring from the regulator to a female Molex KK socket; this will plug onto male pins mounted in the printed wiring board, 0.100" centers spacing just like the LM78xx.

Those regulators are available in fullpak (epoxy insulated tab), metal-tab-wth-hole, and short-metal-tab (for soldering to a surface mount "heatsink").

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** The *mounting tab* is definitely metal, just coated in epoxy to insulate it.

** If the originals are held on with a clamp, you need to buy the same again.

If held with 3mm bolts, you can use the ones you have.

BUT you MUST add insulating bushes and mica pads to BOTH - plus a smear of white thermal grease.

Bolting the positive one straight to the chassis is likely to result in ground loop hum.

... Phil

Yes, the tab is for heat sinking and your suggestion of using the Molex plug and socket is very clever.

Thanks for your reply.

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David Farber 
Los Osos, CA

Hi Phil,

The original ones do not have any insulating bushings or mica pads. They are mounted to the chassis with 3mm bolts and some thermal grease. I contacted Digi-Key but they could not tell me if their parts were insulated or not. When I place my order, I will get the insulating pads and bushings just to make sure.

By the way, you can get a schematic here.

Thanks for your reply.

--
David Farber 
Los Osos, CA

I would not agree with using a socket if the regulator flexes slightly. You will run through the insertion limit number of the socket very quickly. Rather, can you solder a flexible loop to the regulator leads - assuming the PCB and the heat sink are not well joined so there is some flex.

I have seen sockets used in similar situations in arcade games and the failure rate is significant. Connectors are the principle point of failure after all...

John :-#)#

--
(Please post followups or tech inquiries to the USENET newsgroup) 
                      John's Jukes Ltd. 
MOVED to #7 - 3979 Marine Way, Burnaby, BC, Canada V5J 5E3 
          (604)872-5757 (Pinballs, Jukes, Video Games) 
                      www.flippers.com 
        "Old pinballers never die, they just flip out."

Huh, we buy either spendy teflon shoulder washers, or cheaper nylon. (with 4-40 screw.)

George H.

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** JRC make "full pack" versions of the popular 7815/7915, TO220 regs.

Types are JRC7815A and JRC7915A.

The pak is called a T220F

Lots of ICs are made like this, with a penalty in the thermal resistance spec.

..... Phil

Spendy vs. cheaper. Very nice. ;-)

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David Farber 
Los Osos, CA

I meant to post the following to the group yesterday

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First, thanks for posting that pic.

I finally found out what was causing the roadblock in my search for the LM7815/7915 in the epoxy case. STMIcroelectronics makes this part and it is available at both Mouser and Digi-Key. The difference lies in the part number for the epoxy case. The letter "M" is dropped from the part number. They are L7815CP and L7915CP. Of course those numbers don't turn up when you look for the original part number.

That being said, I think you were implying it's still better to use the metal tabbed package and use insulating hardware which will improve the thermal transfer. Is that correct?

Thanks for your reply.

--
David Farber 
Los Osos, CA

Just a wild guess here... But I suspect the epoxy coating may have less thermal resistance than a mica washer with the bare metal tab.

--
"I am a river to my people." 
Jeff-1.0 
WA6FWi 
http:foxsmercantile.com

I suspect the difference will have minimal effect on the operation of the mixer. I'll opt for the epoxy coated device.

Thanks for your reply.

--
David Farber 
Los Osos, CA

Fullpack transistors generally have a much lower wattage rating than their metal backed counterparts for that very reason.