Installation of Schottky diode in Die Cast Case

[dpbarry]

Hi Folks

Just looking a bit of advice/clarity on the installation of a Schottky Diode in a Die Cast Case. The ciruit is basically a battery backup to the main PSU due to the fact that if power fails to the diesel heater without it going through its cool down cycle, it will damage the circuit board in the heater.

This is the whole video:

Based on the circuit and the way it looks installed in the die cast case, would the case technically be at supply voltage (+14v or +12v) if it is bolted to the case with pin 2 being the Common Cathode. My worry would be that if the case touched anything connected to any negative aspect of the circuit/heater, sparks would fly.

I've posted a question on his Youtube channel to query. I'm probably looking at this the wrong way but better to ask and be educated.

Regards

Declan

 

[Nigel Goodwin]

The diode is designed to be mounted to a heatsink (the case should do), and you simply bolt it in place using a suitable insulating kit - they used to a mica washer and a plastic spacer, but now are usually a rubber washer and a plastic spacer.

It would be quite dangerous to bolt it directly to the case.

I notice you've listed RS part numbers, I was unable to locate any at RS last time I wanted some, and had to get them from Farnell.

Here's some on Amazon:

https://www.amazon.co.uk/Easycargo-Insulator-Mounting-Rubberized-Silicone/dp/B07H1C7JP3

 

[sagor1]

The picture clearly shows an insulating spacer/washer under the diode, so it is not connected to the case electrically

 

[dpbarry]

The picture clearly shows an insulating spacer/washer under the diode, so it is not connected to the case electrically

I do see what looks like a rectangular piece of plastic but the nut/bolt kind of made me question what I was looking at until I looked at Nigel's response above. There must be an insulated washer going through the die cast case so as not to make it electrically connected to the case.

It's all a learning curve.

Declan

 

[dpbarry]

The diode is designed to be mounted to a heatsink (the case should do), and you simply bolt it in place using a suitable insulating kit - they used to a mica washer and a plastic spacer, but now are usually a rubber washer and a plastic spacer.

It would be quite dangerous to bolt it directly to the case.

I notice you've listed RS part numbers, I was unable to locate any at RS last time I wanted some, and had to get them from Farnell.

Here's some on Amazon:

https://www.amazon.co.uk/Easycargo-Insulator-Mounting-Rubberized-Silicone/dp/B07H1C7JP3

Hi Nigel

This circuit is from a third party. Actually, the Schottky Diode code is RS Stock No.:743-5911

Its just that I am putting one of these heater unit in my shed and wanted a battery backup - just in case main power did get interupted (Can't afford to run the electric heater at current prices! LOL!!)

Just looking at the circuit diagram and not understanding how the Schottky Diode was connected to the case made me a bit uneasy given that its rated at 20A.

Excuse my ignorance and lack of knowledge but if I mount it with a heatsink (or even use the case) how does it transfer heat away if it gets hot? From initial readings, once the heater is starting up, it can require up to 10 Amp to get going.

Strangely, the insulating case you sepecified doen't ship to my address but this one does:

https://www.amazon.co.uk/WayinTop-Insulator-Transistor-Rubberized-20mmx15mmx10mm/dp/B081GS15N6/ref=dp_fod_sccl_2/261-1437173-3103148?pd_rd_w=heeMw&content-id=amzn1.sym.6121fe57-41ad-4d6a-a0d3-900996f46830&pf_rd_p=6121fe57-41ad-4d6a-a0d3-900996f46830&pf_rd_r=6J21WG0XB5VZ49QV8PH6&pd_rd_wg=pxwu4&pd_rd_r=9a3aae4c-7e13-49fe-b543-d587cb81a08c&pd_rd_i=B081GS15N6&psc=1

Declan

 

[dpbarry]

Ahhh! Did a bit more digging and think I may have found the answer to my question in relation to thermal conductivity:

https://aosmd.com/res/application_notes/package/AN101_TO220_Guidelines.pdf#:~:text=The%20insulator%20is%20thermally%20conductive%20and%20transfers%20the,the%20heat%20sink%20to%20ensure%20good%20thermal%20contact

 

[Nigel Goodwin]

I do see what looks like a rectangular piece of plastic but the nut/bolt kind of made me question what I was looking at until I looked at Nigel's response above. There must be an insulated washer going through the die cast case so as not to make it electrically connected to the case.

It's all a learning curve.

Declan

If you think there's an insulator there, then just test it with a multimeter on ohms - it shouldn't read from tab to enclosure.

Heat sinks are simply radiators, they radiate the heat to the air around them - plenty of fins makes them much better, as does blowing air through the fins. But even a plain diecast case like that will greatly increase the capability of the rectifier, on it's own it will easily over heat.

 

[dpbarry]

If you think there's an insulator there, then just test it with a multimeter on ohms - it shouldn't read from tab to enclosure.

Heat sinks are simply radiators, they radiate the heat to the air around them - plenty of fins makes them much better, as does blowing air through the fins. But even a plain diecast case like that will greatly increase the capability of the rectifier, on it's own it will easily over heat.

Cheers Nigel.

Parts are on their way so once I put it all together I'll be able to test it. All I was going on was the third party video and circuit diagram but with the help from yourself and Sagor1, I now understand what is going on and for that, Thanks to both of you.

Declan

 

[rjenkinsgb]

There must be an insulated washer going through the die cast case so as not to make it electrically connected to the case.

The insulator should be a flanged insulating bush though the diode tab, so the bolt is electrically connected to the case.

Insulating "kits" should have both parts, eg.

W4605 KIT | Winslow Heatsink Transistor Mount Kit for use with TO-220 | RS

uk.rs-online.com

 

[Tony Stewart]

TI also makes an Ideal dual diode IC for FET switches with an internal charge pump. There are a few developers, one I found with an Alum substrate board rated for 60A 80V

Two IN's Out & Gnd.

https://www.aliexpress.com/item/1005005708517730.html?src=google&aff_fcid=82f8dd48fa4d43cfb0a6cf642c5d3548-1698179154706-09578-UneMJZVf&aff_fsk=UneMJZVf&aff_platform=aaf&sk=UneMJZVf&aff_trace_key=82f8dd48fa4d43cfb0a6cf642c5d3548-1698179154706-09578-UneMJZVf&terminal_id=63a0f4db7a6445ed97b42a958a9c290b&afSmartRedirect=y

 

[dpbarry]

The insulator should be a flanged insulating bush though the diode tab, so the bolt is electrically connected to the case.

Insulating "kits" should have both parts, eg.

W4605 KIT | Winslow Heatsink Transistor Mount Kit for use with TO-220 | RS

uk.rs-online.com

Hi rjenkinsgb.

I'm a little confused.

If the bolt is going through the flanged insulating bush, how is it 'electrically' connected to the case. Do you mean mechanically connected?

Once I fit the insulating kit and test it with a meter, going by what Nigel Goodwin has said, there should be no connectivity between tab and case - if I have fitted the insulating kit properly.

Declan

 

[Nigel Goodwin]

Hi rjenkinsgb.

I'm a little confused.

If the bolt is going through the flanged insulating bush, how is it 'electrically' connected to the case. Do you mean mechanically connected?

I think you've misunderstood him - the insulating bush is on the diode, so the bolt is electrically connected to the box but not to the diode. You 'could' put the bush on the case side, so the bolt is connected to the diode, and insulated from the case - but that would be silly - as the head of the bolt could accidentally be shorted to the case on the outside.

It's fairly universal that the insulating bush is on the component side - with occasional special case exceptions.

Once I fit the insulating kit and test it with a meter, going by what Nigel Goodwin has said, there should be no connectivity between tab and case - if I have fitted the insulating kit properly.

Declan

Exactly.

 

[dpbarry]

I think you've misunderstood him - the insulating bush is on the diode, so the bolt is electrically connected to the box but not to the diode. You 'could' put the bush on the case side, so the bolt is connected to the diode, and insulated from the case - but that would be silly - as the head of the bolt could accidentally be shorted to the case on the outside.

It's fairly universal that the insulating bush is on the component side - with occasional special case exceptions.


Exactly.

Ahh! Apologies.

I get what you and jenkinsgb means now. As the bolt is being passed through from the outside, metal on metal makes it part of the box so in effect, electrically connected.

I was thinking more physically - if that makes sense

Cheers

​

 

[dpbarry]

The insulator should be a flanged insulating bush though the diode tab, so the bolt is electrically connected to the case.

Insulating "kits" should have both parts, eg.

W4605 KIT | Winslow Heatsink Transistor Mount Kit for use with TO-220 | RS

uk.rs-online.com

Apologies. Misunderstood what you meant and Nigel filled me in.

Declan

 

[schmitt trigger]

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