Reverse Polarity and Over Voltage Protection Design

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Brian
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Re: Reverse Polarity and Over Voltage Protection Design

Post by Brian »

Good point, fair enough.

Brian.
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jharvey
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Re: Reverse Polarity and Over Voltage Protection Design

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Um get me part numbers so I can get datasheets, then I can tell you my thoughts. I'm going to use digi 296-21594-5-ND as the vendor 1 thru hole reg, unless someone objects.

How much reverse voltage can a part handle, and for how long. Simply don't know right now.
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Fred
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Re: Reverse Polarity and Over Voltage Protection Design

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jharvey wrote:I'm going to use digi 296-21594-5-ND as the vendor 1 thru hole reg, unless someone objects.
WTF is that? LOL, you START with an actual MANUFACTURER part number such as LM2940 then spec out a digi part number, can you tell us what that actually is please?
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jharvey
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Re: Reverse Polarity and Over Voltage Protection Design

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When I go to digikey.com, and put in 296-21594-5-ND, I get a Texas instrument "LDO REG POS-VOLT 5V TO220-3" I now have a pinout I can use to figure out what pin is pin1, and a datasheet that tells me how I can expect the part to perform.

I see continuous reverse voltage of -15v and reverse spikes of -50v. I also see the forward OV protection is 28v, and max is 60v. Also comes in SMD packaging. Now I have a starting point.

EDIT
This digikey number is the above mentioned TL750M05CKCSE3
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Delta
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Re: Reverse Polarity and Over Voltage Protection Design

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Fred wrote:That is all very well and good for the regulator, but if we are to have some caps on board for filtering, it is not well and good for them. Tantalum are very sensitive to even short term reverse voltage and can go with an almighty crack. Likewise electro, though it may handle a touch in reverse, hold it there for more than a split second and bang you have fluid and paper everywhere. For the over voltage side of it, just using 50 or 60V caps will solve it, but for reverse hookup something is required if they are not to explode. Thoughts?

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Non-polarized capacitors
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Fred
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Re: Reverse Polarity and Over Voltage Protection Design

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Jared, why have you chosen an obscure TI part for an international project? Is it pin out compatible with say LM2940? If it is, please spec the LM2940 part and choose to use the nicer(?) TI part yourself. I'd never heard of that part before and we are vaguely trying to keep things accessible and common and easy to find at an ordinary hobby shop where possible.

Non polarised caps = 4x the board space/weight for the same value and quite possibly worse frequency response too, but that's just a guess. If you "build your own" NP caps then you need 4 to make the same value as one is. If you don't then they are not common to find. It's a clever answer, but probably not a practical one as far as I can tell. Correct me if I'm wrong.

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jharvey
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Re: Reverse Polarity and Over Voltage Protection Design

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I don't object to the LM2940 looks pin compatible, so I'll speck Newark's 41K4613 as vendor2 thru hole.

I think I'll also aim for vendor 1 as US/this side of the hemisphere, and vendor 2 as over the big drink. I seem to recall Newark is associated with Farnell. I know the Newark number shows up correctly under Farnell. So I think Newark numbers are good for you folks over the big drink. Does that sound right?

Looks like both come in similar SMD packages that should be close enough to each other. I see TO-263 for the LM2940 and KTT for the TL750m08. I'll stew those over for a bit and generate digikey and Newark number for them to populate the smd fields.
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Delta
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Re: Reverse Polarity and Over Voltage Protection Design

Post by Delta »

Fred wrote:Jared, why have you chosen an obscure TI part for an international project? Is it pin out compatible with say LM2940? If it is, please spec the LM2940 part and choose to use the nicer(?) TI part yourself. I'd never heard of that part before and we are vaguely trying to keep things accessible and common and easy to find at an ordinary hobby shop where possible.

Non polarised caps = 4x the board space/weight for the same value and quite possibly worse frequency response too, but that's just a guess. If you "build your own" NP caps then you need 4 to make the same value as one is. If you don't then they are not common to find. It's a clever answer, but probably not a practical one as far as I can tell. Correct me if I'm wrong.

Fred.
We only need two of them - one at the input of each regulator. I suppose it comes down to - do we use a suited regulator with reverse protection and eat up space with two non-polarised caps (8x normal if you like) or do we use a non-suited regulator with little protection and eat up space protecting it......same diff if you ask me. Although the non-pol caps and good reg will use less power and blow the fuse less often......
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Fred
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Re: Reverse Polarity and Over Voltage Protection Design

Post by Fred »

SMD simply will not work for the regulator in a linear type, it will cook and worse, it will cook the board as that is where SMD power devices sink their heat. If you want an smd regulator it will need to be switchmode and you will need to be careful etc. I'd recommend that you stick to keeping some things only TH, reg being one of them.

Can you even get NP tantalums? I'd like to see 3 caps on each input and output. Ceramic, tant, electro. If they are polarised they take up normal space and perform normally. I'd rather ditch reverse protection all together than go with semi exotic parts (NP is usually audio circuits only stuff)

Fred.
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Brian
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Re: Reverse Polarity and Over Voltage Protection Design

Post by Brian »

The datasheet for the LM2940 simply says a 0.47uF cap is required on the input side of the regulator if the regulator is located "far from the power supply filter". So what exactly is the power supply filter in this case, an electrolytic cap, an inductor, the vehicle's battery, and how far is far? However the datasheet is very specific about the output capacitor size and ESR rating.
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