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Power line filtering in a vintage car

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floobydust:
Would you trust your $500 cellphone/tablet to a generic car USB charger?
For a few bucks, I'll add a TVS. I find most chargers are very cheap and using old IC's with little headroom on Vin.

The TVS I mention is prolific in the auto industry, it's in many if not all ECU's and BCM's, ABS and in my car. I will confirm the 27V part but it's odd SM8A27 specs compared to a 26V SM8S26A. The boost is a 24V spec and I think in the SAE standards, not ISO. Where do you see it? In reality many automakers have their own vehicle transient standards, not all following ISO or SAE. It's perfectly fine to pop the fuse here with load dump or 24V boost, which are extremely rare occurrences in the life of a car.

A problem with going to a 33V TVS is most voltage regulators (LDO or buck) can't take the 53V clamp (vs 40V), and the TVS dissipates more energy. It's a tradeoff. You want the lowest possible clamping voltage, including over automotive temperatures (a TVS tempco is large).

Many people want to run an Arduino, Raspberry Pi, ESP32/8266 in their car.  There is a need for a basic recipe.
But any TVS is going to clamp fairly high and that eliminates all of the usual jellybean regulators out there.

https://www.emtest.com/what_is/glossary/standards/ISO-7637.php
https://www.teseq.com/product-categories/automotive-electrical-disturbances.php

jfiresto:

--- Quote from: DBecker on January 08, 2020, 06:31:58 am ---... Many of the cheap lighter-to-USB buck converters are quite robust.  They are designed to work with either 24V or 12V systems, with margin....
--- End quote ---

It may not be as much as one might hope. I asked a manufacturer about their 12/24V vehicle USB charger before I added it to a car that has no central overvoltage (load dump) suppressor like the OP's. Up  to 36V was the number they gave me.

The best I could do was to add a fused MOSFET switch, upstream, that cuts out at 22–24V, followed by a 5kW 5KP24A TVS diode if the switch doesn't open or is a little slow. I can dig up and post the circuit if someone needs an example of how (or how not?) to do it.

DBecker:

--- Quote from: floobydust on January 08, 2020, 08:06:23 am ---Would you trust your $500 cellphone/tablet to a generic car USB charger?
For a few bucks, I'll add a TVS. I find most chargers are very cheap and using old IC's with little headroom on Vin.

--- End quote ---

Your lead-off argument is an appeal to emotion, rather than a technical point.

Billions of people trust their phones to car USB chargers.  The production volume of those chargers is so high that most are using recent purpose-designed chips.  They are optimized to work in harsh conditions with the cheapest external components, and they do an excellent job of it.

The specific TVS I listed, the SM15T33/33C, is an appropriate one to use with auxiliary equipment.  ST numbers their TVS as the nominal breakdown voltage, not the operating voltage.  The Vishay chart listed above shows that their "28" part actually has a 33V typical breakdown voltage, with a minimum breakdown of 31V, so it's quite close.

ISO 16750-2 is a short, easily read standard.  It's worth a quick read-through.

floobydust:
Your technical point relies on faith unless you disassemble the car USB charger and see what chip is used and if there is a TVS. I've seen enough of them to know they aren't always good products. My philosophy is not to trust it unless I've taken it apart.

Billions of people are not driving cars from 1949 with mechanical regulators.  Modern alternators incorporate zener diodes for rectifiers, the system transient voltages are lower.

One employer has product using a 78M05 on 24V truck electrical systems, with a SMBJ33A TVS. How do you think it would do? Vin max. is 35V and the TVS clamps well above that. About 20% of the power supplies failed in the field. Many with the Vreg IC dead or 22uF 35V tantalum input cap shorted.

If noobs can understand to make sure the TVS highest clamping voltage is below their voltage reg's maximum input voltage, then they are mostly covered. Another series diode to cover reverse-polarity spikes and best to have a some line resistance in the form of a polyfuse or resistor, to limit TVS current.

m3vuv:
my friend had a 1935 derby bentley,the fuel pump was electric on that made by su.

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