Load dump protection

[Gaudeamos]

The subject of surge protection in this thread caught my attention. I'm brand new here... wasn't sure about posting my question in that thread. So, here I am starting this thread.

I'm planning on making a few electrical modifications to my Iron-Age Chevy van ('92 Astro), some of them involving electronics. So I'm wondering what transient protection measures are in place to cover electronic components. I didn't see any centralized protection in the wiring manual. My guess is that each module has its own transient protection measures, since there are only a handful of them.

Anybody with any insight how (if?) load dumps would be handled early 90's GM light trucks?

 

[gophert]

The subject of surge protection in this thread caught my attention. I'm brand new here... wasn't sure about posting my question in that thread. So, here I am starting this thread.

I'm planning on making a few electrical modifications to my Iron-Age Chevy van ('92 Astro), some of them involving electronics. So I'm wondering what transient protection measures are in place to cover electronic components. I didn't see any centralized protection in the wiring manual. My guess is that each module has its own transient protection measures, since there are only a handful of them.

Anybody with any insight how (if?) load dumps would be handled early 90's GM light trucks?

I would read and follow this.
https://www.ti.com/lit/an/snva681a/snva681a.pdf

Or I would just cross my fingers and hope a lot.

 

[Mickster]

This doesn't directly address your query about load dump handling with regard to your particular vehicle, but it may be more helpful for you to outline what it is that you intend to do with your electrical modifications to the Astro.
For instance, if you are simply buying and installing well-known brand, off-the-shelf components to build a kick-ass audio system, or adding other common automotive-targeted products, there's probably not much need for concern as pretty robust measures have most likely already been engineered-in by the product manufacturers.
However, if you are scratch-building your own expensive micro-driven, safety-critical, radar/camera-aided pedestrian avoidance system, that's a different matter.

 

[Diver300]

Central load-dump protection is inside the alternator so you won't see it in a wiring diagram. I think it is done by making the main rectifiers into zener diodes.

There are three ways common ways to protect against load dump.

Firstly, design or specify the devices to stand 100 V for 1/4 of a second. If you have 12 V relay, it will survive a load dump. A 200 V transistor will survive as well.

Secondly, the most common way is to build in some sort of transient suppressor to absorb the current and keep the voltage low. It's not very easy as there can be a lot of current, and the voltage my rise quite high even with the suppressor. Also, longer term voltage surges to lower voltages may overheat the suppressor.

Thirdly, you can have an active circuit, including a transistor or MOSFET, to turn off the circuit if the voltage rises to high. https://www.flexautomotive.net/EMCFLEXBLOG/image.axd?picture=/EMCLAB/PNG/Transients/Cold_Crank_Reverse_Polarity_Load_Dump_Protection.pn

 

[Gaudeamos]

I've been away for a few days... thanks for your replies everyone.

I would read and follow this.
https://www.ti.com/lit/an/snva681a/snva681a.pdf

Or I would just cross my fingers and hope a lot.

That example is one of several ways of protecting against load dumps, or overvoltage in general:
1) crowbar (SCR) + fuse to disconnect the protected device, requiring service to replace the blown fuse; this is frowned upon in modern automotive practice
2) clamp (TVS diode or MOV) to limit overvoltage to the protecting device's clamping voltage; there remains an overvoltage range between the device's working standoff voltage and its clamping voltage
3) series disconnect (MOSFET) to isolate the protected device during overvoltage, as illustrated in the above link
4) regulation through the voltage surge (DC/DC converter) to stabilize voltages for the protected device.

"Modern" vehicles have centralized load dump suppression (so I've read) either integrated within the alternator or installed close to it. This clamps any load dump voltage to 35-40V. Beyond that individual electronics or subsystems provide their own protection. For critical components that must remain functional option (3) is out of the question; they'll likely have voltage stabilization - option (4). Other components will likely be protected by an overvoltage disconnect as per option (3). I've observed that many automotive qualified IC's are spec'ed to tolerate 40V or so... just about the clamping voltage of a load dump suppression device.

So far I'm assuming that these "modern" protection measures don't fully apply to my '92 GM van. There are after all just a handful of electronic modules. I'm guessing that each of these modules would be self-protected from power transients incl. load dumps.

 

[Gaudeamos]

... it may be more helpful for you to outline what it is that you intend to do with your electrical modifications ...

I'll be installing an auxiliary deep cycle battery with solar/AC/alternator charging. I'll elaborate on request, and I plan on discussing details in other threads. My approach will generally be to build from generic off-the-shelf and home-rolled modules, rather than install the usual components from the RV marketplace.

Instead of outlining the grand scheme, I'll just start with a trivial example that lead to my original post:

I foresee frequently disconnecting the vehicle battery in the immediate future. I'd like to splice a 9V battery into the radio's persistent power line to maintain clock and station presets. This requires a pair of cathode-connected diodes (**broken link removed**) for isolation.

I haven't yet measured current draw on this line, but let's assume that I'm designing for 10A just because the circuit is fused for that. So, 10A through a Schottky diode produces 5+W in heat - not too much for a decent heat sink. That would be the practical approach. No special measures required for load dumps, spikes, reverse polarity, etc. But... Alas... I don't always lead the most practical of lifestyles ...

A more elegant approach would be using an ideal diode circuit:
https://www.ti.com/lit/ds/symlink/lm5050-1-q1.pdf, https://www.ti.com/lit/ds/symlink/lm74610-q1.pdf
This would put an open NFET in the power path instead of a Schottky diode. I could use this opportunity to familiarize myself with these potentially useful devices. But... suddenly I'm considering IC's that may need more serious protection than simple Schottky rectifiers.

My observation was that low-power TVS diodes like the ones suggested in the linked data sheets will not handle the energy of a load dump... and it's not reasonable to include load dump rated TVS diodes in every little gadget that I might want to wire into my vehicle. So, let's make sure that there's centralized load dump protection present somewhere in the vehicle! That would limit system-wide (positive!) transient voltages to a definite maximum that any added electronic device must be prepared to face.

Realistically, I'll most likely run with the practical straightforward option ... and maybe for not quite 10A... but that worst-case-load-dump thing lingers on my mind as I consider other electronic enhancements. What is the worst-case SHTF scenario with my vehicle's current wiring? Hence, my original post.

 

[Gaudeamos]

There are three ways common ways to protect against load dump

Apologies Diver! I paraphrased you in my msg to gophert before reading your reply.

I'm grateful to you and other respondents for tips re. protecting installed devices. At this point I'm inclined not to assume anything about the protection present in my vehicle; I'll take care to appropriately harden any electronics that I install.

First of all, I'll install a beefy load dump rated TVS diode in a "central" location. It may be redundant, but I'll know it will globally limit any overvoltages to 40V. 2 x SLD11-018 are on order.
https://www.littelfuse.com/~/media/...es/littelfuse_tvs_diode_sld_datasheet.pdf.pdf

After that, there'll be local protection (if necessary) against other transients:
> up to 24V steady state overvoltage (AAA courtesy jump starts)
> any other transients 24-40V; this is the range where the TVS diode is only partially active.
> down to -15V steady state reversed voltage (reversed jumper cable with donor's alternator running)
> any other transients <-15V

 

[Gaudeamos]

Btw how would I change the visible text representing a link to anything other than the URL?
Edit: OK... I see that I managed to do it in my first post, but I've forgotten how... I guess this is one of the privileges of aging.