OT?: Power supply over voltage protection

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Hi all

I know its slightly off topic here, but plenty of people here to help. Whats the industry general practice for over voltage protection on the input to your dc supplies? Ie my AVR is running from a simple 7805. I have spec'd the DC input for 8-32VDC, but want to protect the circuit if someone applies say 48V to it.

Matt

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Some discussion:
https://www.avrfreaks.net/index.p...

By the way, is your circuit ready to cope with the heat dissipated by the 7805 with an input of 32V? How much current does your circuit consume?

Embedded Dreams
One day, knowledge will replace money.

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The 7805 has a heatsink attached, and I'm only running 100mA-200mA max through it depending how many LEDs are on. The circuit has been in use for a while now with no problems. Just had the boss ask me what happens if someone accidently connected 48V to it. Had to admit I have no idea. We normally only use it with our 12V batteries so it hasn't been an issue previously.

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A 300mA fuse and a 33V Transorb or even a beefy zener will give some protection but the fuse will die.

John Samperi

Ampertronics Pty. Ltd.

www.ampertronics.com.au

* Electronic Design * Custom Products * Contract Assembly

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I would prefer something that didn't kill the system. Its ok if it goes offline while the fault exists, but should ideally restore once the overvoltage has been removed.

Guess I should mention that the actual DC input also charges an internal Li-polymer battery and needs to withstand 35W (1-5A) continuous current. The charger can handle up to 100V though so I'm not too worried there. If you connect that much up to it then it deserves to die a horrible death and we can supply a new one! I'm more worried about the 7805 and what happens on 48V which is common in our industry.

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How does your product work at 32V? That is what you spec'ed it at so it must work happily with that input (at the ambient temperature you specify).
Above that, I'd go along with JS, but remember TVS devices have a knee in their characteristics.
If you spec to 32V then your product should be quite happy with an input voltage a fair bit higher (within reason). Perhaps this is what your boss is fretting over.

Edit: can an 7805 take 48V?
Look at the Nat Semi automotive regulators (eg LM2937)

Last Edited: Mon. Nov 10, 2008 - 03:21 AM
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Quote:
but should ideally restore once the overvoltage has been removed.
Then a polymer fuse may do the trick.

John Samperi

Ampertronics Pty. Ltd.

www.ampertronics.com.au

* Electronic Design * Custom Products * Contract Assembly

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Here's my take on this...

mbotherway wrote:
I have spec'd the DC input for 8-32VDC, but want to protect the circuit if someone applies say 48V to it.

You've specified the maximum input voltage, or rather; the data-sheet has.

Beyond the normal input protection (I.E. fuse, transient suppression, etc...) who's responsibility is this sort of miscalculation?

Regardless of what your boss thinks, it's unreasonable to protect stupid people from their own stupid foolishness.

Over protection is simply a waste of time, money, effort and other resources. Ask your boss how you should protect the device from 120 volts!

This is what I call Kerr-ism!!!

The story goes... We had a 2-1/2 digit DMM, back in the early 1980s. So we set our power supplies to exactly 5.00VDC. Some time later we got a 3-1/2 digit DMM, our boss insisted that we set our power supplies to exactly 5.000VDC. And again, about two years later we purchased a 4-1/2 digit DMM. Then too, the boss wanted us to set the power supplies to exactly 5.0000VDC. Should we carry this on to Ad nausea?

The TTL specification was 5.00VDC +/-5%. That translates to 4.75VDC to 5.25VDC. Having a 4-1/2 DMM, and setting the power supply to exactly 5.0000VDC add no benefit or value to the product. This is Kerr-ism...

Neither does safe-guarding stupidity from itself, justifying protection beyond that which is practical.

The harder you try to safe-guard things from the stupidity of the stupid, all the more stupid they become.

Tell your boss that adding that additional amount of protection to the product will increase its price by an additional 25%, and then observe how fast he/she backtracks, deciding that it's just not justifiable.

You can avoid reality, for a while.  But you can't avoid the consequences of reality! - C.W. Livingston

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Might have to connect one up to 48V tommorow and just see what happens. I've run it off 35V without problems.

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If the datasheet says 35V maximum, I wouldn't even bother to test it at a value higher than that. The biggest risk of that kind of test is to believe the device can handle it if it doesn't get burned during the test.

Embedded Dreams
One day, knowledge will replace money.

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Quote:

Might have to connect one up to 48V tomorrow and just see what happens

Cease and desist. :shock: Don't go tempting fate. What if the regulator fails in such a way that the high voltage is passed on to the load?

If you really have to ensure that the device will operate at higher voltages, add a pre-regulator (simple transistor+zener type will do) that will drop the extra voltage. You can go up to several hundred volts with that technique.

If you think education is expensive, try ignorance.

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mbotherway wrote:
Whats the industry general practice for over voltage protection on the input to your dc supplies?

A sticker spelling out the permissible voltage range.
mbotherway wrote:
I have spec'd the DC input for 8-32VDC,
So in your case a sticker reading "Input: 8 - 32 VDC".

Stealing Proteus doesn't make you an engineer.

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ArnoldB wrote:
So in your case a sticker reading "Input: 8 - 32 VDC".

We already have lexan stickers printed with that exact phrase right next to the input. I guess if the general consensus is forget it then I'll do just that. At the end of the day we just end up selling more units!

Cheers all - Matt

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Quote:

What if the regulator fails in such a way that the high voltage is passed on to the load?

I actually had one of those (similar, in any case). The track/pad for the ground lead of the regulator broke, so the regulator quite justifiably stopped regulating. What a mess. Spent all day replacing components. :P

If you think education is expensive, try ignorance.

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You could use the zener and a PTC resetable fuse from LittleFuse. There are SMD parts and I do not think they can handle your battery charge current. So I would put the fuse in front of a zener and before the regulator for the pcb with the AVR. Somehow you have to handle the battery charging and protect the main board electronics. The PTC fuses automatically reset themselves without any human being actions.

As stated above, you could use a standard fuse and the zener. It does not seem unreasonable to blow the fuse until the user stops blowing fuses and corrects the problem with the 48 VDC into a product intended for a lower input range.

Al Welch

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I don't think the problem here is technical.
We can all agree it's feasible, it just costs extra money.
What you're buying with that money is a more bullet proof product. If your industry is full of 48V systems, people will eventually hook it up to that 48V. Unfortunately those people won't all confess their stupidity and will say the product just stopped working. This is bad advertisement for your business.

There is a reason all electronic modules in a car will stand reverse battery voltage and double battery voltage. Customers will get pissed if their stupidity costs them money.

Just ask your boss whether it is worth the extra money.

Igor