Expensive piece of hardware in my project is meant to run on 8.4v NiMH battery.. I want to power the whole project from a cordless drill battery.
Can use a DC-DC converter to provide 8.4v, but if it is to fail closed circuit part will be destroyed.
My question is: how to detect overvoltage condition?
Cant find a dedicated IC on RC Components website, 8.4v voltage references are available only in production quantities and limitations of zener diodes makes them unsuitable.
The maximum voltage of a 7-cell (8.4V nominal) NiMH battery is around 9.8V (1.4V/cell) when fully charged, if that makes it any difference.
Steve
A crowbar may work for you.
Do an search on "crowbar circuit".
Thank you Steve, sure, there is always a tolerance with battery powered devices.. RS Components stock voltage references in 7.5, 8.2 and 10v. 8.2v variant offered on a reel of 3000!
There are of course adjustable references but somehow I dont feel suspicious about those - ageing and all that.. Might be just paranoia though? 
Thank you for your reply Idahowalker, crowbar circuits usually rely on a fixed or adjustable zener.. I thought about using a voltage reference and a comparator opamp, but closest 8.2v references only available on huge reels.
Problem with zeners is temperature drift and and increase of zener voltage with age..
With a bit of luck there is a chance that it will remain within tolerances as Steve suggested, but somehow I dont feel lucky that it will not let me down in 5-10 years time.
Safest approach might be to use a separate supply of 6 alkaline D cells
A voltage "reference" is probably not what you want. For power, you want a voltage regulator. And, you can get adjustable voltage regulators if you can find the one you want. (Typically, adjustable voltage regulator chips are used with fixed resistors to "program" the voltage, so although the chip is adjustable the final circuit is not.)
A reference is just a low-current reference, it's not designed to power anything. It could be used as part of a protection circuit or part of an over-voltage detection circuit, or part of a regulated power supply, using a comparator to compare the actual power supply voltage with the reference. If you can't find the exact reference you want you can use one with a higher voltage with a voltage divider (2 resistors) to get the reference voltage you really want. Or the actual voltage can go through a voltage divider before going into the comparator, etc.
I want to power the whole project from a cordless drill battery.
What's the battery voltage? What's the maximum voltage your circuit can take?
Can use a DC-DC converter to provide 8.4v, but if it is to fail closed circuit part will be destroyed.
A good DC-DC converter should be just as reliable as anything else. If you buy something cheap from an unknown supplier/manufacturer, maybe you should take some extra precautions...
Thank you fro your reply, DVDdoug!
Great idea to sense supply voltage through the voltage divider (since already got few voltage references for another project).
As for the DC-DC converter - I live in Glasgow, Scotland, which is on the fringe of civilised world. RS components and ebay are only sources of supplies here, so DC-DC converter is a direct import from China.
Probably should make a DC-DC converter myself, will not be of any better design then what I can get from China, but at least there will be certainty that it's not made with recycled components!
And include crowbar part in this circuit - with resistive divider, voltage reference, op-amp comparator and triac.
This is an XY problem. You did not say what input voltage the "expensive hardware" expects. If it is expensive it is likely it will tolerate some supply voltage variations and will have some protection inside.
AlfaOmega:
Can use a DC-DC converter to provide 8.4v, but if it is to fail closed circuit part will be destroyed.
Yes. Use a DC-DC converter. If it fails then all bets are off. You cannot protect against everything.
Make sure it does not fail. Use a reputable supplier. Choose a unit with many times more capacity than you need. Buy a few and test them to destruction: find out what fault conditions cause the type of failure you are concerned about.
Consider using a fuse to protect against over-current faults.
Nothing wrong with using a zener diode as a reference for over-voltage detection - this is not in any way a precision application. In a device is spec'd for 8.4V then 9V won't hurt it, 10V is probably OK too, a 9.1V zener will be fine.
Adjustable references - you are worried for absolutely no reason, they are fixed references with the voltage divider brought outside the unit. Modern resistors do not drift in value (metal film).
And yes a DC-DC converter can fail closed circuit, but its very unlikely - unless you buy a crap one off eBay, then all bets are off. Branded reputable one will be very reliable, manufacturer reputations rest on this, but you do need to heed the absolute maximum ratings in the datasheet.
You could use voltage regulators like LM317, you will need to design a small circuit with variable resistor and can set it's output voltage to 8.4V
