Intrinsically Safe Radios and Phones

[Comcokid]

Some of the discussion in thread248-169911 is making me ask a question I've always wondered about. I design low power circuits that are tested to intrinsic safety standards. This is not a problem when the circuit requires only a few milliamps at 5 Volts or less.

However, I see advertisements for "Intrinsically safe" radios and and cellular phones.
Example

http://www.hazardousareadirect.com/Products/ECO-EX-HANDY04.htm

Given that Cellular GSM transmssion requires a 577 usec (North American GSM) or 1154 usec (European GSM) transmission burst of up to 1.6 to 1.8 amps maximum, that the power amplifier stage must have a stable supply voltage with low impedance (<150 milliohms) to the power source. Or, if the battery has a high effective impedance, you need a large capacitor (>2000 uF) mounted very close to the final Power Amplifer. The way I read the IS specs, you need a large resistor in series with any battery (several ohms rated at tens of watts), and you cannot have a capacitor large enough to store the energy for the TX burst.

How do they possibly get such a device through the IS regulations?

Or are there special IS regulations for communication devices?

 

[itsmoked]

They keep the voltage low.

For instance,(if I remember correctly), when I last designed an I.S. device the charts for capacitance verse voltage essentially went unlimited for allowable capacitance at 5 volts,(they were down like 0.1uf by 10V). So if you can get the radio to work at 3-4Vs you wouldn't have much of a current limit. Furthermore what we did was run the loads thru PTCs. They, of course, take a bit of time before they actually trigger being thermal in nature. So you can pick ones that would trip at your transmitting current but because the transmissions are short the PTC never can quite trip.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[JLSeagull]

More electronic stuff is available these days than ever before. How many technicians use IS multi-meters? I only noticed that Fluke began selling IS VOM's ecently. Cell phones, cameras, radios etc. are available with IS ratings. I don't design them - but I use some of these devices in plants.

 

[ScottyUK]

e-com seem to be re-badging test instruments and mobile phones with the addition of an ATEX certification. Their multimeter is a Fluke 80 series, the handheld radio is a Motorola, and their mobile looks awfully like a Nokia product. I am not sure what the differences are between these and the standard products.

http://www.ecom-instruments.co.uk/products.htm

I use an I.S. multimeter when needed. They're useful but have limitations: I think it is Metrix who require that once the instrument is re-tested after being used on a non-I.S. circuit in order to maintain its certification. They're only I.S. up to about 60V too: I know it's improbable that a voltage greater than 60V will be present in an I.S. circuit, but how do you verify this prior to using the instrument? With your tongue?


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