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M7 Heat Sinking and Duty Cycle

The M7 transceiver has a 5-watt RF power output rating.  In a typical application the units is in Standby or Receive mode most of the time.  A small fraction of the time, it is transmitting.  But when it transmits, the M7 begins heating up, dissipating about 8 watts of heat.   This depends upon the RF power output setting and the DC input voltage.

As a precaution to protect the M7 radio modem from over-heating, the M7 has thermal protection built into it.  When the internal circuits exceed 70 degrees Celsius, it will automatically turn its output power down by about 3dB.  If the transmissions continue and the heating continues, at around 75-80 degrees Celsius, the RF power amplifier will shut down to protect it and other circuits.  It only will remain shut down till the next transmission attempt, and at that time, if it has cooled off below 70C, it will begin transmitting again. 

The temperature of the M7 enclosure must be kept below 60 degrees Celsius, (140 Fahrenheit) for proper operation of the unit.  For GPS transponder operation, there is no problem doing this, because the duty cycle is low.  But, if the M7 is used to send data, and is on the air a large percentage of the time, then the enclosure’s temperature will begin to rise.  The following chart shows the case temperature at 25% and 50% Duty cycle.

M7 Duty Cycle

 
M7 Duty Cycle

You can see in the chart, that the M7’s enclosure temperature gets hotter if the DC input voltage is higher, or if the duty cycle is higher.

For example, if the DC input voltage is 10V, and the unit is operated at 25% transmit duty cycle, then the enclosure temperature would be about 42 degrees C.  Given the same duty cycle, the enclosure temperature would be 46 degrees if the DC input were to be 14 volts.

Raveon offers a heatsink option for the M7.  The heatsink is large finned heatsink that covers the top of the M7, and is secured on with thermally-conductive epoxy.  When this heatsink is attached, the M7 will stay cooler.  The following chart illustrates this:

m7dutycycle-heatsink

The above data is the M7’s enclosure temperature with a heatsink secured to it.  The heatsink covers the top of the enclosure and uses normal air convection (no fan).  It reduces the case temperature by about 4-8 degrees.

If a CPU cooling fan or similar fan were added instead, the case temperature rise would be only a few degrees above ambient.

For technical information about Raveon’s UHF data radio modems, <click here>

For technical information about Raveon’s VHF data radio modems, <click here>

The M7 data radio transceiver with the optional heatsink attached is shown below.

Filed under: Hardware Information, Installation Information | Posted on March 23rd, 2009 by John Sonnenberg

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