I wired up the bitx40 and made 1st contact with it at 5w. Nice! I noticed the transistor thats on the radiuno boarsdgets hot. Theres no heat sink on it, is this normal or a symptom of something?

That is a linear voltage regulator (7805).

How hot is hot? Can you keep your finger on it?

The temperature increases with the voltage applied. I use mine at 13.8V power source voltage and is warm, not hot. I can take a measurement of the temperature after 30 minutes and let you know how my regulator is doing.

Good luck,
--
Ion

VA3NOI

With 78xx series linear regulators, they 'Throw away' the excess power as heat.

approximately;

( Vin - Vout ) x Current = Wasted heat (in Watts)

so, the bigger the difference between input voltage and output voltage and the bigger the current draw, the more heat you'll generate.

After many burned fingers and a few burned out regulators, I now always fit a heatsink! (Was the first thing i did to my Raduino!)

Leon, M6LPK

开云体育

>>After many burned fingers and a few burned out regulators, I now always fit a heatsink! (Was the first thing i did to my Raduino!)

I've also heard that one can regulate voltage in two stages to spread the heat between two regulators. For instance, from 12 to 6V by first using a 7809 regulator followed by a 7806. Theoretically, I guess, the same amount of heat would be wasted, but only half as much in each device. A heat sink *might* be the simplest solution, unless you don't have one but you *do* have the two regulators.

73,

Todd K7TFC

Yes, cascading regulators like that works well to spread the dissipated power between them. It's still being tuned into heat but not all in one package. A very usefull technique even with heatsinking, if you're drawing some decent current (1A+).

I've run the audio stages of a few rigs i've built on a 7809 and then into a 7805 to supply the digital stuff. with the added bonus of a little bit more decoupling to help keep the digital noise out of the AF/RF circuits.

Leon, M6LPK

So, last night I did some meaurements.

Room temperature at 18 °C. LM7805 goes as high as 52°C with 13.8V at power feed.

Temperature drops to 49°C when at 12.8 V (that is the votage of a fully charged lead acid battery)

Temperatue rise of 34°C. If you will enclose the board and ambient temperature is summer hot you can expect temperature of the case to get close to 100. A heat sink will be required.
--
Ion

VA3NOI

Following possibilities are to be considered.

We can divert? the back-lit circuit of LCD to unregulated voltage with higher series resistor.

The datasheet for7805 indicates around 3V . Now we have 7 or 8V and the load current? might be around 1 amp.We can mount? it on a heat sink.
Thus we can also have a input 12 or 13.8V through a 5 ohm 5 watt resistor? and it helps reduce the input/ output differential of the regulator.

regards

?sarma

?vu3zmv

Hi Sarma!

The load current is way lower, about 0.15A (what the current draw on the +5V line is).

That results in a power dissipation of 1.25 to max 1.35W.

TO-220 package has a thermal resistance Jonction to ambient in free air of 23.9 C/W

1.3W * 23.9 C/W = 31 C, which is very close to what I have measured on my board
--
Ion

VA3NOI