Commercial circuit, 1210 resistor dissipating 1 Watt

amdx · 2017-08-18T14:35:12+00:00

After replacing a defective 9V regulator, an output Mosfet a transistor, and a 100 resistor that looked like it had been hot, I have a working FM transmitter. But then I noticed the 100 ohm 1210 smd resistor running hot. The same 100 resistor that I had previously replaced. The resistor has 8.7 Volts across 100 ohms or 0.76 Watts. (a 1210 smd is a 1/2 watt device) Before I even started the repair I bought another FM transmitter of the same model. (The model does have same updates to the circuitry. But I'll leave that for later discussion.) I got out the new FM transmitter and measured the voltage across the 100 resistor (btw, it also feels very hot) the voltage is 9.8V. 9.8V across 100 ohms is 0.95 Watts. Voltage measured with 10Meg input DVM. The original unit worked for about 4 years averaging 4 hrs a day. So the questions! Do Leave it as is? Replace the 1210 smd with a 1 watt part? 2 Watt part? The 100 ohm is a voltage dropping resistor to the output Mosfet when the low power mode is selected. I always run in low power mode, cause the resistor to dissipate high power. Here's the updated circuit. Thanks, Mikek

Aug 18, 2017 26 Replies

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John Larkin 8 years ago

The upper 100 ohm resistor dissipates about a watt in low-power mode.

John Larkin Highland Technology, Inc picosecond timing precision measurement jlarkin att highlandtechnology dott com http://www.highlandtechnology.com

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amdx 8 years ago

Yep, the plot thickens. The output part was labeled K2. I had several recommendations as a cross, but the best came from here, from Jorge? He had a K2 part in an Icom, the manual listed that part as a RD01MUS1. I don't know if it is a P channel or an N channel, because google has pages showing the arrow going in both directions. Here's a spec sheet, see page 4 for a symbol of the device I am using.

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kevin93 8 years ago

On Friday, August 18, 2017 at 2:28:57 PM UTC-7, amdx wrote: ...

...

You would think they would say N or P-channel, very unclear datasheet.

I don't think I've ever seen a P-channel RF transistor. Mobility is higher in N-channel so they are usually better for a given cost.

I would think N-channel. That fits with the circuit.

kevin

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amdx 8 years ago

So what does the direction of the arrow signify? Thanks, Mikek

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amdx 8 years ago

After more googling, I see many more N channel Mosfes with the arrow pointing in. So I agree that I have an N channel Mosfet. I also think I have it drawn as an N channel Mosfet. So I'm not sure were P channel started.

Thanks, Mikek

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amdx 8 years ago

Corrected updated schematic,

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tabbypurr 8 years ago

or,

nsmitter

voltage across

Whenever I've looked at power resistor specs the stated Pdiss has been for

1,000hr life. I've even seens power Rs where the rated diss is for operatio n at -100C!!

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