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Gate voltage
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Newbie question here. I read that the IRF510 gets unstable and breakdown at 3-4 volts.
When setting RV2&3 to 100ma each , when i check the gate voltage it is 3.60 and 3.99-4V(2 different irf510 mosfets), amps are about 1.6A @100ma for rv2&3 ?on a tone Should I drop the RV2&3 down so as gate voltage is 2.5v, however I only get 50ma on PTT, but a loud bellow results in? 1.3 amp draw. Thanks Alan |
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Alan? mm0wxt Gate voltage only determines where on the voltage versus current curve the device is operating.? As you mentioned, different MOSFETs will show more or less gain at a particular gate voltage.? MOSFETs do not draw gate current, so they operate a bit differently from older tube type circuits where it was possible to draw grid current at some bias settings and at some signal levels.? Think of an IRF510 as a variable resistor between source and drain, and with the gate voltage determining the resistance value.? Purpose of adjusting the gate bias is to set quiescent current at a location on the drain current curve where operation in push-pull will result in linear output.? Setting idling current too low will cause distortion because it makes the IRF510 operate on the non-linear part of the drain current curve.? Setting the idling current too high will result in non-linearity due to drain current operating near the top of its curve and potentially flat-topping or running out of dynamic range.? Setting the idling current too high will also cause excessive heat to be generated in the IRF510 device.? In an IRF510 based push-pull RF power amplifier stage it is critical to match the antenna to the transceiver output impedance (nominally 50 ohms).? If the antenna terminal impedance is too low it causes the impedance that is reflected back to the IRF510 devices to be too low and allows them to draw too much current on peaks and results in overheating.? If the antenna terminal impedance is too high it is reflected to the IRF510 devices as an excessively high impedance which allows the rather high internal capacitance inside the device to throw it into oscillation.? You do not want your RF power amplifier stage to self-oscillate because then it is not being controlled by your transceiver, and can generate excessively high currents and/or voltages, resulting in component breakdown and failure.? Remember that the uBITX and BITX40 designs are not yet old enough to smoke. Arv? K7HKL _._ On Sat, Mar 3, 2018 at 1:17 PM, <mm0wxt@...> wrote: Newbie question here. I read that the IRF510 gets unstable and breakdown at 3-4 volts. |
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Alan MM0WXT Very strange.? There seems to be several anomalies involved.? (1) You indicate 200 ma of drain current but no voltage on the drains.? (2)? 200 MA on PTT activation and 1.8 amps on CW key-down would indicate that your RF power amplifier is operating properly, but you are not seeing any RF on your watt meter. (3)? Your watt meter is seeing zero output but IRF510 current indicates power output should be present.? Lets go through some possibilities: (1)? Are you measuring the drain voltage of the ground connection on the source pin on those IRF510's?? Do you measure any voltage on any of the IRF510 pins... Source = ?? Drain = ?? Gate = ? ?? Thinking here that the ground lead for your voltmeter might not be making a good ground connection. (2)? Do you have a way to measure RF volts on the drain of each IRF510?? Can you measure RF volts on the input to the output filter?? Can you measure RF volts on the antenna end of the output filter???? If you do not have a scope or RF probe, it is easy to build a simple RF probe as shown in N5ESE web page.? ? for the probe are not particularly critical because all we need to know is presence or absence of RF voltage. (3)? Is the watt meter making reliable connection to the antenna end of the output filter?? When you disconnect the watt meter from your transceiver and connect an ohm meter across the input of the watt meter it should measure within a couple ohms of 50 ohms.? Using an RF probe or oscilloscope you should be able to trace RF voltage from input (gate) of the IRF510 devices, to output (drain) of the IRF510's.? Then you should be able to trace this RF voltage from output of IRF510's to input of the output filter, and through the output filter section to the antenna connector. This next part is going to be a bit more complex, mostly because my uBITX is a beta version that has only two LPF filters on the output.? Yours should have 4 filters.? The schematic for production units is at this URL. The uBITX IRF510 output is through transformer T11.? This transformer couples RF from the IRF510's to the filter selection relays, through the selected filter, and through the transmit-receive relay K3 to the antenna connector.? Any break in this stream can cause no RF to be present at the antenna connector.? Transformer T11 also blocks any DC voltage from entering the filters and antenna connection.? The filter selection relays (KT-1, KT-2, and KT-3) are operated by control voltages marked [TX-A], [TX-B], and [TX-C] on the schematic.? These voltages turn transistors Q-17, Q-18, and Q-19 ON/OFF to select the proper filter for your operating frequency. ?Relay? ?K3 switches the antenna connector between receive and transmit.? Its is operated by voltage labeled [TX] on the schematic. Schematic depictions of the collector circuits for Q-17, Q-18, and Q-19 looks a bit weird.? It was done that way because space was at a premium for showing the protection diodes D-11, D-12, and D-13 as being across the relay coils.? It is correct but just looks strange to me.? Lets see how far that gets you in finding the problem.? Don't try to over-think the situation.? This stuff is not rocket science or brain surgery. Arv? K7HKL _._ On Sun, Mar 4, 2018 at 3:05 PM, <mm0wxt@...> wrote: Hi and thanks arv. I did some checks and getting zero volts on dummy load, not even warm. Checked drain voltage on tx and zero. Ptt still draws 200ma at PA and 1.8 amps on a cw carrier |
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