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Servo power busses?
03-31-2004 | 10:43 AM
#26
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RE: Servo power busses?
ORIGINAL: patf
Is there any concern for how much draw you are pulling across the receiver boards? (as Eddie alluded to) using a kingcat for and example there are some 10(?) digitals all pulling amperage across the receiver board. That is where I would be most worried.
Is there any concern for how much draw you are pulling across the receiver boards? (as Eddie alluded to) using a kingcat for and example there are some 10(?) digitals all pulling amperage across the receiver board. That is where I would be most worried.
I just went into the lab and attempted to measure the resistance of the PC board traces on the receiver for the power connections. None of the (fairlyexpensive/accurate) multimeters we have could accurately measure the resistance - the connections between the multimeter leads and the pins themselves were much greater resistance than the traces themselves. However, I did measure up to as much as 0.2 ohms of resistance between power pins, so let's say that this was all in the PC board traces. Power = I^2 * R, so let's say we're drawing 20 amps continuous (way high!). That means we might have to dissipate as much as 40 watts in that PC board trace. Now I can hold a 200 watt soldering iron on that trace all day, and I might singe the epoxy laminate, but I'm not going to melt the copper trace. I don't think that running that much current (or the much less value it really is) through the receiver boards is a problem in that respect.
Where it may be an issue is the spikes of current draw as the servos get loaded or un-loaded and how that gets translated into power or ground bounce noise inside the receiver. I think that's the reason for the noise filters on the extensions in the KingCat, and they are fairly effective at cutting down high-frequency noise, but I'm not sure how much they effect this type of noise that would likely be lower frequency.
Bob
03-31-2004 | 11:57 AM
#27
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From: Cape TownCape, SOUTH AFRICA
RE: Servo power busses?
Bob,
Another method to measure resistance for very low ohmic values is to plug a lab supply into the BATT end of the RX and a spare lead into THROT. Solder a 5 Ohm 10W resistor to the lead and set the supply to 5V. The current through the traces will now be approx 1A. Verify the current via the lab PSU. Use a uV meter and measure the voltage drop from either end of the PCB trace. My gut tells me you wil calc less than 10 miliohm. More revealing would be the drop across the std servo leads and extensions....
The RX sees the supply of the batt pack minus the batt lead resistance. It follows that the Batt lead and switch is going to be the biggest factor in voltage drop.
Cheers
Andre
Another method to measure resistance for very low ohmic values is to plug a lab supply into the BATT end of the RX and a spare lead into THROT. Solder a 5 Ohm 10W resistor to the lead and set the supply to 5V. The current through the traces will now be approx 1A. Verify the current via the lab PSU. Use a uV meter and measure the voltage drop from either end of the PCB trace. My gut tells me you wil calc less than 10 miliohm. More revealing would be the drop across the std servo leads and extensions....
The RX sees the supply of the batt pack minus the batt lead resistance. It follows that the Batt lead and switch is going to be the biggest factor in voltage drop.
Cheers
Andre
