What current can be drawn from the servo signal?

[FredericG]

Does anybody know what the typical current is one can/should be draw from a receiver's servo signal? I suppose a servo does not have a high impedance as it would be vulnerable to noise...

Thanks,
Frederic

[jeffs555]

It depends on the receiver. The servo outputs on a receiver are typically driven from a cmos decoder, so the drive current is normally somewhere around 5ma, but could be even lower.

[AndyOne]

Some of the more expensive receivers use an R-C filter in all the servo signal output lines so this will reduce the output even more than those that don't use filtering.

Andy.

[andrew b]

A few years ago I regularly ran 3 Futaba S148's off a single output on the receiver with no problems.

[FredericG]

I did a small test with a Futaba PCM and a Schulze Alpha 435 receiver. Both generate a 3V pulse.

Connecting the Schulze via a 680 ohm to the led of an optocoupler, the voltage drops to 2.6V, the Futaba signal drops to 1.5V. With 1K5 the voltage is 2V on the Futaba.

I suppose the voltage drops are due to protecting resistors in the receiver and that I cannot harm the receiver at these light loads. Right?


Thanks
Frederic

[Acetronics]

This question has already been answered on this forum ...

DO NOT exceed 5 mA for CMOS decoders, 10 mA for NE 5044 decoders and .1mA for old µP decoders ( some have ... 10k resistors in series !!! - Old MC18 Graupner i.e.)

1mA seems a good value for modern receivers ...

For my gadgets ... I always use 47k in series with the decoder output !!! sure no bad surprise involved !!!


Alain

[MatC]

My hitec HFS-05MS uses a PIC to decode the signal, so that gives around 20mA max current.

[henrik04]

I did a small test with a Futaba PCM and a Schulze Alpha 435 receiver. Both generate a 3V pulse.

Connecting the Schulze via a 680 ohm to the led of an optocoupler, the voltage drops to 2.6V, the Futaba signal drops to 1.5V. With 1K5 the voltage is 2V on the Futaba.

I suppose the voltage drops are due to protecting resistors in the receiver and that I cannot harm the receiver at these light loads. Right?


Thanks
Frederic
To connect the LED of an opto I would simply use a 20 cents FET (2N7000) for example and avoid loading the receiver. So simple.. :D