Matchbox multiple servo controller

[Piers Acland]

[Nick Somerville]

I have a question about these JR matchbox units. The manual quotes an operating voltage of between 4.8-6v. It can either take power from the receiver or a separate battery and switch harness can be directly plugged in. I have purchased a Life battery to power the unit and it has been operating just fine using a servo tester and a 1s lipo for signal to set up my four flap servos.

 

Now, I have HV servos in the fuselage of my model and HV aileron servos and as I have a pair of 2s lipo receiver batteries from a previous model I would like to use these rather than buy a new pair of life batteries.  So my question is, will the signal from the receiver carry the full voltage to the JR Matchbox that is powering it’s four servos from the lower voltage Life battery? I have noted that there is a JR HV Matchbox available. Perhaps I should have bought one of those. Or would a cheaper option be to find a simple voltage regulator to go between receiver and Matchbox; though at first looks the servo voltage regulators are not that cheap.

[Martin McIntosh]

Nick, I too intend to use a Matchbox on 2s LiFe which works OK when bench testing. A 5s NiMh would of course exceed this voltage when fully charged. I would suspect that so called HV servos simply have a regulator inside since I have a bipe with two HV slim wing and two normal of the same make. There is no difference in speed between them. (All I could get at the time).

I have a small Hurricane, converted to i/c, which at one time gave problems with a servo not liking the BEC 5.5V so to be safe I added a 5V UBEC to the i/c version (2s LiFe).

[Paul De Tourtoulon]

Sorry but no idea what you are talking about, but can you use a 7805 ( 5 = 5v ) 1 ampere  voltage regulator, they cost less than £1

[Nick Somerville]

Thanks for the replies Martin and Paul. The 7805 is unreliable below 7v apparently, so although voltage from 2s lipo should be above that it is a small risk. I phoned a model shop just now and they suggested simply connecting the JR Matchbox with just the negative/earth and signal wire. Apparently it will work fine like this. Electronics are out of my knowledge window so any thoughts appreciated before I start trying it out.

[Paul De Tourtoulon]

I have been using them for 30+ years and never had a failure, and I still have a couple lowering the voltage on the ignition boxes on my Rcgf engines, and

on my 250 year old  Marutaka Spitfire using 5  nihm elements on my 4.8v Graupner servos, and all without heat sinks.

[Frank Skilbeck]

2 hours ago, Martin McIntosh said:

 I would suspect that so called HV servos simply have a regulator inside since I have a bipe with two HV slim wing and two normal of the same make. There is no difference in speed between them. (All I could get at the time).

 

Maybe on the electronics but not on the motor, if you look at the torque specs they go up as the voltage increases which is what you'd expect. But normal and HV servos working on the same voltage would most likely give the same speed and torque, just that on high voltage the regular servos may let out the magic smoke.

[Martin McIntosh]

PdT, what use is a 1A 7805 when you are running eight or more digi servos?

 

Nick, I may have forgotten to say that I shall be using a Failover Switch to safely couple two LiFe packs together. They are rated at 5.9V o/p, 5A continuous. I have had several of these in use for quite a while and the only failure was with one which smoked but still gave an output on the good side.

[Paul De Tourtoulon]

One on each servo if he doesn't want to spend £300 + on a powerbox,,,,

[Andy Stephenson]

If using a 5V regulator a much better one is the LM2940 it has a "drop-out" of 0.5V and providing it has a big enough heatsink it can handle up to 26V input.

The interesting thing is that they use the term "drop-out" which for an 7805 is true because the output does actually drop to zero when the input is below the threshold, however, the LM2940 maintains a 0.5V difference between the input and output as the input drops. This leads to a much safer situation if one is using a regulator in a fail-safe application.

[Martin McIntosh]

From what I remember about the 2940 is that it is a low drop out device, meaning that it will maintain the o/p voltage when the i/p is nearing 5V. Very good but again only 1A I think so not much use in a model.

Not really looked at my Matchbox that carefully yet but I believe that they can be powered separately from the Rx and servos, in which case a low current regulator would be fine.

[Andy Stephenson]

If you need a regulator with a higher current rating, the easiest way is to simply buy a UBEC from one of the many suppliers.