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Author Topic: How do you drop a volt or two?  (Read 6944 times)

Colin Bishop

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How do you drop a volt or two?
« on: December 13, 2010, 06:55:24 PM »

Any electrics wizards out there who can help?

I am building a small model powered by a couple of 3 volt nominal 280 type motors. It's a bit bigger than 'Plastic Magic' but not by much!

To save weight, I have bought a couple of ACTion P52A speed controllers plus a P82 mixer. The rudder control is a Hitec  a HS5 micro servo.The way all this works is that you connect the single power supply of 4.8 volts ( (in this case 3800 mah NiMH cells|) to the RX. The mixer then plugs into the rudder channel and the two ESCs plug into the mixer with their outputs to the motors. So the motor juice ultimately comes from the power pack via the RX.

The main limitation, as you might expect, is that the maximum permitted power drawn by each motor is 1 amp. My problem is that the motors on 4.8 volts draw around 1.5 amp on full load plus the speed is probably likely to make the boat plane which is not a desirable scale feature. So I need to slow the motors down. If I go down to 3.6 volts on the main power pack the RX is marginal and the rudder servo will probably drop out as it will be below its rated level. So I need to reduce maximum voltage to the motors to keep the current down to below 1 amp.

I have had some encouraging results by introducing either a 12v 0.5 amp  SES bulb or a 12v 5 watt car festoon bulb in the motor lines. but I'm not altogether sure why this should work. As the load on the motor goes up, the bulb illuminates but the power drawn stays well below 1 amp, even when the motor is stalled. Why is this?

Also, is there a more scientific method of dropping the max motor voltage from 4.8 volts to around 3.5 volts?

An alternative option is to program the TX to restrict the throttle travel but I'd like to know if there is an alternative.

Any suggestions most welcome.

Colin
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Martin [Admin]

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Re: How do you drop a volt or two?
« Reply #1 on: December 13, 2010, 07:03:12 PM »

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Shipmate60

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Re: How do you drop a volt or two?
« Reply #2 on: December 13, 2010, 09:26:28 PM »

Colin,
What does each motor draw in air connected to the shaft.

Bob
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flashtwo

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Re: How do you drop a volt or two?
« Reply #3 on: December 13, 2010, 09:53:35 PM »

Hello Colin,

An ordinary resistor has a linear relationship between the voltage drop versus the current flowing through it - Ohm's Law.

The light bulb is not linear, since as it gets hotter its resistance increases, i.e. the higher the current, the hotter it gets, the resistance goes up and we get a larger voltage drop. Thus, with the bulb in series with the motor, the motor draws lots of current through the intially cold bulb, the bulb gets hot and reduces the current flow; ultimately an optimum point is reached where the current just maintains a temperature of the bulb pertaining to that current.

A forward biased power diode can give an approximately constant 1.2 volt drop over a range of current. Typical diode is a 1N5401 which can take 3 amps. (see Maplins part no. QL82D @ 29 pence.)

Ian.
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Colin Bishop

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Re: How do you drop a volt or two?
« Reply #4 on: December 13, 2010, 10:36:07 PM »

Bob, connected to the shaft on the bench each motor draws 0.25 amps.

Flashtwo, a diode has been suggested by somebody else but they only work one way so it gets a bit complicated when you want to reverse the motor - as you do!

Colin
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DavieTait

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Re: How do you drop a volt or two?
« Reply #5 on: December 13, 2010, 10:47:22 PM »

You can put 2 diodes in parallel on each + and - wire Colin , just make sure they are set opposite directions , that will mean you will have a voltage drop only 1 direction each way and the motors will run as normal. You have to put the 2 diodes on each wire though so you'd need 8 diodes for the set up your running.
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flashtwo

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Re: How do you drop a volt or two?
« Reply #6 on: December 13, 2010, 10:51:38 PM »

Hi Colin,

To make it reversible, you just have two diodes connected in parallel, "pointing" in opposite directions with the cathode of one diode connected to the anode of the other. One will be forward biased "on", and the other will be reversed biased "off" - this will give you about 1.2volt drop in either direction.

There are devices called DIACs, but the forward bias voltage would be too high for this application.

Ian.

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flashtwo

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Re: How do you drop a volt or two?
« Reply #7 on: December 13, 2010, 10:52:45 PM »

Davie beat me to it!

Ian.
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DavieTait

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Re: How do you drop a volt or two?
« Reply #8 on: December 13, 2010, 10:54:27 PM »

Am I right in thinking its best to do it on both connections or can it be done with just one cable ?? Its been 25 years since I did my Electronics ONC lol

Thinking about it it should be ok to only have 2 diodes on the + side as I don't think it would really matter where on the circuit it is as the voltage drop would affect the motor regardless or am I wrong again ( wouldn't be the first and definitely won't be the last lol )
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Colin Bishop

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Re: How do you drop a volt or two?
« Reply #9 on: December 13, 2010, 11:01:45 PM »

Reversed diodes - brilliant! Why didn't I think of that?

Need to give it a bit more thought - tomorrow!

Thanks guys!

Colin
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DavieTait

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Re: How do you drop a volt or two?
« Reply #10 on: December 13, 2010, 11:04:29 PM »

It might be an idea to check with Ian if you need to do this on both cables or just the + side Colin , as I said its been 25 years since I did my Electronics ONC and to say i'm "rusty" gives rust a bad name lol
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Shipmate60

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Re: How do you drop a volt or two?
« Reply #11 on: December 13, 2010, 11:17:39 PM »

Colin,
If the motors are too fast could you consider a geared drive which should reduce the current consumption.

Bob
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Rex Hunt

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Re: How do you drop a volt or two?
« Reply #12 on: December 13, 2010, 11:41:40 PM »

It might be an idea to check with Ian if you need to do this on both cables or just the + side Colin , as I said its been 25 years since I did my Electronics ONC and to say i'm "rusty" gives rust a bad name lol

Does the +ve side not become the -ve side when the motor polarity is reversed...........or am I missing something?

Rex
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SteamboatPhil

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Re: How do you drop a volt or two?
« Reply #13 on: December 13, 2010, 11:46:38 PM »

Humm, my version  little batteries....... {-) {-) {-)

       But what do I know......dark magic    %)

      OK I'll keep quiet and get back to  my chuff chuff chuffing.... O0 O0

      Good luck with the project Colin, which should work without my hints and tips   O0 O0 O0
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nick_75au

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Re: How do you drop a volt or two?
« Reply #14 on: December 14, 2010, 09:12:07 AM »

You would be better boosting the voltage to the receiver with this gadget, much more elegant than diodes ok2

http://www.hobbyking.com/hobbyking/store/uh_viewItem.asp?idProduct=11784

Providing the esc will run on the lower voltage. You would have to be a little fiddling with the esc wiring to power direct off the battery but allow the Rx signal to be sent to it.

Perhaps a P78 and the gadget would work better as the P78 works from 2 volts to 12

Nick
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malcolmfrary

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Re: How do you drop a volt or two?
« Reply #15 on: December 14, 2010, 10:14:18 AM »

My vote is with the "pair of diodes in series with each motor" lads, but it might need more than one pair to get the required drop.  A bit of suck it and see, there.  Had there been just one ESC I would have just stuck the two motors in series, but I assume from the description that differential control is intended.
1N4001 diodes are smaller and a bit cheaper, and with their rating of 1A (32A surge) should do the job.  You could always bung a couple of ohms worth of dead electric fire element in parallel with the diodes - this would take some of the current away from the diodes, but the diodes would limit the maximum voltage dropped and bypass the "spare" current away from the resistor.
Stepping the voltage up to the RX is more elegant, but I would want a definitive reply from ACTion as to how well the ESC output stages would perform on such a low voltage.
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roycv

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Re: How do you drop a volt or two?
« Reply #16 on: December 14, 2010, 03:34:57 PM »

Hi, I have not worked this out but what about making the diodes Zener Diodes rated at 3.5 volts, have them mounted in pairs i.e. opposite connections, but each pair connected across each motor with a resistor to bleed off the volts above 3.5.  I think this would limit the voltage to 3.5 for each motor.  The speed controller is connected to the motor and then to the end of the bleed resistor maybe 0.5 ohms at 5 watts.

The esc is supplying power at say 4.8 volts but chopped up to give the speed control, so I think the zener diodes would just limit the amplitude of the esc output to 3.5volts for the motors.  Above this the resistor gets warmer.

This has caused considerable pain to my fading abilities as I did my electrics over 50 years ago, but what do I know!
Roy
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Colin Bishop

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Re: How do you drop a volt or two?
« Reply #17 on: December 14, 2010, 05:33:08 PM »

Well, a bit more to report:

I was in Squires today and they had some IN5401 diodes at 14p each so I bought 8 of them. Having got them home I experimented with them using the ACTion 280 motors and also a couple of Squires 3v motors of the same can size that I bought a while back. Results are:

ACTion Motors
With two diodes in series: 1 amp on load in the water 2 amp stall
With three diodes in series: 0.75 amps on load 1.4 amp stall

Squires Motors
With two diodes in series: 0.6 amps on load 2.5 amp stall (more than the ACTion motor!)
With three diodes in series 0.5 amps on load 1.75 amp stall

I suppose I could protect the ESCs with a 1 amp or 800 ma fuse which would blow in a stall situation and then adjust the travel on the TX electronic throttle travel for belt and braces.

However, with regard to Roycv’s suggestion I have had a PM from another member suggesting a 3.9 Ohm 2.5 or 5 watt resistor in the motor line which is very similar but omitting the zener diodes.

A small amount of ‘central heating' in the boat won’t matter much as the main power pack is 3300Mah.  Intriguing isn’t it?

Colin
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malcolmfrary

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Re: How do you drop a volt or two?
« Reply #18 on: December 14, 2010, 08:16:57 PM »

Quote
Hi, I have not worked this out but what about making the diodes Zener Diodes rated at 3.5 volts, have them mounted in pairs i.e. opposite connections, but each pair connected across each motor with a resistor to bleed off the volts above 3.5.  I think this would limit the voltage to 3.5 for each motor.  The speed controller is connected to the motor and then to the end of the bleed resistor maybe 0.5 ohms at 5 watts.
No.  The zener diodes are a voltage drop at the zener voltage with the current one way and a forward conducting diode the other, so basically, two in reverse parallel are effectively a short circuit.  If they are in nose-to-nose series, they become a bipolar zener.  It just remains to calculate the value of resistance needed, and the appropriate wattage for both resistor and zeners.  The addition of zener diodes in parallel with the motor would limit voltage to the motor by shunting excess current away from it, but would increase overall current from the ESC,  which is not the object of the exercise.
Colin seems to be well on the way with his 1N5401's at a sensible price.  Simplest is usually best, and the non-linear resistor in series (or, to give it its technical name, "light bulb"), might be the next best solution.  Next best because, in operation, it will be being pulsed at ESC frequency, and might not like it.  In the telecoms trade, barretters were never a good idea in a pulsed circuit, but were a good reliable line current regulator at the non-pulsed end of a transmission bridge.
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Colin Bishop

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Re: How do you drop a volt or two?
« Reply #19 on: December 14, 2010, 08:57:51 PM »

Well I'm still experimenting! Circuits involving Zener diodes seem to be aimed at stabilising the voltage and I don't actually want to do that. The main objective is to limit the current, the voltage drop may be a by product of that. I have ordered some power resistors around the recommended range and will see how that goes. Bottom line is to use the electronic restrictions on the TX throttle.

One thing I have noticed is that at these very low voltages, the resistance in the mechanical setup is a crucial factor. If the two drivelines are not exactly matched in terms of free running then the current absorbed by the one with marginally more resistance is quite disproportionate to what you would expect. I suspect that at higher voltages you would not notice the difference to anythiong like the same extent.

In a sense, I have painted myself into a corner here by imposing certain absolute values in that the ESCs are limited to 1 amp but the RX and the rudder servo both really require 4.8 volts. The problem is the motors. At 4.8 volts they are operating on at least 75% of the ESC capacity while on load and considerably exceed that if stalled which will certainly let the smoke out. What is really needed are 3v nominal 280 type motors which draw appreciably less than the ACTION or Squires types but until you try this setup you don't appreciate the potential problems.

With hindsight, a separate motor supply and more robust ESCs would probably have been better but there you go! I am still hopeful that I can make this setup work if I can just limit the current drawn by the motors downstream of the ESCs.

Colin
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roycv

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Re: How do you drop a volt or two?
« Reply #20 on: December 15, 2010, 12:19:13 PM »

Hi Malcolm thanks for putting me straight re zener diodes.

Colin, why not replace the drive motors with a couple of motors taken out of old servos, they will run at the right voltage and should be powerful enough.

regards Roy
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Colin Bishop

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Re: How do you drop a volt or two?
« Reply #21 on: December 15, 2010, 01:25:06 PM »

Roy,

Because I have already got everything set up for 280 size motors including mounts, two expensive couplings etc. and I'm not convinced that the servo motors would be powerful enough for what I want anyway although these have been suggested as possibilities. Same reason for not trying Bob's suggestion of gearing although that wouldn't reduce the stall current.

I just thought that squeezing the amount of current/voltage to the motors would be all that was necessary but it seems to be extraordinarily hard to do! I will see whether the power resistors I've ordered do the job, if not then I will have to have a rethink!

This topic is certainly throwing up some interesting points though!

Colin
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BigA

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Re: How do you drop a volt or two?
« Reply #22 on: December 15, 2010, 04:38:57 PM »

Colin - That's a great website! Never heard of Squires before - yet another supplier! Thanks.
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Colin Bishop

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Re: How do you drop a volt or two?
« Reply #23 on: December 15, 2010, 06:15:58 PM »

Yes, Squires is a bit of a sweetshop to say the least!

Some practical tests tonight in the bath. With 4 diodes daisychained, max speed was OK'ish pulling a total of 0.5 amp on both motors but there didn't seem to be all that much in hand for stormy weather. Might be OK but nothing in reserve. However, reducing to three diodes caused the boat to aspire to being a Club 500 so ideally something between the two is needed towards the lower end. I shall have to try out the resistors when they arrive. I didn't try two diodes as I'm not building an aeroplane...

The diodes got slightly warm but seemed happy enough.

Photos below show the test rig and the motor installation.

Colin

 
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RaaArtyGunner

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Re: How do you drop a volt or two?
« Reply #24 on: December 15, 2010, 11:31:22 PM »

Colin - That's a great website! Never heard of Squires before - yet another supplier! Thanks.

Can't locate it  <:(  can you post the link
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