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Any electronics experts out there?

Moonshane

By Moonshane
in DIY Astronomer

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Moonshane

Moonshane

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Hi all

I am struggling with a way to reduce my 12v battery to drive a motor at the correct speed for my http://stargazerslounge.com/diy-astronomer/127311-equatorial-platform-dobsonian-telescopes.html and wonder if anyone can point out where I am going wrong please?

I am keen to just have the one batter to power my fans and when cooled my EQP. this means reducing my battery to say 6.75v to run the motor after the cooldown is over and I want to start observing.

using Volts out = Volts in x (Resistor value 2 / Res Value 1 + Res Value 2) http://tronixstuff.files.wordpress.com/2010/03/vdformula.jpg I have a few options with regard to resistors and can play about with them to get the right voltage and then use that.

I found a really simple diagram http://tronixstuff.files.wordpress.com/2010/03/voltdivide.jpg

and tried this tonight with two 330 ohm resistors to see if the motor runs at half speed (I am powering a 12v DC motor).

here's some pics which show a roughly put together set-up.

I suppose my question is how (or rather where) do I connect to get the 6v to the motor?

This seems really simple but I cannot fathom it at all and it's becoming very frustrating!

Thanks in advance

Shane

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Photosbykev

Photosbykev

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A problem I can see is the output is directly proportional to the input so as the battery voltage drops the motor speed will also drop. I think you would be better off with an adjustable regulator circuit so you can set the output voltage and it will be independant from the input voltage.

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JamesF

JamesF

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I did an electronics AO level in my L6th year, got an A grade and can remember almost nothing about it :glasses1: I shan't let that minor detail get in the way though.

If you're making a voltage divider out of two equal value resistors then the voltage measured between the resistors will be half the voltage across the pair, so putting two 330 ohm resistors across a 12V battery will give you 6V measured across either resistor.

Unfortunately in the real world it's not that simple as once you connect a load across the resistor it will have its own internal resistance and can change the ratio of the voltage divider. With a motor it might even be the case that the internal resistance varies with the speed at which its turning, but I just can't remember.

I feel sure a voltage divider is a very poor way to achieve a reduction in voltage such as you're after anyhow. Some sort of voltage regulator would probably make more sense. Or, if you're prepared to stump up £10, you just buy one of these:

Caravan and Leisure In-Car Charger

James

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sixela

sixela

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You can find some variable voltage regulators that accept 12-13.8V in and output all kinds of voltages at the other end; they usually have a car cigarette lighter plug. You'll find these at most car accessory centres. There are also dedicated 12v-6.5v DC/DC converters with higher amperage.

The circuit you have isn't realistic. By definition, it only works when most of the current is wasted and almost none goes through whatever you put in parallel to one of the resistors (because if current goes there, it changes the divider).

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Nillchill

Nillchill

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use this

Image15.jpg

this will give you the output voltage and current and regulate the supply

this circuit uses a 7808 8volt regulator. This increases the output current of the regulator, it also introduces a 0.7-volt drop caused by the transistors base-emitter junction. This results in the actual output voltage being approximately 5.3 volts, if a 7806 is used. By using the 7808 instead you get a 7.3 volts output. This is at the upper end of what a 6-volt auto electrical system should have when the generator is running, so 7.3 volts is fine. If You would rather have 6.7 volts instead, you can drop the voltage 0.6 volts by adding a 1N4002 diode in series with the input to the base of the transistor

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Mikea

Mikea

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You are on the right track, but the simple divider will not work here because the current drawn by the motor is much greater than the current through your divider (18mA). As soon as you add the load, the voltage at point B on your diagram will fall way below 6V because the effective resistance between B and C is now much less than 330 Ohms. Using smaller resistors would be one solution, but not a good one because you will be wasting battery power heating up resistors! Kev and James both mentioned regulators, which is a better idea. A switching regulator will give you the best efficiency - an analogue regulator will be less efficient as it will also waste some power as heat.

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Moonshane

Moonshane

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wow thanks guys! some excellent pointers there! I am quite happy to buy something that will work, I just don't want to keep buying things which don't until I find one that does - it's a costly way to build rubbish in a drawer :):glasses1:

based on some of the comments I have found this AX3022 Based Switching Regulator Adapter, Step-Down on eBay (end time 07-Mar-11 07:10:13 GMT) which seems to fit the bill. it seems to be able to cope with the amps drawn by the motor (about half an amp with no load) and I think the blue box with a cross head screw is the 'rheostat' for controlling the output voltage up and down?

this looks to be as simple as connecting + and - in and out and then tuning the voltage to get my correct speed of turn on my motors for my tracking platform.

am I correct with this? if so I'll order a couple :p

I hope so as it will solve a big problem for me.

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skybadger

skybadger

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A dew controller probably switches at too low a frequency - hence the pulsing. A DC motor PWM controller will run at 1000 Hz or above to control the speed.

I would also recommend the LM317 adjustable voltage regulator and a rheostat to adjust the output voltage. Lots of diagrams on the web.

Mike

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Mikea

Mikea

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ps

I did try a PWM device (dew controller) but this created too much pulsing vibration. would a switching regulator do this too?

As Skybadger says, the dew controller - if it's a PWM device - will switch at a lower frequency, and this will be "seen" at the heater terminals. But a heater doesn't care if it sees a nice DC level or a choppy square wave, whereas your DC motor will get upset with anything other than DC. The switcher that you and Martin mentioned should be fine as the output is filtered (smoothed) by C6.

LM317 is a nice analogue circuit with no choppy waveforms or noise and would be fine also.

Do you know what the current draw of the motor is?

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Moonshane

Moonshane

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Im a bit confused as to what you are trying to drive...is it a fan or a drive motor?

hi Martin

I'll be running a fan initially at 12V during cooldown. when cool and ready to observe, I will disconnect the fan and the run the motor at a lower voltage

cheers Martin - looks good.

If you want 6v, why not just use a 6v battery?

Hi Roger. of course if I was using 6v or a regular 'set' voltage (like 1.5, 3, 4.5, 6 or 7.5v then this would be fine. my problem (or wish) is to be able to fine tune the voltage to reduce the speed of the motor. the voltage used is unknown at the minute but expected to be 6.75v. it may however be 6.2, 7.6 or any other voltage required to get the motor running at the right speed to effect good tracking speed.

A dew controller probably switches at too low a frequency - hence the pulsing. A DC motor PWM controller will run at 1000 Hz or above to control the speed.

I would also recommend the LM317 adjustable voltage regulator and a rheostat to adjust the output voltage. Lots of diagrams on the web.

Mike

brilliant. I am starting to understand this and wrongly assumed that all PWMs 'ran' at the same speeds.

maybe the easiest and most appropriate item would be something like this? 12V 15A DC Motor Speed Control PWM HHO RC Controller on eBay (end time 26-Feb-11 23:58:41 GMT)

As Skybadger says, the dew controller - if it's a PWM device - will switch at a lower frequency, and this will be "seen" at the heater terminals. But a heater doesn't care if it sees a nice DC level or a choppy square wave, whereas your DC motor will get upset with anything other than DC. The switcher that you and Martin mentioned should be fine as the output is filtered (smoothed) by C6.

LM317 is a nice analogue circuit with no choppy waveforms or noise and would be fine also.

Do you know what the current draw of the motor is?

the draw of the motor is about 0.5A under no load so I presume this will rise a bit when under load? I don't envisage it straining too much though as it's running via a threaded rod.

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wayne weedon

wayne weedon

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If you really want to DIY may I recommend you look up the PICaxe chips.

These are little PIC microcontrollers that cost a couple quid each, can be programmed in BASIC or with a Flow Chart.

Implementing a little PWM controller would be a doddle with one of these, and other than a Mosfet or darlington driver you would need little else.

Look them up, they are useful and fun to play with.

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Moonshane

Moonshane

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cheers Wayne and Martin

I am not too bothered about DIY for this (I didn't make the motor :)) I just want something that will work. I think the link to the DC Motor PWM controller on ebay would work based on Martin's sheet?

12V 15A DC Motor Speed Control PWM HHO RC Controller on eBay (end time 26-Feb-11 23:58:41 GMT)

I'd sooner spend £14 and get it right than spend lots of weekends and even more money getting it wrong.......

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martin_h

martin_h

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That looks like an interesting little box! I presume it switches a lot faster than the dew heater controller. Any idea what the HHO bit stands for?

Hybrid Hydrogen Oxygen - I believe that the units are used for injecting into inlet manifolds

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