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Old 11-29-2012, 04:47 AM
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Default Power Circuit Help Needed



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I have an Argent Data Simplex repeater, it runs on 3vdc, I am going to put it together with a 12v solar panel, charger controller and a 12vdc gel cel battery.

I need a circuit to drop the 12vdc down to 3vdc for the repeater module and I am somewhat electronically challenged.

I am thinking I need a transformer circuit to reduce the voltage, however I am concerned that the battery will see up to 14vdc from the charge controller and don't want to overwhelm the 3vdc circuit.

I am also concerned that the 3vdc reduction circuit will use more power than necessary, I need to keep power usage as efficient as possible.

In theory I should be designing a circuit that also has a 12vdc low voltage shut off as well, so the radio and repeater both lose power if the battery drops below 10.5vdc so as not to damage the battery.....


Anybody here good with circuit design?
Old 11-29-2012, 06:27 AM
JeffreyGlover JeffreyGlover is offline
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Harbor freight has a 45 watt solar kit. That particular controller has a 3 volt power outlet among others.
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Old 11-29-2012, 08:14 AM
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For the voltage reduction, you need a step-down converter. Here's a diagram for a 12V->5V converter (5V should be close enough for your 3V input).

http://www.electroniq.net/power-supp...g-adp2323.html

For the low voltage cutoff, I have one of these on my ham station.

http://www.westmountainradio.com/pro...s_id=pwr_guard
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Old 11-29-2012, 08:20 AM
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Default

Quote:
Originally Posted by fullofit View Post
I have an Argent Data Simplex repeater, it runs on 3vdc, I am going to put it together with a 12v solar panel, charger controller and a 12vdc gel cel battery.

I need a circuit to drop the 12vdc down to 3vdc for the repeater module and I am somewhat electronically challenged.

I am thinking I need a transformer circuit to reduce the voltage, however I am concerned that the battery will see up to 14vdc from the charge controller and don't want to overwhelm the 3vdc circuit.

I am also concerned that the 3vdc reduction circuit will use more power than necessary, I need to keep power usage as efficient as possible.

In theory I should be designing a circuit that also has a 12vdc low voltage shut off as well, so the radio and repeater both lose power if the battery drops below 10.5vdc so as not to damage the battery.....


Anybody here good with circuit design?
buy this

http://www.pololu.com/catalog/product/2110

its only 14.95 and will do the trick for you.
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Old 11-29-2012, 02:15 PM
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The reason they are talking converters is because you cannot step down DC voltage with a transformer. That only works with AC voltages. Any of the suggested converts should work for you just fine.
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Old 11-29-2012, 04:24 PM
technoprepper technoprepper is offline
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You need a fuse, lightning/EMP arrestor, EMI filter, low voltage disconnect, a DC-DC converter, and another EMI filter. The first four items can be shared with radio.

There are some very cheap DC-DC converters on ebay but they tend to be made with counterfeit parts. Here is one that does't look too bad.

Digikey sells many DC-DC convertor modules(over 200,000) 12V car cigarette lighter DC adapters will also work and may be available nearby; you can remove them from the lighter plug if you like.

Capacitor plague is likely to be a problem on DC-DC convertors. Consider replacing the caps with Nichicon brand high ripple current caps purchased from a reputable supplier such as digikey and having spare caps and/or spare convertors.

Because the design of this repeater controller is susceptible to power supply noise, you should consider running 6V in to the onboard regulator. This will waste power but the repeater controller doesn't use much. They claim it will run on as little as 4V but 78L33 has a typical dropout voltage of 1.7V (added to 3.3V this means dropout happens around 5V typical). Because VCC is connected directly to R6 and VREFH, power supply noise could significantly affect the audio.

Power supply noise can also be radiated through the 12V wiring and the antenna as well as internal wiring which is why I suggest EMI filtering before and after the DC-DC converter. This noise can be particularly bad on the HF bands.

Given the apparent low current draw of the repeater controller, you might just run your 12V into the onboard regulator. This will quadruple the power consumption but it may still be very low compared to the radio.

Low voltage disconnects tend to be expensive ($80-200) but you can get a bulky used one on ebay for $10. Here is a small $70 one: http://www.shop.powerplanted.com/pro...2&categoryId=3
And a $100 one:
http://www.digikey.com/us/en/ph/cryd...red%20Products
powerwerx:
http://www.powerwerx.com/emergency-v...nnect-lvd.html
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Old 11-30-2012, 03:55 AM
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I emailed Scott at Argent Data and got an immediate response and as it turns out the external power plug will handle up to 24vdc so I don't need to add a reduction circuit.

Great posts and learning experience though, thanks guys!
Old 12-13-2012, 01:50 PM
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All you need is a small voltage divider circuit. Since you need to convert from 12V DC to 3V DC, you can use two resistors with values of ratio 3/1. Choose the exact values according to the current rating off your circuit.

PCB assembly
Old 12-13-2012, 02:41 PM
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Quote:
Originally Posted by georgestan View Post
All you need is a small voltage divider circuit. Since you need to convert from 12V DC to 3V DC, you can use two resistors with values of ratio 3/1. Choose the exact values according to the current rating off your circuit.
You necrobumped the thread to share that bad an idea?

The current drawn changes the voltage so the ratio needed could be other than 3:1 ratio and the current changes. Voltage regulation is awful unless the resistor values are ridiculously low (at which point they do end up being about 3:1). And you waste power in this approach, especially when you try to get some semblance of voltage regulation. You are not only wasting 3/4 of the power in the drop across the top resistor (which would also be the case with a linear vs switching regulator) but you are wasting all of the power from the current that flows through both resistors which could be significantly higher than the load current. Not only are you wasting way more power than the other options, you create a thermal management problem. And resistors that can bolt to heat sinks to get rid of the heat tend to cost more than voltage regulators that can.

Suppose the minimum current is 1mA, the maximum current is 100ma, and the average current is 10ma. In order to get 10% voltage regulation (which is poor), you need 1A flowing through the bottom resistor (ten times the current fluctuation in the load). So you are wasting about 100 times the power in the bottom resistor (plus 300 times in the top one) than is actually being used by the circuit. Resistor divider 0.25% efficiency, linear regulator: 25% efficiency, switching regulator: 60-99% efficiency. Get your current estimates wrong and you could blow up the repeater controller or cause it to malfunction.

And there is a good chance that repeater users would find themselves listening to the current fluctuations in the circuit instead of the person transmitting.
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Old 08-21-2013, 11:06 AM
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This is fairly simple; the charge controller is connected to the battery which feeds it 14V DC. That is when it is charging. The charging voltage has to be greater than the potential between battery terminals. You can either use a REF voltage IC which gives you a very stable voltage source depending upon the resistor network and can give you exactly 3V DC from 12V DC. Or you can use a car charger. Place the 12V from the battery terminals on its input and it’ll give you around 4V at the output. You can place a small voltage divider after this to make it exactly 3V without much loss of current. Hope this helps.

printed circuit assembly
Old 08-21-2013, 09:30 PM
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Buy a ready made switching regulator off ebay, they're only a couple dollars, you won't save anything doing it yourself. Don't use a linear regulator, the difference between 3v and 12v (all 9v of it) multiplied by the current will be dissipated as heat, so it's pretty inefficient for all but the smallest of loads.

Quote:
Originally Posted by georgestan View Post
All you need is a small voltage divider circuit. Since you need to convert from 12V DC to 3V DC, you can use two resistors with values of ratio 3/1. Choose the exact values according to the current rating off your circuit.

PCB assembly
It's load dependent, so the equation changes with current drawn. It's an ok method for voltage sensing, reference voltages and similar near zero current applications though.
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