| Author |
Message |
    
Jameslarson
Intermediate Member
Username: Jameslarson
Post Number: 249
Registered: 4-2006
| | Posted on Friday, July 07, 2006 - 1:59 pm: | 
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Hi. I have a power source that is 14.6 volts DC. The item I need to power up only requires 12.3 volts. What value resistor can I use to lower it to 12.3 volts? It won't be drawing alot of current, so a 1/4 watt resistor is fine. I just need to know the ohms value. Thanks. |
Chad
Advanced Member
Username: Chad
Post Number: 967
Registered: 11-2004
| | Posted on Friday, July 07, 2006 - 3:32 pm: |
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Resistors limit current, you need to know the exact draw of the device in amps and use ohm's law. There is no way to calculate it without knowing this. For example if you put 14.6V into a 1M resistor you will get 14.6 out with no load, if you load it even slightly you will quickly be approaching 0 volts.
Note: the resistor approach will only work for constant load devices such as "lamps" be it filament or LED. Or things that have a steady current draw minus inrush current. If it will have varying current you will need to use a voltage regulator circuit be it a simple Zener network or an adjustable regulator like an LM317.
Chad |
Tech237
Moderator
Username: Tech237
Post Number: 427
Registered: 4-2004
| | Posted on Friday, July 07, 2006 - 4:13 pm: |
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James,
To be honest you are better off not just using a resistor. Yes a proper regulation circuit.
However having said that, most devices that are designed for nominal 12.6V will handle the 14.6 without much trouble - they may run just a littel bit warmer.
To use just a resistor it needs to drop 2.3V at whatever current is being drawn. Say you are drawing 10mA that woulds give a resistor value of 230 ohms. Now this resistor would need to disipate 230mW so a 1/4w resistor would work but would get rather warm.
If you were to draw 1A the resistor would need to be 2.3 ohms and would carry 2.3W of heat.
Now I think I have this correct - it's been so long since I have thought of Simple Ohms law applications. |
Tech291
Moderator
Username: Tech291
Post Number: 408
Registered: 11-2004
| | Posted on Friday, July 07, 2006 - 10:30 pm: |
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Jameslarson,
It would be simpler to just use a lm7812 3 terminal regulator.bias the negative terminal with a 1n4148 diode and you can bump it to 12.6 vdc.
TECH291
CEF#291/CVC#6
KC8ZPJ |
Ferd_burfell
Junior Member
Username: Ferd_burfell
Post Number: 14
Registered: 6-2006
| | Posted on Saturday, July 08, 2006 - 5:17 pm: |
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Most "power sources" show 1 output voltage and another when presented with a load. Attatch your "item" then check the voltage - it will likely be lower. As someone else said, that voltage difference won't be a problem anyway. |
Jameslarson
Intermediate Member
Username: Jameslarson
Post Number: 254
Registered: 4-2006
| | Posted on Sunday, July 09, 2006 - 2:03 am: |
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Hi Tech291. How owuld I actually do that? I am handy building electrnoic parts if someone can simply sjow me the way. In other words, tell me where I put the positive lead, what bias the negative terminal means, etc.. Thanks. |
Jameslarson
Intermediate Member
Username: Jameslarson
Post Number: 256
Registered: 4-2006
| | Posted on Monday, July 10, 2006 - 4:17 pm: |
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Here's some more info that may help you guys answert his for me.
The reason why I want to lower the voltage is because I have a DigiScan UFO elite (REDCO is the company) that is hardwired into a Cobra 2000. This is basically an expansion of channels with a frequency counter built in. It is a small rectangular box with switches on it to change frequency. Quite neat,,,But anyway, read on...
The back of the case of this UFO elite gets really hot. I took the cover off, and that is where a 3 legged black transistor is mounted to. I put a meter on it, and it is getting fed 14.6 volts DC. I was hoping to lower the voltage a little so it would not get so hot. I mean it really gets hot on the back metal plate it is attatched to. The transistor says this on it:
A-945
El Salvador
7805C
Any thoughts on why this transistor is getting so hot? I mean I know it should be warm, but I'm telling you, the back metal plate it is attached to gets very hot. To the point where you can't touch it. I checked specs online, and it should be fed 13.6 volts. Even if I feed it that, it still gets hot. Any thoughts? |
Tech291
Moderator
Username: Tech291
Post Number: 409
Registered: 11-2004
| | Posted on Monday, July 10, 2006 - 7:03 pm: |
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James,
those #'s show that is a 5 volt + regulator.If possible you might try putting a larger heatsink on the back of it.It sounds like(if powered from the 2000 power supply) the power may be pulled off the rectifier before the regulator.try drawing power from the orange wire on the center pin of the ac/dc switch on back of the radio.If you are seeing 14.6 vdc at that point,there is a pot on the regulator board that you can turn it down to 13.8 vdc(what it should be stock).
TECH291
CEF#291/CVC#6
KC8ZPJ |
Jameslarson
Intermediate Member
Username: Jameslarson
Post Number: 258
Registered: 4-2006
| | Posted on Monday, July 10, 2006 - 7:13 pm: |
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Hi Tech291. Thanks for the input. So is it OK to feed this part with 13.8 volts even though it is a 5 volt + regulator? Thanks.
TECH291
yes,it is ok!The purpose of the 7805 regulator is to control the 5vdc needed for the ttl level ic's in the digi-scan. |
Kid_vicious
Senior Member
Username: Kid_vicious
Post Number: 1900
Registered: 9-2004
| | Posted on Monday, July 10, 2006 - 11:16 pm: |
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james, the positive side of C172 would be a great place to draw the power for the digiscan from.
make sure its the positive side!
that 7805's job is to provide a stable 5 volts even if the input voltage is not constant.
there is a "range" of input voltages that it can handle.
somewhere around 11-16 volts, but im not sure.
that digiscan is cool!
matt |
Jameslarson
Intermediate Member
Username: Jameslarson
Post Number: 263
Registered: 4-2006
| | Posted on Monday, July 10, 2006 - 11:45 pm: |
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Yeah,,,Hey Kid. yeah, the digiscan is cool alright,,,,BUT the back of it gets very hot. i mean literally to the point that it will burn skin. I temporalily through in a 25 ohm 3 watt variable rheostat resistor and lowered the 14.6 volts down to 8 volts with this variable resistor...The Digi scan works fine and cool at that voltage, but now it is the Variable Rheostat that gets hot,,,Can't win... |
Tech237
Moderator
Username: Tech237
Post Number: 433
Registered: 4-2004
| | Posted on Tuesday, July 11, 2006 - 12:11 pm: |
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James ,
That is what i was trying to tell you about using a resistor - the resistor has to dissipate the heat created by the 2.3V plus that you are trying to drop.
Not sure what value of actual resistance was in line but if you are dropping 6.6 V that means that you are dissipating a couple of watts of heat - depends on the actual resistance.
FYI W (in this case the heat) = I2 x R or Current squared x Resistance. |
Jameslarson
Intermediate Member
Username: Jameslarson
Post Number: 266
Registered: 4-2006
| | Posted on Tuesday, July 11, 2006 - 1:24 pm: |
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Hi Tech 237. OK, so what should I do then? If I have t build a circuit, please show me the details. I have no problems getting some perf board and building something. Plese let me know the exact parts and deatils. thanks again. |
Tech237
Moderator
Username: Tech237
Post Number: 434
Registered: 4-2004
| | Posted on Tuesday, July 11, 2006 - 1:53 pm: |
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Give me a chance to sit down tonight and I'll sketch it up for you. |
Tecnicoloco
Member
Username: Tecnicoloco
Post Number: 78
Registered: 4-2003
| | Posted on Tuesday, July 11, 2006 - 2:00 pm: |
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Hey Amigo:
Follow the recomendations to look for a diferent power source.
ie:Like Tech 291 mention,"Try to draw power from the AC/DC Switch,is the orange wire conected to the center of the switch.
Right there you should get a regulated voltage coming from the main power supply to the ON/OFF Switch in the front of your Radio.
That Orange wire is the Main Line to supply power to the whole Radio Amigo,after been regulated by the power supply.
To make things easier for you,go to an electronic store and buy a new 7805 IC Regulator with more current capacities.
Make shure that you use "thermal compound" very gentle in the back of the new IC to transfer the heat to the heatsink.
Follow this 2 recomendations,I have 3 Digiscan's too and they run a little hot sometimes.
Last recomendation,run power to the Digiscan with an External Power Supply.
Tecnicoloco
73 de |
Jameslarson
Intermediate Member
Username: Jameslarson
Post Number: 267
Registered: 4-2006
| | Posted on Tuesday, July 11, 2006 - 7:37 pm: |
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Hi Tecnicoloco. Yes, it is wired there.
tech237. I really appreaciate that. Thanks. |
Chad
Advanced Member
Username: Chad
Post Number: 979
Registered: 11-2004
| | Posted on Tuesday, July 11, 2006 - 9:40 pm: |
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I'm feeling stupid... What is a Digiscan?
Chad |
Jameslarson
Intermediate Member
Username: Jameslarson
Post Number: 268
Registered: 4-2006
| | Posted on Tuesday, July 11, 2006 - 10:22 pm: |
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Chad, a digi scan is a little rectangular black box that basically expands the frequency of the 2000 without having to drill holes, mount switches, etc...
Basically, the original 2000 PLL chip is completely removed and the digiscan is hardwired in. It's range is 25.995 to 28.
Instead of using the channel selector on the 2000, you use this little rectangular box to change the channels. It changes channels in 5 khz steps rather than 10. It also has a way so that each time you power up it automatically goes to your favorite channel.
I suppose I simplified it a bit, but I think you get the picture. |
Kid_vicious
Senior Member
Username: Kid_vicious
Post Number: 1905
Registered: 9-2004
| | Posted on Wednesday, July 12, 2006 - 12:09 am: |
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the digiscan came out in the early eightys or the late seventies.
its basically its own PLL that is prgrammable for the chassis you put it in.
i think they used the MC145106 PLL that alot of export radios are using.
matt |
Chad
Advanced Member
Username: Chad
Post Number: 980
Registered: 11-2004
| | Posted on Wednesday, July 12, 2006 - 8:53 am: |
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Ahhhhh, I see. I can see where there would be concern for proper voltage regulation 
Chad |
