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Author Topic: Stepping down voltage?  (Read 872 times)
K9TRW
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Posts: 4




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« on: May 20, 2002, 08:07:53 AM »

I bought one of those emergency car starter gadgets.  It's a 12 volt gel cell.  I've already used it to run my mobile 10M rig.  However, I'd like to use it to power my HT for emergencies, field day, etc.  My HT's battery is 7.2 volts.  How can I step down the voltage from 12 Volts to 7.2 volts in order to do this?

Thank you
Ted Wagner, K9TRW
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VK2GWK
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Posts: 196


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« Reply #1 on: May 20, 2002, 08:39:52 AM »

There are two methods. One is by using a regulator IC like the LM317 - if necessary (how much current draws your HT on TX?) with a power transistor to reduce the voltage to 7.2 V. By reducing 12V to 7.2V = losing 4.8V by say 700 mA you produce 3 - 4W of heat. So you will need a big heat sink and you waste energy.

Another method is using a "chopper" IC - like the IP33036 - that drives a transistor. This way you increase the efficiency, because the voltage reduction is done by taking only the necessary "chops" of energy from the 12V battery  to make the 7.2V by the necessary current. less heat and no waste.

Here in Oz Dick Smith Electronics has this sort of voltage reducers with the IP33036 for sale. May be you will be able to find a similar supplier in the US.

But more fun is making it yourself.....
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N8FVJ
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Posts: 692




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« Reply #2 on: May 20, 2002, 08:57:06 AM »

Purchase a 3 terminal variable voltage LM-350 regulator (TO-3 case) from Radio Shack and mount on a small heat conductive case with an insulator kit & heat conductive compound. You will need a small adjustable resistor to set output voltage and a 200 ohm 1/2 watt fixed value resistor. Also, a 'safety shield' is needed as the TO-3 case is 'hot' with 7.2 VDC and can not be allowed to touch grounded metal in your vehicle. I believe the instructions for wiring the regulator are wrong from Radio Shack, check your ARRL Handbook for proper wiring. If I can explain this, hold the LM-350 looking at the back side with the two pins facing you. Rotate the device so the distance from the two pins to one of the mounting holes are shortest in the 'top position' orientation, that is pointing 'up'. The pin on the left is 'voltage adjust'- install a 500 ohm adjustable resistor from this pin to 12VDC negative . The pin on the right is 12VDC positive input. The case (2- mounting holes) is the 7.2VDC output. These 'mounting holes' must be insulated from the 'mounting screws' with the insulating washers provided in the 'mounting kit'. Install a 200 ohm, 1/2 watt resistor from 12VDC + to the adjust pin as well. If needed, 'RF Proof' the regulator with .01uf & .001uf ceramic capacitors on the input & output (+ to -). That is, across the 12VDC input and across the 7.2VDC output. Current capacity is 3 amps, thus use a 3 amp fuse in-line on the 12VDC positive lead. If you need the full 3 amps, use a 3 amp 'Slow-Blow' type fuse. A lack of a fuse could cause a fire under the right circumstances during a short circuit condition- send Murphy's Law 'down the road' and use the fuse!
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KC7GF
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Posts: 19




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« Reply #3 on: May 21, 2002, 11:42:31 PM »

Depends on what type of HT you are using but many will work with +12v applied directly (through a proper size fuse of course) My Yaesu VX5 and Kenwood TH79 are both this way. The VX5 regulates it internally and the TH79 puts out more power on 12v than it does on it's standard 6 or 7.2 volt internal batteries. Check with the manufacturer. If not one of the above ideas will work well. I built a 9v (actually 8.7v) regulator inside a cigarette lighter shell to power my Alince DJ-C5 (an 8 volt regulator with a diode in series with the ground lead to raise it .7v more)(and a fuse of course.) The DJ-C5 is only 300 mW so it draws very little current on TX.

Art - KC7GF
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