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# DC Voltage Conversion

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#### NitroIgniter

##### New Member
First, I'd like to say hi. I'm new and I'm on the learning end of electronics.

I'm looking to build or purchase something that can convert power from a 12 volt car battery into 1.5 volts. I'm sure such a device can be purchased, but I have no clue what I'm looking for. Also, I don't know if any other variables apply to what I want to do with it.

I run nitro engines in radio controlled cars and trucks. These engines work like diesel engines where the user applies power to a glow plug to make it start.

I currently use 1.5v 1500 mAh rechargeable batteries to make my plugs glow. It takes 5-10 amps to make the plug glow enough to fire up the engine. Therefore, I could drain one of these batteries in 10 minutes if I left it attached the glow plug.

With my above idea, I could plug my glow plugs into my car battery. Or could I? Would the enormous power behind my 12v car battery cause the element in my plug to die? I'm sure a car battery has more than 10 amps behind it.

My questions:

1.) what could I buy or build to convert 12v dc to 1.5v dc? ( +- 0.3 volts is fine )
2.) does the power behind a large 12v car battery have the potential to ruin a 1.5v 10A system even if I've already converted the 12v to 1.5v?

Obviously, I'm clueless on the subject. Thanks in advance for any education you are willing to give.

-NI

First - nice job on describing the problem! Clearly you are not clueless.

If you figure that 14 volts might be available from the battery you could use a resistor in series with the plug BUT it's got to be one rather hefty resistor. If you need 10 amps then 10 amps will flow thru the resistor. The entire circuit will consume 140 watts (10 amps times 14 volts) and the plug uses only 15 watts (10 amps times 1.5 volts) - the resistor will dissapate 125 watts - a 1.25 ohm, 125 watt resistor is mighty big. Now 13 resistors, 10 watts at 0.1 each in series might be attainable at low cost.

An alternative might be a couple of headlights in parallel - they are about 40 watts or so, I think.

A simple linear regulator or power transistor circuit is a little more elegant way to do it but becareful with sizing. With paragraph 2 in mind - you've got to get rid of 125 watts.

I've given you some down and dirty ideas. I am sure others can help with more elegant solutions.

Another thought is a simple timer on your 1.5 volt pack to shut it off - if forgetting to shut it down is more the problem. That might be more portable. Then ask the gang here for ideas on a simple charger to recharge from the car battery.

Again, nice job with describing the problem.

stevez, thanks for the information. You didn't say anything about converting the 12v dc to 1.5v dc. Will putting these resistors in change the voltage too?

I know you guys have a visual diagramming system, but I have no clue how to read it. This is my silly attempt to make sure I understand how to wire 13 resistors in series.

-NI

You've got the resistors drawn correctly.

As you suspect, there is no real conversion going on - the resistors just waste the excess voltage. A linear power supply or basic transistor supply would do the same - just act as a resistor.

Worth noting here - the resistance of most materials changes with temperature. It is possible that the "cold" resistance of the plug is radically different than the "hot" resistance. Good to know if for some reason you wanted to measure the resistance of the plug with a meter then use that info.

Linear Technology LT1575 high efficiency regulator
IOUT (Max) 10A Input Voltage 1.36V to 22V Output Voltage 1.5V
https://www.linear.com/prod/datasheet.html?datasheet=357
fixed output voltage version CN8-1.5 [dip] or CS8-1.5 [surface]
first page of the datasheet has typical application
use a hefty external N-channel MOSFET and heatsink
I'd use a little more capacitance on the output of the regulator

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