18 to 12vdc reducer

[AusGuy]

Hi guys new to the forum and in need of some help

I need to reduce n 18vdc battery to 12vdc to run a 12v spotlight. Have called my local electronics shop and they were not much help and have done a forum search with out much luck.

It looks like I need to build this device so need help with this as im not to savy with electronics but I am an electrician so have basic understanding and have done a fair bit of electronics as a kid but that was 25 years ago

Any help on this would be much appreciated

Cheers

 

[Pommie]

If your not worried about efficiency then you just need a resistor. The resistor will need a wattage rating of half the bulb and will get hot. The value of the resistor will be R=72/W.

Mike.

 

[AusGuy]

If your not worried about efficiency then you just need a resistor. The resistor will need a wattage rating of half the bulb and will get hot. The value of the resistor will be R=72/W.

Mike.

OK

Sorry to be a pain but the lamp will be a 75w so what do I ask for at the shop and is that just in line with the +

Thanks

 

[Pommie]

That's a big lamp. You would need a 36W 1Ω (ohm) resistor. This is a rather large resistor and I don't think you will find one at a local shop. You could buy 4 off 3.9Ω 10W resistors from Jaycar and wire them in parallel and place it in the + supply line. They will need to be well ventilated.

Mike.

 

[AusGuy]

Thanks Pommie

 

[Hero999]

Another way would be to pulse it at 100Hz 66.67% duty cycle which will use virtually no power.

 

[stevez]

AusGuy - I'd do as Hero suggests and use PWM. This is my second attempt at a response - not sure what happened here a moment ago. I am a hobbyist and would expect the experts to comment or clarify.

PWM will allow you to turn the 18 volts on for an amount of time you desire. In your case, if you can turn the power on 100 times per second - and each time you have it on for 2/3 of that - the net power to your lamp will be the same as if you applied 12 volts constantly.

I'd break this up into two parts - the switch - which could be a power MOSFET or just a normal NPN transistor. You'll turn the switch on and off with a PWM signal - thats the second part - the means to generate the PWM.

When this transistor switch is fully on or fully off it won't dissapate much power. You'll need to be sure your PWM circuit can come close to turning the transistor fully on or you'll generate some heat (which isn't the end of the world) and have some voltage drop. For starters I'd stick with a power MOSFET - maybe an IRF520. You won't need a lot of current to turn it on.

You'll need to generate the PWM signal but you'll only need to set it to one setting. I would not worry about any kind of feedback or regulation - the load is constant. A 555 timer can be used for this. I've also constructed PWM generators - first generate a sawtooth wave then run in into a comparator to get the pulse width you want.

The way I would approach this - I'd open the book I just bought on PICAXE. It has the circuit already described and even addresses the issues of turning the FET on all the way. A PICAXE is an inexpensive microcontroller - the rest of the parts are easy to find too. IM me and I'll share what I can about what is described in the book.

 

[Hero999]

The problem with using a 555 is the power supply is higher than 15V so a regulator like the LM7812 is needed and the same is true for a PIC in which case you'll need an LM7805.

If you want to use a PIC, you'll need to use a logic level MOSFET and a BJT will need a driver or need to be a darlington.

A 555 is probably the best option since it can easilly run at 12V and drive any MOSFET or BJT.

 

[mneary]

Uh, the PWM idea is a good one, but 66.7% won't do it. You'll be applying 1.5 times its rated voltage to the lamp for 66.7% of the time, during which it will draw about 1.5 times its rated current. That's 225% of its rated power for 66.7% of the time, giving an RMS power of 150%. Your duty cycle needs to be about 45%.

 

[Speakerguy]

Texas Instruments PTN78020. ~95% efficient conversion from 18 to 12V, up to 6A output.

https://www.ti.com/lit/gpn/ptn78020w

They will send you 1 free sample. If you need more they are ~$20 from Digikey.