Hi all,

I'm a power electronics engineer but currently doing a project which will involve driving a scalextric car using a pedal generator, for a school fair STEM/engineering demo.
As part of this I need a way to step down the generator (DC brushed) at a fixed ratio of 2:1, as the generator is rated 35V, but the track needs 15V maximum otherwise the cars will be way too fast. I want it to operate down to low voltages and the converter to be unregulated, acting like an AC transformer would. I.e faster you pedal the generator, faster the car goes.
I could build a fixed duty cycle step down buck in CCM but I want to save time and to make it more polished, buy in a converter which will do it for me.
Trouble is the only ones I can find are either low power (<3W) or designed for IBMs and have low voltage cutoffs. I need around 10-20W, 2:1 ratio step down, which operates down to as low voltage as possible. I mean a 5-35V or more input voltage range. Basically a DC version of an AC transformer.

Anyone have any ideas??

Thanks very much for your time

 

Buck converters do not work that way. They will only give the expected output, based on duty cycle, when running in continuous mode. And that will not happen with an input and load varying the way you need.

 

Buck converters do not work that way. They will only give the expected output, based on duty cycle, when running in continuous mode. And that will not happen with an input and load varying the way you need.

I haven't thought about the buck Idea much yet as I've been looking for a converter I could buy in. Don't have a lot of time to spend on the project. But my idea would be to try and keep it in CCM even at low power by varying switching frequency. I'm not too bothered about efficiency or it working well in the lower voltage/power end though. In CCM gain isnt load dependent so can keep a fixed 50% duty cycle and the voltage will always halve, therefore fixed ratio. But yes I'd need to look more into load response for a DC brushed motor load, I'm not experienced with this. I'm sure there's a way to make it work with a controller though.

 

That might be possible, though I don't think you can keep it always in continuous mode over a wide range of loads without somehow swapping out the inductor and capacitor. But I doubt that you can find a commercial product that does that since I can think of no scenario where that would be wanted. Why do you think you always want the voltage halved? It seems a very odd requirement to me. Typically, buck converters do not operate in continuous mode, unless specifically designed to do so with a known input and output voltage and current.

 

If you have the time and inclination, a simple switching circuit with a center tapped step down transformer feeding a bridge rectifier can supply half the voltage, with the benefit that the voltage will vary with the generator output. That could even be a transformer output audio amp with a small oscillator providing some convenient frequency , and the generator providing the output. Then a variac providing the step-up to feed a bridge rectifier to deliver whatever DC you need.

 

It can be done with 555 generated 50% duty feeding one mosfet. Coil and diode can be inserted in power path to smooth out the motor current.

 

It can be done with 555 generated 50% duty feeding one mosfet. Coil and diode can be inserted in power path to smooth out the motor current.

You might be right. All he needs is a PWM fixed duty cycle, to halve the power to the cars. I was thrown off by his post that asked for a DC to DC converter. That is unnecessary.

Nice suggestion.

 

Double the size of the pulley on the generator, so that it runs at half the speed.

 

link
Hope the link works.
DC to DC buck.
Input range up to 38V which is close to you 35V.
Output can be set, in your case 15V or 20V.
Input must be 1.5V above out.
Current <5A. These little boards often are rated too high, so you 1.5A should be really good.
Motor startup current is much higher than 1.5A.

I know this does not do 2:1. That will take more parts. But it will get you from a 36V battery to a 15V motor. It can be used for a speed control.

 

The original concept was voltage increasing with pedal speed, which a voltage regulator will not do. I was suggesting using the generator output as the supply for what would be functionally an amlifier with a push-pull output stage., and a separate driver supply..

 

The original concept was voltage increasing with pedal speed, which a voltage regulator will not do.

It can be used for a speed control.

Remove the voltage pot from the supply in post #9 and replace it with a 'gas pedal'.

 

Remove the voltage pot from the supply in post #9 and replace it with a 'gas pedal'.

The user is already pedaling a generator, and steering so a gas pedal would not work.