Need help with this.

Thread Starter

jecker10

Joined Jan 20, 2015
9
I am currently woking on trying to run a power supply to a device at 10, 25 and 100% duty cycle. The only way I have found that I cando this is through PWM, if so can anyone help with a basic circuit ithas to be able to handle 13.5v. If there is another way to do this I am all ears.
Thanks
 

Evil Lurker

Joined Aug 25, 2011
116
I am currently woking on trying to run a power supply to a device at 10, 25 and 100% duty cycle. The only way I have found that I cando this is through PWM, if so can anyone help with a basic circuit ithas to be able to handle 13.5v. If there is another way to do this I am all ears.
Thanks
Duty cycle is measure of time proportionate to time on and time off. In theory you could use a electromechanical relay to turn a device on for 6 minutes and on again in another 54 minutes and you would have a duty cycle of 10%. The same could be said if your relay was turned on for one minute and off for 9 minutes. However if you want to go faster than a mechanical switch can turn on and off or if you are turning something off and on again a lot your gonna have to use PWM.
 

Thread Starter

jecker10

Joined Jan 20, 2015
9
The power supply is a DC power supply, I am not sure what the device is at the moment(this is for work and another guy has the request I am just supposed to build something to help with the power setup), the current is whatever the device takes which I am not sure what it is. The tme period of the cycle has to vary, I realize there are a lot of unknowns and variables but they want something that can affect the duty cycle of a power supply that is adjustable so that th duty cycle can be altered as required.
Thanks
 

Alec_t

Joined Sep 17, 2013
14,280
Is the 20Hz already available (e.g. as a time reference), or does that have to be generated too? If you need to generate it, how accurate must the frequency be?
 

Thread Starter

jecker10

Joined Jan 20, 2015
9
The power has to be on
-13.5V DC for 4 min @ 100% duty cycle
-13.5V DC for 26 min @ 25% duty cycle
-13.5V DC for 30 min @ 10% duty cycle

I will have to generate the 20Hz signal in whatever setup I have to make
 

Alec_t

Joined Sep 17, 2013
14,280
With that specific timing this project cries out for a micro-controller, which would also be able to handle the PWM generation and drive a MOSFET to switch power to the load.
 

Thread Starter

jecker10

Joined Jan 20, 2015
9
I have a controller to output voltage so if I have to make multiple devices and have the controller switch between them then that is what I will do. Was hoping for something that would simply control the duty cycle but if i have to use a micro controller I do have my Arduino at home I can bring to work.
 

Alec_t

Joined Sep 17, 2013
14,280
if i have to use a micro controller
There's no have to. Just more convenient. If you're a glutton for punishment you could do all the timing, the PWM generation and the duty cycle control/switching with discrete logic :).
 

Thread Starter

jecker10

Joined Jan 20, 2015
9
Right, I am just gathering all of the information I need so that I can present the options to my boss. This seems easy enough using an Arduino and a MOSFET to run the duty cycle.
Thanks
 

wayneh

Joined Sep 9, 2010
17,496
The power has to be on
-13.5V DC for 4 min @ 100% duty cycle
-13.5V DC for 26 min @ 25% duty cycle
-13.5V DC for 30 min @ 10% duty cycle

I will have to generate the 20Hz signal in whatever setup I have to make
Since you are not looking for continuous control of the duty cycle (like most folks want went, for instance, dimming LEDs) you could use a 4017 counter to get preset levels with good precision. Choosing any single digit at the output would get you 10% duty cycle. ORing together 2 digits would give you 20%, and so on.

I can't think how you would get 25%, but 20% or 30% would be trivial.

Oh wait, if it's only counting to 4, then a single digit is 25% duty. So you could hit whatever values you need by just setting DIP switches.
 

crutschow

Joined Mar 14, 2008
34,281
....................................
Oh wait, if it's only counting to 4, then a single digit is 25% duty. So you could hit whatever values you need by just setting DIP switches.
But counting to 4 would not give a 10% duty cycle.

You can get a 25% duty-cycle while counting to ten if the input clock has a 50% duty-cycle, by using an added D flip-flop, clock inverter, and triple OR-gate.
Connect the inverted clock to the FF Clk input and the counter output O1 to the FF D input.
ORing Q0, Q1, and the FF Q output gives a 25% signal.
 

wayneh

Joined Sep 9, 2010
17,496
But counting to 4 would not give a 10% duty cycle.
Right, that's why I was thinking you would need DIP switches to allow setting the count-to value as well as choosing the outputs. If the OP only needs those 3 values plus 100%, it shouldn't be hard to figure out the switch settings.
 

crutschow

Joined Mar 14, 2008
34,281
Right, that's why I was thinking you would need DIP switches to allow setting the count-to value as well as choosing the outputs. If the OP only needs those 3 values plus 100%, it shouldn't be hard to figure out the switch settings.
That would work if the change in PWM frequency with duty-cycle is not a problem (or the clock frequency can be changed along with the change in the count).
 

Thread Starter

jecker10

Joined Jan 20, 2015
9
Looking at all options but I think the MOSFET with the Arduino should be easiest, if I am right. I have most of the items I would need for it already, if i am wrong and someone knows an easier way then I would be grateful for that info.
Thanks for all the help
 
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