Obtaining varying voltage from a constant voltage power supply

I have a 24V 20A power supply. 24V is constant not variable.

I want to use a heating wire such as nichrome one for an experiment.

It means I need to pass around 4.5 or 5A through a pure resistive element. I need to find a way to adjust the current through the heating wire. I can do it by varying the voltage across it. The heating wire has resistance about 1.5 ohm.

So I guess I have two options: First one is to add a series power potentiometer which can handle currents up to around 10A.

And the second option could be using an opAmp and 24V as its rails.

Since I cannot find such power potentiometer how can I implement a 0-12V varying output with max current upto at least 6A from the power supply I have?

Or any other ideas to vary the current through the resistor?

• I'd like to see the Op-Amp that supplies 5A.
– JRE
Commented Sep 2, 2016 at 7:46
• Alright so it seems not possible by using an opAmp? Commented Sep 2, 2016 at 7:50
• For that current, an LDO might be prohibitive, so a buck converter is what you'll probably need. This means an extra mini PCB, because, fortunately, these days there are dedicated chips that perform very well. The passive components, OTOH... well, they depend on your dedication to this problem, I think. Commented Sep 2, 2016 at 7:50
• Have you looked up how much current an Op-Amp can deliver? It is usually in the mA range. You can increase this using transistors, but there are chips made for this kind of task.
– JRE
Commented Sep 2, 2016 at 7:51
• @JRE I can use transistors but I dont know how to implement it in this case. I would be glad to know about the chips. Commented Sep 2, 2016 at 7:53

Use a PWM generating chip like the LTC6992 and drive an N channel MOSFET (a type with low on resistance) on and off from the output (or via a driver chip). The MOSFET source can be grounded to make life easy and, the nichrome wire can connect between drain and 24V supply.

The fact that you are hitting the nichrome wire with high frequency PWM should not be a problem if you are just using it to heat something up.

LTC6992: -

If your wire is long (i.e. has significant inductance) then it's prudent to use a flyback diode to protect the MOSFET from back-emf flybacks.

MOSFET: -

Replace the lamp with the nichrome wire. +Vin is from the PWM generator which can also be a 55 timer circuit.

• out will go to mosfet but what about input? it says analog pwm duty cycle control. is that the input for adjustment? Commented Sep 2, 2016 at 8:26
• @user16307 Correct, you can control PWM duty cycle from 0% to 100% with a 0V to 1V DC signal from (say) a potentiometer. If you do feed the MOSFET directly then don't try and run at too high a PWM frequency - this ensures that the turn on and off times of the MOSFET do not significantly waste power. Maybe 10 kHz is a good figure to aim for. Commented Sep 2, 2016 at 8:28
• i have a circuit where i can vary freq. and pulse width form 0 to 100% can I use that instead? is frequency around 10kHz ok with resistive application? i cannot find LTC6992 at rscomponents site. Commented Sep 2, 2016 at 8:38
• You can certainly use what you have described but just make sure it can drive the gate capacitance of the MOSFET reasonably and produces enough gate voltage to properly turn it on so that the on-resistance of the MOSFET is in the milli ohms range. Commented Sep 2, 2016 at 8:42
• For this kind of task you don't even need more than 1KHz for PWM. Commented Sep 4, 2016 at 18:36

The simplest, most efficient, and most flexible route into the future is to buy a DC to DC converter, aka Buck converter, aka SMPS. They're relatively inexpensive and available.

Any dissipating solution, like a series resistor, power rheostat, or linear regulator, will burn a lot of power.

I bought one a few months ago from fleaBay and have been very impressed. The input range is 4-32v, ideal for one or two car batteries, or 5v, or a 19v laptop supply, my 18v drill pack, or indeed your 24v PSU. The output voltage is adjustable from 1.2 to 32v (as long as it's lower than the input voltage), and they claim it's good to 15A output current, though I've not pushed it beyond 5A yet. The cost today, £5.90. Search for 'DC/DC buck'