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Question about PWM vs ULN2803A

Started by kurt-olsson, May 29, 2021, 11:45:11 AM

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I have a question about PWM lights vs Switches.

I know i should provide wiring diagram but will try to ask the question as "discussion" and not as my problem since i have solved it with Mosfet modules instead of PWM on ULN2803A.

My Korry light works great on my ULN2803A but all my switches as A/T OEM and Landing Gear Solenoid dont work well. Lots of noises and problems. But when using Mostef moduls with pin to switch on/off it works great.

I think this has to do with "Active low" or something on the korry lights. The switches dont work that way since they just is a current switch.

Dont know if i even can formulate a question well for you to understand. hehe

Anyone else using ULN2803A with Switches and not only Korry Lights?


What is your actual question? Try a simple outline and work from there.

The ULN2803A is a Darlington transistor buffer/driver. You can use it with a low-voltage, low-current signal (such as from a microcontroller) to switch high-current, high-voltage loads. The same is true for a MOSFET. There's a lot of documentation out there to help you decide which is better, but for a lot of applications it either doesn't matter, or the choice is already made for you (e.g. it's whatever you had in your junk box).

The transistor and MOSFET can be turned on and off, thus turning the load on and off. You get the benefit of solid-state fast switching of higher voltages and currents, although in many cases you could use a relay. However, a relay generally needs a transistor buffer to drive it anyway, and the relay contacts change state orders of magnitude slower.

That's fine for things that need to be on or off, however, if you can do that rapidly you can vary the *power* that is transferred to the load, which is the technique of pulse-width modulation (PWM). This is most obviously used for dimming LEDs, but can also be used for motor speed control and other loads.

So, the first step is to decide what you are switching, and whether the driver has sufficient voltage and current handling. The second step is to decide if you are switching it on and off, or if you want to use PWM to vary the power to the load. There is a third option whereby you might vary the current in a transistor circuit to vary the current in the load, or vary the supply voltage, but generally these are more complicated and PWM is simpler and easier.

If you are trying to solve a problem then draw a schematic (to show what you have and how things are connected), then describe what you expect to happen (or what you are trying to achieve) and what is actually happening. This is basic information that anyone would need to try and help you, and it applies to pretty much any problem in any area.


Thanks for your answer and you answered my question without me asking one correctly.

After reading your text its clear to me that if you want on/off logic the mosfets do a better job, faster and can handle more current better.

Thats the answer i was looking for, good enough for me.

I did not provide a wiring diagram since i didnt have a setup related to my question.

Thanks anyway, i did get my answer. :)


Quote from: kurt-olsson on May 29, 2021, 11:36:31 PMAfter reading your text its clear to me that if you want on/off logic the mosfets do a better job, faster and can handle more current better.
Er, that's not what I said at all.

A transistor, or rather a BJT (Bi-polar Junction Transistor) and a MOSFET (Field-Effect Transistor) can both be used for switching purposes. They are both transistors. They both have specific applications where one or other device works best, and general applications where either device can be used.

What you are doing is not at all complex, so either should work. If something is not working as you expect then you are probably doing something wrong. For example, if you have a solenoid that is not working well with a ULN2308A (a bunch of BJTs in a Darlington configuration) you should find out why, because it's a very simple application. If there's something you've overlooked you should try to understand it so that future projects will be more successful. Just changing a transistor for a MOSFET (or vice versa) and finding it works better is not helpful or useful in the long run. You might just be masking a bigger problem. It is important to know why something is like it is, and why (therefore) changing something is going to fix it.


Agree. I will investigate and check the documentation more on the ULN2803A but i have read it before.

The mosfet work as plan and i understand 100% why it does. But the ULN2803A should work in theory but doesent. My biggest clue is that it has max amp draw of 500ma if each channel is runned bybitself. The solenoid has amp draw of 480ma so its close to max limit.

I just was looking for if anyone else experienced the same thing.

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