# Connecting two LED stripts with individual power sources to one MOSFET

I have two LED strips (IKEA's Ledberg white), each with their own 24V power supply and I want to make them dimmable with an Arduino.

I was wondering if it is safe to connect both of them (their negative pins) to a single MOSFET? Or do I need two MOSFETs? I've tried and it works, but I'm not sure if it is OK. If it is, would I need any protection (diode?) to prevent current from flowing from one supply into the other? I'm assuming I wont need that because the LEDs would do that.

Also, assuming each strip consumes 44mA at 24V, how much current and voltage would pass trough the MOSFET?

• Add a schematic, if possible. I'm not clear to me what you mean by "each with a $24V$ supply," for example. Does this mean that these strips have their own separate $24V$ power supply and you want to continue to use both power supplies, however you'd like to use a single MOSFET for dimming them via a single MOSFET? Or? – jonk Aug 28 '16 at 2:04
• Yes, each strip has their own power supply and I want to continue to use them, but have a single MOSFET controlling both. – amfcosta Aug 29 '16 at 10:11

simulate this circuit – Schematic created using CircuitLab

Figure 1. OP's scheme.

Would I need any protection (diode?) to prevent current from flowing from one supply into the other?

You have identified one problem and you are probably correct in your thinking that with both supplies on that the LEDs themselves would prevent backfeed in the event that one supply was a little higher than the other.

You also have to consider other potential failure modes:

• If one supply fails you will have a long-term 24 V reverse bias on that chain of LEDs.
• At start-up and shut-down the supplies will rise and fall at different rates. This won't be as bad as a permanent difference.

A separate diode in each LED string leg might give some added peach of mind at little cost.

Of more concern - and I don't know the answer - is that the Ikea power supplies are switched mode and you are adding a PWM load to them. I would imagine that the PSUs are optimised for their designed steady load with the minimum component count, etc., to achieve that reliably. By using PWM control of the load there may be a chance that you affect the stability of the PSU. You might get away with it forever, for a time or not at all depending on whether or not you are close to the switching frequency. If you're fortunate the PSU will shut down. If not it might overheat.

Others may have more experience with this.

Also, assuming each strip consumes 44mA at 24V, how much current and voltage would pass trough the MOSFET?

Peak current will be the sum of both, 88 mA.

• Thank you for your answer! The schematic is correct. Is there anyway to know if the supplies will behave ok or not? Also, what type of supply can I get to replace them? – amfcosta Aug 28 '16 at 20:37
• Sorry, I don't know what's in the PSUs. You can solve the whole issue by driving two MOSFETs from the same micro-pin. One MOSFET per strip. – Transistor Aug 28 '16 at 21:17
• But wouldn't driving two MOSFETs be the same in terms of affecting the stability of the PSU? I would still be changing their load. – amfcosta Aug 29 '16 at 9:31
• Yes, sorry about that. It would only deal with the cross-feed problem. – Transistor Aug 29 '16 at 9:52
• So, no real way of evaluating if using these power supplies is a good ideia (apart from opening them and looking at the components)? – amfcosta Aug 31 '16 at 22:37

The more mosfets you connect in parallel, the more current you can handle, making them work as a single switch. You should calculate the sum of currents for the two LED stripes, and check if it is a safe level for your mosfet in its datasheet. Regards.

• This question is about multiple loads and one MOSFET, not about one load and multiple MOSFETs. – user2943160 Aug 28 '16 at 15:16
• The only limitation I see to his design is the current that his mosfet device can handle. – user2807874 Aug 28 '16 at 15:30