NerdKits - electronics education for a digital generation

You are not logged in. [log in]

NEW: Learning electronics? Ask your questions on the new Electronics Questions & Answers site hosted by CircuitLab.

Basic Electronics » MOSFET Problem on Constant current source

November 28, 2009
by soundman003
soundman003's Avatar

I was wondering about a problem I have with the switched constant current source at this site http://www.hoelscher-hi.de/hendrik/english/led.htm The circuit is the one around the bottom of the page to the right. I tried it and it works well when I use 12V input but when I try 24V input on the LM317 using a string of LED's they stay on and the PWM from the ATMEL does nothing. I have 6 led's in series which should be around 18V. I put a 10K resistor from Gate to ground and there is still a faint glow on the LED's and then the PWM doesn't work correctly. I am not sure put I think it is saturating the mosfet somehow and making it come on. Thank you for any help on this.

November 30, 2009
by mrobbins
(NerdKits Staff)

mrobbins's Avatar

Hi soundman003,

This does sound strange. So you're powering the microcontroller from 5V (the 7805 output), and connecting one of the I/O pins directly to the gate of the MOSFET, is that correct? And on 12V, you're able to switch the LEDs on and off from the microcontroller, but on 24V you're not? Just want to make sure I understand everything.

This is surprising / confusing. If you had the MOSFET's drain and source reversed, the connection would appear "always on", but then your PWM shouldn't work at 12V. If the gate were connected to your LED string, then no current could pass. The only other configuration in which something might happen would be if the gate were inadvertently connected to ground, the source terminal to the LEDs, and the drain to the microcontroller pin... which would definitely get you some odd behavior.

Is this a 2N7000 for the MOSFET, or the BUZ11 as shown in the schematic?

Mike

November 30, 2009
by soundman003
soundman003's Avatar

Hello Thank you for a response on this. I am powering the DMX receiver with a AC wall adapter 12v then I am using the driver circuit around the bottom of the page to the left for Luxeon LED's. Then I use one of the outs from the DMX receiver to fade it. When I use an ATX power supply at 12v to the in of the LM317 on the driver circuit it works great but when I use anothe power supply that is 24v the LED's turn on and stay on the fade (PWM) does nothing. I tried a 10k resistor from gate to ground and it turned them off but then the PWM doesn't work wright because the voltage eventually goes to ground through the 10k resistor on the gate. The only thing that I am not doing is the power supply for the driver is separate from the power supply for the DMX circuit and I am using the ground from the driver supply for the led's and am not connecting the ground from the DMX circuit I was afraid that it might hurt it since it was 2 different supply's. I haven't ruled out that it might be something to do with the 24v power supply for the driver circuit.

November 30, 2009
by soundman003
soundman003's Avatar

Also it is the BUZ11 MOSFET. Thank you

November 30, 2009
by soundman003
soundman003's Avatar

Here is a link to the power supply I am using for the driver. http://www.mpja.com/prodinfo.asp?number=17436+PS Thank you

December 01, 2009
by soundman003
soundman003's Avatar

Mike I ran a wire through a 10k resistor to earth ground and it works. It must be somthing to do with the 24v power supply I have I found this by accident. I had a 10k resistor connected to a clip and to ground and when I touched it the LED's went out so I ran it to the green wire on the ac plug which should be earth ground and it works. I did notice a little flicker on the LED's when dimmed real low but that was it. What do you think caused this to work? It also works with out the 10k resistor there and straight to earth ground. Thank you

December 03, 2009
by mrobbins
(NerdKits Staff)

mrobbins's Avatar

Hi soundman003,

I do think you have an issue with grounding and the different power supplies. In general, if you have two supplies that are both separately isolated from the 120VAC line power, then they're going to be "floating" with respect to each other. You can't just connect one signal wire (your PWM signal) between them and expect things to work properly. In general, it should be OK to connect the two grounds together, and this will make both halves of your circuit define "0" as the same voltage. The only situation where this would not be OK is if both sides are actually non-isolated and are already grounded, and then there is some mis-wiring in your home / power strip / etc that causes ground to actually be not ground!

Hope that helps.

Mike

December 21, 2009
by BobaMosfet
BobaMosfet's Avatar

Something to consider-- BJTs are used for switching current, FETS are used for switch VOLTAGE.

December 21, 2009
by BobaMosfet
BobaMosfet's Avatar

Grounding--- you cannot keep your grounds separately. You must tie your grounds together cleanly, and directly, otherwise you can create what is known as "ground loops" and these can make it very difficult to find problems.

Post a Reply

Please log in to post a reply.

Did you know that spinning a permanent magnet motor makes a voltage? Learn more...