LM3524 PWM pin 10 shutdown voltage

[gim461]

I am trying to fix a 12V/240V inverter which uses an LM3524 PWM chip to do the switching. I need to know what voltage is needed on pin 10 to shut the chip down. The datasheet says a current of 1mA will shut the chip down, but I need to know what voltage that equates to.

Thanks.

 

[KrisBlueNZ]

Hmm, I see what you mean. The data sheet isn't very helpful.

It does show the internal circuitry though, and based on that, and the suggested current of 1 mA into pin 10 to shut the device down, a voltage between +2V and +5V on pin 10 will shut it down.

 

[gim461]

Many thanks to KrisBlueNZ.

I am only getting about 0.7 volts on pin10 so the chip stays turned on and the inverter won't go into standby mode. Now that I know the necessary voltage I can try to figure out why it's not getting through to pin 10.

 

[KrisBlueNZ]

0.7 volts is suspiciously close to the base-emitter forward voltage of the two transistors connected to that pin.

You asked what voltage "is needed" on that pin, and I suggested 2~5V. My meaning was that if you want to shut it down, applying 2~5V to that pin will do that. I did not mean to say that anything less than 2V will NOT shut it down.

A voltage of 0.7V on that pin MIGHT be enough to shut it down. To ensure that it doesn't shut down, the voltage on that pin must be 0.5V or less. 0.7V is on the borderline.

Can you measure the current going into that pin, instead of the voltage on it? You'll have to cut the track (or desolder the pin and isolate it from the pad) and connect a current meter across the break.

Is that the problem you're having? The inverter won't shut down?

Can you trace out the circuit diagram of the components connected to that pin?

 

[gim461]

Can you measure the current going into that pin, instead of the voltage on it? You'll have to cut the track (or desolder the pin and isolate it from the pad) and connect a current meter across the break.

Is that the problem you're having? The inverter won't shut down?

Can you trace out the circuit diagram of the components connected to that pin?

The inverter just dropped dead one day. I tested all the components on the board except the three ICs. Everything checked out OK so I replaced the ICs. They are LM3524 PWM, LM324 Quad OpAmp and NE555 Timer (it was the LM3524 which was defective). The inverter came back to life but quickly burnt out a resistor. I replaced the resistor and changed the ICs again and now all is good except that I can't use the remote start feature, as the inverter won't go into standby mode.The resistor that burnt was in the +12 supply to LM324 and NE555. The LM324 had a near short across it's power supply pins when I pulled it out. I don't know how that happened, as it was a new IC.

I have already traced out some of the circuit, so yes I can finish that off and post it. Several pins of the LM324 and NE555 are connected to pin10 via various diodes and resistors. The inverter has over voltage shut down, over current shut down and possibly low voltage shutdown. I presume it all works through pin 10.

And yes I could measure the current into pin 10. Thanks for your help and interest. I will post the circuit trace later today.

 

[KrisBlueNZ]

Have you searched for information about the inverter? It's possible (but unlikely) that a schematic might be available online, or there might be notes on other faults and repairs. What is the brand and model number?

Thanks for the description of your repairs so far. I would like to know about the resistor that burnt out. Can you trace out that part of the circuit as well? Usually when a resistor burns out, it's because of a problem somewhere in the circuit. It seems odd that just replacing the resistor, and replacing the ICs again, cleared the fault and stopped the resistor from burning up again. That's not how it usually goes. Is it possible that when you replaced the ICs the first time, you left a solder whisker shorting something out? That might explain it.

Any more information you can post on the inverter could be helpful.

 

[gim461]

I measured the current into pin 10. It is 0.13mA when the inverter is switched to standby. It is zero when the inverter is switched on.

The inverter is a WEA (Wind Energy Australia) Model 1206. It is a 600VA square wave inverter. I have had it for 25 years and it wasn't new when I got it. It has never failed before. I also use new switch mode inverters and they certainly don't have the reliability of this old beast. They usually only last for a couple of years. Mind you they are very cheap to buy. I can't find any information at all about WEA inverters on the net. I presume they stopped making them years ago.

It is possible that I left a solder whisker on the board. I searched for shorts after the resistor burnt out, but found nothing. The resistor was charcoaled, so I didn't know what value it was. But it was a 1/4 watt resistor, and it burnt quickly so I figured it might have got about 1/2 watt power. At 12V, 1/2W equates to 288 ohms. I have put in a 330 ohm resistor. Lowering the resistance would increase the voltage on pin 10 so I tried 100 ohms earlier, but the inverter still didn't shut down, and also it didn't sound right, and the overload LED flickered on.

I just put 1V onto the shutdown pin using a flat dry cell. The inverter still didn't shut down and again the sound changed and the overload LED flickered on. I'll post my circuit drawing soon and maybe you'll be able to make sense of what I've described.

 

[KrisBlueNZ]

OK.

Can you also post some photos of it?

The inside of the whole unit, viewed from above and below, and a close-up of the main circuit board, from above and below, could be very helpful.

Use natural light if possible, and take the photos looking straight down or at a slight angle. If there's direct sunlight, make sure there is no reflection in the photo.

 

[gim461]

Here's the schematic and photos. Apologies for the poor quality.

 

[KrisBlueNZ]

OK that's a good start.

To really understand the design, I would need a complete schematic diagram of the PCB and the off-board components it connects to. The ICs need to be represented as rectangles, apart from the LM324 which needs to be represented as four op-amp symbols (see the data sheet for the pin allocation). Generally this requires multiple passes, gradually refining the schematic from a "this connects to this" diagram to a drawing that really shows the function of each part of the circuit. This is a fair bit of work for you, and I don't know whether you'll be prepared to do it.

Also, I'm interested in the voltage rail on pins 8 and 4 of the 555. This rail is tied in with the resistor that lost its smoke, and with the connection between the remote start switch and the LM3524 shutdown pin. Can you make sure that you draw every component connected to that rail? Is there a zener diode connected between that rail and the 0V rail, to regulate that rail voltage? I suspect the resistor that lost its smoke was a much lower value than 330R. What voltage do you measure on that rail now?

The photos are reasonable quality but I would like to see both sides, with views directly downwards on the board or at a slight angle, and with the board square in the frame.

 

[gim461]

There is one mistake in my schematic. In the top horizontal line there is only one diode, not two.

I just tried the dry cell trick again, this time with a slightly higher voltage, between 1.1 and 1.2 volts, and the inverter went into standby. So it looks like I need 1.2 volts on the shutdown pin. So you may be right about 330 ohms being way too high. If I put 10 ohms in there I should get 1.14V on the shutdown pin (at 12V supply) which might just work, but it seems to be cutting things fine to me.

I have drawn every component connected to the supply rail (I think! It gets very confusing at times). There is no zener diode connected to that rail, other than the one I've shown connected to pin 2 of the LM324. It is pin 8 of the 555 and pin4 of the LM324 which are connected to that rail. They are the +Vcc supply pins to the respective chips.

 

[gim461]

I changed the 330 ohm resistor to 10 ohms and it's working, sort of. It will go into standby mode so long as the supply voltage is above 12.3V. However if the inverter is then started remotely, then stopped remotely, it won't go back into standby unless the voltage is above 12.5V. I suspect there is still something not quite right with the circuit, but I don't know what.

I'm really not happy about that as sometimes the battery voltage goes down to 12V or even a bit lower. I am thinking about making a modification to the circuit by changing the 100 ohm resistor from the shutdown pin to earth to a 120 ohm resistor. That would give me 1.35V on the shutdown pin at 12V supply.

 

[KrisBlueNZ]

OK. I agree you should increase the 100 ohm from the shutdown pin to 0V. I would increase it significantly, say to 220 ohms.

 

[gim461]

The inverter is now back in service, which is good for us as it is the only remote start inverter we have, and means I don't have to walk out to the battery shed to turn the inverter off at the end of the night.

Many thanks to KrisBlueNZ for his ongoing interest and help. There were moments when I despaired of being able to fix the inverter myself, but the support from Kris gave me the confidence to keep trying.

PS I did a quick check of output voltage and frequency before I hooked the inverter back in. Voltage was 250V RMS, and frequency was 70Hz, at no load. At some time in the future I will check them under load, and make adjustments if needed. I think 70Hz is a bit on the high side.

The original PWM chip was an LM3524N. The replacement is LM3524D. It is a new improved version which is said to be pin compatible. But maybe the difference in the chips could explain why the pin 10 to earth resistor needed increasing.

 

[KrisBlueNZ]

Well done!

Thanks for your kind words.