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/diy/ - Do It Yourself

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File: 69 KB, 1240x583, circ1.png [View same] [iqdb] [saucenao] [google]
1513661 No.1513661 [Reply] [Original]

My Dryers heater coil is not turning on.
>240vac from wall is OK.
>heater coil, thermostat, thermal cutoff sensor all ok.

Moving on to the main board, the relay that sends one leg of 120vac is not turning on. Checking voltage on the relay its getting a constant 12VDC & no ground. If I ground it myself the relay will energize and switch properly allowing the heating element to come on. I move on from relay to find source of ground signal which is this darlington transistor array in the image. I first assumed he was bad but I'm not to sure now, I started thinking he was actually OK after removing & testing with curve tracer.

So this next guy in the chain is a Renesas 78K0-KD2 MCU (78K0525A).
I was kind of assuming he would be the one to change signals but Currently:
when element should be on he is sending a ground signal.
When element should be off he is sending a ground signal.

I don't really know much about this stuff. Anyone here can help me understand? Would the switching signal come from this mcu or on the COM line?

>> No.1513663
File: 162 KB, 1258x709, dryer1.jpg [View same] [iqdb] [saucenao] [google]
1513663

>>1513661

>> No.1513664
File: 188 KB, 1258x709, dryer2.jpg [View same] [iqdb] [saucenao] [google]
1513664

>>1513661

>> No.1513667

My recommendation? Buy a new dryer.

>> No.1513673

>>1513661
>curve tracer.
how do you have a curve tracer and not know what you're doing?

if the input is high, the output should be grounded (element on, relay on)
if the input is low, the output should be high (element off, relay off)

>> No.1513675
File: 120 KB, 497x447, diagram.png [View same] [iqdb] [saucenao] [google]
1513675

I guess from looking at this, the COM circuit closes and then the mcu input is passed through the transistor the ground is passed through to the relay?

>> No.1513678

>>1513673
I know some basics and do a lot of repairs based around some other work I do.

>> No.1513680

>>1513667
Its a $800 dryer vrs maybe a simple part.
I have a replacement board coming in but I like to tinker.

>> No.1513682

>>1513673
This was my assumption but the output from mcu is not changing. So your opinion is bad mcu (or mcu is not seeing what it wants to see to allow the switch)

>> No.1513685

>>1513682
>This was my assumption but the output from mcu is not changing. So your opinion is bad mcu (or mcu is not seeing what it wants to see to allow the switch)

if you really want to check, cut the trace somewhere convenient and check the signal

while you're at it, feed the darlington input with whatever the MCU supply voltage is and see if the relay clicks

>> No.1513719
File: 53 KB, 940x627, 9460336-3x2-940x627.jpg [View same] [iqdb] [saucenao] [google]
1513719

>>1513685
Thank you that was exactly the advise I needed. I lifted the pin on transistor and gave it 5v. Relay switched.

I guess the mcu is not recognizing a certain requirement or the I/O has gone bad on that pin. Not really worth digging around any further.

>> No.1513763

>>1513680
Buy a better dryer that won't break on you.