Broken Royal Paper Shredder

[fzabkar]

Re: Broken Royal Paper Shredder

Also once the relay is on the coil drop out Voltage can be as low as 50% or less of the rated coil Voltage

I don't understand why we need to be concerned with coil drop-out voltage. Assuming that the coil voltage is regulated at 12V by the 7812, then the coil current will always be 30mA.

When the relays are off, the rail voltage must get pretty high.

When the relays are off, the rail voltage does not rise to 35V. That's because ZD1 begins to conduct, turning on the transistor and causing dummy load current to flow. This "load balancing" circuit ensures that the loop current remains fairly constant.

I have a feeling that the dummy load circuit had failed to keep the V rail in the safe range.

My suspicion is that the transistor may have insufficient gain to switch on the dummy load as hard as it needs to. This would cause the zener and base resistor to burn up.

[budm]

Re: Broken Royal Paper Shredder

All I can say right now is to replace the Transistor, 18V Zener, the Base resistor, and then take the DCV reading to prove the theory on how it works.

Example of using Zener for tripping circuit:
http://www.radio-electronics.com/inf...s-circuits.php
I use similar setup for latched over voltage shutdown to trip the relay.
Similar setup is also used the Zener for the AC_DET in the TV to disable the power supply if the incoming AC is too low.

[fzabkar]

Re: Broken Royal Paper Shredder

I would remove the zener and connect a 15V lab supply to the input of the 7812. Then I would test the functioning of the circuit (without AC) before replacing the suspect components. This is to eliminate the possibility that some other part of the circuit is causing the load regulation circuitry to work too hard.

[genchaos]

Re: Broken Royal Paper Shredder

Thank you all for the wonderful and helpful discussions. I have been following the comments (in the midst of trying to get paperwork done), and trying to understand it, as it is above my knowledge base. However, this is providing a wonderful learning opportunity, as I have been reading, re-reading, looking things up on the internet, trying to gain a better understanding, with much of this starting to make better sense.

What budm stated in post #10, that "I also do not expect the BASE current flowing through the 18V zener (18V drops on the Zener), 300 Ohms (Vr300 = ???? V) resistor and the E-B (0.6V) junction to be that high to cause the burn parts unless EB junction is shorted out" was interesting because I had not considered the transistor malfunctioning.

Fzabkar's statement (post #21) that, "My suspicion is that the transistor may have insufficient gain to switch on the dummy load as hard as it needs to. This would cause the zener and base resistor to burn up."

Based on these arguments and suggestions, I will replace the transistor 9013, the 18V Zener (will replace with higher 1W instead of original silkscreened 0.5W), and base resistor, after which I can make the DCV readings and post what those are.

Thank you all again for the wonderfully educational discussion, and all your time and help, and I will post once I get all the parts ordered and replaced!

[fzabkar]

Re: Broken Royal Paper Shredder

I'm wondering whether you could connect a series string of 4 x 5.6V 1W zeners across the 7812 input. That would limit the voltage to 22.4V. A max current of 100mA would would result in a total dissipation of 2.2W.

[Behemot]

Re: Broken Royal Paper Shredder

Don't think so, not without bleeder resistor and who knows how that would work. None of them will be exactly the same, some will start conducting sooner, the voltage would divide disproportionally and some may burn, than the rest will get even more voltage…even batteries in series are actually a problem - I often seen things like one VRLA got 12.5 V, other 15 V. Guess which will sooner go bad…

[budm]

Re: Broken Royal Paper Shredder

Current is already limited by the Xc of the cap.
https://cdn.badcaps-static.com/pdfs/...a4b3f33193.pdf

https://cdn.badcaps-static.com/pdfs/...5df14516be.pdf

Right now the power is already wasted on the 12V reg and the 200 Ohms (@ 12V 60mA).

[japlytic]

Re: Broken Royal Paper Shredder

Could the Rubycon units on this board be fakes?
I recommend taking the 2.2uF capacitor out of circuit (after discharging it first) and checking its capacitance with a capacitance meter - X-class and Y-class capacitors (as well as other AC capacitors) can lose capacitance throughout their electrical lives (when power is applied).

[stj]

Re: Broken Royal Paper Shredder

why would they need to be fakes? i'v seen loads of rubycon's lose capacitance with age.
specially ZL's - there is something about those.

[stj]

Re: Broken Royal Paper Shredder

looking at the pcb foto again, has the fuseable resistor been metered?

[genchaos]

Re: Broken Royal Paper Shredder

Quick question re: transistor substitution:
This weekend, I was trying to find replacement transistor 9013, but having difficulty. Looking for an alternative, I came across PN2222A. Both 9013 and PN2222A are BJT TO-92, with V-CBO=40V and 75V, respectively, I-C=500mA and 1000mA, and P-D=625mW (same for both).

The datasheet for the 9013 transistor is here:


The datasheet for the PN2222A transistor is here:
https://cdn.badcaps-static.com/pdfs/...14ac8f0249.pdf

I am thinking I should be able to substitute PN2222A for the 9013. Am I correct?

As for stj's suggestion of metering the fuseable resistor, it measured a bit off at 0.98M ohm (vs rated 1M Ohm with 1% tolerance). I don't think it makes much of a difference. If after the transistor, 18V zener and base resistor are replaced, and there appears to be problems with the sensor, I will check on LM324...thanks stj. I'll also get DCV measurements, to try to resolve how this works. Thanks again.

[Behemot]

Re: Broken Royal Paper Shredder

Yeah if they have same specs that is perfect. Usually you only find close substitution, or than superior.

[budm]

Re: Broken Royal Paper Shredder

'As for stj's suggestion of metering the fuseable resistor, it measured a bit off at 0.98M ohm (vs rated 1M Ohm with 1% tolerance).' That is not fusible resistor, not 980K Ohms resistor.

[stj]

Re: Broken Royal Paper Shredder

the fuseable resistor is the gray one under s5

it looks like it may be 200ohm

[genchaos]

Re: Broken Royal Paper Shredder

Thanks budm and stj for the correction. The fuseable resistor (gray one under S5) meters at 306 ohms (banding: orange/black/brown/gold (300 ohms 5%), so that seems okay. Thanks.

[budm]

Re: Broken Royal Paper Shredder

So it is 300 Ohms not, 200 Ohms, so it is fed by regulated 12V so the current draw will be only 40mA (0.48W) in standby mode.