Broken Royal Paper Shredder

[genchaos]

I was given a broken paper shredder which I was reluctant to discard since it seems so wasteful, and so I am trying to repair it.

It has a sliding Panel Switch for rev/off/auto (see image P2-Shredder-Extrnl). The shredder's symptoms are as follows: (1) The rev works; (2) Auto does NOT work as the shredder will not come on when paper is placed into the paper slot; (3) The Panel "Ready" Light also does NOT illuminate at any time.

I opened the case, and the shredder consists of: (1) Motor/Shredder Blades, (2) Mechanical Safety Switch, (3) Transformer, (4) Panel Switch, (5) Panel "Ready" Light, (6) Optical Paper Sensor, and (7) Main Circuit Board.

Since the shredder motor works in rev, I am presuming the motor and transformer are good. I checked the Mechanical Safety Switch, and that is good. I cleaned the Optical Paper Sensor thinking this might be the problem, but that did not seem to help. The 2 relays SRU-S-112L are for the forward and backwards drive of the shredder motor.

I looked at the Main Circuit Board, and noticed there was discoloration/burn marks on the board in the area of ZD1 (Zener Diode 1, at bottom of images P0-MainBoard-Frnt and P1-MainBoard-Rvrs). ZD1 is rated at 18V/0.5W, as marked on the circuit board. The adjacent resistor also appears to be discolored, but tests correctly at 300 ohms. The fuse is good.

So I am presuming the ZD1 Zener Diode provides the current for auto(forward) shredding and also power for the Panel "Ready" Light. With ZD1 presumably having overheated and gone bad, I am thinking this would explain the non-illumination of the Panel "Ready" Light and why the shredder does not work for auto(forward).

There is another Zener Diode on the circuit board which rated for 6V (located between resistors R16 and R17), which I am presuming is for the rev circuit which works.

So my plan is to replace the ZD1 (18V/0.5W) Zener Diode. However, I am posting here because I had several questions, that hopefully someone more knowledgeable may be able to assist me with:
(1) Is there a way to test the Zener Diode out of circuit, or should I just replace it since it has discoloration/burn marks? Should I also replace the adjacent discolored resistor, even though it still tests correctly?
(2) Assuming that the ZD1 overheated, would you consider replacing ZD1 with an 18V/1W Zener Diode instead of the recommended 18V/0.5W?
(3) Does my repair plan sound reasonable and explain the shredder's symptoms, or would you recommend any additional testing such as testing the voltage regulator L7812CV which is also adjacent to the discolored/burned ZD1?
(4) What might have caused ZD1 to overheat...trying to shred too many papers with motor requiring too much current draw?
(5) Any other suggestions?

Thanks very much for your suggestions and help with my repair questions.

[stj]

Re: Broken Royal Paper Shredder

i would replace the zener and resistor inline with it, and uprate the wattage on both,
resolder all the connectors,
and consider replacing the chip - it's an LM324

[Behemot]

Re: Broken Royal Paper Shredder

The voltage must oscillate heavily, it seems to me the diode is there on the 12V regulator input. According to datasheet it can go as high as 35 V. Or maybe no-load voltage is too high so the diode has been burning when it was in stand-by all the time.

[fzabkar]

Re: Broken Royal Paper Shredder

These transformerless PCBs usually reduce the 240V/120V mains supply to a low DC supply voltage by way of a series capacitor (2.2uF 250V) and resistor (0.2R 8W). AIUI, the function of ZD1 and R21 is to switch on a dummy load so that the 7812 never sees an excessive DC voltage at its input.

   2.2uF             +-----------+
   250V 0.2R          |      |
       8W   KBP206     |  ____  |
    ||  ___   .-----.     | |  |  |
 ACL o-||--|___|----|~  +|-----+---+--|7812|---)----------+--o
    ||    +--|~  -|--+ |+   |____|  |     |
         | '-----' | ===    |   |     .-.
         |      | ---    |   |     | | dummy
 ACN o-----------+      | | 220uF |  18V|     | | load R
               | |    | 0.5W|     '-'
               | |    |   z     |
               | |    |   A     |
               | |    |   | ___  |/
               | |    |   +-|___|--|
               | |    |        |>
               | |    |    300R   |
               +--+--------+-----------------|
                             ===
                             GND

(created by AACircuit v1.28.7 beta 02/28/13 www.tech-chat.de)

[Behemot]

Re: Broken Royal Paper Shredder

Oh I haven't noticed the big picture looking at the small…

Yes, also check the 2.2uF capacitor if you have meter, they always tend to go bad in these reactivity circuits.

Than replace the zener with bigger one and as the 7812 may get up to 35 V input, I think zener around 25 V would be better. It will not burn so much power as it will start conducting much later.

[Agent24]

Re: Broken Royal Paper Shredder

How nice of them to silkscreen the zener voltage on the board! You never normally see that.

Even if the resistor tests OK you should still replace it as it will probably be damaged and could fail sooner rather than later.

[selldoor]

Re: Broken Royal Paper Shredder

Just as an aside to the main repair flow, I worked on one of these some time back. Electrically only the fuse had gone in the plug however I was pleased I didnt spend a lot of time on it as with the fuse replaced it was obviously a
mechanical problem. Whilst the metal cutting teeth were good the comb that removes the shreds was broken in a few places. After very little use the paper
shreds built up and broke more teeth off the comb. Still have the moter and the switches (for spares haha) but the rest is binned.
So all I am saying is check the comb is good with no broken or bent teeth before you put a lot of effort into the electrics.

[stj]

Re: Broken Royal Paper Shredder

i like the high-torque ones that eat cd's

[genchaos]

Re: Broken Royal Paper Shredder

Thanks for all the responses.

I will definitely proceed with replacing the Zener with a higher voltage/wattage such as 25V/1W, and uprate the 300 ohm resistor wattage (current resistor look like only 0.25W). Not knowing much about the Operational Amplifier LM324 IC, I will read about it, and consider replacing it too as stj suggests.

It makes sense that if the function of ZD1 and R21 is to switch on a dummy load so that the L7812 input does not receive excessive DC voltage (with the consideration that excessive DC voltage going through ZD1 caused it to overheat), then increasing ZD1 from 18V/0.5W to 25V/1W should allow it to process higher voltages and power while still staying under L7812's maximum 35V DC input voltage. Thanks Behemot and fzabkar (fzabkar's diagram is very helpful in simplifying things).

In terms of checking the 250V/2.2uF capacitor, unfortunately I do not yet have an LCR meter, but have been planning to purchase one at some point. But I will keep in mind that this might be/go bad.

I was surprised too (but very grateful!), that the board manufacturer silkscreened the zener voltage (and also component codes/resistances on the circuit board), but then it also makes me wonder why they did not utilize a higher-rated zener.

I will take a closer look at the comb that removes the paper shreds prior to doing the actual repairing as selldoor suggests, since I did not really look at this with the shredder only working in reverse. But that is a very helpful suggestion, as it would be annoying to fix all the electronics only to have the mechanical teeth/comb not work properly.

Provided the teeth/comb look good, I will probably proceed and order the replacement parts online, and will update this thread once I receive and install the new parts. Thanks so much for all your helpful responses and suggestions!

[budm]

Re: Broken Royal Paper Shredder

'I will definitely proceed with replacing the Zener with a higher voltage/wattage such as 25V/1W,'

If that diagram is correct you should verify the input Voltage to the IC or check the V drops across the IC to make sure it will not be dissipating too much power. The VDC of that feed the circuit has to be higher than 25V for the dummy load to come on, it was made to come on at 18V before so using 25V may be too high which mean the dummy load will not be present and cause the input to the Reg IC to be higher than designed.
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.

[fzabkar]

Re: Broken Royal Paper Shredder

I would stick with the 18V zener. If you replace it with a 25V zener, then the input voltage to the 7812 will increase from 18.6V to 25.6V, resulting in increased dissipation in the regulator.

AISI, the purpose of the zener is to hold the input voltage at 18.6V, irrespective of the load. It does this by transferring some of the load current to the dummy load resistor. In this way the voltage drop across the 2.2uF 250V capacitor is maintained at a constant value, even when both relays switch on and off.

BTW, each relay coil dissipates 0.36W at 12V. That's 30mA each. The impedance of the 2.2uF capacitor at 60Hz is about 1250 ohms [= 1 / (2 x pi x 60 x 2.2E-6)]. This means that the loop current is about 100mA at 120V. If both relays switch off, then the current through the 2.2uF capacitor would change from 100mA to 40mA, resulting in a reduction in the voltage drop of around 75VAC. This additional voltage would then appear at the input to the 7812. The dummy load resistor needs to shunt additional current to ground so that the load seen by the capacitor remains constant. This also ensures that the maximum dissipation in the 7812 is only 0.6W [= (18V - 12V) x 100mA].

SRU-S-112L, Sanyou, relay, 12V, 0.36W:
https://cdn.badcaps-static.com/pdfs/...8802008dad.pdf

[Behemot]

Re: Broken Royal Paper Shredder

Oh you are right, they are linear, not switched. So it may be worth putting a strip of metal on the regulator too so it is somewhat cooler…although I doubt the circuit will draw more than maybe quarter an ampere.

fzabkar: if the voltage is too high the zener will try but be overloaded, overheat and melt or short; it is simpler to add a piece of metal to the regulator than get huge zener, especially when they are limited in size (and they won't even fit in the holes)

[stj]

Re: Broken Royal Paper Shredder

my thinking on the LM324 is that it's probably related to amplifying the signal from the optical sensor that turns the shredder on.

[budm]

Re: Broken Royal Paper Shredder

The way the Zener is connected as shown in the diagram is for Biasing the E-B junction of the transistor, the Voltage has to be higher than 18V for Zener to conduct due to Vr300 drops and VB-E of the transistor.
When the relays are on the output from the rectifier will drop down below 18V causing the transistor to turn off so the dummy load is disconnected automatically.
It is just a simple way of switching the dummy load in and out off the circuit.
The dummy load looks to be 200 Ohms (or may be 300 Ohms, cannot tell if that is Red or Orange in the first band), since it is fed by 12V so the the current of about 60mA is on the dummy load when the relays are off.
BTW, it will not be regulated to 18.6V due to 300 Ohms resistor in series, as the input Voltage goes up the Vz will stay at 18V, the VB-E will stay at 0.6V but the Vr300 will go up (or down if the Vin drops in value) as more current will flow due to increases in Vin.

[fzabkar]

Re: Broken Royal Paper Shredder

Let's assume that the maximum current in the dummy load matches the 60mA combined coil current. Let's also assume that when both relays are on, the input voltage is 15VDC. This means that the zener is not conducting and the dummy load current is zero.

Now let's switch off both relays. The input voltage rises, the zener conducts, and the 60mA dummy load is switched on. This means that the input voltage should drop to 15V as before. However, as soon as this happens, the zener stops conducting and the dummy load drops out. But if the dummy load drops out, then the input voltage rises, the zener conducts, ...

Clearly the dummy load current is linearly controlled, not simply switched. The 7812 input is regulated to the zener voltage. Moreover, if the two relays are not switched in unison, then we need to accommodate a 30mA load change in addition to a 60mA change. A loop current of 30mA results in a voltage drop of around 40VAC across the 2.2uF capacitor.

As for the base resistor, if the collector current is 60mA, the base current would be typically less than 1mA, assuming a hfe of 60 or greater. The voltage drop would then be insignificant.

[budm]

Re: Broken Royal Paper Shredder

But the Zener is not connected as shunt Voltage regulator.
Measure the Vout of the rectifier with the correct 18V Zener.
If you connect 18V Zener in series with the resistor and then connect that series circuit to the output of the power supply, it will not maintain 18V due to the resistor in series with the Zener.

[budm]

Re: Broken Royal Paper Shredder

'As for the base resistor, if the collector current is 60mA, the base current would be typically less than 1mA, assuming a hfe of 60 or greater. The voltage drop would then be insignificant.' That means the Vout from the bridge is 18.9V to get 1mA of BASE current, I expect it to be around 20V in standby mode.
So assume the output of the bridge is 20V, then the Vdrops on the 18V Zener will be 18V, the VB-E is 0.6, that leave (20-18-0.6) 1.4V on the 300 Ohm Base resistor which = 1.4/300 = 4.7mA of Ibe x Beta (est. 100) of the transistor = 0.47A Ic which mean the transistor is fully saturated.
BTW, you also need current for the LEDs and the IC to run. Also once the relay is on the coil drop out Voltage can be as low as 50% or less of the rated coil Voltage, the spec you have is only 5% min! very low! (most of them are in 15~25% range for power relays) Pull MIN is 9VDC.
OP can easily take the Voltage readings once he install the 18V zener to see what we have for real.

[redwire]

Re: Broken Royal Paper Shredder

When the relays are off, the rail voltage must get pretty high.
My math 300R resistor R21 and ZD1 would each dissipate ~1W with a 35V rail (max. LM7812 can take is 35V). ZD1 would see 54mA and base-drive for Q (really high).
So we have 1/2W and 1/4W parts getting 1W, when this shredder is idle- as its design basis.

To troubleshoot, I would check the LM7812 voltages are reasonable.

The jam (motor overcurrent) detector looks like the opto P621 and 8WR2J power resistor? There should also be a couple timer functions. A jam timer and after the optical sensor is clear the motor usually runs for a few more seconds.

[budm]

Re: Broken Royal Paper Shredder

BTW, if the base current is only 1mA then I cannot see how the 18V Zener and the 300 Ohms BASE resistor got so hot.

[budm]

Re: Broken Royal Paper Shredder

When the relays are off, the rail voltage must get pretty high.
My math 300R resistor R21 and ZD1 would each dissipate ~1W with a 35V rail (max. LM7812 can take is 35V). ZD1 would see 54mA and base-drive for Q (really high).
So we have 1/2W and 1/4W parts getting 1W, when this shredder is idle- as its design basis.

To troubleshoot, I would check the LM7812 voltages are reasonable.

The jam (motor overcurrent) detector looks like the opto P621 and 8WR2J power resistor? There should also be a couple timer functions. A jam timer and after the optical sensor is clear the motor usually runs for a few more seconds.

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