# Help understanding simple boot dryer circuit

Hi, I'm trying to repair and hack a boot dryer, but I need help understanding the circuit to figure out what's wrong.
Here is the circuit diagram:

I think the one of the 10W resistors is the main heating element, and the second is used to drop the voltage down from 120V to the 12V needed by the motor, together with the resistors on the LEDs, but in that case the voltage would be very dependant on the load, that will vary for that motor (specs) . I'm not sure how that can work to do not damage the motor.
I thing the diods are there to allow current flow only in one direction so the motor can work properly spining in one direction. But I don't know why to put two diods in series. Also, I don't understand the role of the zenner diod on the right of the diagram.

This is the board that contains the diods and the small resistors.

The problem is that the motor seem to lack power, so after understanding the circuit I hope to figure out which one is the faulty component.

Thanks a lot!!

After the fuse left side branch is for the heating element and thermal switch to control heat not more than 105'C. The right side branch to run the motor, just half wave rectified DC voltage reduced to 12V using a zenor diode. Two diodes in series is just to increase the PIV. One LED is to show power "ON" and the UV LED is to kill bacteria.

kesirajus:
diodes in series is just to increase the PIV.

May also be for safety , in case one fails short

The problem is that the motor seem to lack power, so after understanding the circuit I hope to figure out which one is the faulty component.

Thanks a lot!!

Motor could be shorted , You do after all have an overheating problem.

Possibly the heating resistor , is that value what is printed on it or a measured value.

from 120V to the 12V needed by the motor, together with the resistors on the LEDs,

You have a voltage divider made up of the 680 resistor the Zener and motor.
The zener is there to protect the LED's

Using a resistor-zener divider to power a motor direct from mains voltage would normally be stupid,
but here the resistor can be part of the heating element, so its heat-dissipation doesn't go to waste.

The Zener limits the voltage to prevent flash-over on the low voltage motor, as well as protect
the LEDs. It will also generate heat of course, but the series resistance will have been chosen to
minimize that when the motor is running normally. If the motor goes open-circuit the zener may
overheat - but that's no really a problem, the series resistor protects from gross failure.

Sometimes re drawing the circuit can clarify things

I suppose the zener orientation is irrelevant in view of the AC input ( ussually the cathode is on top ) . In any case , if the wattage is < or = to 10W , the load current is < or = to 0.833 A. (833 mA). (equiv resistance 14 ohms (at 12V). Did you measure the motor coil resistance ?

Stall current for the motor is 710 mA.

Checking figures i make it 83 mA so with 1/2 wave rectification i figure a total current of approx 35 mA in normal operation.

From stall current i make the resistance to be 17 ohms.

After looking at motor specs I concur.

Thanks everyone for your answers. I'm understanding the circuit much better now.
However, I'm still struggling to make sense on how the zener regulates the voltage.

I understand the operation in a circuit like this:

Where the zener can drive as much current trough R to keep the voltage in the load equal to the zener diode voltage.

But in the case of this boot dryer circuit, the current would be limited by the DC motor that is in series with the zener. So I don't understand how in this case the current gets regulated to the right value to keep the 12 volts for the motor.

Now, I see this as a learning experience and I really appreciate your help.

For the problem, it seem that it was wearing of the brushes of the motor, I'm getting a spare to see if that fixes the issue.

After that I can start with the hacking and putting the arduino into the picture

Cheers

It would help if you measured the motor current by inserting a meter in series ( in current mode with the red lead plugged into the 300 mA jack)
Your schematic isn’t correct because the motor isn’t across the zener. It is across the zener in series with two diodes plus a 680 ohm resistor, all of which drop voltage . Measure the voltage across the resistor to calculate the series current (I = V/680).

raschemmel:
It would help if you measured the motor current

The problem is that the motor is currently barely moving and often unable to start spinning by itself, so any measurement I do won’t be representative of normal operation. Approximated figures can be estimated from the motor specs.

Once I get the spare motor (which I think is the problem after examining the state of the brushes), if the system resume working properly I’ll do the measurement to understand how the circuit works.
Thanks

Trouble shooting is the process of collecting data from which conclusions may be drawn, not making assumptions. Knowing if the current is > or < the calculated value is a step in the right direction. Fauly brushes should result in lower current whereas shorted windings should result in higher current. Knowing how much lower than normal provides additional information from which to draw a conclusion. It is better to take measurements than not.

I'm still struggling to make sense on how the zener regulates the voltage.

The Zener is just limiting the voltage across the LED plus 100 Ohm resistor circuit to 5 volts, thus limiting the current through the LEDs. It shouldn't have much effect on the motor.

The motor is current limited primarily by the 680 Ohm 10 W resistor. The voltage across the motor is not, strictly speaking, regulated.

You may want to also check the motor itself - often as these things get older, the cheap sleeve bearings they use in them dry out and it goes slower and slower (pulling more current at the same time). Make sure it spins freely - if the bearings are dry, you need to fix that problem first.

Try to ignore the LED, take them out of the diagram.
They have no effect on the total current draw of the motor.

All that happens is the current gets divided between the leds and the Zener depending on the exact zener voltage.

With the leds removed ALL the current passes through the Zener..