# PSU Question - (Newbie)

#### Dodgydave

Joined Jun 22, 2012
9,939
Why don't you run the fan on Halfwave using a diode in series with the fan across the Transformer, the same for the other fan using the diode on the other half cycle.

#### CCinBR200

Joined Oct 4, 2020
80
Why don't you run the fan on Halfwave using a diode in series with the fan across the Transformer, the same for the other fan using the diode on the other half cycle.
Thank you for the idea. I am not familiar with those concepts. Where can I learn how to do that? (I also decided to go with just one larger fan - 330mA ). I surely must have been wrong about the current draw for the smaller fan.

Carlo

#### CCinBR200

Joined Oct 4, 2020
80
Why don't you run the fan on Halfwave using a diode in series with the fan across the Transformer, the same for the other fan using the diode on the other half cycle.
I did some reading on this. I can't figure out how far this design will drop the voltage (from 39v?)

#### CCinBR200

Joined Oct 4, 2020
80
You can turn a regular 1/2W or 1W zener diode in to a power zener by using it with a power transistor.
Here's what I came up with. My thinking is that a 4k3ohm resistor will provide 9mA (39V/4300). How am I doing?

#### Dodgydave

Joined Jun 22, 2012
9,939
You can run it using Half wave supply like this, you can put two fans one on each cycle.

#### CCinBR200

Joined Oct 4, 2020
80
You can run it using Half wave supply like this, you can put two fans one on each cycle.

View attachment 222819
I've got some 1N4004 diodes that are rated at 400V so I think those can work. How does this design know to supply the 12V to the fan rather than some other voltage?

#### Dodgydave

Joined Jun 22, 2012
9,939
It uses only half of the AC cycle, so the voltage will be lower around half of the supply , probably about 12 to 14V with the capacitor.

#### CCinBR200

Joined Oct 4, 2020
80
It uses only half of the AC cycle, so the voltage will be lower around half of the supply , probably about 12 to 14V with the capacitor.
Ohhh. That sounds good. Just to be safe, I'll try putting the L7812CV between the 1N4004 and the fan. Thanks for the ideas!

#### dl324

Joined Mar 30, 2015
12,800
Here's what I came up with. My thinking is that a 4k3ohm resistor will provide 9mA (39V/4300). How am I doing?
It's the base-emitter junction that needs to be turned on, so the calculation is
$$R=\frac{V_{BE}}{I_{D1}}=\frac{0.7V}{9mA}=77.7\Omega$$
The closest standard 5% value would be 75 ohms.

You're missing a connection dot on the cathode of C2 (critical) and the anode of C3 (cosmetic). The secondary voltage should be on the secondary, not by the switch. We don't wrap circuits like that. The preferred direction is left to right and top to bottom. There are exceptions to that rule, but this isn't one of them.

EDIT: 1N4745 is a 16V zener, so you're not understanding the circuit.

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#### CCinBR200

Joined Oct 4, 2020
80
You can run it using Half wave supply like this, you can put two fans one on each cycle.

View attachment 222819
I gave this a try with the 1N4004. I have the following voltages (all with no load):
at anode of C2: 40V
at cathode of Zener: 39.9V
at anode of Zener: -1.8V
at pin 1 of L7812: -2.0V

When I added the fan, the Zener burned up. What am I doing wrong?

#### Dodgydave

Joined Jun 22, 2012
9,939
What!! You don't need any Zeners they will conduct both ways.

, just use the 1N4004 diode .

#### CCinBR200

Joined Oct 4, 2020
80
What!! You don't need any Zeners they will conduct both ways.

, just use the 1N4004 diode .
Sorry. I got things confused. I am using 1N4004, not Zener.
I remeasured. What is see now is the volts at cathode are inching up from about 300mV. When it gets to 400mV, it drops again. Cycle repeats.Same at pin 1 of the L7812.

#### CCinBR200

Joined Oct 4, 2020
80
What!! You don't need any Zeners they will conduct both ways.

, just use the 1N4004 diode .
And the cap is .1uF 50V.

#### Dodgydave

Joined Jun 22, 2012
9,939
You need to put a load on across the capacitor , say a 100 ohms resistor the voltage across it will be around 14 V, and the fan should be able to spin .

#### CCinBR200

Joined Oct 4, 2020
80
You need to put a load on across the capacitor , say a 100 ohms resistor the voltage across it will be around 14 V, and the fan should be able to spin .
I added a 100ohm resistor parallel to the cap. New results:
at anode of C2: 40V
at cathode of Zener: 40V
at anode of Zener: 39.2V
at cathode of cap: 39.2
at pin 1 of L7812: 39.2
at pin 3 of L7812: 3.5 (I assume it's in self-protect mode as 35V is it's max)
no spin

What am I doing wrong?
Thanks for all your patient help!

#### Dodgydave

Joined Jun 22, 2012
9,939
Can you draw or post a picture of the circuit.

#### CCinBR200

Joined Oct 4, 2020
80
Can you draw or post a picture of the circuit.
Note that after C2, there is the main power circuit branching off (not shown).

#### Attachments

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#### Dodgydave

Joined Jun 22, 2012
9,939
So have you got the regulator the right way round.?

Try the method I gave in post#25 using just one diode .

#### Papabravo

Joined Feb 24, 2006
15,789
Note that after C2, there is the main power circuit branching off (not shown).
That regulator is going to be dissipating nearly 7 watts and will cook itself to death in short order. Are you familiar with what happens to an IC when you let the magic smoke out?