555 Timer is hitting 200F+ Can't figure out why

Thread Starter

cgw94

Joined Jun 11, 2020
5
Hi all,

Took a stab at copying a basic 555 Timer circuit found here

I replaced the mosfet with a motor driver found here

Here is my terrible circuit layout
1591926310667.png



The only thing i can think of is that i dont have anything between the motor driver and the timer? I am going to redraw the circuit so it makes more sense but I want to start from scratch so I can see if I overlooked something.

First time doing design to PCB and it was fun! More fun if it worked though!

Thanks!

Powering with a 9v battery*** I turned the circuit on with no motor connected*

Components are

POT 10k
R1 2.2k
C1 C2 100nF
 

dl324

Joined Mar 30, 2015
10,987
Welcome to AAC!

Instead of posting links, post the relevant information on this forum.

Neither of your schematics are readable. It will take me some time to make any sense from them.

I don't know what schematic editor you're using, but that timer symbol is retarded. I made one that made more sense for the editor I use (inputs on the left and top and the output on the right).
 

jpanhalt

Joined Jan 18, 2008
9,754
Your second schematic doesn't show clearly a connection between pins(2,6) of the 555 and the pot for charging.

The logic input to IN1 of the motor drive is rated at only 100 uA and a maximum of 7 V. I don't see how the normal current can be a problem; however, you may have have exceed the maximum voltage of the driver and created a short . Does the 555 get hot if the DRV8870 is disconnected? How much current is the motor pulling? A typical 9 V battery cannot supply much current.

If it doesn't get hot with the DRV8870 disconnected, try a separate supply with common ground for it. You still need to reduce the 555's output voltage.
 

Thread Starter

cgw94

Joined Jun 11, 2020
5
Welcome to AAC!

Instead of posting links, post the relevant information on this forum.

Neither of your schematics are readable. It will take me some time to make any sense from them.

I don't know what schematic editor you're using, but that timer symbol is retarded. I made one that made more sense for the editor I use (inputs on the left and top and the output on the right).
Thanks for the advice. I was using KiCad to create the schematic. Ill post the info next time!
 

Thread Starter

cgw94

Joined Jun 11, 2020
5
Your second schematic doesn't show clearly a connection between pins(2,6) of the 555 and the pot for charging.

The logic input to IN1 of the motor drive is rated at only 100 uA and a maximum of 7 V. I don't see how the normal current can be a problem; however, you may have have exceed the maximum voltage of the driver and created a short . Does the 555 get hot if the DRV8870 is disconnected? How much current is the motor pulling? A typical 9 V battery cannot supply much current.

If it doesn't get hot with the DRV8870 disconnected, try a separate supply with common ground for it. You still need to reduce the 555's output voltage.
ahhhhh i see I thought i had the 555 circuit wrong. I will try this setup next. I ordered 50 of these PCB's because I was quite sure my circuit was ok. Live and learn i guess.
 

JohnInTX

Joined Jun 26, 2012
4,113
Also, in the schematic in post #2, Pin 7 is connected directly to the supply +. The discharge transistor on the pin shorts the supply when it is turned ON (when the 555 output is low). That would account for the heat. I believe it should be on the other side of R1 for your PWM circuit but double check that. It can't be wired as shown, though.
 

Thread Starter

cgw94

Joined Jun 11, 2020
5
Also, in the schematic in post #2, Pin 7 is connected directly to the supply +. The discharge transistor on the pin shorts the supply when it is turned ON (when the 555 output is low). That would account for the heat. I believe it should be on the other side of R1 for your PWM circuit but double check that. It can't be wired as shown, though.
1591935124999.png

This is the circuit i based my on off of. I i just tested my circut and I did indeed run that over to source by mistake.....whoops!
 

crutschow

Joined Mar 14, 2008
25,126
Yes, the poor little discharge transistor is trying to pull the Vcc supply to ground, which it can't.
Result is high current and a lot of heat.
 

Audioguru again

Joined Oct 21, 2019
1,595
I guess KiCad makes wires all over the place the same as Multisim. The text is tiny because the parts are too far apart.
I also spotted discharge pin 7 shorting the supply voltage and causing the heating.
 

jpanhalt

Joined Jan 18, 2008
9,754
For cutting the track on your pre-made PCB, I use a small burr (ball cutter) in a Dremel. You can also use a drill -- 135° included angle (often called an aircraft or aluminum drill) also works well but a makes a bigger spot.

For cleaning solder resist (which is probably an epoxy-like material), I use a fiberglass scratch brush or #11 blade to get started, then the brush. It makes a neat, clean spot.

1591985722769.png
 
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