So the other day, my family toaster which is roughly as old as myself (25yrs), burned down.

When I took a look at the circuit, I found out that some components melt down completely.

One of them is (or at least i think it is) the load resistor of the diode bridge.

I made a schematic from the components i could still read: Toaster Circuit

So the unknown resistor is RL. J3 is connected to the huge toaster wires (the ones that actually toast). those wires are then connected to the mains (230V 50Hz)

Would anybody have any idea on how to estimate the value of the RL?

(Also, Kudos to whoever can give me some hints for R1 and R2 as well)


  • \$\begingroup\$ If you are attempting to fix a toaster, that is off topic. If you are attempting to understand how this circuit works, then maybe ... \$\endgroup\$ – Mattman944 Feb 15 at 15:15
  • \$\begingroup\$ In engineering, we describe things using numbers rather than relative terms like large, small, high, low, and "(very) old." Although you violated that principle in your title, you managed to mitigate that error by mentioning that the toaster is about 25 years old. I mention that because the toaster that I have been using nearly every day for about 50 years was about 20 tears old when I got it. The one that it replaced predated pop-up toasters. americanhistory.si.edu/collections/search/object/nmah_324392 \$\endgroup\$ – Charles Cowie Feb 15 at 16:33
  • \$\begingroup\$ You need to show the full circuit from J3 via the heaters to the incoming AC mains. \$\endgroup\$ – Andy aka Feb 15 at 17:39

RL is just a bleed resistor to discharge C2. So, when someone disassembles the toaster, they don't get zapped.

To discharge to a safe level in about a minute: 27k.

Power = 160^2/27k = ~ 1W, this is probably too much power to waste. Maybe 100k?

Need to know the coil resistance to determine appropriate values for R1 and R2.

Edit 1: I wouldn't call J3 "AC in", to me that means line (mains). Andy is right, it must be a current sense. I will try again when I get a good schematic.

Edit 2: New guess as to what the overall circuit looks like.

The relay is probably either a 5V or 12V relay. If it is a 120VAC 1000W heater, then the heating element is about 14 ohms when hot. RL needs to be chosen to get the appropriate voltage to Q1-E. Somewhere around 0.4 to 1.0 ohm. Lots of assumptions made.

R1 needs to create enough base current to saturate the transistor, need to know the relay voltage and resistance. R2 = 10 * R1 is often a good choice.

Edit 3: Previous version can't be correct, too much current in the bridge rectifiers. Maybe the heater is split into 2 pieces. R11 produces just enough voltage to power the circuits.

To further analyze, we need to know the characteristics of Bridge_Out. Is it smooth and it is used to power the circuits? Then R6 (RL) is maybe 100 ohms. But, then Counter_Rst doesn't make sense.

The physical size of R6 (RL) might give some clues. If we knew the power, the we could estimate the resistance better.

If R11 opens, the circuit is toast LOL. Maybe this is what happened.


simulate this circuit – Schematic created using CircuitLab

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  • \$\begingroup\$ Look at the diodes in the bridge and ask yourself if they can withstand full AC voltages? More than likely, the resistor is quite low in value and possibly used as a current sensing element or more likely as a clock generator for the timer. \$\endgroup\$ – Andy aka Feb 15 at 17:42
  • \$\begingroup\$ @Andyaka - good point \$\endgroup\$ – Mattman944 Feb 15 at 17:50
  • \$\begingroup\$ Thanks a lot for the help! Sadly i cant give you the actual size of RL since it completely burnt down (there's only a small chunk of plastic attached left) but from the footprint i would say it was either a classic 1/2 Watt through hole resistor, either a diode (unlikely). About Counter_reset, it is actually the output of the counter IC. It goes to VCC when it finished counting. My guess was that when that output goes up, the voltage on the pnp is insufficient and that blocks the current that flows to the relay. I'm not sure about the use of counter reset though \$\endgroup\$ – TouchTheFishy Feb 16 at 10:06

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