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Comment by sgarland | Hacker News Reader

I have wondered for the longest time what the BAGEL button actually did, because on my cheap toaster, it seems to do very little.

Unfortunately I still have more questions after reading the schematic.

SW4 (BAGEL) allows current flow through R9/LED3 (BAGEL) which raises Vbe on Q2, forward biasing it, allowing current flow through Q2, dropping Vcs.

Meanwhile, power has been flowing through the main switch, with RL3/RL4 making a voltage divider. That main power flows to D1, forward biasing it, and then hits the cathode of D4, reverse biasing it, and allows Relay 1 to hold main power closed. The cathode of D4 also tees off to inputs on Relay 2, and the cathode of D5 (whose anode is fed by both another input on Relay 2, and the source of Q2).

So, when Q2’s Vcs drops due to it turning on, D5 presumably reverse biases, so the only input felt on Relay 2-2 is from D1. And Relay 2 does… something. If anyone else can explain, I’d be grateful.

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The bagel button makes only one side of the slot get hot (or at least makes one side hotter than the other). Your toaster probably has a bagel icon on the top showing you which way to put the bagel in (cut side towards the center in every toaster I have seen).

Source: I'm not married to toaster moguls but I do peer into operating toasters to see which wires turn red. Feel free to replicate my research and post your observations.

Mine doesn’t have a bagel icon, but I do put them cut-side towards the center.

Looking closely during operation, the elements facing out from the center didn’t appear to change color with the bagel button depressed. It toasts the bagel just fine (as well as bread), though.

I should get a wattmeter and see if power draw changes.

I don't think I've ever before been party to any conversation where the words "toaster mogul" ever appeared in the exchange, and yet, here they are, in all their golden brown and delicious glory!

My theory is that the bagel button does one thing only - it illuminates the bagel button when you press it. I've seen toasters with functional bagel mode but my latest cheap crap toaster does precisely nothing different when the bagel button is activated.

Okay then. What does the frozen bread button do?

Ok, so that I didn't know. Here's from the Oster website: "When this feature is selected, the toaster will automatically defrost your food and then toast it in one easy step." So the temperature is lower for a little bit to thaw the food then higher to toast it.

The example toaster has 2 toast slots 3 heating elements (EDIT: Technically 4, as the last element is split into two: RL3 and RL4).

Relay 2, activated when the "bagel" light is lit through the gate of Q2, disconnects neutral from heating element 2 (the one in between slot 1 and 2) and 1 (the one on the outside of slot 2).

This means that there is only power to the outside heating element of slot 1, allowing you to toast only one side of a bread or bagel.

Then what is RL4? If it were just a resistor for a voltage divider, I’d think it might be labeled differently than the other three.

RL3 and RL4 are both heating elements in slot 1 - likely just one heating element with a tap somewhere - that indeed acts like a voltage divider.

The power to the circuit flows from in between those heating elements, through a single rectifier (D1), through a 1W resister (R1), before being clamped to 5.1V by a zener (VZ1). The use of the heating element as a voltage divider is just to get closer to the target voltage (<15V considering the rating of C1), using the fact that a heating element is just a resistor with an enormous power rating. No one cares about the power efficiency of a toaster control circuit.

However, the schematic is missing a connection to VCC from the 5V rail it created - it only connects to the high side of the mains relay/toast magnet as well as the bagel button. It was supposed to connect to VCC, which in that schematic is only connected as to the clock circuit.

(I was not using the RL numbers when I numbered the heating elements in my previous comment originally - my bad. I updated the comment.)

Thank you, makes sense.

I have a different experience than the other commentator with my toaster’s bagel button. Both sides still get heated. My bagel button makes the toasting cycle last longer than normal, which is perfect for defrosting a frozen bagel and then toasting to the setting the wheel is set to.

Is it labelled "bagel", or is it a symbol that could be interpreted? What you describe is what the "defrost" feature of the linked circuit would do.

I have certainly misinterpreted iconography completely before...

It's labeled bagel, and there is a separate defrost button. According to this[0] manual from Cuisinart, "The bagel feature adds extra time to the toasting cycle to allow for thicker breads." So there's our answer!

[0] https://www.cuisinart.ca/on/demandware.static/-/Sites-ca-cui...

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