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I'm trying to understand the circuit in this PCB taken from a cordless hot glue gun. Battery voltage is 18-19v. I've been able to identify most of the components but not all. And most importantly I would like to understand what the circuit is doing.

Front Side of Ryobi P305 PCB Flip Side of Ryobi P305 PCB

What I Think I Know about the Circuit

  • Q1 Fairchild 4486 TrenchMosfet
    • Pin 1 is Gate (top left)
    • Pin 2,3,4, are Source
    • Pin 5,6,7,8 are Drain (entire right side)
  • R1 31 Ohm
  • R2, R3, R4 1 Ohm
  • R5 20 Mega Ohm

The main ic is a Fairchild Semiconductor 30V P-Channel PowerTrench MOSFET enter image description here

This circuit sits between the battery and heating element. I think it just makes sure the drain on the battery isn't too fast and that it isn't shorted. Please correct me or confirm my theory.

I'd like help to identify the unknown components then understand what the circuit is doing. My ultimate goal, if possible, is to control the heating element on/off with an Arduino or similar control circuit to maintain a specific temperature via a thermister added near the heating element.

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    \$\begingroup\$ You can investigate the components by looking up their numbers on sites like www.s-manuals.com, where you can see that D1 appears to be a Zener diode. (That's a SOT-23 package.) \$\endgroup\$ – Andrew Morton May 3 '18 at 17:41
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    \$\begingroup\$ I learned a great deal in my early days by reverse engineering circuits. You've done the hard part so I recommend that you trace out the circuit and create a schematic in your question using the built-in CircuitLab editor. It all appears to be single-sided so you won't even have to flip the board! \$\endgroup\$ – Transistor May 3 '18 at 17:49
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    \$\begingroup\$ It looks like some of SMD resistors ar code with EIA-96 code. For example, R1 is marked as 01E is 1Meg, R2->01D = 100k; and R3 and R4 looks like 01C = 10k or 01D = 100k kiloohm.info/eia96-resistor/01C R6 is 304 = 300000 = 300k \$\endgroup\$ – G36 May 3 '18 at 19:07
  • \$\begingroup\$ @Transistor there is one jump on the back side of the pcb. from a leg of Q2 and one side of R6. I started a circuit online but didn't finish. I don't think I had the right resister values and I'm still not sure what Q2 is. I looked for a while to identify the marking "PK 6". Thanks everyone for the help so far! \$\endgroup\$ – Brian Boatright May 3 '18 at 21:55
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    \$\begingroup\$ No problem. Draw the schematic. Double-click the components to enter the value or remove the value if you don't know what it is. Guess the transistor type and if it doesn't make sense someone will provide some suggestions. \$\endgroup\$ – Transistor May 3 '18 at 21:57
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Draw the schematic, and use your multimeter or oscilloscope, to see what it does, as you vary Vin, and as the gun heats up.

Likely functions might be:

  • low voltage cutoff
  • chopper to keep average Vout fairly constant as battery discharges
  • timer

As far as controlling it goes: I understand that they often use PTC heating elements for glue guns. The heating element itself is self-regulating, moreso if the supply voltage is constant.

This also means that you can sense its temperature by measuring its resistance - you do not need to add an external sensor. You do however need to work out its R-T curve.

If you measure V/I of the glue gun in-circuit, you have R. An ADC will do division for you, so you can arrange the signals to get the ADC N to be I/V or V/I, without any further computation.

schematic

simulate this circuit – Schematic created using CircuitLab

(n.b. In this circuit Fet1 must have low R as it forms part of measured R1 i.e it is an error term)

One thing to note about PTC heaters, is that they are good to temperature control to the design temperature, because they will have the correct R and I. So electronic control can easily make them more accurate.

However you can't make them much hotter - the resistance just goes up, and current stops flowing.

You can have trouble making them much colder, because the R goes down, and the current goes up - you end up with low duty cycle big current pulses. Also at some temperatures (e.g. 50C) the R-T curve can be NTC or flat. Control is impossible in this area.

This is probably irrelevant to a heat gun, which is only useful in the working zone anyway.

But, when sizing your FET you need to watch what the max current during warmup is. The lowest R may be at (say) 50C not at cold or hot, so max I over whole temp cycle needs to be measured.

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  • \$\begingroup\$ this project just got a bit more interesting. thanks for the answer! \$\endgroup\$ – Brian Boatright May 3 '18 at 21:56
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I also recommend you to draw a schematic in the CircuitLab editor. Then you can simulate this and understand it. For what I can see here, this is some simple output voltage regulator. The gate of the mosfet is driven according to the current output voltage.

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For me your circuit looks something like this:

schematic

simulate this circuit – Schematic created using CircuitLab

And the circuit looks like a strange version of an LDO voltage regulator with a series pass transistor.

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