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Reverse engineer this?

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Tipsy

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I have a home-made (not by me) device called a Throttle Position Sensor (TPS) switch. It is a small circuit board attached to an automotive coil relay (12v), as shown below.

The white wire is the TPS switch input. The pot is used to fine-tune the exact point the TPS signal trips the switch.

Sorry for my poor photography skills, hopefully you can just make out the tracks.

Apparently, most car's TPS devices output 0-4.9V (idle-full throttle) dependant on throttle position. But mine has to be different and only outputs 0-3.9V. I am told this is the reason the TPS switch will not work for me.

Can anyone analyse the circuit and tell me how it works. Additionally, can it be modified to work with my car?

I have tried to draw the circuit from what I can see but being such a novice cannot determine the correct diode and transistor symbols or connections.

The INPUT TRIGGER is the TPS (not shown) output (0-3.9v) seen as the white wire.
The LOAD connects to the Load's negative terminal, the positive (not shown) direct to battery via fuse.

I hope that was clear, but I'll try and answer any questions.

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It should work fine. If 3.9v is not high enough to operate it, then replace the pot with a pot of a higher resistance value ie 2x or 5x the resistance etc.

So if it is a 10k pot, replace with a 20k or 50k pot.

Thank you Mr RB, I have a selection of pots to play with.

When you say "not high enough to trigger it" what is it exactly? The transistor or the Relay or something else?

I think he meant it by voltage divider rule. A voltage across a resistor is the current times the resistance, ohm's law. By using higher valued pots, your adjustability works in higher values of voltage when the pot is connected to a constant-valued resistor.

Vizier87, that's most helpful but I'm still unsure what the voltage is failing to trigger at this point. Ultimately, we want the relay to operate which are normally powered by 12v.

FYI, the POT in this case is marked 470K - that is quite high already?

"Trigger it" refers to the fact it needs 0.6v on the base of the transistor with respect to ground so the transistor turns on, which turns on the relay.

Now you said the pot is 470k, I have a bad feeling your transistor is blown. The bottom image, near the bottom of the PCB, shows what looks like black carbon residue where it might have shorted out, and the tracks that shorted go direct to the transistor.

Thanks Mr RB! So if the transistor only needs a small voltage, the failure to switch on could be due to the relay?

Please ignore any black residue, it is remnants of a protective resin the creator added to the whole circuit as it's designed to be used in the hostile environment of an engine bay. Additionally, I noticed the centre leg (if that's the correct term) of the transistor was not soldered to the track pad (it still isn't but I can rectify that).

But I can assure you that is not the reason it won't work as I was given a second replacement unit which I tested and does work with ~5-6V. What I don't have is an adjustable power supply to test an output as low as 3.9v - can't afford one either.

But I can assure you that is not the reason it won't work as I was given a second replacement unit which I tested and does work with ~5-6V. What I don't have is an adjustable power supply to test an output as low as 3.9v - can't afford one either.

You can always build your own adjustable power supply with an LM317 adjustable regulator. If you are serious about troubleshootin electronics, I would advise you to build one...it is a basic tool that should be in every hobbyst bench, along with a multimeter.

Please post all the resistor values (shown on the schematic) and the transistor number. It should be easy enugh to recommend new resistor values and/or a new transistor type to get it running for you.

I looked into the LM317 and realised I'd have to create another circuit to run it off the mains 240Vac. But then realised, being the hoarder I am, there's a box full of mains power adapters I can hack. If only I knew what I was doing!

As for the component values - I don't have the schematic, I drew the circuit as my best guess. I've also forgotten my resistor colour code and the order of reading the banding. And as they're blue, cannot distinguish the colour bands so easily, least of all due to the residues and position but this is what I think I can see.

R1 (inline with the TPS white wire) Brown (or Black or Red), Black, Green (or Blue), Red.
R2 (inline with -12v and POT) Red, Red, Yellow, Brown.
T1 11c (or 13c) 517 M004

If that's no good I can desolder them.

Take a multimeter, and simply read each resistance value with it.

Sounds like R1 is a 1 megohm (or 10 megohm?) resistor, which explains the high 470k for the pot.

Try replacing R1 with a lower value resistor like 100k or 220k. That should give you plenty enough adjustment to get the transistor to turn on with a 3v input from the throttle sensor.

Ok, I had time to desolder them (not sure on the wisdom of measuring in-situ) and the results are in: Full marks to Mr RB!

R1 (that acts on the input trigger voltage) is indeed showing 1000Kohm on my cheap multimeter while R2 (that acts on the POT?) is 220K.

It looks like I'm going to have to wait until I can whip up some power supplies to simulate the application and therefore I have a question regarding this.

I assume it's not enough to supply the 12v (for the relay) from one source and the 3-4V input trigger from another - something to do with common ground is it? If so, how can I get create a second ~4V off the same 12v source? Is it ok to just resistance? I tried before but could barely get the voltage down to 9-8V.

I have a home-made (not by me) device called a Throttle Position Sensor (TPS) switch. It is a small circuit board attached to an automotive coil relay (12v), as shown below.

The white wire is the TPS switch input. The pot is used to fine-tune the exact point the TPS signal trips the switch.

Sorry for my poor photography skills, hopefully you can just make out the tracks.

Apparently, most car's TPS devices output 0-4.9V (idle-full throttle) dependant on throttle position. But mine has to be different and only outputs 0-3.9V. I am told this is the reason the TPS switch will not work for me.

Can anyone analyse the circuit and tell me how it works. Additionally, can it be modified to work with my car?

I have tried to draw the circuit from what I can see but being such a novice cannot determine the correct diode and transistor symbols or connections.

The INPUT TRIGGER is the TPS (not shown) output (0-3.9v) seen as the white wire.
The LOAD connects to the Load's negative terminal, the positive (not shown) direct to battery via fuse.

I hope that was clear, but I'll try and answer any questions.
Perhaps the transistor symbol is shown wrongly. It could be a PNP device.

The collector is connected to Negative.and emitter through relay coil to positive.

T1 11c (or 13c) 517 M004

Perhaps the transistor symbol is shown wrongly. It could be a PNP device.

The collector is connected to Negative.and emitter through relay coil to positive.

Hi mvs sarma

Yes, I was purely guessing which type and how the pinouts might be. Do the markings I quoted on the transistor not give us any clues? I'll look into myself when I've got more time. Need to get on with making my power supplies first.

One possible candidate for conversion is a Mains DC Adapter that outputs 18Vdc ~ 400mA. It used to be the charger for a cordless drill so I opened it up and found this...

It appears the manufacturers took a standard adapter and added a small circuit to the positive side consisting of a diode, a resistor, an led and what I'm thinking is known as a ballast resistor? Is that right? I can't fathom what use this circuit is (other than having an LED and failsafe diode) as with or without it, I'm reading roughly the same 18.9V with my multimeter.

So my questions are;
Would this charger/adapter be suitable for my power supply
What does this extra circuit do and...
Do I need it or...
Can I use it elsewhere?

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Perhaps the transistor symbol is shown wrongly. It could be a PNP device.
...

I don't think so, I just assumed that he drew the schematic wrong, the Emitter of the NPN should go to ground, where he has it drawn connecting between the pot and R2.

Basically the 3 resistors just make a voltage divider so the NPN turns on at the right input voltage, then there's just a relay and diode in the NPN collector circuit.

Tipsy- Now you have exact resistor values it makes more sense, you need a new R1 of about 470k.

Also if you want to build an adjustable DC power supply from scratch it's probably best to start a new thread, you may get more help that way.

Thank you kindly Mr RB, I will start a new thread as you advise. I will revisit this one when I've got myself the power supply I need.

Thank you everyone that has contributed.

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