Chinese Plasma Cutter

[tytower]

I have a Chinese CUT 50 brand plasma cutter.

When I pull the trigger a relay clicks a buzzer sounds and an LED for overheating comes on so no go plasma cutter. The machine is completely cold.

I'm guessing a temp sensor is broke and so turns the thing off.

Where would the temp sensor or sensors be placed ? How can I find and test them?

[wotnot]

I have a Chinese CUT 50 brand plasma cutter.

When I pull the trigger a relay clicks a buzzer sounds and an LED for overheating comes on so no go plasma cutter. The machine is completely cold.

I'm guessing a temp sensor is broke and so turns the thing off.

Where would the temp sensor or sensors be placed ? How can I find and test them?

Typically they mount a thermistor to the heatsink that carries the (fast) output diodes (some may have another thermistor mounted to the heatsink carrying the SMPS switchers)...but normally an overheat warning relates to the output section.

On the Unimig plasma cut 40, they only have one mounted to the output section heatsink (the SMPS section is monitored/protected by current measurements on shunt resistors in the SMPS circuitry)

Image of the thermistor on the output section heatsink on the Unimig....note; on these you have to do a near complete teardown and remove controller board to get at it)






See ->

[tytower]

[wotnot]

I didn't see it in those images, but, where do this pair of white wires go? (out of the connector)



edit: just thinking aloud, they could've cheated real horrorshow...(seen this with cheap chinese audio amps) -- instead of mounting a thermistor to the heatsink proper, I've seen them use little glass bead thermistor on the main PCB adjacent/under where the heatsink mounts to the PCB

[VroomVroom]

is there significant warping on those PCBs ? doesnt look great

[tytower]

@VroomVroom

No warping just they are not supported there as I was trying to get a look under and around each board so I have it pulled apart.

[tytower]

@wotnot

17 years ago I built me a lab power supply from a Silicon Chip Magazine design and kit. The temp sensing was done with transistors. As it got hotter the tranny switched more current and a comparator circuit flipped when it was too hot.

So I'm looking for that possibility too.

[wotnot]

Images are useful though - this all looks very familiar -->>

The temp sensing was done with transistors.

Yeah, but the chinese way is as cheap as possible -- transistor junctions cost heaps more than thermistors =)

[tytower]

Man thats a good reference . If I don't get it from there then I am simple. Thanks heaps .

A little story . Bought an Audi A4 about 2003 model. I would start it in the morn and go to the PO about 15 minutes at 100K park clear the box and come back to start it and it would not.
It was flooding I reckoned . Wait 1/2 hour or more to cool down and it started again?
Checked a bit but the flooding gave it away . Fuel out the exhaust.
It had 2 of those cheap sensors ,made in china ,in a holder that plugged into the motor. $ 85 AUD in Aus and about $250 to fit it .
Ebay had them for $28

What was happening was the one sensor worked and fed the instrument panel. The second didn't, so the car had some sort of default setting on that sensor and the cpu decided it was bloody cold so it set the choke for -20C . In Aus my ambient is about 25C up to 40C Motor was probably 120C . So Flooding. Must admit replacing it in the engine was a bugger of a job though.

[wotnot]

A little story . Bought an Audi A4 about 2003 model. I would start it in the morn and go to the PO about 15 minutes at 100K park clear the box and come back to start it and it would not.
It was flooding I reckoned . Wait 1/2 hour or more to cool down and it started again?
Checked a bit but the flooding gave it away . Fuel out the exhaust.
It had 2 of those cheap sensors ,made in china ,in a holder that plugged into the motor. $ 85 AUD in Aus and about $250 to fit it .
Ebay had them for $28

What was happening was the one sensor worked and fed the instrument panel. The second didn't, so the car had some sort of default setting on that sensor and the cpu decided it was bloody cold so it set the choke for -20C . In Aus my ambient is about 25C up to 40C Motor was probably 120C . So Flooding. Must admit replacing it in the engine was a bugger of a job though.

By the time I'd read the bit in bold, I knew to check CTS (coolant temp sensor) ; they used to run two sensors...the CTS on a 5volt reference, and the dash temp gauge sender (usually a 10v reference via regulator in dash cluster) ; more modern cars just use CTS, and send a drive signal to the dash temp gauge. Modern EFI engines do not have a choke ; when cold they increase the injector pulse duration to make engine fuel mixture rich -- likewise, the don't have a throttle stop screw to adjust idle rpm. and idle air is controlled by IACV (idle air control control valve), and nor do they have a fast idle cam for when the engine is cold (some use a cold start air admittance valve, some use an auxiliary air control valve, and others an air regulator with a bimetallic strip that closes the regulator as the engine heats up)

I've had a couple of coolant temp sensors fail over the years ; I carry a 3K9 .5watt resistor in the glovebox, so I can unplug the failed sensor and bridge the terminals with the resistor (3K9 makes the ECU think the engine is up to normal operating temp) -- normally they still start cold like this, because ECU has a reading on IAT (intake air temp), and usually provides some enrichment in response to that...but this trick is a handy workaround until one can source/fit a new sensor =)