Need help identifying an early 90s PCB thru-hole transformer and a few other questions

Question

Hi. I need help identifying an early 90s PCB transformer. It's on an auto-idle control board from an early 90s gas-powered generator from a company that has since gone out of business. I'm not familiar with components but think I can identify all of the other components to remake another board in case of a catastrophic failure. The problem I'm having is identifying the size and type of what I think is a transformer. I've included a few pictures. It's located at the bottom of the board and has a yellow and red wire wrapped around it. The red wire comes from the generator head and goes out to a breaker to power the receptacles, and the yellow wire comes from a terminal lug next to another terminal lug which accepts the other (yellow) wire coming from the generator head. And can anyone tell how big the small capacitor is that's located to the right above the transformer? I can't find any marks on it other than the temperature rating. I also have to replace a 5W 82ohm resistor that blew due to a bad Schottky diode that I've found and will replace. Would it be safer to replace the blown resistor with a 12W 82ohm resistor? And can I use an aluminum encased surface mount resistor on a thru-hole PCB? Any help would be appreciated! 
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jw0752 · Answer

Hi Chris,

The small capacitor is probably marked on the side that is glued down. You may have to pry it loose carefully. You can up the wattage of a resistor without any consequence except that of a larger foot print. The transformer is being used to measure the current that is being delivered. As the current goes up so too the load on the motor and therefore the need to increase the throttle to maintain proper RPM. I would reuse the transformer if possible as they are quite hardy and not likely damaged.

John

jw0752 · Answer

Hi Chris,

If you mount a power resistor on the board make sure it is in a raised position so air is free to flow around it. This will keep it from damaging the board. You may not get the full wattage dissipation from an aluminum encased unless it is mounted to a heat sink but if you are going from a 5 watt to a 25 watt I am sure it will be capable of handling 5 watts. The transformer was probably OEM and manufactured for this application and not available except from the manufacturer of the generator. It does not look like a stock transformer and would be difficult to duplicate without the design specifications of the circuit.

John

three-phase · Answer

I would agree with jw0752 that the small transformer is a current transformer measuring the load on the sockets to allow the engine speed to be controlled based upon the load the generator is supplying. As John as said they are usually very robust. If you do have concerns about it, they can be tested by injecting a current through the primary and measuring the output current on the secondary, depending on your circuits the output current may be converted into a voltage. If you take a few readings at different current inputs you will be able to plot the ratio, which should be consistent across the range. This of course depends on what test apparatus you have and the setup you can achieve. If you have a failure, coils can generally be rewound by dismantling them and identifying the wire size an number of turns, quite a slow painstaking job for something that small. Kind regards Donald

jw0752 · Answer

Hi Chris, Here are some very close equivalents: http://www.newark.com/search?st=BT149 Seems that the MCR100-8 are no longer being produced. John

mcb1 · Answer

There is one available to purchase. 100105-01 | Desa International* | Circuit Board REPAIR Not sure if this one is valid or not http://www.uhtastore.top/space-heaters-c-42_1633_3697_3699/desa-auto-idle-circuit-board-10010501-nos-p-7315.xhtml Sounds like these are not as reliable as they should be, and maybe you can start a repair business... The manual shows how they interconnect. Desa GENERATOR User's Manual so the transformer seems to be detecting the load and throttling back whenever the load drops off. Mark

jw0752 · Answer

Hi Chris,

You are correct it is a .01 uF and the Z indicates a 20% tolerance. From your picture it appears that there is a line drawn under the 103Z and this would indicate 50 / 100 volts according to the literature. If there is no line then it is suppose to be 500 V. No one seems to be too concerned with the voltage on ceramic disks and it is extremely rare for them to degrade or fail. As far as the size being an indicator of the voltage I would go with thickness as a better indicator of higher voltage.

John

shabaz · Answer

Hi Chris, The ceramics look good, they don't really need changing. Physically bigger ones would be fine if you really want to, but it is not going to fix anything, because the originals won't be faulty. Can I ask, where are you getting your replacement parts from? The reason I'm asking is because if I google " CERA-MITE .01uF 250v 103Z 12mm disk Radial Ceramic Capacitor " then I get hits on Amazon and ebay. That's not really a good source for parts. You might get lucky with them, but if you're obtaining your semiconductors, i.e. the SCRs from there too, that rings some warning bells. From personal experience, there is a significant risk of getting fake or pre-used parts disguised to look like new. Fake/pre-used and real semiconductors are difficult enough for any engineer to distinguish visually (and sometimes even with test tools), so it really isn't worth the risk unless perhaps it is for some low power hobby experimentation (which isn't the case for your repair).

Need help identifying an early 90s PCB thru-hole transformer and a few other questions

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