Help Identifying SCR

Thread Starter

tgarrity

Joined Jan 26, 2009
10
Hi, I hope this is the right forum for this question. If not, please point me in the right direction.

I have what I believe to be a blown SCR in my car battery charger, due to me reverse connecting it to the battery. Oops. The left leg of the SCR is blown off completely. The identifying marks on it are:

MCR 265
4
M 9221

The bottom M is actually the Motorola logo. The small circuit board it is on is printed "SCR1" where it is attached. I have done internet searches on all of the above info, with no hits whatsoever. Can anyone help me identify this, or lead me to a replacement? Is this something I can get at Radio Shack?

Thanks,

Terry
 

eblc1388

Joined Nov 28, 2008
1,543
That part# MCR256-4 appears in my 1986 Motorola Master Selection Guide.

It is a black TO-220AB plastic case style SCR. The pins are K,A,G (left to right) if you view the part number from front with the pins pointing downward.

It has a On state (RMS) current of 55A, Vdrm or Vrrm of 200V and I(gt) 50mA, Itsm 550A and Vgt 1.5V. The datasheet is here:

Datasheet MCR265-4 manufacturer Motorola

You'll need to find a replacement with rms current of 55A or higher and 200V or higher. It might be difficult to get a part with that spec in a TO-220 style package.

Ask here if you can't locate a replacement.
 
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Thread Starter

tgarrity

Joined Jan 26, 2009
10
Thanks very much for your reply, I appreciate it. I haven't worked with this stuff for many years. I did a search on Digi-Key and Newark, and there didn't appear to be any substitutes. I left off the package type and also tried typical vs. max spec values.

I did a global search on the internet using your P/N and found one on eBay for about $10 incl. shipping. Is this my best bet? Is there another way I could approach this, or any other way to substitute? How about just replacing the SCR with an inline fuse rated above 55A or a circuit breaker? I believe it is just being used as a "crowbar" circuit in this application.

Thanks for your help.

Terry
 

eblc1388

Joined Nov 28, 2008
1,543
I did a global search on the internet using your P/N and found one on eBay for about $10 incl. shipping. Is this my best bet?
I love eBay but usually the postage is higher than normal. I think the seller is taking some of that as revenue too. So I just factor that into my consideration.

Is there another way I could approach this, or any other way to substitute? How about just replacing the SCR with an inline fuse rated above 55A or a circuit breaker? I believe it is just being used as a "crowbar" circuit in this application.
Can't easily decide on that without seeing a schematic. But a fuse don't usually offer as good a protection as a crowbar circuit with reversed battery polarity.

Usually, a crowbar circuit has a fuse or circuit breaker upstream so that the fuse will brown or breaker operates instead of the crowbar SCR being destroyed. This is also required to prevent starting a fire. Why is there no fuse fitted in your case?
 

Thread Starter

tgarrity

Joined Jan 26, 2009
10
L. Chung,

I agree, I think people on eBay are making their margins on the shipping rather than the product itself. I figured the SCR should be about the price of a transistor or something, somewhere around fifty cents or a buck, so I thought $10 was highway robbery. But I managed to do some more digging and cross referenced it to another part, NTE54000, but it turned out to be around the same price w/shipping, so apparently the SCRs cost that much.

Unfortunately, no schematic, no manufacturer or anything else to go by other than what I'm seeing in the case according to my detective work. So I'm shooting from the hip, as we say here in the Midwest. ;-)

There isn't an inline fuse, which is why I think it blew the SCR. But there is, what I think is a bi-metal circuit breaker. But if reverse polarity, may not prevent blowing the SCR? This is actually the first time in the 15 years I've owned it where I inadvertantly connected it backwards. (Senility already??)

Don't know if this helps, I've attached a couple pics, a wide view, and a closeup of the SCR and associated components. The yellow connector on the right is attached to the bi-metal breaker, which is on the bottom side of the view(s), which is attached to the anode of the SCR, I believe, according to your datasheet.

Just so you know, I bought a replacement battery charger, so this project has become an exercise in edification for me at this point.

Thanks again so much for your assistance. I have more questions on this circuit if you have time/interest.

Regards,

Terry
 

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eblc1388

Joined Nov 28, 2008
1,543
Looking at the images you have posted I doubted that the SCR is for reverse voltage crowbar protection.

The circuit board has four small transistors and a IC, so it is trying to control something, in this case the charging of the battery.

I can identify a center-tapped full wave transformer secondary going to two diodes mounted on heatsink and their common cathode then goes to the SCR. I'm pretty sure that the SCR is some sort of firing angle phase control to regulate the current going to the external battery during charging.

The bi-metal switch is there to protect overcurrent to the battery should the SCR fails to a short circuit.

Looks like you have to buy the exact part number for replacement. It is also possible with a very slight chance that the reversed polarity connection earlier also damage the electronic parts on the circuit board besides blowing off the SCR.

I'm away for a few days so would someone who own or had owned a similar charger please offer some insights for the OP?
 

Thread Starter

tgarrity

Joined Jan 26, 2009
10
OK thanks. I'm not very familiar with SCRs, must have missed that week in electroncis school. From the stuff I read on them, the crowbar seemed like the most likely scenario to me, so thanks for the analysis.

I'm attaching a pic of the front, showing the different functions. It has switches on the front for Regular/Deep Cycle batteries, 2/10 amp output, and normal/engine start (55 amp) charging.

The components on the circuit board look perfectly fine to me (nothing appears fried), but I know they could just appear that way. When it blew there was a pop and a quick puff of smoke, and that was it. I susect that was the leg of the SCR being blown off. I soldered a lead from a resistor onto it to replace it, but the charger only shows very low DC output (mV) rather than 12-13 VDC. SCR continuity tests show:

Gate to Cathode: shorted both directions
Anode to Cathode: open both directions
Gate to Anode: open both directions

My understanding is that is should read like a normal diode between anode and cathode (open one direction, shorted the other).

The thing that concerns me is, the outputs from the transformer to the rectifier diodes show about 13.5VAC. But at the recitifier output it only shows about 11.5VDC. My understanding has always been that a car battery needs about 13.8VDC for charging. I guess I expected the rectifier output to be nearly the same as the transformer output. The rectifier diodes measure correctly, so I think they are good.

If I could get it working for $10 by replacing the SCR I'd be happy with that. Otherwise, I'll just pitch the thing.

Thanks once again for your help on this, I appreciate it.

Terry
 

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