Author Topic: Transistor question  (Read 5874 times)

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Offline snipersquad100

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Transistor question
« on: September 14, 2013, 08:39:49 pm »
Hello electrical engineers.
Ive got a power supply pcb kit whish required a bc557 transistor, I only had a c557b. Both are pnp's but are they the same?
I cant find a datasheet for the c5557b. The problem im having is that the base and the emitter of the c557b is getting 18v but it's not switching on the collector that drives the relay. Any help would be appreciated. thank you

Offline Bertho

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Re: Transistor question
« Reply #1 on: September 14, 2013, 08:51:28 pm »
My guess is that you have a BC557B. The difference between BC557 and BC557B is the current gain (Hfe), which is higher in the B-version. Without knowing the circuit, I can only guess, but probably should work; especially because you say it is only used as a simple switch.

Without knowing the schematic it is kind of difficult to know which voltages are supposed to be where. Can you post a (snippet of ) the schematic?
 

Offline casper.bang

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Re: Transistor question
« Reply #2 on: September 14, 2013, 08:51:44 pm »
Hello electrical engineers.
Ive got a power supply pcb kit whish required a bc557 transistor, I only had a c557b. Both are pnp's but are they the same?
I cant find a datasheet for the c5557b. The problem im having is that the base and the emitter of the c557b is getting 18v but it's not switching on the collector that drives the relay. Any help would be appreciated. thank you

A, B and C designations usually has to do with HFe (gain) and if I remember we're talking 150 vs. 200 vs. 250 gain of current between basis and collector. Usually not very critical, adjust your basis resistors accordingly. Do you have a drawing of the circuit?
 

Offline snipersquad100

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Re: Transistor question
« Reply #3 on: September 14, 2013, 09:01:20 pm »

The markings on the transistor is c557b, I got a few of them off ebay. The transistor in question is the one on top above the relay.

Offline Andy Watson

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Re: Transistor question
« Reply #4 on: September 14, 2013, 09:16:17 pm »
Does it look like one of these:
http://www.ebay.com/itm/300589082539

?
 
... I only had a c557b. Both are pnp's but are they the same?
I cant find a datasheet for the c5557b.
Try prefixing the number with 2S, i.e look for 2SC557B.
My ancient and crusty transistor data book says that the 2SC557 is an NPN. You could verify its polarity by ohms/diode test with you multimeter.
 

Offline snipersquad100

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Re: Transistor question
« Reply #5 on: September 14, 2013, 09:19:17 pm »
yes, the same

Offline Bertho

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Re: Transistor question
« Reply #6 on: September 14, 2013, 09:29:29 pm »
If the transistor/relay is not turning on then the comparator is not going low.
 

Offline snipersquad100

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Re: Transistor question
« Reply #7 on: September 14, 2013, 09:31:44 pm »
do you mean the lm741 on the left side of the schematic, I know the print is a bit small for you to read.

Offline Andy Watson

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Re: Transistor question
« Reply #8 on: September 14, 2013, 09:40:15 pm »
Bleep it out with your multimeter. I think you will find it is an NPN.

Japanese transistors have a 2Sxxxx numbering system, from the days when a small blob of plastic with three legs was almost certainly a transistor - therefore the "2S" is often omitted from the label. The letter after the 2s denotes (according to Tower's International Transistor Selector, 1980):
2SAxxxx PNP high frequency.
2SBxxxx PNP low frequency
2SCxxxx NPN high frequency
2SDxxxx NPN low frequency

Almost any any  PNP will do the do the job, unfortunately, an NPN is not close enough ;)
 

Offline snipersquad100

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Re: Transistor question
« Reply #9 on: September 14, 2013, 09:43:32 pm »
thanks andy, I'll change it in the morning.  :-+

Offline casper.bang

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Re: Transistor question
« Reply #10 on: September 15, 2013, 09:11:28 am »
Does it look like one of these:
http://www.ebay.com/itm/300589082539

?
 
... I only had a c557b. Both are pnp's but are they the same?
I cant find a datasheet for the c5557b.
Try prefixing the number with 2S, i.e look for 2SC557B.
My ancient and crusty transistor data book says that the 2SC557 is an NPN. You could verify its polarity by ohms/diode test with you multimeter.

Negative; 547/548 are NPN and 557/558 are their PNP counterparts (first bipolar transistor pairs people gets to play with in the GMT/UTC part of the world).
 

Offline snipersquad100

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Re: Transistor question
« Reply #11 on: September 15, 2013, 08:15:55 pm »
Thanks Andy, you were right. The c557c is a npn.  I replaced it with a pnp and the relay is clicking on. Now im getting 0 to 32 volts. However the relay should kick in at around 12v to bring in the 30v supply tap but its coming in as soon as I switch on the power supply.
Not a problem thou 0 - 32 is better than 0 - 18.
 Thanks again for your help.


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