Transistor question - Page 1
Forum section: electronics / beginners

author: snipersquad100
author_rank: Frequent Contributor
timestamp: September 14, 2013, 08:39:49 pm
content: 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

author: Bertho
author_rank: Regular Contributor
timestamp: September 14, 2013, 08:51:28 pm
content: 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?

author: casper.bang
author_rank: Frequent Contributor
timestamp: September 14, 2013, 08:51:44 pm
content: Quote from: snipersquad100 on September 14, 2013, 08:39:49 pmA, 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?

author: snipersquad100
author_rank: Frequent Contributor
timestamp: September 14, 2013, 09:01:20 pm
content: 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.

author: Andy Watson
author_rank: Super Contributor
timestamp: September 14, 2013, 09:16:17 pm
content: Does it look like one of these:http://www.ebay.com/itm/300589082539?Quote from: snipersquad100 on September 14, 2013, 08:39:49 pmTry 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.

author: snipersquad100
author_rank: Frequent Contributor
timestamp: September 14, 2013, 09:19:17 pm
content: yes, the same

author: Bertho
author_rank: Regular Contributor
timestamp: September 14, 2013, 09:29:29 pm
content: If the transistor/relay is not turning on then the comparator is not going low.

author: snipersquad100
author_rank: Frequent Contributor
timestamp: September 14, 2013, 09:31:44 pm
content: do you mean the lm741 on the left side of the schematic, I know the print is a bit small for you to read.

author: Andy Watson
author_rank: Super Contributor
timestamp: September 14, 2013, 09:40:15 pm
content: 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 frequency2SCxxxx NPN high frequency2SDxxxx NPN low frequencyAlmost any any PNP will do the do the job, unfortunately, an NPN is not close enough

author: snipersquad100
author_rank: Frequent Contributor
timestamp: September 14, 2013, 09:43:32 pm
content: thanks andy, I'll change it in the morning.

author: casper.bang
author_rank: Frequent Contributor
timestamp: September 15, 2013, 09:11:28 am
content: Quote from: Andy Watson on September 14, 2013, 09:16:17 pmNegative; 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).

author: snipersquad100
author_rank: Frequent Contributor
timestamp: September 15, 2013, 08:15:55 pm
content: 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.