{"id":394,"date":"2020-03-04T23:26:26","date_gmt":"2020-03-04T20:26:26","guid":{"rendered":"https:\/\/tomorrow82.ru\/?page_id=394"},"modified":"2020-03-04T23:26:26","modified_gmt":"2020-03-04T20:26:26","slug":"the-protection-circuit","status":"publish","type":"page","link":"https:\/\/tomorrow82.ru\/?page_id=394","title":{"rendered":"The Protection Circuit"},"content":{"rendered":"\n

Figure 1 shows a simplified version of the driver and output stages \nwith the protection circuit added. Only two of the output transistors \nare shown. Q10 monitors the voltage across R45 and Q11 monitors the \nvoltage across R46. Normally, these transistors are cut off. Excessive \ncurrents in R45 or R46 will cause Q10 and Q11 to cut on. This removes \ndrive signal from the bases of Q14 and Q15 to limit the maximum current \nin Q18 and Q19. R28 through R31, R37, and R38 set the current limit \nthreshold which is a function of both the load voltage and the load \ncurrent.<\/p>\n\n\n\n

\"Figure<\/td><\/tr>
Figure 1. Simplified driver\/output stage with protection circuit.<\/td><\/tr><\/tbody><\/table><\/figure>\n\n\n\n

Figure 2 \nillustrates the the operation of the protection circuit. The figure \nshows the amplifier output current versus output voltage. The unshaded \nregion is the region in which the limiter circuit does not activate. The\n current at which the circuit limits is determined by the output \nvoltage. The figure shows load lines for load resistances of 2, 4, and 8\n ohms, all of which stay inside the unshaded region. The graphs in the \nfigure are based on theoretical equations which require estimation of \nthe threshold voltages at which the transistors and diodes in the \ncircuit turn on.<\/p>\n\n\n\n

\"Figure<\/td><\/tr>
Figure 2. Operating range of the VI limiter.<\/td><\/tr><\/tbody><\/table><\/figure>\n\n\n\n

To prevent \nthe current limit function from operating on fast transients, capacitors\n can be added in parallel with R28 and R29. To suppress parasitic \noscillations in the protection circuit, capacitors can be added in \nparallel with R37 and R38. With all four output transistors, separate \nresistors connect from the emitter of each to the bases of the \nprotection circuit transistors. In a previous version of the amplifier, I\n had a capacitor in parallel with each of these resistors. However, one \namplifier built by a student had a strange problem with the protection \ncircuit that we eliminated by removing two of these capacitors. I never \nreally understood this. The full circuit diagram shows only two \ncapacitors.<\/p>\n\n\n\n

D5 through \nD10 are signal steering diodes. These diodes must be either fast \nswitching diodes or fast recovery rectifiers. If they are not, the \nprotection circuits can be activated on zero crossings of the ac output \nvoltage to cause what looks like crossover distortion on the waveform. \nD11 and D12 protect Q18 through Q20 from voltage transients which can be\n induced at the output by inductive loads if the amplifier is driven \ninto clipping. These diodes are normally reverse biased and have no \neffect on the operation of the amplifier. I once had a student who put \nD11 and D12 on the circuit boards backwards. In this case, there is a \ndirect short circuit between the positive and negative power supplies, \nand the amplifier will smoke. The power supply traces were blown off his\n circuit board.<\/p>\n\n\n\n

When the \nprotection circuit is activated, either Q10 or Q11 can saturate. This \nconnects the collector of Q12 or Q13 directly to the loudspeaker output.\n To protect Q12 and Q13, their maximum current must be limited. This is \naccomplished by and Q8 and Q9 shown in Figure 3. Excessive currents \nthrough R21 and R22 will cause Q8 and Q9 to cut on. This removes the \ndrive signal from the bases of Q12 and Q13 to limit their maximum \ncurrent.<\/p>\n\n\n\n

\"Figure<\/td><\/tr>
Figure 3. Current limiters for Q12 and Q13.<\/td><\/tr><\/tbody><\/table><\/figure>\n\n\n\n

I have had \npeople ask me about omitting the protection circuits from the amplifier.\n Although I don’t recommend this, it can be done by omitting Q8 — Q11, \nR28 — R31, R37 — R40, C17 — C20, and D5 — D8. A fast blow fuse should \nthen be added in series with the output. I recommend a 3 to 5 A fuse. If\n you wish to remove the rotection circuit from an amplifier that has it \ninstalled, all you need to remove is Q10 and Q11.<\/p>\n\n\n\n

I once had\n a student who omitted the protection circuit from his amplifier. One \nchannel would periodically go dead. He would bring the amplifier into \nthe lab and we would find a fuse blown and nothing else wrong with it. I\n suspected that one of the bias diodes (D1 — D4) was cracked and was \nintermittently becoming an open circuit. The student noticed one night \nthat the amplifier went dead when he sat on his bed. Upon investigation,\n he found the loudspeaker wire for one channel went under the carpet and\n under a bed post. The weight of the bed had broken the insulation on \nthe wire and he had an intermittent short circuit on the amplifier \noutput. He was lucky that he only blew a fuse because power transistors \nusually blow faster than fuses. If he had put the protection circuit in \nthe amplifier, he would not have blown the fuse.<\/p>\n\n\n\n

Another \nstudent, who built a higher power version of the amplifier, had an \ninteresting experience with the protection circuit in it. This amplifier\n was the «Double Barrelled Amplifier», which I also published in Audio \nmagazine. The student let his fraternity use the amplifier in their \nchapter room until he graduated. He told me that the amplifier had been \nin use for some time when he noticed a some distortion. He investigated \nbehind the amplifier and discovered that the loudspeaker wires were \nintermittently touching on one channel. He said it had probably been \nthat way for some time because the current had almost eaten through the \nwires, and they were fairly heavy gauge cables. If he had not put the \nprotection circuit on his amplifier, he could have lost some expensive \noutput transistors.<\/p>\n","protected":false},"excerpt":{"rendered":"

Figure 1 shows a simplified version of the driver and output stages with the protection circuit added. Only two of the output transistors are shown. Q10 monitors the voltage across R45 and Q11 monitors the voltage across R46. Normally, these transistors are cut off. Excessive currents in R45 or R46 will cause Q10 and Q11… <\/p>\n

\u041f\u0440\u043e\u0447\u0438\u0442\u0430\u0442\u044c \u0441\u0442\u0430\u0442\u044c\u044e \u0446\u0435\u043b\u0438\u043a\u043e\u043c<\/a><\/p>\n","protected":false},"author":1,"featured_media":0,"parent":0,"menu_order":0,"comment_status":"closed","ping_status":"closed","template":"","meta":{"footnotes":""},"class_list":["post-394","page","type-page","status-publish","hentry"],"_links":{"self":[{"href":"https:\/\/tomorrow82.ru\/index.php?rest_route=\/wp\/v2\/pages\/394","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/tomorrow82.ru\/index.php?rest_route=\/wp\/v2\/pages"}],"about":[{"href":"https:\/\/tomorrow82.ru\/index.php?rest_route=\/wp\/v2\/types\/page"}],"author":[{"embeddable":true,"href":"https:\/\/tomorrow82.ru\/index.php?rest_route=\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/tomorrow82.ru\/index.php?rest_route=%2Fwp%2Fv2%2Fcomments&post=394"}],"version-history":[{"count":6,"href":"https:\/\/tomorrow82.ru\/index.php?rest_route=\/wp\/v2\/pages\/394\/revisions"}],"predecessor-version":[{"id":403,"href":"https:\/\/tomorrow82.ru\/index.php?rest_route=\/wp\/v2\/pages\/394\/revisions\/403"}],"wp:attachment":[{"href":"https:\/\/tomorrow82.ru\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=394"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}