![\"20130512-190527.jpg\"](\"https:\/\/i0.wp.com\/www.bartola.co.uk\/valves\/wp-content\/uploads\/2013\/05\/20130512-190527.jpg\")
<\/a><\/h1>\nWhen using fixed-bias you don’t want to take any risks and damage your output transformer. What happens if your output valve (e.g. a transmitting power valve) makes false contact at the grid or the bias supply fails to start for whatever reason. Well, your valve will conduct fully and probably melting the anode and if not damaging the primary winding of your OT. Both things are catastrophic and you don’t want to try it…too expensive!<\/p>\n
You may think there is a simple solution: a fuse. We all know fuses are\u00a0inaccurate\u00a0and can be dangerous if they don’t blow properly. Also HT fuses are expensive and they don’t come up with a wide range of values.<\/p>\n
Rod Coleman pointed me out time ago to include a classic crowbar protection circuit using an SCR device to shunt the main HT supply and blow a fast fuse securely. This will be very quick and avoid any damage at all at the output stage.<\/p>\n
The circuit is very simple to implement and to tweak to your own requirements. The sensing resistor has to be placed at the return loop of the cathode of the output valve. The sense resistor (R3 below) will develop a voltage which is proportional to the anode\/cathode current. The R4\/C1 RC stage provides a low pass stage to ensure the SCR is not triggered by any small variation in the current. Also there is a 12V zener connected between the SCR gate and cathode to protect it. When the SCR is triggered, then the R2 (a big 7W wirewound one) will shunt more than 500mA instantly and blow the fast fuse (which is not shown below) but is in series between the raw supply and R2.<\/p>\n
I’m using an IXYS high-voltage thyristor which is the CS19<\/a> (800V\/20A). \u00a0Worst case scenario the shunt resistor (R2) will blow away and if you use on of the white ceramic ones, they will not catch fire.<\/p>\n My design is optimised to trigger at 200mA DC. The 4-65a is biased at 100mA and will peak at 170mA maximum. Is quite unlikely that a very low 20Hz tone will have that level as will probably kill my speakers but if that is the worst case scenario, then we can see that a transient signal of period 50ms has to be at least of 220mA peak to trigger the circuit:<\/p>\n Impact in sound is insignificant as there is no current flowing through the gate in normal operation and interaction with the RC capacitor is minimum.<\/p>\n <\/p>\n <\/p>\n <\/p>\n","protected":false},"excerpt":{"rendered":" Protecting your valuable output transformer When using fixed-bias you don’t want to take any risks and damage your output transformer. What happens if your output valve (e.g. a transmitting power valve) makes false contact at the grid or the bias supply fails to start for whatever reason. Well, your valve will conduct fully and … Continue reading “Crowbar protection”<\/span><\/a><\/p>\n","protected":false},"author":1,"featured_media":0,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"_jetpack_newsletter_access":"","_jetpack_dont_email_post_to_subs":false,"_jetpack_newsletter_tier_id":0,"_jetpack_memberships_contains_paywalled_content":false,"_jetpack_memberships_contains_paid_content":false,"footnotes":"","jetpack_publicize_message":"","jetpack_publicize_feature_enabled":true,"jetpack_social_post_already_shared":true,"jetpack_social_options":{"image_generator_settings":{"template":"highway","default_image_id":0,"font":"","enabled":false},"version":2},"jetpack_post_was_ever_published":false},"categories":[150,5],"tags":[137,397],"class_list":["post-1553","post","type-post","status-publish","format-standard","hentry","category-4-65a-se-amplifier-2","category-valves","tag-4-65a-se-amplifier","tag-crowbar-circuit"],"jetpack_publicize_connections":[],"jetpack_featured_media_url":"","jetpack_sharing_enabled":true,"jetpack_shortlink":"https:\/\/wp.me\/p2r2tK-p3","jetpack_likes_enabled":true,"jetpack-related-posts":[],"_links":{"self":[{"href":"https:\/\/www.bartola.co.uk\/valves\/wp-json\/wp\/v2\/posts\/1553","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/www.bartola.co.uk\/valves\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/www.bartola.co.uk\/valves\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/www.bartola.co.uk\/valves\/wp-json\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/www.bartola.co.uk\/valves\/wp-json\/wp\/v2\/comments?post=1553"}],"version-history":[{"count":6,"href":"https:\/\/www.bartola.co.uk\/valves\/wp-json\/wp\/v2\/posts\/1553\/revisions"}],"predecessor-version":[{"id":1562,"href":"https:\/\/www.bartola.co.uk\/valves\/wp-json\/wp\/v2\/posts\/1553\/revisions\/1562"}],"wp:attachment":[{"href":"https:\/\/www.bartola.co.uk\/valves\/wp-json\/wp\/v2\/media?parent=1553"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.bartola.co.uk\/valves\/wp-json\/wp\/v2\/categories?post=1553"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.bartola.co.uk\/valves\/wp-json\/wp\/v2\/tags?post=1553"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}<\/a><\/p>\n<\/a>This will clearly give sufficient headroom to protect the output stage from false triggers. If something goes wrong, the anode current will go up and when 200mA are reached, then the circuit will trigger and cut the HT supply by blowing a 500mA Fast Fuse. A fuse is a very decent price to pay in case of an accident.<\/p>\n