Re: Bipolar Power Supply

[Fred Townsend]

Syd H. Levine wrote: Thanks for that information.  One last question; does it matter if it is a bidirectional or a unidirectional TVS (transorb)?  I am guessing the bidirectionals are necessary for AC circuits, but I am also guessing you can use either on a DC circuit??? You are correct. If a DC type is used it serves as reverse polarity (shunt) protection too. Of course a fuse is now mandatory (required by UL if you care). Fred Townsend ----- Original Message ----- From: Fred Townsend To: Multiple recipients of list CHIPDIR-L Sent: Sunday, September 18, 2005 11:09 PM Subject: Re: Bipolar Power Supply MOVs will handle more power but are much, much slower.  Any inductance in the circuit will oppose the leading edge of any transient. This will allow a MOV enough time to go into conduction so a MOV should be placed after the inductor.  Unfortunately the current surge caused by MOV conduction is sometimes enough to destroy the inductor by magnetostrictive forces.  The transorb is much faster and should be placed upstream of the inductor where it will protect the inductor and capacitor. Bear in mind the current through the transorb is limited by circuit resistance. Usually that resistance is caused by wiring and any fuses (?) present. Once you have estimated the resistance and fuse you can calculate the peak current and have some idea how to size the transorb. Fred Townsend Syd H. Levine wrote: Well, I took a look at TVS devices as well as MOV devices.  I am at a loss to know how to select an appropriate one for an automotive power supply.  Any suggestions would be most appreciated.   Syd ----- Original Message ----- From: Fred Townsend To: Multiple recipients of list CHIPDIR-L Sent: Sunday, September 18, 2005 7:34 PM Subject: Re: Bipolar Power Supply I understand your environment very well. I have logged oil wells. It's brutal. I stand by my assessment that you probably do not need additional filtering but I would not want to bet my professional reputation on it. A risk assessment would show you have temperature extremes; explosive, toxic, gases (H2S, CH4 just to name a few); other explosives; severe, mechanical shock; and difficulty to repair to deal with.  The cost of additional filtering is slight. I would also add transient protection too. Plan for the worst and hope for the best. Fred Townsend Syd H. Levine wrote: This thing is a meter that shows when a casing collar locator (CCL) passes a coupling in an oil well. It is a simple amplifier and a microammeter that always runs off raw vehicle DC just fine. But the part that needs the bipolar supply is an auxiliary board that is a simple comparator circuit driving a Sonanlert annunciator when a collar is detected. This problem arises because the original manufacturer put one of those potted "brick" bipolar supplies in the gizmo that runs off mains line AC power (actually off of a generator or inverter in the truck). The way the thing is rigged, it is profoundly unsafe since explosives are dangling below the CCL. I have to get that line power out of the unit, hence the DC / DC converter. In the abscence of the comparator circuit, I would just run it on the vehicle 12 volt supply, but that dern comparator needs the bipolar supply. Sorry for not being more specific in the original post. Syd ----- Original Message ----- To: "Multiple recipients of list CHIPDIR-L" <CHIPDIR-L@fatcity.com> Sent: Saturday, September 17, 2005 7:19 PM I have no way of determining what will or will not problem because you have not defined the sensitivity of your application. 'Little circuit' doesn't tell me much. However the converter scheme you describe is about as good as it gets. I would make sure that primary and secondary grounds are NOT connected together. Otherwise I would not expect a problem. Fred Townsend Syd H. Levine wrote: I have a little circuit that runs off the 12 volt power supply in a vehicle. Part of the circuit needs a bipolar supply, and I was planning to use one of the cute little DC / DC converters to output maybe 5 watts off + and - 15 volt DC. The converter I have selected has a PI filter on the input, accepts a wide input range (8-18 Volts), and is regulated. My question is whether alternator hash will be a problem, Will I need a choke and capacitor on the front end of this gizmo like we used to put on car radios and the like? Or will the converter do its job OK with raw vehicle DC? Thanks in advance. Syd H. Levine AnaLog Services, Inc. Phone: 270-276-5671 Telefax: 270-276-5588 E-mail: [EMAIL PROTECTED] URL: www.logwell.com -- Author: Fred Townsend INET: [EMAIL PROTECTED] Fat City Hosting, San Diego, California -- http://www.fatcity.com --------------------------------------------------------------------- To REMOVE yourself from this mailing list, send an E-Mail message to: [EMAIL PROTECTED] (note EXACT spelling of 'ListGuru') and in the message BODY, include a line containing: UNSUB CHIPDIR-L (or the name of mailing list you want to be removed from). You may also send the HELP command for other information (like subscribing).