By changing the values of R33 & R31 to make them a voltage divider you may be able to loose Z31 - at the expense of some temperature variation. What do the Anodes of D31, D32 connect to externally? It seems they form a near short circuit through Q11 Q21, collectors (if turned on) & Q13, Q23 bases. If limited by external resistor then OK, otherwise you may sink excess current into this node. Or is it just a test point? RP On 17/08/05, Russell McMahon wrote: > Criticism sought. > :-) > > A month or so ago I was talking about H bridges. > Object was to drive a motor from 4 x C cells at about 150 mA - 300 mA run > current. > Motor ideally needed 4v+, less still worked within reason. Motor was unhappy > with more than say 6 volts and supply CAN rise to 8v or even 10v when a > mains adaptor is added.. Existing bridge dropped 2v plus across bridge. Also > had no voltage limiting. > Low component cost was an object. Operation on as flat as possible batteries > important. Off current should be under say 10 uA. > Commercial H bridge ICs tended to suffer from higher cost, poor saturation > voltage, too high minimum operating voltage. > > It's worth noting that at the currents concerned (about C/10 for Alkalines) > a battery pack with open circuit or low current voltage of over 5 volts may > drop to 4 volts or less on load - ie internal impedance at this level of > load can be very poor. > > ____________ > > This is what I came up with. > It's more complex than may be expected but not as bad as it feels at 1st > glance. > Original had 6 transistors. This has 9. Plus 3 more in optional brake which > wasn't part of original. > It has been suggested that the voltage regulator may more elegantly be able > to be incorporated in the bridge proper. I agree, but it proved hard to do > while also maintaining essentially zero off current. > > It works superbly. > > Resistor values may not be final values. > > Circuit (14 kB gif) is at > > http://russell.servepics.com/temp/r100b.gif > > Motor Brake at top right is an optional extra. It works very well. R42 is > pivotal for proper operation. 100k values in brake could be lower but easily > met timing needs. > > H bridge proper is at bottom. > Its job is to provide two saturated transistors for the motor drive when > driven on. > It does. > > The "voltage limiter" is in fact a voltage regulator. It's job is to prevent > motor voltage from rising too high when supply is above spec. It does NOT > limit motor current except in its role as a voltage limiter. Rather than > grounding Q33 and Q32 emitter they are taken to effective ground by D31 or > D32 when the bridge is driven on. When the bridge is off the emitters float > and this reduces idle current to zero. The regulated voltage need not be > precise - just not overly high ever. > > The motor brake has two MOSFETS in opposing series. When the bridge is off > the FETs are driven on by R41. Q43 is off. When the bridge is driven on Q43 > is on and the FETs are off. The result is that the motor is shunted by R45 > when the bridge is off and is not shunted at all when the bridge is on. R45 > can be selected to suit required brake severity. > > Q12, Q13, Q22, Q23, Q31 are something like BC337 / 2N2222. > Q31 dissipation needs to be watched. > Q41, Q42 need to be logic FETs to allow low battery voltages. The FETs are > the most expensive components. > > Overall quiescent current is about zero. (Sub uA). > > Q12/13/21/22 are NOT provided with base pulldowns as they have no voltage > applied when the bridge is off. > > R33 is essential for correct voltage regulator action. > R35 is a good idea. > > R42 is needed for good braking in both directions. Without it the floating > load makes FET clamping suspect in one motor direction. > > It would be nice to think that an IC would do this job more easily and > cheaply. It would have to provide low Vsat (under 0.5 v total at say 300 > mA), < 3mA input drive, very low off current (< 10 uA, motor voltage > limiting. Brake is an option which would be nice in an IC. ` > > > Criticism invited. > How would I do it better while meeting the requirements ? > > > > Russell McMahon > -- > http://www.piclist.com PIC/SX FAQ & list archive > View/change your membership options at > http://mailman.mit.edu/mailman/listinfo/piclist > -- http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist