Share defend encyclopedia of yuan of understanding of parts of an apparatus in circuit
[introduction] the ceaseless progress as the society, the development of content couplet net, setting applies outdoor electronic product, continuously fast growth, electronic product received extremely wide application, no matter be common cause, still be business is used civil perhaps, had arrived deep each domains, of environment of the diversification that this also caused product function, application complication.
As ceaseless progress of the society, the development of content couplet net, setting applies outdoor electronic product, continuously fast growth, electronic product received extremely wide application, no matter be common cause, still be business is used civil perhaps, had arrived deep each domains, of environment of the diversification that this also caused product function, application complication. As product function increasing, its function interface is richer and richer also, for instance: Network interface (take POE function) , imitate video interface, frequency interface, call the police interface, RS485 interface, RS232 interface is waited a moment.
Communication product is in applied process, the overvoltage that forms as a result of the reason such as be struck by lightning and cross electric current to be able to be caused to equipment port damage, because this ought to be designed,defend accordingly circuit, the different place that type of environment of market of position of its product breed, network, target, application, signal and implementation become each port basises to wait for a variety of elements originally defends circuit also differs correspondingly.
1, gas discharge tube
Graph the principle graph symbol of 1 gas discharge tube
Gas discharge tube is parts of an apparatus of a kind of switch protection, working principle is gas discharge. When polar a voltage is enough big when, extremely puncture of clearance general discharge, by position of former electric conduction of insulation condition translate into, similar short circuit. Below electric condition polar a maintained tension is very low, be in commonly 20 ～ 50V, because this can have the effect to circuit of the class after protection. The main index of gas discharge tube has: Answer voltage of puncture of time, dc, concussion puncture voltage, shed capacity, insulation resistance, extremely capacitance, follow on current shuts off time.
Of gas discharge tube answer time to be able to achieve hundreds Ns down to to count Ms, in protecting parts of an apparatus, be the slowest. When the short circuit of puncture of gas discharge tube in making prevent Leiqi when the overvoltage of be struck by lightning on wire cable, initiative puncture voltage is the concussion puncture voltage of gas discharge tube basically, discharge canal puncture guides after connecting polar maintaining voltage drops 20 ～ 50V; On the other hand, a flow-rate ratio of gas discharge tube pressure quick resistor and TVS canal want big, the protection such as gas discharge tube and TVS should make cross electric current for the most part to carry release of gas discharge tube when parts of an apparatus is operable.
Because this gas discharge tube is used at defending commonly the foremost class of circuit, ever since of class defend You Yamin resistor or TVS canal comprise circuit, the noise of these two kinds of parts of an apparatus is in season very fast, the protective effect of circuit of the class after be opposite is better. The insulation resistance of gas discharge tube is very tall, can reach the volume level of thousand ohm. Extremely the value of capacitance is very little, it is commonly under 5pF, extremely leakage current is very small, for NA class. Because this gas discharge tube receives online grade,go up what to won’t form basically to circuit to affect.
The follow on current of gas discharge tube is shut off is an issue that design circuit needs to consider mainly. The place before be like is narrated, gas discharge tube is in commonly in maintaining voltage of the follow on current below electric condition 20 ～ 50V, when applying in circuit of direct current source, if two space voltage exceeds 15V, can not apply discharge to be in charge of directly in two space.
When be used in circuit of 50Hz alternating current source, although alternating voltage has had at 0 o’clock, the follow on current that can realize gas discharge tube is shut off, but gas discharge tube kind parts of an apparatus is in after passing electric puncture for many times, its follow on current shuts off ability to will be reduced greatly, after be being used for a long time, pass to also cannot realize follow on current at 0 o’clock in what communicate circuit shut off; If electric current and voltage are phasic,still be being put in a kind of circumstance is abhorrent, bring about follow on current possibly also to cannot be shut off.
Because this is in,the look line of circuit of alternating current source is opposite line of protective ground wire, look is right gas discharge tube is used alone between 0 lines and look line improper, use single-phase power supply when the equipment that use phone and line of the look in cannot assuring to apply actually and central line are nonexistent when receiving the possibility that turn over, central line also is improper to protecting ground wire to use gas discharge tube alone, right now use gas discharge tube needs and press quick resistor series connection. In the look line to central line protection in circuit of alternating current source do not use gas discharge tube basically.
In the design of the circuit that prevent thunder, the dc puncture voltage that should pay attention to gas discharge tube, concussion puncture voltage, connect those who spread the parameter cost such as the capacity to choose. Install the discharge on common communication circuitry to be in charge of, cannot move inside the fluctuant limits that asks it runs voltage to reach its to allow normally in circuit make, criterion its dc spark voltage should satisfy: Min(ufdc)≥1.8UP. The Ufdc in type sheds puncture voltage continuously, min(ufdc) represents the least value of dc puncture voltage. UP runs the peak value of voltage normally for circuit.
Main applicable is in gas discharge tube the right protection of line of look of mouth of alternating current source, central line; Shed the protection between RTN and protective ground continuously; The protection of signal opening line to the ground; The protection of line of core of line of make a present of of mouth of day make a present of to screen layer.
The failure mode majority of gas discharge tube is open circuit below the circumstance, when because circuit designs reason or other element to bring about discharge,burning out, also can cause the failure mode of short circuit. Service life of gas discharge tube is opposite shorter, the function after be being pounded for many times can drop, at the same time other discharge canal is being used for long can have flat be no longer in force the circumstance of this kind of natural be no longer in force, the existence after because this prevents Lei Qi by what gas discharge tube forms,be being used for long is safeguarded reach changing issue.
2, pressure quick resistor
Graph 2 pressure quick the principle graph symbol of resistor
Pressure quick resistor is parts of an apparatus of a kind of voltage limiting protection. Use pressure quick of resistor nonlinear characteristic, go out to be pressed now when overvoltage quick of resistor polar, pressure quick resistor is OK voltage clamp be worth relative to fixed voltage to, realize the protection of circuit of the class after be opposite thereby. Pressure quick the main parameter of resistor has: Pressure quick voltage, connect when shedding capacitance of capacity, knot, noise to answer etc.
Pressure quick the noise of resistor is in season a class that it is Ns, faster than air discharge canal, a bit a few slower than TVS canal, the overvoltage that is used at electronic circuit usually protects his to answer speed to be able to satisfy a requirement. Pressure quick the knot capacitance of resistor is in commonly the hundreds of quantitative class limits to thousands of PF, unfavorable and immediate application is below a lot of circumstances in the protection of circuitry of high frequency signal, when applying in the protection of alternating current road, because of its knot capacitance relatively congress increases leakage current, defend need when circuit in the design mature. Pressure quick of resistor shed a capacity bigger, but smaller than gas discharge tube.
Pressure quick of resistor pressure quick voltage (Min(U1mA) ) , connect when circuit is designed, shedding a capacity answer to consider mainly. In dc loop, ought to have: 2)Udc of Min(U1mA) ≥(1.8 ～ , the Udc in type is the dc in loop rated job voltage. In communicating loop, ought to have: 2.5)Uac of Min(U1mA) ≥(2.2 ～ , the Uac in type is the communicates working voltage virtual value in loop.
Afore-mentioned principles extraction a cost basically are to assure to press quick resistor is in in power source circuit when application, proper safe abundant is spent. When signal loop is medium, ought to have: Min(U1mA)≥(1.2 ～ 1.5)Umax, the Umax in type is the peak value voltage of signal loop. Pressure quick of resistor shed a capacity to answer according to preventing the design of thunder road index is decided. Generally speaking, pressure quick of resistor shed a capacity to want to be more than those who be equal to the circuit that prevent thunder to design to connect shed a capacity.
Pressure quick resistor basically can be used at line of direct current source, alternating current source, low frequency the road, day make a present of that takes feed circuit.
Pressure quick the failure mode of resistor basically is short circuit, pass when what pass when electric current is too big, create a powerful person or family possibly also piece be cracked by scamper and open circuit. Pressure quick resistor service life is briefer, the function after be being pounded for many times can drop. The after preventing Lei Qi to be used for long presence that accordingly You Yamin resistor makes is safeguarded reach changing issue.
3, transient state of voltage clamp bit-type restrains diode (TVS)
Graph 3 TVS provides principle picture
TVS (Transient Voltage Suppression) it is parts of an apparatus of protection of a kind of voltage limiting, action and pressure quick resistor is very similar. Also be those who use parts of an apparatus is nonlinear character general overvoltage clamp the protection that is worth circuit of the class after implementation is right to an inferior voltage. The main parameter that TVS is in charge of has: Retrorse puncture voltage, most tongs capacitance of power of voltage, instant, knot, answer time to wait.
Of TVS answer time to be able to reach Ps level, it is the fastest in parts of an apparatus of protection of voltage limiting surge. When the overvoltage that is used at electronic circuit is protected, its answer speed to be able to satisfy a requirement. The knot capacitance basis that TVS provides of workmanship different, can divide roughly for two kinds of types, loud ties capacitance TVS is in commonly the quantitative class of thousands of PF of hundreds of ～ , the knot capacitance of small knot capacitance TVS is in commonly the quantitative class of a few PF of ～ of a few PF. The knot electric capacity of TVS of general component type is higher, the watch sticks two kinds of types in type TVS canal to have. In the protection of circuitry of high frequency signal, answer to basically choose the TVS canal of small knot capacitance.
What TVS is in charge of is nonlinear character comparing is pressed quick resistor is good, when should passing what TVS is in charge of to cross electric current to increase, the clamp that TVS is in charge of voltage ascendant speed is compared pressure quick resistor is slow, because this can be obtained than pressing quick the residual voltage output with idealer resistor. In the electronic circuit that protects subtly in a lot of need, applying TVS to be in charge of is better choice. TVS is in charge of connect shedding a capacity is the smallest in protector of voltage limiting surge, use at commonly most the careful protection of end class, because its know flow small, general need not the protection at circuitry of alternating current source, when the circuit preventing thunder of direct current source uses TVS to be in charge of, return commonly need and pressure quick a parts of an apparatus of generous drifting a look such as resistor cooperate to use. TVS canal facilitates compositive, suit to be used on veneer very much.
Another advantage that TVS has is can choose parts of an apparatus of one-way or two-way protection neatly, in the signal circuit of odd polarity and circuit of direct current source, choose one-way TVS canal, can obtain lower residual voltage.
The retrorse puncture voltage of TVS, connect when circuit is designed, shedding a capacity should consider mainly. In dc loop, ought to have: Min(UBR)≥(1.3 ～ 1.6)Umax, the UBR in type is the retrorse puncture voltage of dc TVS, umax is the voltage peak value in dc loop.
The protection preventing thunder that TVS canal basically can use at circuitry of circuitry of direct current source, signal, day make a present of.
The failure mode that TVS is in charge of basically is short circuit. But pass when what pass when electric current is too big, cause TVS possibly also the canal is cracked by scamper and open circuit. The service life that TVS runs is relatively longer.
4, voltage switch transient state restrains diode (TSS)
The principle graph sign that graph 4 TSS provides
Voltage switch transient state restrains diode (TSS, thyristor SurgeSuppressor) as identical as TVS canal, also be the parts of an apparatus of voltage limiting protection that uses semiconductor technology to be made, but its job principle and gas discharge tube are similar, and with pressure quick resistor and TVS canal are different. When the overvoltage that manages two end when TSS exceeds the puncture voltage that TSS is in charge of, TSS is in charge of will overvoltage clamp to lower than puncture voltage immediateness on the level of 0V, TSS canal lasts later condition of this kind of short circuit, cross electric current to fall till what had shed TSS canal after critical value is the following, TSS regains open circuit position.
TSS is voltage switch transient state restrains diode, emerge namely restrain transistor, perhaps call conductor discharge canal, solid puts fulgurite to wait a moment. The brand such as LangTuo. TSS canal is the protective parts of an apparatus that uses semiconductor technology to be made, basically use at the protection preventing thunder of signal circuit. Cannot use in power source port. Of TSS parts of an apparatus stream the capacity is general and highest can achieve 150A (8/20uS) .
TSS canal and TVS canal are the parts of an apparatus of voltage limiting protection that uses semiconductor technology to be made, TSS canal is voltage switch model. TVS is voltage clamp bit-type. TSS canal is answering time, knot capacitance respect and TVS canal are identical characteristic, make a watch stick parts of an apparatus easily, suit to be used on veneer very much. TSS canal agrees with the signal circuit protection with signal higher n.
TSS canal is answering respect of capacitance of time, knot to have as identical as TVS canal characteristic. Make a watch stick parts of an apparatus easily, suit to be used on veneer very much, after TSS provides an action, be worth overvoltage from puncture voltage around next pulling go to the level that approachs 0V, at this moment the knot of diode is pressed fall small, use at signal n so taller circuitry (for example: Imitate subscribers feeder, ADSL) flow-rate ratio TVS is connected when protection the canal is big, protective effect also has been been in charge of than TVS. TSS agrees with the protection of the signal circuitry with signal taller n.
An issue that needs an attention when use TSS is in charge of is: TSS canal is in below overvoltage action after puncture, when the electric current that has shed TSS canal the value falls after critical value is the following, TSS canal just regains open circuit position, when because this TSS canal is in line means,be being used, the normal electric current of line means should be less than the critical extensive telegram in reply that TSS provides to flow. Those who close is different and the type that critical extensive telegram in reply sorts a value to be in charge of along with TSS and design application field are different, when using, should notice to prove the exact value of date of place appliance bodily form in manual of parts of an apparatus.
The puncture voltage that TSS is in charge of (Min(UBR) ) , connect when circuit is designed, shedding a capacity should consider mainly. When signal loop is medium, ought to have: Min(UBR)≥(1.2 ～ 1.5)Umax, the Umax in type is the peak value voltage of signal loop.
TSS provides the more application protection preventing thunder at line means.
The failure mode that TSS is in charge of basically is short circuit. But pass when what pass when electric current is too big, cause TSS possibly also the canal is cracked by scamper and open circuit. The service life that TSS runs is relatively longer.
5, temperature coefficient thermal resistor (PTC)
PTC is parts of an apparatus of protection of a kind of current limliting, it has temperature of a movement to be worth TS, when the temperature inside its noumenon under TS when, its block value is maintained basic and constant, at this moment block value calls cold resistor. When noumenon of temperature coefficient resistor when TS of that temperature prep above, its block value increases quickly, the value of the largest block that can reach can be hit than cold resistance too 104 times the left and right sides. Because its block value can follow temperature to increase elevatory and quickly, so general series connection uses as on the line of transient big electric current shed protection too.
PTC response rate is slower, be in commonly millisecond class above, accordingly its nonlinear resistor character crossed electric current not to produce effect originally through time base in be struck by lightning, the normal resistor that can press it only (cold resistor) come estimation its current limliting action. The action of thermal resistor more body are touched in line of such as power now touch wait for the circumstance that appears to shed protection for long too, commonly used in the protection at subscribers feeder road.
At present PTC basically has PTC of high polymer material and PTC of pottery and porcelain two kinds, among them the overvoltage of PTC of pottery and porcelain is able to bear or endure the PTC that suffers ability to compare high polymer data is good, but the PTC of high polymer material answers speed faster than PTC of pottery and porcelain. Normally PTC of pottery and porcelain cannot realize low block value, what the PTC that low block is worth all uses is the material of high polymer.
6, switch of insurance canal, fuse, air
Switch of insurance canal, fuse, air belongs to protective parts of an apparatus, use at equipment in-house occurrence short circuit, flow too wait for breakdown circumstance to fall, can disconnect the short circuit load on circuit or shed load too, prevent electric fire and the safe character that assure equipment.
Insurance provides the protection that is used on veneer commonly, switch of fuse, air can be used at overall protection commonly. The use that simple introduction is sure to be in charge of below.
Be in charge of to be being discharged by air on power source circuit, pressure quick canal of resistor, TVS comprises defend circuit, must deserve to insurance canal undertakes protective, produce safe problem in order to prevent the after defending circuit damages facility inside equipment. Graph two 5 example that gave out to be sure to apply, defend in A circuit among them circuit and advocate loop is common an insurance, should defend when invalidation of circuit short circuit advocate loop power supply can disconnect at the same time, in B circuit advocate loop and defend circuit has respective insurance, should defend defend the insurance of circuit disconnects when circuit invalidation, advocate loop still can work normally, but right now when port reappear overvoltage, port the damage that because lose,may defend and causes in-house circuit. Two kinds of circuit have advantages and disadvantages each, can choose according to need in designing a process. The necessity that the protection of circuitry of the signal circuitry that does not have feed, day make a present of uses insurance pipe is not big.
Graph two example of 5 insurance application
The character of insurance canal basically has: Rated electric current, rated voltage. Among them the branch that rated voltage has dc and communication.
Tag the voltage rating on frit silk to express this frit to silk is equal to in voltage or be less than its in the circuit of rated voltage completely OK ground of on the safe side interrupts its rated short-circuit current. Voltage rating series includes to be in N. E. In C regulation, and a requirement that also is insurance business lab, as the safeguard that prevents fire hazard. To silk of frit of most small size and miniature frit silk, frit silk manufacturer people rated voltage is adoption level 32, 63, 125, 250, 600V.
Wraparound and character, frit silk can be in be less than its to be used below any voltage of rated voltage and do not damage its fusing character. Defend the insurance in circuit is in charge of, appropriate chooses explosion proof slow fusing to be sure to be in charge of.
7, inductance, resistor, lead
Itself of inductance, resistor, lead is not protective parts of an apparatus, but defend in what combination of parts of an apparatus of many different protection forms in circuit, can have assorted effect.
Defend in parts of an apparatus, the characteristic of gas discharge tube is a discharge big, but answer time voltage of puncture of slow, concussion is tall; A flow that TVS provides is little, answer time the fastest, voltage clamp characteristic and best; Pressure quick the character of resistor uprights at this both between, defend when circuit asks whole knows flow big, when can realizing careful protection, defend circuit often needs these a few kinds to defend parts of an apparatus cooperates to rise the protection with better implementation is characteristic.
But these defending parts of an apparatus cannot simple paralell connection rises use, for example: Will connect discharge to be pressed greatly quick resistor and connect the TVS with little flow to provide direct parallel connection, had been in below voltaic action, TVS canal can produce damage first, cannot develop pressure quick resistor opens the dominant position with large flow. Because this defends in a few kinds parts of an apparatus cooperates assorted occasion, often need inductance, resistor, lead to wait to defend undertake assorted between component in what differ. These a few kinds of component face to undertake introductory respectively below:
Inductance: In series connection type direct current source defends in circuit, cannot have on feed line bigger pressure fall, accordingly extremely of circuit cooperate to be able to use hollow inductance, pursue as follows:
Graph 6 with inductance implementation two class defend of parts of an apparatus cooperate
The action that reachs since electric induction: When defending circuit achieves a design to know flow, the biggest passes electric current to should not achieve TVS to be in charge of discharge on TVS, because this inductance need provides the adequate current limliting capability that passes electricity to be struck by lightning.
In power source circuit, a few problems that the design of inductance should note:
1, when inductance coil been sheddinging the full allotment of labour of equipment to make electricity, can work normally and won’t overheat;
2, use hollow inductance as far as possible, the inductance of the core that take magnetism has been in below voltaic action can produce magnetism saturation, the inductance in circuit is measured can be calculated in order to do not have the inductance quantity when magnetism core only;
3, coil should be circled as far as possible make monolayer, do the parasitism electric capacity that can reduce coil so, can enhance coil to be opposite at the same time of transient overvoltage be able to bear or endure get ability;
4, circle the insulation layer that makes inductance coil lead go up to should have enough ply, what encircle in order to make sure in transient overvoltage action gets offline is v/LIT all over the ground do not send produce puncture short circuit.
In power source mouth in defending circuit is designed, inductance takes a value to be 7 ～ 15uH normally.
Resistor: In line means, received component is strung together to want to the inhibition of high frequency signal on circuit as far as possible little, accordingly extremely cooperate to be able to use resistor, pursue as follows:
Graph 7 with resistor implementation two class defend of parts of an apparatus cooperate
The effect that resistor should have and before the function that narrates inductance is basic and same. Above graph is exemple, the extraction of resistor is worth computational method to be: Measures so that air puts fulgurite concussion puncture voltage is worth U1, the I1 of the biggest discharge that checks manual of TVS parts of an apparatus to get TVS is in charge of 8/20us impulse current to fall, and TVS is in charge of highest clamp voltage U2, criterion of resistor the smallest take a value to be: R≥ (U1-U2) / I1.
In line means, a few problems that the use of resistor should note:
1, the power of resistor should enough big, had avoided the resistor below voltaic action to produce damage;
2, use linear resistor as far as possible, the influence that makes resistor is transmitted to normal signal as far as possible small.
Lead: Certain hand in / the full allotment of labour of direct current equipment makes electricity very big, exceed 30A, defend below this kind of circumstance of circuit extremely cooperate to use inductance to be able to appear the question with too big volume, to solve this problem, can will defend circuit cent is two parts, class defends the class after mixing defends before do not design going up with a circuit board, the wire of make a present of that formulary length can use between two class circuit at the same time will do cooperate.
Graph 8 cooperate with what lead realizes two class to prevent parts of an apparatus
This kind of combination forms defend in circuit, provide the function since place of length feed line, the action with inductance is identical, because the inductance quantity of 1 meter of long lead is between 1 ～ 1.6uH, feed line achieves certain length, can rise to coordinate action goodly, the line means of feed line can work according to matching completely voltaic size is chosen neatly, overcame use inductance to do extremely the defect that electric current of very big job cannot have shed on the inductance when cooperating.
8, transformer, smooth Ou, relay
Itself does not belong to transformer, smooth Ou and relay protective parts of an apparatus, but the ability that the segregation character that these parts of an apparatus can use to have in the design of port circuit will come to raise port circuit to fight overvoltage.
Port be struck by lightning in all modular protection design has two kinds of methods:
1, circuit installs voltage limiting protector rightly, when circuit introduces overvoltage of be struck by lightning, voltage limiting protector becomes short circuit condition to put flow passing report the earth;
2, the design on circuit keeps apart component, the circuit that keeps apart component both sides not in all, when circuit introduces overvoltage of be struck by lightning, this instant overvoltage brings to bear on the both sides that keeping apart component. During wanting to keeping apart component in overvoltage action only, segregation component itself is isolated from puncture, and keep apart line of the high pressure before component puncture of incorrect and other low-pressure part, the overvoltage of be struck by lightning on circuit cannot enough translate into crosses electric current to enter equipment interior, the in-house circuit of equipment also got protective.
Need to design the protection that need a standard only on circuit at this moment, defend circuit can simplify greatly. For example the protection of aether net mouth can use this kind of train of thought. The component that can achieve effect of this kind of segregation basically has: Transformer, smooth Ou and relay.
The transformer here basically is to point to those who be used at signal port all sorts of signal transmit transformer. Transformer has commonly first / sub the index of an insulation compression, the concussion compression of transformer is worth (apply to be struck by lightning) can be worth according to dc compression or communicate compression to be worth conversion to come out. Roughly estimation formula is: Concussion compression is worth =2× dc compression to be worth =3× communication compression.
Graph 9 realize segregation with transformer
Graphic representation gives a kind of signal port that combines transformer inside to defend circuit is designed on. When be struck by lightning, device is external on wire cable but pair of ground of the induction in all modular overvoltage action is in of transformer primary and sub between, if pursue 9. As long as first / sub do not produce insulation puncture, the overvoltage on cable of equipment dispatches from foreign news agency won’t translate into crosses electric current to enter equipment interior. At this moment port needs to do the protection that need a standard only, the segregation that uses the parts of an apparatus such as transformer is characteristic, be helpful for simplifying the circuit preventing thunder of port.
Using this kind of method to design those who need an attention is: The insulation compression ability of the component itself such as transformer, smooth Ou and relay should very tall (pound compression to be more than 4kV for example) , issue puncture of very incidental insulation in the action of overvoltage otherwise, cannot rise to improve the function of port compression.
Additional, when the segregation that uses transformer is characteristic, need to notice transformer first / sub have distributinging electric capacity, certain circumstance issues peripheral wire cable to go up in all modular overvoltage can pass distributinging capacitance to arrive from primary coupling sub, in entering in-house circuit thereby, isolate the effect with respect to what destroyed transformer so, because this answers to be chosen as far as possible,contain the first time extremely the transformer of screen layer, it is terminal of transformer screen layer inside veneer ground connection, if pursue,9 are shown.
At this moment the effective insulation compression of transformer became primary with screen ground connection the insulation compression between end is worth. Use in all another needs an attention issue that modular segregation designs is circuit of the other on primary circuit and veneer, ground print a line to answer on veneer depart leaves, have enough insulation space. General, two of 1mm of apart of the brim on printed board print a line, ability suffers 1.2/50us to pound voltage 4kV left and right sides.
In defend be opposite in the process of type selecting of parts of an apparatus of circuit gas discharge tube, pressure quick the switch of canal of resistor, thermal resistor, insurance, fuse, air parts of an apparatus that wants to choose to have safe attestation.